Wednesday, November 27, 2013

On the origin of the thunderstorm symbol (R)

This blog post has a special significance to me, as it is the result of wondering about this for years now and finally getting around to digging in and figuring out the story here.  It has to do with the symbol that meteorologists use on weather maps to represent thunderstorms.  You may be familiar with this symbol--a stylized "R" shape with an arrow:
The symbol has acquired quite a following among meteorologists and it is widely used as a "cool" weather emblem.  The University of Oklahoma School of Meteorology even uses it as a part of their logo:
However, I have long wondered---where did this symbol come from?  Who developed it?  What is it actually trying to represent?  The story, as I found it, is as colorful as I had hoped it might be.

The symbol traces its initial origin back to the 1870s.  The late 1800s were a blossoming time for meteorology as a science.  The advent of the telegraph made it possible for weather observations to be rapidly disseminated across countries and continents and made observations of the large-scale structure of the atmosphere (the synoptic scale) possible for the first time.  In 1870, the War Department created a branch of the US Army Signal Corps to collect and publish weather information, and the precursor to our weather service was born.  They started issuing 2-3 times daily weather maps that summarized the temperature, pressure wind direction and sky cover reported in from major telegraph stations across the US.  Our weather mapping was born.  Several other countries also had developed weather services and each country developed their own way of displaying weather information on the maps.  Each country also had different priorities for what sorts of weather they wanted to show on the maps.

It became apparent quickly that given the large scale of synoptic weather features, international cooperation would be needed to get the full picture of the weather.  As such, a series of international conferences were held in Europe to help set international standards for how weather should be observed and recorded. The narrative that follows here is adapted from a report on one of these conferences (the 1973 International Meteoroloical Conference in Vienna) given to the Royal Meteorological Society by Robert H. Scott.  A digital copy is available through Google Books at this link.

Our attention here first turns to the 1872 International Meteorological Conference in Leipzig.  At this conference, a sub-committee was formed to address guidelines for "Hail, Thunderstorm and Cloud".  One of the questions put before this sub-committee was:

"Is it desirable to introduce for Clouds, Hydrometeors and for other extraordinary phenomena, symbols which shall be independent of the language of particular countries and therefore universally intelligible?"

The sub-committee, formed by E. Ebermayer (Bavaria), H. Schoder (German Empire) and C. Sohncke (German Empire) [along with support from H. Wild (Russia), J. Lorenz (Austria) and E. Plantamour (Switzerland)] addressed this question in an interim year and presented their report on the question at the 1873 International Meteorological Conference at Vienna.

 Their procedure for developing symbols was to poll all the nations that had organized weather reporting and mapping and learn how each of them annotated various weather phenomena.  Most nations simply used the first letter of the word that described the phenomenon in their own language.  For instance, in the US we put "R" for rain and "S" for snow (and that was about it).  Since it was expressly noted in the question for the sub-committee that these symbols should be "independent of the language of particular countries and...universally intelligible", the committee sought out more "iconographic" representations of the weather.  In their report, they present the following two tables, which show all of the different nations' weather symbols and the symbols that the sub-committee proposes to be used (click the image to get a bigger view).
There, at the bottom, we see what I believe is the first publication of the thunderstorm "R".  So where did it come from?  Examining each nation's symbols shows us a likely scenario.  If we look at the column for "thunderstorm", most nations don't bother annotating it.  Of those that do, three (Prussia/Saxony, Wurtemburg and Switzerland) all use the letter "G" (or "Gew") which is the first letter of the German word Gewitter, meaning "thunderstorm".  This doesn't match well with the requirement that the symbols be "independent" of the local language.  Austria uses a double-ended arrow for thunderstorm with a single-ended arrow for lightning. But it's the Russian symbol that is the most intriguing.  It is comprised of two components--a zig-zag line for lightning, which makes a lot of intuitive sense, and then the Cyrillic character г on the left.  If you look up the Russian word for "thunderstorm," it's гроза or "groza".  So I believe that again we have a nation just using the first letter of its native word for "thunderstorm" for the symbol.  I'd hypothesize that the sub-committee decided to take the most stylistic of the symbols (the lightning zig-zag is a really nice, intuitive touch) and merge them together to make our thunderstorm symbol  (Even if it included part of a Russian word). The addition of the arrow looks to come from the Austrians who used the arrow to indicate "lightning".  And, voila! Our thunderstorm symbol is born.

While my focus is on the thunderstorm symbol here, you'll note that many of our other symbols we use were also proposed here.  The filled circle for rain was used by France and Russia (and Austria for mist) as was the asterisk/star for snow.  The filled triangle for hail seems to be a hybrid of the open triangle from the Austrians and the filled squares from France and Russia.  Grauple is an extension of that with an open triangle.  But as to mist, hoar frost and dew?  Those aren't so clear...

Regardless, these symbols were proposed at the meeting, but the total number of categories of how many symbols were needed was debated.  The sub-committee later published a more extensive list of proposed symbols for a wide variety of weather categories.
But there was still debate on how many of these symbols to officially adopt, so the conference as a whole postponed the approval to a later meeting.  It wasn't until the 1891 International Meteorological Conference in Munich that the symbols were finally adopted by the committee.

The US seemed to approve of these standards and responded accordingly.  In 1893, amidst a flurry of reports about the climates of a variety of cities and how to observe thunderstorms and whatnot, the US Weather Bureau (having moved to the US Department of Agriculture in 1890) re-published the list of symbols approved by the International Conference, showing that these are the symbols they would start using.

In the meantime, in 1892 the Weather Bureau launched a project to investigate the possibility of forecasting thunderstorms using rapid telegraph messages.  They tried having local telegraph operators telegraph stations to their east if they were experiencing a thunderstorm with the idea that the stations further east could then warn the local population that the thunderstorm was approaching (we're talking thunderstorm warnings in the 1890s here...which I think is amazing...).  They tried it in three regions--the northeast, the upper midwest, and Ohio--and got reports back on the effectiveness.  The findings are summarized in Bulletin 9 of the Weather Bureau (which also is digitalized on Google Books, though you have to scroll way down to get to Bulletin 9 toward the end of the document).  While some of the reporters felt that these warnings were not practical and didn't give enough lead time (things we still work on to this day), the results were generally encouraging.  (As a side note, it probably helped that they focused on June-July-August across the northeastern quadrant of the country, where linear and quasi-linear MCS storms are the dominant mode of convection.  These lines of storms are pretty coherent and can last a long time--even overnight.  This makes them a bit easier to predict in this way.)  The Weather Bureau seems to have realized after this experiment that it could be useful to include thunderstorm information on their weather maps.

Finally, the US Weather Bureau (which was still issuing daily weather maps) upgraded their map printer from a 'milliograph' stencil-duplicator to a faster and more efficient "chalk-plate" process in 1896 (as described in this interesting paper).  All the maps from that time are archived through the NOAA Central Library Data Imaging Project.  There is a clear change in the style of the maps on June 15, 1896.  Here is the title bar and legend before the change:
No mention of thunderstorms--just sky cover, rain or snow, and by "storm track" they mean the path of the low pressure center over the previous few days.  "Storm Signals" indicate if a station reported storm-force winds.  Here's the title bar and legend after the map change:
It appears they hadn't switched over to the new "Rain" and "Snow" symbols (probably because there remained a conflict with the filled dot for "cloudy".  But we see the addition of a "C.W." annotation for "cold wave" (they didn't have the Norwegian cyclone model yet, so the concept of a "cold front" did not exist yet).  And, in the second line from the bottom, we see our R for thunderstorms reported during the previous 12 hours.  Here's a snapshot of the map on June 15, 1896 showing the reports from the central US, including the first thunderstorm symbols on a Weather Bureau map.
Looks like they had thunderstorms in Dodge City, Kansas, Des Moines and Keokuk, IA and Springfield and St. Louis, MO.  Another batch of storms impacted Milwaukee, WI and Grand Rapids, MI.  And so our thunderstorm reporting began.

I had a lot of fun researching this topic.  There's more to come for sure.  A lot of the other symbols we use were defined at a later time.  For instance, when did the extra "kink" in the thunderstorm symbol get added for "heavy thunderstorm"? And what of all the different symbols for each type of cloud?  There's a history there to be sure.

Monday, November 25, 2013

Strong winds and rain in the east; snow in the northwest?

In my last blog post I talked about the blast of arctic air that was forecast to move across the country last weekend and into this week in association with a cut-off low that would slowly creep across the southern US.  Here we are on Monday and today's 500mb pattern from the ECMWF still shows that cut-off low over Texas and New Mexico.

The colder air and lift that are accompanying this storm have already contributed to snow and ice throughout New Mexico, Texas and Oklahoma.  In fact, the weather map from the Oklahoma Mesonet today shows several sites that are not able to report the wind speed or direction (the red dots on the map below) because the anemometers and wind vanes are frozen!

This storm is expected to  slowly chug eastward over the next few days before ramping things up along the east coast.  Look again at the 500mb chart above.  See that other trough in the northern Great Lakes / northwestern Ontario?  That trough is forecast to merge with the cut-off low sometime during the next few days.  One one hand, this means that the cut-off low will no longer be cut-off---back in the normal flow, it will accelerate and move on out of here.  Before it does, though, the cold air it's bringing behind it is going to run into the warm Atlantic Ocean, and that means rain--heavy rain--for the eastern US.  Here's the ECMWF forecast rainfall around 15Z Wednesday morning:

Heavy rain up and down the east coast on a day when lots and lots of people will be travelling.  Not good.  Fortunately by Thanksgiving day itself, it appears that the precipitation will be on its way out (as this storm accelerates off the coast).  Here's the ECMWF precipitation forecast for Thursday morning at 15Z:
Only light precipitation for parts of the northeast.  A lot of this near the lakes may be lake effect snow as strong northwesterly winds will occur behind this storm.  See the tight gradient in the blue contours on the above map?  Those are the surface pressure contours, and the tighter the pressure gradient the stronger the winds.

This brings up another question that I know many people are dying to hear the answer to--will the Macy's Thanksgiving Day parade balloons be able to float with this weather?  I'm told that if the winds gust higher than 35 mph then the balloons are a no-go for the parade.  So what are the models saying?

It's probably going to be close.  Our models mostly forecast sustained winds, that is, the average wind speed over a certain amount of time (usually a few minutes).  Wind gusts are instantaneous wind speeds, and the wind gusts can be significantly higher than the sustained winds.  Here is the ECMWF sustained wind forecast over New York City for 15Z on Thursday while the parade is going on:

You can see the winds really pick up speed off the coast.  But over the city itself, the bluish-greenish colors indicate winds of 15-18 knots or 17-21 mph.  Again, those are the sustained winds--gusts will be higher.  The GFS and NAM have slightly stronger sustained winds, getting up to more like 25-28 mph--again, with higher gusts.  So, as of right now, it's going to be borderline.  We'll have to watch the model runs as we get closer to get a better idea of how strong the winds are going to be.

And finally, just as a teaser for everyone out here in the Pacific Northwest, the really long range models for next Sunday-Monday have a strong cold front coming through in association with a sharp trough coming down the Pacific coast from Alaska.  Here's the ECMWF 156 hour forecast (!) of 850mb heights and temperatures for next Sunday.

A blast of cold air headed our way.  The ECMWF has some low-level instability following this cold front and is throwing out some convective snow showers in its wake.  Though we should never, ever believe models this far out into the future, for fun here is the ECMWF snowfall accumulation forecast for next Monday morning:
Two inches of snow in the lowlands east of Everett?  Eh...maybe.  Don't take my word for it yet.  But it's definitely something to keep an eye on...

Wednesday, November 20, 2013

From calm to cold with cut-off lows

Looks like we're in for a major synoptic pattern change over the next week or so.  This morning's 12Z analysis from the ECMWF model at 500mb finds us with a nice zonal (east-west) pattern.  There are a few minor shortwave troughs but nothing very large in scale.
By tomorrow morning (Thursday), that is forecast to change.  The weak troughing over the eastern Pacific and western US is forecast to deepen rapidly into a strong trough.
And by Friday morning, the trough has deepened to become cutoff low--completely separated from the main jet stream winds (shown by the enhanced colors on the map) that are blowing to the north.
It's pretty amazing how over the course of 48 hours the upper-level pattern can change so quickly.  As I noted in previous blog posts, cut-off lows like this tend to stick around for a while.  Because they are separated from the main jet stream winds, there's nothing to really force them to move along from west to east.  So they usually just sit there.  This low will hover around for just a few days though, before transitioning back to an open-wave trough by next Monday as seen here:
But then, yet another cut-off low is forecast to re-develop off the west coast by next Wednesday!
I should also note that this particular upper-air pattern forecast for next Wednesday is bad news for the east coast--you have a deep trough covering the northeastern quarter of the country and then another shortwave trough racing through the southeast US and about to emerge over the Gulf Stream.  Nor'easter type storms are often born with this sort of setup as cyclogenesis occurs off the Carolinas.  This is still a long way out in the forecasts (168 hours), but it's something to watch...

Another aspect of this cut-off low moving across the US this week into next is that it will open the door for colder air currently over Canada to sneak down into the US.  Compare this morning's 12Z surface temperature analysis from the GFS:
With the forecast for Friday morning's low temperatures:
Much colder air moving in.  Pretty strong front at the leading edge of it too.  There are chances for some severe weather tomorrow as the front crashes through the plains, followed by a round of wintry precipitation. We're going from tornadoes this past Sunday to winter weather by the end of the week.  Such is autumn...

Monday, November 18, 2013

Severe, tornadic storms and upper-level forcing

Just thought I'd quickly share this one screen capture I grabbed yesterday during the unusual November tornado outbreak over the Midwest.  I find myself more and more often using Weather Underground's "Wundermap" tool to overlay weather model output on current observations, satellite and radar to try and both see how well the model is doing and make connections between model features and what's actually happening.  Here we see the composite radar image over the continental US overlaid on top of the ECMWF 300mb forecast for around 1 PM CST on Sunday.  The 300mb wind speeds are shown in color with the 300mb height lines shown as the white contour lines.

You can really see how these lines of convection in the radar imagery formed along the exit region of the upper-level jet streak.  This is a favorable location for divergence aloft, particularly when the jet streak has cyclonic curvature as it does here.  Furthermore, the surface cold front associated with this storm was actually back over central Illinois at this time.  It was upper-level support out ahead of the cold front---this strong jet streak coming around the base of the trough---that provided a lot of the forcing to get this convection going.  Of course, once these storms organize into line segments they can be self-reinforcing in generating enough lift to keep themselves going.  But once again, this emphasizes how understanding what's going on in the upper-levels (something our models forecast fairly well) can be both useful and important for understanding when and where convection is going to develop (something our models don't do nearly as well).

Thursday, October 3, 2013

Snow, severe weather and a tropical storm...all in the next few days

It's an active weather period for the continental United States.  Our relatively tranquil summer weather pattern has been shattered in the past month as the flow has become far more amplified. Autumn is here.  Take a look at this morning's 500mb analysis from the University of Wyoming:

A fairly deep trough is digging through the intermountain west.  As this trough begins to cross the Rockies today, strong pressure falls are forecast in the lee of the mountains in the central high plains.  Take a look at this morning's 12Z NAM surface analysis:

And then the forecast for tonight at 00Z:

The low pressure center in the central US is forecast to deepen and pressure gradients are on the increase.  This means stronger winds.  In addition, you can see that there is a sharper contrast between the warmer air to the southeast of the low and the cooler air to the northwest.  This is showing us frontogenesis---the strengthening of horizontal temperature gradients into sharp fronts.  This points to a continually deepening cyclone, as mid-latitude, extratropical storms like these derive their energy from strong temperature gradients.

What does this mean for weather?  The colder air and northeasterly (upslope) winds on the northwest side of the low point to snow for Colorado, Wyoming and the northern Plains.  In Colorado, that's not very good for a place still recovering from devastating floods less than a month ago.  As the storm moves east over the next 48 hours, that low is really forecast to deepen.  Here's the NAM forecast of 500mb heights and 1000-500mb thickness for Saturday morning:

You can see really tight height gradients around that upper-level low, indicating strong winds aloft.  These winds are going to drive severe weather chances on Friday into Saturday.  Furthermore, strong winds on the back side of the low combined with ongoing snow should deliver blizzard conditions to the northern plains.  However, I showed the thickness map to point out that most forecasts still keep the lower atmosphere too warm for significant snowfall as this storm moves away from the Rockies.  That solid blue line off on the northern fringes of the map is the 5400m thickness line, usually a good indicator of the rain-snow divide.  That's well to the north, though there are so colder pockets near the low.  I'm not expecting major snow with this in the midwest.

However, severe weather is definitely on the ticket.  The SPC has slight risks for severe weather out for parts of the central plains and into the midwest for today and tomorrow (with a "see text" on Saturday) and even a moderate risk for Iowa tomorrow:

Lots of thunderstorms are expected, and with a height gradient like you see in the thickness map above, there should definitely be enough wind shear to support severe weather.

Finally, not to be outdone in this lackluster tropical year, we have a tropical storm (Karen) that has developed in the Gulf of Mexico.  Unlike pretty much every single tropical storm that has formed this year, the upper-level conditions are marginally favorable for development of this storm as it drifts north, though there still is great uncertainty as to where the storm will make landfall and how powerful it will be.  It's looking to hit either as a strong tropical storm or a weak hurricane at this point.  Here's the HPC forecast track:

Though there remains disagreement even among our best hurricane models.  The Hurricane-WRF model run from this morning has the storm making landfall in the Florida Panhandle as a strong tropical storm:

However the GFDL hurricane model has the storm making landfall over in Louisiana, again as a strong tropical storm.
We'll have to watch this as it approaches over the next day or so.  It will also be interesting to see what will happen after this storm makes landfall and starts interacting with the trailing cold front from the low-pressure center that's going to bring the snow and severe weather to the central US.  Longer-range forecasts from the GFS hint that by next Monday, the combination of tropical moisture from Karen's remnants and lift provided by that trailing cold front could bring heavy rain to the mid-Atlantic states.  Just what Washington needs...

Thursday, September 26, 2013

Sea-level pressure is horribly complicated

Yes, it's going to rain a lot this weekend in the Pacific Northwest.  Let's get that out of the way first.  If you want more details on that, see Cliff Mass's blog here.  I may post more on that event this weekend as it unfolds.

Right now I want to talk about pressure.  Atmospheric pressure.  This is the first part of a two-part blog series I'm writing on surface pressure observations and how you can help make surface pressure observations better if you own a weather station.  Yes, you.  But to talk about that, we need to get some basics out of the way first.

Atmospheric pressure is one of the more vital observations that we make in meteorology.  We usually think of pressure as the "weight" of all the air in a column above a certain point.  This makes pressure unique among our surface weather observations in that it is strongly connected with the entire depth of the atmosphere.  By observing changes in surface pressure, we're looking at the sum of changes throughout the depth of air above your head.  This makes pressure really valuable--and it shows.  Mariners closely watch their barometers--they know that falling pressure often signals approaching stormy weather while rising pressure indicates clearing skies.  What other single surface observation can so thoroughly describe the changes in the weather?

Mariners have an advantage, though.  They're all on boats at sea-level.  Changes in elevation mean changes in surface pressure.  Here's an example.  This is a map of the raw surface pressure analyzed over the Pacific Northwest:

Pressure values are given in a variety of ways.  The international standard in meteorology is to use a unit called hectoPascals (hPa), which is hundreds of Pascals.  Fortunately, another unit we're more familiar with in the US--the millibar--is exactly equivalent to the hectoPascal.  So, a pressure of 1000 mb is also a pressure of 1000 hPa. Another unit you may be familiar with if you're a pilot or you watch the evening news a lot is "inches of mercury".  I don't use that unit a lot, but it's common among non-meteorologists.  Standard pressure at sea-level is about 1013 mb/hPa or 29.92 in. of mercury. We'll talk more about pressure units in a later blog.

Anyhow, does that surface pressure map above look vaguely familar?  For comparison, here's a map of the land surface elevation over the same area:
The surface pressure pattern looks nearly identical to the elevation map.  By far, the most dominant signal in surface pressure is the signal of terrain.  As you go up higher in the atmosphere, there's less air above you, so the "weight" of the air decreases and the pressure at these higher altitudes is lower.  This poses a bit of a problem for trying to find the weather signal from surface pressures.  The difference between high and low pressure centers in the weather can be on the order of  only a few millibars...maybe tens of millibars for deep lows and strong highs.  Yet we see on the surface pressure map that the pressure change just from going from sea-level to the peaks of the mountains is on the order of hundreds of millibars!  This swamps our weather signal.

So what is a meteorologist to do? We try to filter out the signal of the terrain in some way.  One of the most common ways is to compute the mean sea-level pressure (MSLP), also called "reducing" the pressure to sea-level.  To compute this, we have to make assumptions about the lower atmosphere.  It turns out that the rate at which pressure decreases with height is very predictable, provided that you know the temperature as you go up in height.  If we knew the temperature in the atmosphere between the top of the mountains and sea-level, we could extrapolate what the pressure would be at sea-level if we filled the mountains with air.

The problem is--there is no atmosphere underneath the mountains.  So we can't know what the "air" temperature is between the top of the mountains and sea-level because all we have there is rock.  So how do we get around this?  There are a number of ways.  Some ways involve guessing what the temperature profile would be by using the temperatures along the slopes of the mountains.  Other ways use nearby weather balloon observations launched from lower elevations to help guess what the temperature profile would be.

The most common method, however, is to use an idealized atmosphere.  A long time ago, most of the international community agreed upon a standard "averaged" atmospheric temperature profile to tackle this very problem.  Well, most of the international community.  There are two basic forms: the US Standard Atmosphere  and the International Standard Atmosphere.  From Wikipedia, here's what the US Standard Atmosphere looks like:

The temperature curve is the red line in the middle of the diagram, and you can see how the standard atmosphere changes with altitude. You'll note that for the lower part of the atmosphere there is a constant "lapse rate"--the rate at which temperature decreases with height.  This is about 6 degrees Celsius per kilometer.  Using that, if we know the temperature at the surface of the terrain, we can just keep adding 6 degrees Celsius for every kilometer we go down in elevation until we reach sea-level.  This lets us extrapolate the "average" temperature profile from the terrain elevation to sea-level to let us get our reduced pressure value.

However, the temperature at the surface of the terrain can be influenced by a lot of things--local ground cover, instrument siting, etc.  It seems foolhardy to use the instantaneous temperature measurement at a point to begin this computation. Temperature is a very local phenomenon--it varies drastically over very short distances. Furthermore, temperature varies widely throughout the day right at the surface--it gets cold at night and warm during the day--simply because the land surface is right there.  Yet, once you get above the ground, these wild variations in temperature rapidly die off.  So our temperature we use should be more like the temperature we'd expect at that location if the ground were not right beneath it. Frustration! So now what do we do?

The National Weather Service standard used for airport observations across the country is to take the 12-hour mean temperature at the location and use that for the surface temperature.  This doesn't get rid of any temperature bias due to the instrument's location, but it does help eliminate the wild swings in temperature throughout the day.  The long mean provides a smoother temperature that is a bit more reliable.  They take this 12-hour temperature mean and use the US Standard Atmosphere lapse rate to estimate what the "theoretical" temperature of the atmosphere would be all the way down to sea-level. Knowing this, they then correct the raw surface station pressure reading to a mean sea-level value.  After all this work...finally we get a map that looks like this:

You'll notice that a lot of the terrain is filtered out now.  There are still some hints of it, since all the assumptions made were not perfect.  In particular, you'll notice that over the high terrain of Montana and Idaho there is still an area of lower pressure that seems a bit suspicious.  Those locations have a very high elevation.  The higher elevation means we have a longer distance over which we make all these crazy assumptions about the temperature profile.  Thus, there's a greater chance our assumptions won't work very well.  So, even with this methodology, there's still a lot of problems with mean sea-level pressure over areas of very high terrain. But at least now we can get an idea that there's high pressure off the coast...

Another way of filtering out the terrain is to use a variable called the altimeter setting.  This is commonly used by pilots and all airport observations should give the altimeter setting at their location.  The idea behind altimeter setting is to take the assumptions made in the sea-level pressure reduction one step further--and eliminate the temperature component.  Basically they take the average amount you need to adjust the pressure as a function of the elevation of the station regardless of the temperature, and apply this same correction every time.  No worries about temperature or anything like that.  Different countries use different altimeter equations (surprisingly), but the commonly used one that we use in the US goes something like this:

Altimeter Setting = ((Psfc - 0.3)^(0.190284) + 8.4228807x10^(-5) * Elevation) ^ (1/0.190284)

If we map our altimeter settings, we get a map that looks like this:

Notice that it looks rather similar to the mean sea-level pressure map that we had before--but we didn't have to do any of the fancy temperature estimation.  You just plug the surface pressure and the elevation into the above formula and out comes the altimeter setting.  Because this gets us close to sea-level pressure without the added complication of dealing with temperature, this is the pressure variable I prefer to use in my work.

So there you go--pressure is a powerful variable.  But, trying to get the weather pressure signal separated from the terrain pressure signal is complicated.  In a later blog, I'll talk about how people who have their own personal weather stations need to be aware of surface pressure, sea-level pressure and altimeter setting to be sure their station is working well.

Monday, September 16, 2013

Converting the Flooding Colorado Rain to Snow

The rainfall totals coming out of Colorado over the week have been incredible--upwards of 15" in many places.  One common comparison to "put this into perspective" that I've seen a lot of on TV, online and social media is converting these rainfall totals to equivalent snow depth.  In another blog post, I talked about computing snow ratios--the ratio of snow depth to liquid water precipitation.  One common snow ratio we use for quick, back-of-the-envelope calculations is 10:1--10 inches of snow for every 1 inch of liquid water precipitation.  Thus I've seen a lot of people commenting that the 15" of rain they saw in Boulder would have been 12.5 feet of snow (150")!  While that sounds incredible, could that really happen?  Had this event happened in winter, would this storm really have produced 10 feet of snow in Boulder?

It's extremely unlikely.  There are many factors working against such an event.  First, let's start by looking at the tremendous amounts of moisture associated with this storm.  In my last blog, I talked about how the precipitable water (PWAT) associated with this storm is the highest ever observed in the month of September over Denver.  Here's the annual climatology of average precipitable water values, with the PWAT observed on Thursday evening highlighted.  You can see it was at the maximum observed PWAT value for September.

But what about PWAT values during the winter?  Notice that in December through February, the PWAT values on average are only about 0.25 inches with all-time maxima around 0.5-0.6 inches.  We're currently just about 200% of the normal PWAT for September, so even with the same anomaly in winter that would still work out to be only 0.5 inches.  Certainly a lot for winter, but less than half of the current PWAT.

So why are the PWAT values so much lower in the winter?  Remember that as the temperatures get lower, so do the saturation vapor pressures for water vapor in air.  This means that, by mass, at colder temperatures far less water vapor can be present in air before it starts condensing out.  We often describe this as the air not being able to "hold" as much water at lower temperatures.  You've felt this--even with the relative humidity at 80% on both a hot day and a cold day, the hot day feels far muggier than the cold day.  There's just more water vapor present when the air is warmer.  For snow, we want the temperatures through a good depth of the lower atmosphere to be below freezing, putting more limits on just how high the PWAT values can go.  It's just too cold to have this much water.

Another thing to consider is the matter of what is called precipitation efficiency.  One distinguishing feature of the rain that has been affecting Colorado is that it has had a high precipitation efficiency.  What does precipitation efficiency mean?  It's basically the ratio of how much water is raining out of a storm to how much water is being brought into a storm through advection and evaporation.  If the precipiation efficiency (PE) is 100%, then as much rain is falling out of a storm as is entering it.  If PE is at 0%, then the storm is growing--the cloud is getting bigger--but no precipitation is falling out of it.  PE can theoretically go all the way up to infinite, for a storm that is raining but no longer has any inflow.  Here's a figure from Market et al (2003) showing these different PEs:
These storms in Colorado have had a very high precipitation efficiency--probably close to 100%.  We've had 12+" of rain over 36 hour periods or so in many places with precipitable water values staying very steady around 1.2-1.3 inches over the same period of time.  To get so many inches of rain with these PWAT values the storms have to be very efficient.  One rule of thumb to estimate precipitation efficiency (Scofield et al 2000 and Market et al 2003) is to multiply the average relative humidity from the surface to 500mb by the total precipitable water.  Well, let's look at our Denver sounding from Wednesday evening:
Our dewpoint and temperature are virtually identical up to ~600mb and pretty close above that (aside...we drop below freezing at that point so we have to consider relative humidity with respect to ice...) we are more or less saturated (at 100% relative humidity) all the way up through 500 mb.  With precipitable water around 1.3 inches, the rule of thumb would suggest precipitation efficiency of  130%--probably a bit of an overestimate, but still--very efficient.

This high efficiency is typical of warm rain processes--storms where the rain spends little to no time frozen. Throw in cold processes--including ice and snow--and efficiency tends to drop.  There are a lot of microphysical reasons why this is so, but snowing just is not as efficient of a process.  A study by Hindman et al. (1981) (cited in the Market study mentioned above) showed that for mountain winter storms, typical precipitation efficiencies average between 7%-49%.  Much less efficient than warm rain storms.

So, in summary, if it were cold enough to snow we wouldn't have nearly as much water vapor as this week's Colorado rain storms have had.  And even if we did have anomalously high water vapor to work with, ice and snow precipitation does not occur nearly as efficiently as warm rain precipitation.  We wouldn't be able to capitalize on this moisture.  Unfortunately for Colorado, all this rain had to be rain...

Thursday, September 12, 2013

Epic rainfall and flash flooding in Colorado

Rainfall over the past several days including some extraordinary rainfall last night has led to a critical situation along the Colorado Front Range today.  Unfortunately the forecast for the next couple of days has more rain on the way.  Let's look at what has been going on.

There has been a highly amplified upper-air pattern over the continental US over the past few days. Here's yesterday morning's 500mb analysis from the HOOT site:
 You can see a large, cut-off low centered over the Great Basin region.  Notice where there are lots of height lines (the black contours) close together--off in the upper Midwest and the Canadian Prairies.  That's where the main jet stream is located--the strong, west-east flow that helps guide storms across the country.  The jet stream is well to the north of this low--in fact, the low is comfortably tucked under a very high-amplitude ridge that goes well up into Canada and is centered over the Pacific Northwest.  Here in Seattle this ridge has given us clear skies and record-breaking warmth of the past few days.  Unfortunately that same ridge is keeping the steering flow away from the low over Colorado, leaving it in place.

The green colors in that analysis image show relative humidity at the 500mb level.  There has been a steady stream of moisture being pulled up from the south around this low over the high plains and eastern Rockies.  This moisture can also be monitored in real-time using satellite-derived water-vapor imagery, like this image from last night:
The brighter whites, purples and blues indicate lots of moisture in the upper troposphere.  The CIMSS Satellite blog has an excellent loop of water vapor imagery for this event at their post here.  Water vapor imagery often isn't very good for showing us the low-level moisture; what you're seeing above is mostly moisture aloft.  Checking last night's sounding from Denver we can see that there was far more moisture in the lower levels of the atmosphere:
The dewpoint and temperature profiles are almost right on top of each other from the surface up to around 600mb--that's a very deep layer for the atmosphere to be saturated.  A lot of people have been commenting on how unusually moist this is for Colorado.  One way we estimate the amount of water vapor throughout the entire depth of the atmosphere is through a measure called Precipitable Water (PWAT).  This basically says that if all of the water vapor in the air above your head were to immediately condense into liquid water and fall down as rain, this is how much rain would fall.  On the sounding above, in the lower right corner you'll see the PWAT value calculated as 33.16 mm.  That's 1.3 inches of precipitable water.  For comparison, here is a climatology of the average precipitable water values in Denver throughout the year from the NWS Rapid City page:
You can see in September that the average precipitable water (the 50th percentile) is only about 0.55 in.  Where does 1.3 inches fall?  Above the maximum value (1.25 in.) for September! An extraordinary amount of moisture.

Another thing you'll note on the sounding above is that near-surface winds are out of the southeast.  This promotes upslope flow--easterly winds forced to rise when they meet the mountains.  This constant rising motion in very moist air produces continuous condensation, clouds and rain.  You can see how steady the rainfall was last night in looking at this timeseries of observations from the NCAR Foothills lab on the northeast side of Boulder.

The rain gauge reset itself to zero this morning (hence the big drop in precip down to zero again), but you can see that over just last evening over 6 in. of rain had fallen.  In one evening.  The rainfall was also quite steady over this entire period--no big spurts or jumps in the precipitation that you'd expect if this was due to strong convection.  In fact, not much lightning was reported with this rain (though there was some).  There have been several reports of particular locations in the foothills getting over 8" of rain so far.  This also shows an interesting feature of this event.  Remember our precipitable water above?  It was only 1.3 inches.  If that was the total precipitable water, how could we be getting 6-8 inches of rain?  Remember that there is that stream of moisture constantly being brought up on the south--we saw it on the water vapor imagery.  This keeps replenishing the water vapor in the atmosphere, bringing in more and more moisture as the current moisture rains out.  Still--too get the much rain with these precipitable water values, these clouds have been very efficient at their rain production.

The rainfall overnight consisted of broad areas of stratiform precipitation in some locations, but also (particularly near Boulder) a series of more intense precipitation cells, probably somewhat convective, that repeatedly formed east of town and then moved west over the foothills.  Here's an example radar image from last night:
So what are the consequences of so much water over such a short period of time?  Flash flooding.  Here's the stream gauge data for Boulder Creek at Broadway in downtown Boulder:
There were extreme rises in the stream level overnight.  You'll note that the typical river stage is only a little over 1 foot.  From 6 PM Wednesday through about 3 AM MDT, the creek rose rapidly to just over 7 feet--moderate flood stage.  There was a drop off early this morning as the rain eased a bit, but more rain is developing today and you can see the stream flow on the rise again.  Basically all of the canyons along the northern Front Range hit at least minor flood stage last night and into today.  Here's the gauge data for St. Vrain Creek near Lyons--approaching record levels.  There was also a dam failure last night upstream from Lyons, prompting extensive evacuations.
There continue to be problems with managing this massive amount of water.  Pretty much every road going into the mountains is closed due to flooding or mudslides at this time.  Many streets in Boulder itself are impassible due to high water.  There are evacuations underway in the Big Thompson River canyon (the location of an infamous flood in 1976--which had distinctly different meteorological origin, being due to a parked convective storm as opposed to widespread, more stratiform rain).

Unfortunately it doesn't look like it's time to take a breath just yet.  The rain died down a little overnight, but it's forecast to pick up again today.  We're already seeing that throughout the region.  Here's the latest High-Resolution Rapid Refresh model's forecast for total accumulated precipitation through tonight.
It's predicting another 1-2 inches over the northern Front Range.  It's also alarming to note the 3-5 inch accumulations suggested for the mountains behind Colorado Springs and Pueblo, suggesting an additional flash flooding threat further south.

Friday, July 12, 2013

A retro upper-level low!

This isn't a "major" weather event or anything like that...but it is rather unusual.  Let's take a look at the current 500mb analysis from this morning, courtesy of the HOOT site:

We'll focus on the black contours for now--those are height contours, telling us the height of the 500mb pressure level above sea level.  The green shadings here indicate where there is a lot of moisture, but that's not as important for what I'm looking at.  The basic flow pattern shows a shallow trough over the Pacific Northwest (which is making is more cloudy than sunny in Seattle right now), a big ridge over the central US and a cut-off low over the upper Ohio River valley.  Not entirely unusual...we're seeing some rain along the east coast in association with that low, but otherwise nothing too odd.

Let's see what our models are doing with this weather pattern over the next few days.  Here's the GFS 24-hour 500mb height forecast for tomorrow morning.  Now we've switched to where the color shadings are vorticity, but the black contours are still the height of the 500mb surface.

Mostly the same pattern, but the west coast trough has dug in a little more and the cut-off low over the Ohio River valley has drifted a little to the southwest even more.  Let's fast-forward another 24-hours to Sunday morning's forecast:

Now is where things really are getting unusual.  That cut-off low is moving rather steadily to the west!  Normally our troughs in the mid-latitudes move from west to east--not from east to west.  But here we have a relatively strong cut-off low moving westward.  Another 24 hours, now the forecast for Monday morning:

The low continues to move west.  It's also interesting to observe how this low is forecast to undercut that giant ridge in the central part of the country.  On the larger scale, that ridge isn't really forecast to go anywhere...the low just kind of slides in underneath it.  Tuesday morning's forecast now:

The cut-off low has migrated back all the way to the desert southwest.  Broad ridging is forecast for the eastern 2/3 of the country with that low still holding on down in the southwestern corner.  By next Wednesday the GFS has the low weakening to a remnant trough with is probably being forced by the ongoing "monsoon" season in Arizona.  Enough latent heat is being released with the ongoing convection there to lower the pressure and heights and maintain a "monsoonal trough".

So what does this westward-moving low mean for our weather?  The unusual orientation of the low will mean some atypical wind patterns.  As the low moves westward across the plains, easterly winds will help advect moisture straight toward the Rocky Mountains in Colorado and New Mexico, setting the stage for some heavy rain there late this weekend as the GFS is hinting:

Likewise, extended periods of southerly winds on the eastern side of the low will help to bring up very moist air from the Gulf of Mexico through the southeast, Ohio River valley and into the upper Midwest by the end of the weekend.  This "precipitable water" forecast shows the the forecast total amount of water vapor in the atmosphere above any location.  Lots of moisture in a plume to the east of the upper-level low.
This upper level low is forecast to remain well south of the main jet stream, and there is no strong surface low forecast to accompany it.  As such lots of organized severe weather appears somewhat unlikely with this setup.  Though, that  separation from the main jet stream is actually helping to allow the low to retrograde westward like it's doing.  It turns out that if we had no jet stream, no zonal-mean west-to-east flow to push troughs and ridges along to the east, then all of our ridges and troughs would naturally move themselves westward.  Since this low has developed so far out of the main west-east flow (which is being deflected around it  to the north by that giant ridge over the central part of the country) we get to see the trough exhibit its natural tendency to move westward.

Definitely something fun to watch!