Thursday, July 28, 2011

Heavy rain in the midwest, tropical storm down south

Repeated rounds of storms around the periphery of the still-persisting ridge in the southern US have contributed to rainfall totals that are well above climatology for the last few weeks in some locations.  In Chicago this is the wettest July on record with 9.75 inches so far this month.  Furthermore, this month is now the 9th wettest month of all months in Chicago since records began--pretty impressive.

There's a lot of water vapor to work with in the atmosphere over that region at the moment.  Here's the satellite water vapor image today:
GOES-E water vapor image from 1615Z, July 28, 2011.
The upper-level ridge across the south causes air to advect clockwise around its periphery.  As such, you can see a big plume of moisture stretching from Arizona and New Mexico through the central plains, into the midwest and out through the mid-Atlantic states.  This plume of moisture has been in place over the same area for several days now, and any storms that form in that area follow the same path.  Repeated rounds of storm after storm (MCC after MCC...) following the same path have contributed to the high rainfall totals.

The more moisture there is in the atmosphere, the greater the potential for heavy rainfall.  This seems like a very obvious statement.  To that end, one parameter that meteorologists use to evaluate the potential for heavy rainfall is called the precipitable water value (often abbreviated as PWAT or PWTR).  It's a pretty simple index to derive--basically, all precipitable water does is look at the profile of moisture content over a given location.  It calculates how deep the water on the ground would be if all of the water vapor over a given location were to immediately condense out and fall as rain.  It's saying, if all the water in the air over your head immediately condensed into rain and fell out, how much rain would fall.  Clearly this is never fully realized, as the atmosphere never dumps out all of its water vapor content at once.  But, in general, the higher the precipitable water values, the higher the likelihood for heavy rain.

Most soundings that you see will compute the precipitable water value for that sounding location.  The HOOT website soundings are no different.  Here's this morning's 12Z sounding from Davenport, Iowa, which is in the middle of that moisture plume.
KDVN sounding from 12Z, July 28, 2011.
First, let's look at this sounding for a moment.  Notice how the dewpoint trace (the green line) is rather close to the temperature trace (the red line) from the surface up to somewhere around 430mb.  That's a very deep layer of very moist air.  We'd therefore expect the precipitable water values to be rather high.  You can see the calculated preciptable water value under PWTR above the upper right corner of the sounding.  The black value is the value for the current sounding, and it shows a PWTR value of 2.29 inches.  So that's saying that, based on this sounding, if all the water vapor in the atmosphere over Davenport were to immediately condense out and fall as rain, 2.29 inches of rain would fall.  That's a lot.

But we've already established that this is a record-breaking month for rainfall in this region.  Just how anomalously moist is the atmosphere within this plume of moisture?  The Rapid City, SD forecast office of the National Weather Service has put together a really nice webpage with climatological values of precipitable water for each sounding site.  Here's their yearly graph of climatological precipitable water values for Davenport:
Monthly climatological precipitable water values for KDVN.
There are several curves on this graph, which are explained by a legend in the upper left corner.  Each curve represents a different percentile of the normal precipitable water values based on the averages from 1995 to 2010.  The mean value (the 50th percentile) is given by the red curve.  You can see that for the month of July, Davenport's mean PWTR value is about 1.2 inches.  This morning's value of 2.29 inches is well above that.  In fact, if you look at the range of values for July, you'll see that 2.29 inches isn't quite up to the maximum curve (the light green curve) but it is above the 99th percentile curve (the dark green curve).  That means this precipitable water is in the 99th percentile of all PWTR values seen during July over the past 15 years or so.  That's very high.

Furthermore, the dashed green line shows a value that is two standard deviations above normal.  This value is often used by forecasters to try and separate extreme values from more climatologically normal (but still high) values.  Any time you're more than two standard deviations above normal, it's considered to be a more "extreme" case with a high potential for heavy rainfall.  2.29 inches is definitely above the two-standard-deviations line.  All of this points to the potential for very heavy rain.

Unisys publishes an interpolated precipitable water map for the US after every sounding release time.  So, here's their map of precipitable water across the US this morning at 12Z:
Interpolated precipitable water at 12Z, July 28, 2011.
There's a swath of elevated precipitable water values from eastern Nebraska east through southern Michigan.  This is the corridor where rounds of storms are able to produce very heavy rainfall and bring us these extreme events.

Also notable is the large area of very high precipitable water values right on the Gulf Coast.  They cite 2.4 inches at a point near Lake Charles, Louisiana.  There, too, precipitable water values are in the 99th percentile and thunderstorms with heavy rain are forecast for the next few days.

Of course, a very potent rainmaker looks to be bearing down on the Texas gulf coast in the next day or two.  Tropical Storm Don has formed in the Gulf of Mexico and is moving northwestward.

NHC predictied 5-day path cone of TS Don as of 10AM CDT, July 28, 2011.
It's not forecast to strengthen beyond tropical storm strength, which is good news for the gulf coast in terms of lessening the potential for damaging winds.  What's even better news is the amount of rain this storm could bring to central Texas, which is currently suffering from "exceptional" drought, as seen on the map below.  In contrast, with the extreme rainfall over Chicago recently, I'm pretty sure the little "A" for "abnormally dry" over northern Illinois is going to be disappearing soon...
US Drought Monitor from the Climate Prediction Center/UNL, as of July 19, 2011.

Monday, July 25, 2011

Thunderstorms in Seattle

I awoke this morning to the sound of what I thought were airplanes flying over my house periodically.  It's not uncommon--the main northerly approach to Sea-Tac airport goes right over my house.  And they don't call Seattle "Jet City" for nothing.  But the sound wasn't quite right--and it was occuring way too frequently.  Turns out it was something far more exciting--thunderstorms.

KATX 0.5 degree base reflectivity from 15Z, July 25, 2011.
Lacking my usual Gibson Ridge program this morning, I used an internal utility on the UW computer systems to get this radar image.  You can see a lot of yellows with embedded cores of reds, indicating rather intense rainfall in some locations.  In particular there looked to have been a rather intense cell in eastern Pierce County southeast of Tacoma.

But were these thunderstorms or just heavy downpours?  I did a quick plot of the National Lightning Detection Network data of all lightning strikes registered from 7:30 AM PDT to 9:30 AM PDT.  Here's the result:
NLDN Lightning strikes from 1430Z to 1630Z, July 25, 2011.
Sure enough there have been several lightning strikes in the Puget Sound lowlands, particularly over the Kitsap peninsula.  There has been a lot more activity on the eastern slopes of the Cascades, though. 

The visible satellite image from this morning also shows the billowing, tall clouds identifying this as deep convection.
GOES-W 1km visible satellite image from 15Z, July 25, 2011.
Because the sun had just come up in this image, the sun angle on the clouds was still low.  For this reason, taller clouds cast shadows to their wests.  You can really see the shadow to the west of the cloud in the area of that big storm over eastern Pierce County.  Interestingly that storm began developing right in the vicinity of Mount Rainier.  Perhaps the mountain helped promote the lift necessary to get the storm going...

So why thunderstorms today?  First, we got really warm yesterday.  Here's a meteogram image showing plots of various weather variables on top of the atmospheric sciences building in Seattle over the past day. Temperature is the third panel from the top.  You can see that we almost got up to 85 degrees Fahrenheit yesterday.
Meteogram from 17Z July 24,2011 to 17Z July 25, 2011 from the top of the UW Atmospheric Sciences building, Seattle.
All that heating warmed up the lowest layer of the atmosphere.  At night, the surface and the near-surface layer cooled off again (you can see in the above image that last night the temperature got down to the upper 50s).  However, the air above that retained some of its residual warmth from the day before.

Below is a somewhat complex, but still very informative plot.  This shows several soundings taken using a vertical profiling radar at Sand Point in Seattle.  Every hour, the profiler uses a vertical-pointing radar beam to derive the temperature structure of the lowest 1600 meters or so of the atmosphere--up to about 850mb.  On the plot, the height above ground is given on the y-axis and the "virtual" temperature (which is similar to temperature, only a degree or two different because it factors in the humidity) in Celsius is on the x-axis.  However, coordinates of the graph are tilted--temperature lines are slanted up and to the left and are shown by the light gray lines.  So, it's kind of like a skew-T chart.

The actual temperature profiles are the colored curves.  One is taken each hour, and the legend showing what hour each color represents is on the upper right. 
Virtual temperature soundings from the Sand Point profiler in Seattle for July 2, 2011.
The first sounding is the yellow one, which was at about 10Z or about 3 AM local time.  As time progressed, you can see that the temperature really cooled down, at least below 1000 meters.  The surface temperature drives this--notice that the temperatures at the surface for the first two profiles (the yellow and the light blue ones) are much colder than the air above.  The surface radiates away more energy than the atmosphere above it, so it cools faster.

However, by 12Z (around 5 AM) the sun started to come up (this is now the magenta profile).  You can see that the surface temperature is no longer much cooler than the air above it.  Immediately the surface starts absorbing the solar radiation and its cooling slows down.  It eventually starts warming up.  However, the air above the surface (but below 1000 meters) continues to cool.  Just as the atmosphere cannot cool as efficiently as the surface, the atmosphere also cannot warm up as efficiently as the surface.  It has to wait for warm air in the near-surface layer to mix upwards to really start warming up.  As such, even though the sun has come up, the air between around 500-1000 meters continues to cool even though the surface cooling slows down and actually begins to warm.

However, the cool air at the surface overnight had not deepened beyond 700-1000 meters by the time the sun came up.  You can see that the temperature profiles all abruptly get warmer above the 700-1000 meter depth.  The night was just not long enough for the cold air at the surface to deepen up beyond that height.  Therefore, above this level, the residual warmth from the day before remains.  Because we got so warm yesterday, these temperatures above 700-1000m really are very warm--I mean, it's still 23-25 degrees Celsius up there (around 75 degrees Fahrenheit) while at the surface it had cooled to the upper 50s. 

Why do I go through all of this description of the temperature profiles?  I want to point out that even though at the surface we really cooled off last night, residual heat from our unusually warm day yesterday still lingered just above the surface.  It's that still-warm layer that provided the instability for this morning's storms.

Helping this instability is a trough moving in from offshore.  Here's the 700mb temperature forecast for 15Z this morning:
UW 4km WRF 3-hour forecast of 700mb temperature at 15Z, July 25, 2011.
This forecast is for temperatures above the top of the temperature profile I was just showing.  Notice the cooler temperatures just off the west coast of Washington and the westerly, onshore winds.  These cooler temperature are associated with a trough that is now moving onshore.  So, combine these two ingredients:
  1. A warm layer around 1000m above sea level that was left over from yesterday's very warm day, even though the surface cooled down overnight.
  2. On top of that warm layer, colder air was moving in associated with a trough moving onshore.
And we have cold air moving in over warming air--the right combination to destabalize the atmosphere.  These are not surface-based storms--they are not drawing from warm air at the surface.  Instead they are elevated storms--they're drawing from warm air in a layer above the surface.

That was a look at our fun collection of thunderstorms here in Seattle.  It's a rare event out here, but it makes sense once you look at the setup.  I'm going to bring back those virtual temperature profiles at some point in the future to talk about the development of the capping inversion.  You can really see in those profiles how a capping inversion is developing this morning over the area.  But more on that in another blog.  For now I'll just enjoy the show.

Wednesday, July 20, 2011

The Ridge Continues...

As most people in the central US are well aware, it has been extremely hot and humid for the past several days.  Model forecasts indicate that we'll be staying this way for a little while longer, too.

This morning's 500mb analysis shows the ridge that has been promoting these hot, stagnant conditions rather clearly:
GFS 500mb analysis at 12Z, July 20, 2011.
There is pronounced ridging with the main axis of the ridge centered over the Mississippi River valley.  Troughing is evident over the Pacific Northwest, where cloudy skies and cooler-than-normal temperatures offer one of the few places in the country to escape the heat.

It looks like that trough area in the Pacific northwest will try to move inland this week, causing some fluctuations in the structure of the ridge.  Here's the GFS 48-hour forecast for 500mb heights on Friday morning:
GFS 48-hour forecast of 500mb heights and winds at 12Z, July 22, 2011.
The sharpness of the ridge is diminished and the trough over the Pacific northwest seems to be deepening and making progress inland.  However, this progress appers to be shortlived.  Here's the 96-hour GFS forecast for 12Z Sunday:
GFS 96-hour forecast of 500 mb heights and winds for 12Z, July 24, 2011.
It looks like a shortwave is forecast to advance across the northern tier of the US.  This somewhat modulates the ridge strength, but doesn't dig into the southern plains or southeast where it would really have to go to do anything about the ridge.  The resulting 500mb pattern looks very "zonal"--there's a lot of east-west motion as opposed to strong north-south motion.  Also, the iso-height lines (the dark contours) run mostly east-west.  This is all typical of a "zonal" pattern.  Zonal patterns are highly unpredictable, particularly at forecast times that are far into the future.  Since small, shortwave ridges and troughs can make all the difference in precipitation (and temperature) forecasts, a pattern that's very "flat" is susceptible to small perturbations.  Thus, this forecast is very uncertain.

Still, looking further into the future, there's enough confidence that the Climate Prediction Center outlook for 8-14 days from now still has above-normal temperatures forecast for pretty much everywhere in the continental United States except for the Pacific Northwest:
CPC 8-14 day temperature probabilities as of July 20, 2011.
Another function of this ridge is to direct storm systems and moisture flow in a clockwise manner around the periphery of the ridge.  With the ridge centered over the southen plains and lower Mississippi valley, it's no surprise that that area (in the middle of the ridge) would be expecting below normal precipitation.  In contrast, areas around the perimeter of the ridge will have increased chances of seeing storms and probably have higher chances of precipitation.  This explains the CPC's 8-14 day precipitation outlook:
CPC 8-14 day precipitation probability as of July 20, 2011.
In then center of the ridge (Oklahoma and Texas) the CPC is calling for below normal precipitation.  However, on the periphery of the ridge from Arizona through the Front Range of the Rockies and into the northern plains, the probability for precipitation is above normal.  Unfortunately, this is the same sort of pattern we have been seeing for the last several days.  Places that really need the moisture (like Texas and Oklahoma) still don't look to get much of any as long as that ridge is in place.  However, places that have been repeatedly hit by MCSs over the past few days (like the Dakotas, Minnesota and northern Wisconsin) will probably continue to see rounds of storms.

So, in summary, it looks like more of the same for the next few days, even weeks.  The secret to getting out of the heat?  Head to Seattle... 

Monday, July 11, 2011

The ridge and the week ahead

Last night we had two MCCs merge together over Wisconsin to form a very potent MCC with a leading convective line in southern Michigan this morning.  Here's the IR satellite image from earlier this morning:
GOES-E IR image from 1015Z, July 11, 2011.
You can see the characteristic circular "blob" pattern in the IR imagery that is the hallmark of MCCs.  It seems that the particular route these storm complexes took--from the central and northern plains out through the central and upper midwest--is going to be the favored storm track for the next several days.  Here's today's SPC convective outlook:
SPC Day 1 convective outlook as of 1626Z, July 11, 2011.
And the day 2 convective outlook for Tuesday:
SPC ay 2 convective outlook as of 0600Z, July 11, 2011.
So what's causing the storms to move this way?

It turns out that there is a big ridge aloft over the southeastern US--and it's not going anywhere.  At least, according to the models it's not going anywhere.  Here's this morning's GFS analysis at 300mb:
GFS Analysis of 300mb height (contours) and winds (barbs and colors) at 12Z, July 11, 2011.
Note the high-height area over the southeast with winds flowing clockwise around it.  The jet streaks within the jet stream are somewhat weak and well to the north--mostly in southern Canada.  You can visualize with clockwise flow around this ridge that the prevailing wind pattern would be a big arc stretrching from the desert southwest, up through the central and northern plains, across the central midwest, then out over the mid-Atlantic states.  It's no coincidence that that is the preferred storm path over the next few days.  These prevailing winds aloft carry the storms along with them.

Let's look forward in the GFS model to the forecast pattern on Wednesday morning:
GFS 48-hour forecast of 300mb height (contours) and winds (barbs and colors) at 12Z, Wednesday, July 13, 2011.
The main jet stream is still pretty far to the north.  However, there is a strong, sharp ridge whose axis is forecast to stretch across the northern plains.  If you follow the wind barbs, you can still see that the prevailing flow is up out of the desert southwest, across the northern plains, and down through the central and upper midwest.  So I'd expect a similar storm track to continue.

Now on to Friday morning:
GFS 96-hour forecast of 300mb height (contours) and winds (barbs and colors) at 12Z, Friday, July 15, 2011.
It looks like the ridge has grown even more in this forecast.  Same predominant flow pattern, however.  I'm also interested to see what's going to happen with the trough that has been continually trying to move onshore in the Pacific Northwest all week.  It looks like this trough could be dampening the early summer glory here in the Seattle area by bringing more cloudiness and showers.  In fact, the longer this ridge sticks around the central part of the country, the longer the weather is going to be drizzly in Seattle...

With weak jet streaks moving through, it's clear that storms that fire aren't going to have a lot of support of the flow aloft.  In general, a ridge is not the most conducive area for storm growth and development--at least in a baroclinic sense.  With weak synoptic-scale forcing, I'd continue to expect to see MCCs as the dominant form of storm as this pattern continues.  Remember from my previous posts that MCCs tend to modify the environment more than the environment works to develop them.  So, in the absence of strong forcing from the ambient environment, any instability is probably going to manifest itself in MCC form.

Why should we expect any instability at all?  Two reasons--heat and moisture.  As most people in the central part of the country already know, it's been hot this summer--and sinking, warming air under the prevailing ridge is not going to do much to diminish that.  Heating at the ground level inherently works to destabilize the atmosphere.

The humidity is also a factor.  Not only are we seeing muggy conditions near the ground, but the moist layer can extend to be rather deep.  A lot of this is due to that same, clockwise flow pattern aloft.  It turns out that this particular pattern (with a ridge over the southern US) helps drive the flow of "monsoon" moisture up from the southwest.  This moisture doesn't originally come from the desert southwest--it actually has its origins in the tropical east Pacific.  But the upper-air winds advect that moisture up and around.  It's brought a lot of storminess to the southwest over the last few days.  Here's the RUC analysis of 500mb relative humidity from this morning:
RUC Analysis of 500mb  relative humidity (greens) winds (barbs) and heights (contours) at 15Z, Wednesday, July 13, 2011.
Note a broken stream of high relative humidities stretching up along the western Mexican coast, through the desert southwest, into the central plains and across the upper midwest.  It's that same pattern that we were seeing for the storm track.  So, storms forming in this region have both heat and deep moisture to work with.

It will be interesting to see just how this week plays out.  With such a dominant ridging pattern, the small-scale features will make all the difference.  For instance--what about that small upper-level low analyzed over northwest Texas at 300mb in the first GFS image that I showed?  It actually shows up on the water vapor imagery from this morning as a nice swirl over northwest Texas, so we know this isn't an error in the analysis:
GOES-E water vapor image from 1645Z, July 11, 2011.
That upper-level low has been drifting northwestward overnight and it looks like it's about to start interacting with that plume of higher "monsoonal" mositure coming up from the desert southwest.  Will the added vorticity of this upper-level feature be enough to spin up a warm-core MCC?  What will happen?  We'll have to wait and see.

Ridges don't always have to be boring...

Thursday, July 7, 2011

Aha! An OBSERVED MCV--just the other day

I was pleasantly surprised to see the University of Wisconsin -- Cooperative Institute for Mesoscale Satellite Studies (CIMSS) blog post on July 6th.  It discusses a mesoscale convective vortex that just occured over the southwestern US on July 5th and 6th.  Moreover, they have a stunning GIF animation of visible and IR satellite images that shows how the storms first organized as a mesoscale convective complex then later evolved to a mesoscale convective vortex.

The blog post is at:

And that GIF animation (which is the first image in the blog post) is here.

I encourage you to watch the animation to really get a good idea of the evolution of these sorts of storms.  The synoptic-scale flow aloft was relatively weak, indicating that this complex of storms organized away from any strong upper-level forcing.  What began as clusters of thunderstorms during the day (with very distinct outflow boundaries) congealed into an organized mass as night fell.  You can see in the overnight hours (when the animation switches to the lower-resolution IR images) how the cloud structure does indeed take on the characteristic round shape of a MCC. 

By the morning hours (when the animation returns to the higher-resolution visible images), the remaining circular cloud shield (and a fair amount of the convection going on) dissipates.  But, it leaves behind the warm-core vortex that lies at the heart of the complex.  Internal interactions within the cyclonic flow about this vortex are able to maintain some convection about the vortex center.  However, because this is a vortex, the convective structure has the characteristic "swirl" shape we see in MCVs.  Once again, not unlike a hurricane...

It's incredible how fragile MCVs actually are.  As I mentioned in my last post, studies indicate that on average only three or four MCVs are observed in the US each year.  Any strong upper level winds will tend to shear apart that warm-core vortex long before it gets very organized.  You can tell in the animation that the synoptic flow is weak, though, and atypical of a progressive, strong jet pattern.  The MCV itself drifts westward, which is not the typically direction we see storms move--this shows the relatively "mild" conditions aloft.

Anyhow, I just thought it was pretty amazing that the day after I do a post about MCVs (a relatively rare phenomenon), one happened to be observed.  So, please enjoy this fascinating type of storm...

Tuesday, July 5, 2011

The Mesoscale Convective Vortex (MCV)

For my last post in this series about different kinds of mesoscale convective systems (MCSs), I'm going to briefly talk about one of the rarer types of MCSs--the mesoscale convective vortex (MCV).  These sorts of storms usually form from parent mesoscale convective complexes (MCCs), so we have a full line of mesoscale convective classification:

MCS --> MCC --> MCV

In trying to trace the history of MCVs in the meteorological literature, it seems that the MCV was recognized very soon after Maddox's 1980 paper describing the features of MCCs. It was noted that in some cases, strong mesoscale low-pressure centers would develop in the wake of MCCs.  Johnston (1981) talks about mesoscale vorticity maxima induced by mesoscale convective compexes. Such maxima were theorized to have helped organize the storm that caused the 1977 Johnstown, Pennsylvania, floods by  Zhang and Fritsch (1987).  However, the first actual mention I could find of the term "mesoscale convective vortex" being used to describe this phenomenon came from a paper by Menard and Fritsch in 1989, where they describe a "mesoscale, convectively-generated vortex (MCV)" over Oklahoma and Arkansas.  So you can see that our understanding of this particular phenomenon is pretty young, having only been recognized over the past 20-30 years of reseach.

So what is a mesoscale convective vortex?  In my discussion of MCCs, I talked about how the large area of rising air in the circular MCC had lots of condensation going on.  That condensation of water vapor into rain releases a lot of latent heat, warming the core of the storm.  That warming causes the air to expand, reducing the pressure relative to the surrounding environment.  This, in effect, creates a "warm-core low" with a structure analogous to the structure of a hurricane.  Sometimes, after convection and rainfall associated with an MCC dissipates, this area of low-pressure can actually linger on.  The cyclonic flow associated with the low-pressure center can later help organize new areas of rising motion and convection, often with a charactaristic cyclonic "swirl".

Here's an example of a visible satellite image of this organization going on from the CIMSS satellite page:
GOES visible satellite image from 1615Z, Jul 8, 1997 showing a MCV over western Missouri.  From
Sometimes you'll see MCVs described as a "mesoscale vorticity center".  This is an apt description, and it means essentially the same thing--it just changes the acronym to MVC. You can see on that example image how the MCC has organized itself into a charactaristic swirl-shape.  The analogies between MCVs and tropical cyclones really start to make sense when you see this kind of organization.

Here's a radar image from Patrick Marsh's blog last year when an MCV was observed moving through the Houston area:
NEXRAD base reflectivity composite of an MCV near Houston at 2220Z on June 3, 2010.  From  Annotated by Patrick Marsh.
Patrick has annotated that radar image with streamlines showing the wind patterns and an L marking the center of this mesoscale vortex.

You can see in these two images why these features are considered to be "mesoscale".  Unlike the banded "swirls" or comma-shapes we see with strong synoptic-scale low-pressure centers, these bands do not mark the locations of frontal boundaries.  They are much more akin to hurricane rain bands than frontal bands.  Furthermore, the size of these vortexes is only at most a few hundred kilometers across--often less.  You can see above that these storms are about the size of a state or less.  This still puts them in the "mesoscale" category--smaller than the synoptic scale.

It turns out that, though mesoscale convective complexes are relatively common across the central US, only a very few actually give way to the eventual development of a mesoscale convective vortexBartels and Maddox (1991) did a survey over seven and a half years of visible satellite data and only observed 24 cases of MCVs, working out to around three or four per year.  So, they're a fun feature to see when you can actually find them.

That's just a quick look at MCVs, a rarer, but still important type of mesoscale convective system.  In my next blog, I think I'll finally move away from talking about MCSs and get back to talking about the weather that's currently going on.