More moderate risks...and differences

We're definitely entering the heart of the spring severe weather season now.  In the wake of last weekend's moderate risks in northern Iowa and Wisconsin where several large and damaging tornadoes were reported, yet another potent shortwave is digging into the central part of the country.

The SPC has issued a moderate risk for eastern Oklahoma and western Arkansas today:

Fig 1 -- SPC day 1 convective outlook for April 14, 2011.  From the SPC.

And then as this trough and surface low move eastward, another moderate risk has been issued for an area in central Mississippi and Alabama with a slight risk area throughout much of the southeast.

Fig 2 -- SPC day 2 convective outlook for April 15, 2011. From the SPC.

Based on this morning's 12Z analyses, there is a sharp shortwave across the intermountain west at 300mb:

Fig 3 -- 300mb analysis of winds (colors) and heights (contours) valid 12Z, April 14, 2011.  From the HOOT website.

Now, compare this to the 12Z RUC model (the Rapid Update Cycle model) initialization:

Fig 4 -- 12Z RUC initialization for April 14, 2011.  From the HOOT website.

Overall the two are very similar--and they should be, as the RUC model should be initialized off of the 12Z observations.  But I do notice some small, but important differences.  The most critical is in the placement of the axis of that trough in the west.  In the objective (computer-drawn) analysis of the observations in figure 3, the axis of the trough is placed through far western Idaho and into central Utah.  However, in the RUC initialization, the trough axis seems to be located in eastern Idaho and down into central Colorado--further east than in the objective analysis.  Which do we believe?

One thing we can check is the water vapor imagery from that time.  Water vapor images retrieve radiation that is typically emitted from the upper-levels of the troposphere--around the 300mb level we're looking at in the maps above.  The strong jet streaks up there perturb and contort the water vapor field so that we can rather easly trace the location of most major jet axes aloft on the water vapor images.  Here's the water vapor image from 12Z this morning:

Fig 5 -- 12Z GOES-W water vapor image for April 14, 2011.  From the HOOT website.

See the sharp contrast between the drier air (the blacker shades) and the moister air (the white and blue shades) that stretches from eastern Nevada through southwestern Utah and into northern Arizona and New Mexico?  That roughly marks the center of the jet streak aloft at that time.  I tried to highlight it using my mouse that has a very poor response time on the image below:

Fig 6 -- Same as figure 5, but annotated with approximate location of the center of the jet streak.

If we assume that the winds are more or less parallel to the height contours (which isn't a bad assumption--we should be near geostrophic balance up there...) then we see that the actual trough axis agrees more with the RUC model initialization--it probably stretches into eastern Colorado at 12Z instead of being further west in Utah.  So...the model seems more believable after all.  This once again highlights the need to be cautious when looking at objective analyses--analyses that were done by a computer trying to fit contours to sparse data.  Using higher-resolution observations like the satellite imagery can help to correct any errors.

As the exit region of this jet streak aloft emerges from over the Rockies this afternoon, we've seen a surface low develop which is now in northwestern Oklahoma as of early this afternoon:

Fig 7 -- RUC surface analysis of temperature (colors), mean sea-level pressure (contours) and winds (barbs) for 18Z, April 14, 2011.  From the HOOT website.

 Note the wind field and temperature fields are a little different from what we usually see.  The warm sector to the east of the low is very expansive--the warm front boundary between colder air to the north and warmer air to the south extends northward out of the analyzed low pressure center before turning sharply eastward in northern Kansas and stretching across the midwest into western Pennsylvania.  This seems like an odd structure for a warm front, as usually we don't see them heading northward out of a low-pressure center.  Also the winds to the northeast of the low are more easterly than southerly--this doesn't do much for advecting in moister air needed to fuel thunderstorms.  Therefore, we might guess that the greater threat for severe storms would be further south to the southeast of the low where winds are southerly all the way down to the Gulf of Mexico.  This area probably has more moisture advecting in.  A check of the dewpoint analysis from the RUC shows that this is indeed where the higher dewpoints are located:

Fig 8 -- RUC 15Z dewpoint temperature analysis with wind barbs for April 14, 2011. From the HOOT website.

And we begin to see why the SPC has placed their moderate risk in that particular region.  Why not further south where the moisture is greater? The capping inversion seems to be stronger there and our jet streak aloft seems to be staying further north.  Without that upper air support that comes from the divergence induced by the jet streak exit region, it becomes much more difficult for storms to form.

Upper-level support is fine, but what about forcing near the surface?  We need something to provide lift.  Usually this is accomplished by a frontal boundary, but we saw that the warm front was way too far north and the cold front really didn't seem to have fully developed yet.  However, the dewpoint map shows the primary mechanism being looked to for lift--convergence of the winds along the dryline.  There is a notable dryline present in the dewpoint map from early this afternoon--the sharp boundary between moister air to the east and drier air to the west in western Oklahoma and north Texas is accompanied by winds shifting from southerly to the east and westerly to the west.  This means winds are converging near the dryline, and convergence at the surface typically forces air to rise.  As the dryline mixes eastward across the state of Oklahoma this afternoon, that convergence zone will move with it and storms will form as surface air is lifted.

Why haven't storms fired already?  Our old friend the capping inversion is still in place--but just barely.  The Norman forecast office launched a special sounding at 18Z this afternoon (around 1 PM CDT).  Here it is:

Fig 9 -- 18Z sounding from Norman, OK (KOUN) on April 14, 2011. From the SPC.

A small capping inversion remains right at around 800mb.  However, the lapse rate (the rate temperature is changing with height) below that inversion is very steep--nearly dry adiabatic.  This is a highly unstable layer--and that probably means that a lot of mixing of air is going on in that near-surface layer.  With such a small cap remaining, this mixing will almost assuredly erode the cap away in the near future.  The dewpoint is registered at 56 degrees Fahrenheit at the surface here.  Any small increases in moisture due to advection from the south will be magnified as increases in the convective potential energy.  Already there is a surface CAPE value of some 2064 J/kg.  Very unstable.

And I won't say much about wind shear, but it's rather strong--winds shift from 10 knots out of the south at the surface to 40 knots out of the southwest at 700mb in the sounding above.  Not the largest shear I've seen, but definitely there.  Furthermore, as evening approaches and the low-level jet kicks up, that shear will undoubtedly increase.  However, by then the threat will have shifted to eastern Oklahoma as the dryline will have already mixed through Norman.

One final interesting note about the movement of this low-pressure center.  The RUC model has the low pressure center moving northward along that north-south oriented warm front boundary before settling in at the point where the warm front suddenly turned east.  Here's the forecast surface map from the RUC model for 10 PM CDT tonight:

Fig 10 -- RUC 12 hour forecast of surface temperature (colors), winds (barbs) and mean sea-level pressure (contours) for 03Z, April 15, 2011.  From the HOOT website.

So the low is forecast to move almost due north throughout the day, according to the RUC model.  Also we can see that as the low moves north the cold front is finally beginning to show up, and it is beginning to overtake the dryline from central Kansas down into northern Oklahoma.  At least according to this model.

However, if we look at a map of surface pressure changes in the last three hours, we see something slightly different:

Fig 11 -- 3-hour pressure changes and wind vectors as of 19Z, April 14, 2011.  From the College of DuPage.

The area of strongest pressure falls over the last few hours has been in far eastern Kansas near the Kansas City area.  To me, this implies that the low pressure center is actually moving more easterly than the RUC model would suggest.  This would speed up the timeline of events a little bit as then the dryline would push through Oklahoma somewhat faster. If these strong pressure falls are a sign of the low pressure center's movement, then I wouldn't be surprised to see the risk area extended slightly eastward as the day wears on.  We can also watch the water vapor image to try and trace the location of the jet streak's exit region over time this afternoon.  If it seems that the jet streak is also moving further east, the surface low will probably try to stay with it as that divergence aloft is what's supporting the low.  We'll just have to see.

Regardless, I expect some impressive-looking storms in eastern Oklahoma today.  The tornadic potential will be high, so be alert and listen for warnings if you live in an at-risk area.

Heavy Precip in California

The big weather story for the past few days and right on into today has been the heavy rain and snow falling throughout much of central and southern California.  As noted by Patrick Marsh's (and many other peoples') blog posts and forecast discussions over the past few days, extreme precipitation amounts are being forecast for California.

A check of the most recent hemispheric analysis shows a rather deep 500 mb trough and surface low off of the northwest coast.

Fig 1 -- 12Z analysis of 500mb geopotential height (shaded) and mean sea-level pressure (contoured) on Dec. 18, 2010. From the HOOT website.

The 500 mb trough extends all the way down through northern California and well out into the Pacific.  The height gradient there (as well as the temperature gradient since temperature corresponds to thickness which roughly corresponds to height) hints at a jet stream oriented so as to bring lots of moisture up from the central Pacific and into the California coast.  We can see this on the water vapor imagery.

Fig 2 -- 2330Z GOES-W water vapor image on Dec. 18, 2010. From the HOOT website.

Of course, this image is from nearly 12 hours after the above analysis, but it still shows a strong current of moisture being pulled out of the tropics and into the California coast.  Note the enhancement of the water vapor right after the stream hits the coast--this shows where air is being lifted over the Coast and Sierra Nevada mountains. Since the water vapor channel on the GOES satellites tends to only pick up water vapor in the upper levels of the atmosphere, low-level moisture is often not as visible on these images.  However, when moist air is forced to rise over the mountains, all that very moist air in the lower levels (since, after all, most of that moisture is coming from the ocean surface which is at the lowest possible level...) is lifted up to heights where it becomes more detectable by the GOES imager.  Thus we see an enhancement of water vapor right as the air ascends over the mountains.

A quick look at the radar mosaic confirms large areas of moderately heavy precipitation falling across much of central and southern California.

Fig 3 -- Base reflectivity radar mosaic at 2358Z on Dec. 18, 2010.  From weather.gov.

No wonder the Los Angeles forecast office has flood watches and advisories out for most of their forecast area--southern California doesn't usually get this much rain.  But the ski buffs in the Sierra Nevadas must be loving all this new snow...

Of course, many people (at least, if they were the Seattle media) would blame this large amount of rain on the fact that we're currently in a La Nina pattern with cooler-than-normal sea surface temperatures across much of the equatorial central Pacific.  It's true that the west coast is expecting a wetter-than-average season right now.  The latest images from the Climate Prediction Center show how the sea-surface temperature anomalies have been and are remaining cold over the past few months--a pattern that defines a La Nina event.

Fig 4 -- SST anomalies for several weeks ending the week of Dec. 8, 2010.  Click on the image to see the animation.  From the Climate Prediction Center.

So this current deluge just reinforces the connection between a wetter-than-normal west coast and La Nina conditions over the Pacific.  Is it proof of such a connection in and of itself?  No.  But we'll just say that because we're seeing La Nina conditions, this heavy rain isn't as unexpected as it would have been otherwise.

Warm Core Hurricane

Turning our attention far away from Alaska today, we're going to briefly look at Hurricane Tomas which is now affecting much of Haiti.

Fig 1-- GOES-E Water vapor image of Hurricane Tomas from 1215Z, Nov 5th, 2010

The hurricane hunter aircraft just confirmed last night that the storm had indeed strengthened back to hurricane status, which is bad news for people in Haiti.  I showed the water vapor image to point out one thing that is going to save the US from this storm.  You see the high moisture values trailing off toward the top of the image in a line stretching north-northeastward from the storm?  That is convection along a front that has just moved off the east coast.  Behind the front, over Florida and the Gulf of Mexico, you can see just how dry the air is.  This dry air mass has been rapidly approaching Tomas over the past day or so.  Hurricanes need relatively moist air all the way around them to maintain their strength.  Thus with this dry air moving in, not only will Tomas begin to weaken, but the upper-air winds associated with this front will help to keep Tomas away from the east coast.

Now, Tomas is a tropical cyclone whereas the cyclone we looked at yesterday in Alaska was an extratropical cyclone.  Is there any real difference between the two, aside from the fact that one tends to form in the tropics and the other tends to form outside the tropics?  It turns out there are major structural differences between these storms.  Take a look at this morning's 12 Z sounding from Jamaica, which is about the time that the satellite image above was taken.  Jamaica is located to the southwest of the storm center.

Fig 2--Kingston, Jamaica sounding from 12Z, Nov 5th 2010.  From the HOOT website.

There are many things about this sounding which tell you it's in a tropical location.  For one thing, the fact that the tropopause is way, way, way up at around 110 mb (almost off the chart).  There's also the fact that the temperature profile is almost exactly moist adiabatic all the way from 800mb to the tropopause.  But we're going to focus on the wind structure for now.  We would expect a much more coherent wind profile from a well-organized tropical cyclone.  However, bear in mind that Jamaica is several hundred miles southwest of the storm center and Tomas is only a weak category one hurricane to begin with.

Note how in figure 2, the wind barbs switch their average direction around 200 mb.  Below that level, the winds are generally out of the northwest(ish) direction, while above that they are more easterly. This is a classic pattern found in tropical cyclones (though the switch happens much lower in stronger, more organized systems).  Meteorologists like to define something called the "thermal wind".  The "thermal" wind really isn't a wind at all--it's the difference in winds between two different levels (wind at top minus wind at bottom).  (If you prefer your thermal wind with some light jazz elevator music in the background, try watching this thermal wind YouTube video.  It doesn't actually explain the features of the thermal wind I'm talking about here, but it has light jazz elevator music...).  It turns out (for reasons beyond the scope of this blog for now), that when looking at the thermal wind:

The thermal wind is oriented parallel to the temperature contours with cold air to the left and warm air to the right.

This fancy feature of the thermal wind makes it useful for diagnosing temperature structures based on winds.  It's amazing how everything is connected.  Here is a drawing I put together describing this relationship in the case of our Jamaica sounding:

Fig 3--A conceptual drawing of the thermal wind relationship near a hurricane.

In the drawing above, imagine that the big blue circle is hurricane Tomas with its little eye in the center.  The orange dot represents about where Kingston, Jamaica would be in relation to the storm.  I've drawn two arrows--the green arrow represents the average relative strength and direction of the lower-level winds (below that switch at 200 mb).  The red arrow represents the relative strength and direction of the upper level winds (above the switch at 200 mb).  This means that the black arrow is our "thermal wind", or how the wind changes from the lower levels to the upper levels.  You can see that it connects the end of the low-level wind arrow with the end of the upper-level wind arrow.  Remember this isn't actually a "wind", but a change of winds with height.

Now remember our adage from above--the thermal wind is parallel to temperature contours.  This means that as you follow the thermal wind arrow, temperature is not going to change in that direction.  However, part two of the adage says that the thermal wind is always oriented with colder air to its left and warmer air to its right.  Because of this, we can label the region to the left of the thermal wind arrow as "cold" and to the right of the thermal wind arrow as "warm".

What does this mean about our hurricane? Note how the center of the hurricane is on the "warm" side of the thermal wind arrow.  This implies that temperatures are warmer as you approach the center of the hurricane and cooler as you move away from the center.  This is a defining characteristic of tropical cyclones--they are referred to as warm-core systems.  This means that the center of the cyclone is warmer than the surrounding air.  Fun fact to know.

But what about extratropical cyclones? We'll apply the same analysis tomorrow to an extratropical cyclone to see how their thermal structure works. (If you guessed that they are cold-core systems, you are on to something...)

Curved Jet Streaks--They Do Work!

I figured I might as well start posting some of my thoughts on this blog as that's why I created it in the first case.  I understand some of this may be on a bit more technical level than a post geared toward the general public, but I'll try to provide a mix of different levels to satisfy all who are curious about random weather features.

Fig 1--GOES W Water Vapor Channel from 500Z, Nov. 2, 2010

Today we're focusing on satellite water vapor imagery over the central Pacific from this morning.  A shortwave trough is rather well-defined in this first images, where a quick streamline analysis highlights cyclonic flow around the base of the trough.  Note the relatively "dry" streak along the base of the trough.  This is indicative of a relative wind maximum in the upper-level winds whose moisture content is being sampled by this satellite image.  Such a maximum is commonly called a jet streak.

Fig 2--GOES W Water Vapor Channel from 900Z, Nov. 2, 2010

Fast forward four hours.  Figure 2 shows the same trough (whose axis is denoted in red) with a single streamline shown in yellow to remind us of the flow pattern. Let's clarify a few things from above.  This jet streak is "cyclonically" curved--what does this mean?  You can see in the above image that the wind through the jet streak is indeed curving. We can imagine that if it were to keep curving, it would eventually loop back on itself.  If we think about  that circle for a moment, we'd soon realize that the air moving around the curved circle would be moving counter-clockwise.  For reasons that are even more technical than I want to get into here, in the Northern Hemisphere, counter-clockwise rotation is termed "cyclonic" (i.e. "like a cyclone") and clockwise rotation is termed anti-cyclonic.  In this case, we do indeed have a cyclonically curved jet streak (note also in figure 2 that the "dryness" in the streak has intensified--an indication of strengthening wind speeds, for more complicated reasons...)

Fig 3--GOES W Water Vapor Channel from 1200Z, Nov. 2, 2010

 Now we move three hours later.  One way to to analyze jet streaks is to divide them into quadrants of halves.  In cyclonically-curved jet streaks, the most convenient way to analyze them is to split them in half by drawing a line through the center of the wind maximum and perpendicular to the flow.  In the case above, this line is about where the trough axis (the red, dashed line) is indicated.  We can then talk about the entrance and exit regions of the jet streak.  The entrance region is where air is entering the jet--pretty straightforward.  This would be to the west of the dividing line through the jet streak.  The exit region, where air is exiting the jet streak, would be to the right of this line.  Now for our weather adage of the day...

"Divergence aloft (and consequently rising motion) is enhanced in the exit region of a cyclonically curved jet streak."

By this point you should be gasping in awe at the wonderousness of this statement.  Or maybe not. I'm not going to go into the details of why this is so here, but there is strong support for this statement and patterns associated with other jet streak geometries.  (An early description of some other examples of jet streak-lift associations can be found in R. Lee (1955)). But, we can see this statement in action right now for ourselves in figure 3.  Among other things, remember that rising motion over the ocean should bring very moist air from near the ocean surface up into the upper troposphere (usually through thunderstorms, but there are other ways...).  And here, in the exit region of our cyclonically-curved jet, we do indeed see increases in moisture aloft now as indicated by the green arrow.  Move a few hours later, and it becomes even more obvious:

Fig 4--GOES W Water Vapor Channel from 1600Z, Nov. 2, 2010

Now we see lots of water vapor being lofted up in the exit region of the jet streak.  So, keep this adage in mind when looking at winds aloft and trying to identify areas of rising motion (which is the key to any good forecast).  I would note in figure 4 here that while the trough axis is still represented by the red dashed line, our perpendicular halving-line through the jet streak must move with the wind maximum.  Based on the "dryest" location, this location seems to have shifted eastward and is now indicated by the yellow line.  How jet streaks "move" is yet another fascinating question.  For an interesting early paper on jet streak analysis with regards to their motion, try Riehl and Janista (1952).  For a very nice description of how temperature gradients determine jet streak location, see Patrick Marsh's blog post here.

I apologize for the technical-ness of this discussion, but...I thought it was cool to see this particular theory played out so obviously on the WV channel.  More fun weather observations will come!