Ramblings of a Graduate Student

As mentioned in last night’s (err, early this morning’s) post, a significant severe weather event was forecast for today.  Unlike most events that take start during the day and wind down during the evening and overnight, this event will actually begin to ratchet up overnight and peak tomorrow during the day.

Below are three separate water vapor images from today.  The top image is the original image and bottom image is an annotated equivalent.  Blue lines represent the outline of the jet stream (thick) and jet streaks (thin).  Yellow X’s indicate areas of vorticity maximums.  (This is frequently referred to incorrectly as “energy”.)  The size of the X is a rough approximation of the vorticity maximum’s relative magnitude.

This satellite image is from 12 UTC (7 AM CDT) and indicates a very complicated pattern.  A large, complex mid-to-upper level low was slowly redeveloping eastward from the western United States to eastern Colorado.  Ahead of the large low, divergence aloft was aiding the development of widespread showers and thunderstorms across Kansas, Nebraska, and northwest Missouri.  These thunderstorms originally developed Thursday afternoon in the Texas panhandle and western Kansas as part of an initial shortwave trough that moved through this area.  (Yes, some of these storms produced tornadoes!)

To the southwest of the eastern Colorado maturing cyclone, a strong vorticity maximum was located in northern Mexico, south of the Arizona – Mexico border.  A smaller, weaker vorticity maximum was located in far eastern New Mexico.  Also, an inferred jet streak/potential vorticity anomaly/shortwave trough was ejecting northeast from Mexico through central Texas.

By 18 UTC (1 PM CDT), the eastern Colorado cyclone had not moved much.  However, the jet stream continued to progress eastward over an unstable airmass throughout the south central US.  The eastern New Mexico vorticity maximum was ejecting northeast along the western periphery of the mid-to-upper-level jet stream.  The vorticity maximum south of Arizona began moving eastward and was located just south of the Arizona-New Mexico border in Mexico.

Of note is the development of showers and thunderstorms across portions of northern Louisiana and Arkansas.  These showers and thunderstorms were being aided by the ascent associated on the nose of the inferred embedded jet streak/shortwave trough.  These showers and thunderstorms developed fairly early in the day, preventing wide-spread low-level destabilization of the atmosphere.  This limited the amount of convective available potential energy (CAPE) that thunderstorms were able to use to support vigorous updrafts.  This helped to keep the severe threat and tornado threat a lot lower than it could have been.  If the thunderstorms had developed 3-6 hours later, they would have most likely been even more severe than they were.

Areas to the south and west of today’s convection in Arkansas and Louisiana were experience descent in the wake of the lead shortwave trough that was lifting northeast.  This detrimentally impacted thunderstorm development across a large portion of the warm sector, and also helped limit the severe threat.

By 00 UTC, 24 April 2010 (7 PM CDT, 23 April 2010). the eastern Colorado cyclone had yet to move any distance of significance.  The lead shortwave (embedded jet streak) continued to progress northeastward into Arkansas and thunderstorms persisted across the weakly capped warm sector, which was under the broad divergence aloft.  The thunderstorms in Arkansas and Texas were stronger than they were at 18 UTC (1 PM CDT), but still lacked radar appearances of more robust convection typical in the plains states during severe weather events.  In fact, as the sun continues to set, the intensity of these storms should continue to wane.

The relatively weak vorticity maximum that was located in eastern New Mexico to start the day was now into Nebraska, and was aiding ascent with thunderstorms up there.

The big change in the image is that the vorticity maximum that started the day in northwest Mexico was now located near the Big Bend of Texas and beginning to eject into the United States.  The strong ascent associated with this “primary” shortwave trough was beginning to overcome the earlier descent and aid in the development of convection across portions of central Texas.  As this primary shortwave trough continues to lift northeast during the overnight tonight and into tomorrow, the strong ascent will spread north and east as well.  This will help to ignite new thunderstorms in places that have already seen thunderstorms today as well as in places that have not seen thunderstorms.

Thus, if you live across the south or south east United States, please be aware that even though thunderstorms may not be threatening you at that moment, the threat will continue throughout the overnight hours and into tomorrow.

Last night I posted that it appeared that central Oklahoma (including the Oklahoma City Metro area) would have a good chance at experiencing a dry slot, and that precipitation wouldn’t amount to as much as some of the previous forecasts had indicated.  Examining a regional radar view from 00Z (6 PM CST; displayed above).  It would appear that a substantial dry slot had indeed overtaken the central Oklahoma area.  After all, precipitation had ceased to exist.  In fact, using the warm conveyor belt and dry slot arrows previously used, we could easily construct what would appear to be classic cyclone structure (displayed below).

However, the morning runs of the numerical weather prediction models did not capture such a dry slot in their predictions…at least not one of this size and speed (this thing as moving fast!) nor as early in the storm.

Above is the forecast of the amount of precipitation that would accumulate over a 6-hour period ending 12 hours into the forecast.  It is taken from the Global Forecast System (GFS) numerical weather prediction model simulation that was started at 12Z (6 AM CST) this morning.  In other words, the image above displays how much precipitation the GFS was predicting to fall between 18Z (12 PM CST) and 00Z (6PM CST).  As you can see, the heaviest amounts of precipitation were forecast to fall along the I-35 corridor in Oklahoma and north central Texas.  Certainly not what actually occurred.   In fact, the model didn’t really decrease the precipitation in central Oklahoma until almost 12 hours later (image below).

If we look at the same kind of plots for the North American Model (NAM; 12-hour forecast above, 24-hour forecast below), we find almost the exact same pattens, albeit with different precipitation amounts (possibly related to the different resolutions of the model).

If we think back to the post on dry slots, and examine a 700mb chart, we see that there isn’t a clearly defined dry slot in the GFS 12-hour forecast (above), but there is a hint of one in the 24-hour forecast (below).

I’ve annotated the image above to highlight the possible dry slot, and this can be seen below.

Examining the exact same plots in the NAM model we find, once again, a very similar signal.

Once again, I’ve annotated the image above and it can be seen below.

So, if the models weren’t forecasting such a pronounced “dry slot” then what did the model’s miss? How could they have performed so badly 12-24 hours into the future? While I cannot say for sure, I would like to offer one possibility that is evident when examining a water vapor loop for today. Below, I’ve included a water vapor image taken 3 hours apart. I’ll give you a chance to come up with your own guess before I give mine.

Water Vapor image valid 1145Z on 28 January 2010 (5:45 AM CST on 28 January 2010) .

Water Vapor image valid 1445Z on 28 January 2010 (8:45 AM CST on 28 January 2010).

Water Vapor image valid 1745Z on 28 January 2010 (11:45 AM CST on 28 January 2010).

Water Vapor image valid 2045Z on 28 January 2010 (2:45 PM CST on 28 January 2010).

Water Vapor image valid 2345Z on 28 January 2010 (5:45 PM CST on 28 January 2010).

Water Vapor image valid 0315Z on 29 January 2010 (9:15 PM CST on 28 January 2010).

Any thoughts? This time I’ll add some annotations to the water vapor imagery. The images will be identical to the ones above with the following exceptions:

1. Large “X” is the location of the “main” upper-low / short-wave trough.
2. Small “x” is the location of a considerably smaller upper-low / short-wave trough.
3. As the images advance in time, I leave the previous time step(s) X(s) on the image so you can track it’s movement.

Water Vapor image valid 1145Z on 28 January 2010 (5:45 AM CST on 28 January 2010) .

Water Vapor image valid 1445Z on 28 January 2010 (8:45 AM CST on 28 January 2010) .

Water Vapor image valid 1745Z on 28 January 2010 (11:45 AM CST on 28 January 2010) .

Water Vapor image valid 2045Z on 28 January 2010 (2:45 PM CST on 28 January 2010) .

Water Vapor image valid 2345Z on 28 January 2010 (5:45 PM CST on 28 January 2010) .

Water Vapor image valid 0315Z on 29 January 2010 (9:15 PM CST on 28 January 2010) .

One thing that should have stood out was the the “main” upper-low / short-wave trough did not move much until toward the last few images, and even then it didn’t move quickly. Since this short-wave trough didn’t move much, one could make an argument that the precipitation field shouldn’t have changed all the much either. We certainly wouldn’t expect to see a fast moving dry slot, like what was observed.

The other thing that probably stood out was a subtle short-wave trough / upper-low (and infered upper-high) that rapidly moved from northern Mexico at 12Z (6 AM CST) through western Oklahoma and into Kansas by 03Z (9 PM CST). Without getting into all the physical reasoning and mathematics behind why, meteorologists tend to expect rising motion ahead (or downstream) of a moving short-wave trough / upper-low and sinking motion behind (or upstream) of a moving short-wave trough. I propose that the enhanced precipitation rates that were experienced in western Texas, the development of thunderstorms so early in the day, and the enhanced radar reflectivity on the back-edge of the extensive precipitation shield over much of the southern plains (look at the yellows on the western side of the greens in the radar image at the top of this post) were in response to this aforementioned short-wave trough. However, as the short-wave trough moved through the area, the sinking motion in the wake of the subtle short-wave trough was strong enough to decrease (and temporarily stop) precipitation across western Texas and Oklahoma. Thus, in the wake of the short-wave trough, we had a short-wave ridge that briefly ended precipitation. However, as the short-wave trough and short-wave ridge continued to move away from Oklahoma, precipitation began to redevelop – which is why freezing rain is once again falling in Norman.

Now, if a short-wave trough did move through Oklahoma, and short-wave ridging and sinking motion were occurring, it should be evident on a sounding. Below is a sounding taken at 18Z (12 PM CST) in Norman, OK. Notice how smooth the temperature (red line) and dewpoint (green line) are. This would not indicate sinking motion taking place. (Which is what we would expect since at 18Z Norman was experiencing freezing rain.)

Below is the Norman, OK sounding taken at 00Z (6 PM CST; or six hours later). Again, based on the radar image at the top of this post, Norman was located in what appears to be a dry slot. If we examine the temperature (red line) and dewpoint (green line) in the sounding we see that they are no longer “smooth”. There is a kink in both the temperature and dewpoint at around 500mb (or 6 kilometers). Notice how the dewpoint line becomes relatively far away from the temperature line. This would indicate that something cause the atmosphere to dry out. Now, this drying is located above where we would expect to find a dry slot (remember 700mb?). Based on this drying out and the warming of the temperature at the same level, I suspect this is a subsidence inversion, in other words sinking motion, in response to a short-wave ridge. As I mentioned in the dry slot post, sinking motion tends results in compressional warming…and a drying out of the atmosphere – which is exactly what happened.

If this subtle short-wave trough was not accurately forecast by the models, this would explain why the forecast precipitation and observed precipitation did not match well in time. Now, I have made some hand-waving arguments and over-simplifications, but hopefully you can see how this is a plausible explanation for what has transpired today across Oklahoma. Another explanation combines both the dry-slot and the subtle short-wave trough theory by arguing that today’s dry slot was actually associated with the subtle short-wave trough and not with the main upper-low. Which theory is correct? I’m not sure. All I know is that the model’s did a poor job in forecasting this small, subtle short-wave trough that moved out of Mexico and a lot of precipitation timing forecasts were blown as a result!

Oh, as an aside, as the “main” short-wave trough begins to move into the southern plains, I would expect yet another round of precipitation to develop. Hopefully this round will actually bring central Oklahoma snow instead of ice…