Ramblings of a Graduate Student

The last 72 hours have been a whirlwind for me. It all began with my blog post on Sunday night discussing the uncertainty in where the significant tornado outbreak would occur on Monday. It continued on Monday when a developing tornado nearly hit the VORTEX II Operations Center where I’ve been working for the last two weeks. It persisted on Tuesday with press conferences and interviews regarding VORTEX II’s research on the day before’s tornado outbreak and the work entailed in helping coordinate at least six different damage survey crews. Today, it didn’t let up as I briefed Congressman Tom Cole about VORTEX II’s research and goals. Throughout all of this, I’ve spent at least 12 hours a day running the VORTEX II operation’s center.

As you can probably imagine, I need a break. I could use a day or two of quiet weather to let me get caught up on everything I haven’t been able to get to the last few days. However, the atmosphere has different plans for me.

A large mid-level low is currently located over the central Rocky mountains (easternmost yellow circle). Associated with this mid-level low is a strong mid-level jet streak (northern most blue color fill) that is making its way into the central United States. On the “nose” (i.e., the leading edge) of the mid-level, cyclonically curved jet streak, rising motion is often observed (red circle). It just so happens that there are two separate jet streaks “nosing” into the central United States that are aiding “lift”.

This lift is assisting in the development, and persistence, of thunderstorms in the central United States. These persistent thunderstorms provide VORTEX II ample “targets” for data collection. Thus, VORTEX II was once again attempting data collection tonight. And once again, the atmosphere has toyed with VORTEX II.

Thunderstorms in southwest Oklahoma that were targeted by VORTEX II frequently showed signs of rotation, but tended to produce brief tornadoes in places that VORTEX II could not actually observe them. The armada “chased” these storms all the way from the Texas panhandle-Oklahoma border to the Clinton, OK area. It was shortly before dark, as the storm was racing away for VORTEX II, that the last observed tornado developed. I was able to watch this last tornado develop live on the Internet via storm chaser David Drummond’s live chase feed.

Although I could use a break to get some rest, it doesn’t appear that the opportunity will present itself for the next few days. A second shortwave trough (westernmost yellow circle) is currently just off the California coast, poised to race toward and into the central US over the next few days. Although this is bad news for me, it is extremely good news for VORTEX II as it should provide additional chances to collect badly needed data.

For the past few days, VORTEX II has been hoping that a cold pool aloft (associated with a shortwave trough) would provide enough instability to allow severe thunderstorms to developing across northern Texas during the day today.  The area circled above is the cloud cover developed in response to the lift associated with the aforementioned shortwave tough.  Unfortunately, low-level moisture and wind-fields didn’t appear supportive of thunderstorms with a tornado potential.  As such, the VORTEX II crew decided to pass on targeting northern Texas today.  Instead most of the armada has left Norman for points “unknown” (*wink wink*) to have the first formal gathering of the crew.  Tomorrow morning is the “all-hands” meeting, orientation, and safety reminder.  Hopefully later this week will provide something for the VORTEX II crew to target…

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.

Warning:  This post does contain somewhat technical information.

In preparing tonight’s blog post, which was originally going to be a forecast discussion regarding late week severe weather potential, I came across something that I thought would end up making a better blog post.  Especially when you consider that the severe weather prospects for Friday didn’t look good based on this morning’s 12 UTC model forecasts.

From time to time, meteorologists have to play detective.  What I mean is that numerical model forecasts often do things that at first glance don’t seem all that extraordinary.  However, thorough meteorologists will investigate why a model did what it did, and relying on theory, attempt to glean useful information that isn’t exactly obvious.  A good example of this can be found in examining the 12 UTC Global Forecast System (GFS) forecast for 12 UTC Thursday (7 AM CDT Thursday) through 00 UTC Friday (7 PM CDT Thursday).

The image above is valid 12 UTC Thursday (7 AM CDT), and displays the forecast 500 milibar heights and vorticity (top half) and 500 milibar heights and wind speeds (bottom half).

Through the course of these images, pay attention to the area of southern Colorado and northern New Mexico.  Watch for changes in wind speed and vorticity (both color fills).

The image above is valid 18 UTC Thursday (1 PM CDT).

The image above is valid 00 UTC Friday (7 PM CDT Thursday)

Did you see it?

Notice the model’s forecast wind speeds for northern New Mexico increase dramatically between 12 UTC and 18 UTC and then decrease dramatically between 18 UTC and 00 UTC.  In fact, the maximum seen in wind speeds at 18 UTC are greater than the surrounding magnitudes at 12 UTC and a lot greater than the surrounding magnitudes at 00 UTC.  Did you also notice the same thing happen with the vorticity?  Why might this be?

First, let me explain why the two images are linked together in a single frame.  Typically, meteorologists look for kinks in the 500 milibar height field to identify potential shortwave troughs and/or ridges.  In fact, the dip in the 500 milibar heights throughout the western United States are associated with a (larger) trough.  Troughs are associated with increases in (relative) vorticity and ridges are associated with decreases in (relative) vorticity.  Because increases in (relative) vorticity are associated with troughs, whenever the colors on the top-half of the image increase, one could infer the presence of a trough.  Without going through all the explanation as to why, an increase in wind speed can also be attributed to the presence of a trough, espcially downstream (in this case on the east side) of a trough.  (It has to do with  strengthening lower-level temperature gradient(s)).

In the vorticity images, one can track an area of higher vorticity values from southern Utah (image 1) to northwest Colorado (image 2) to southeast Wyoming (image 3).  This is associated with a shortwave trough breaking off from the southern portion of the trough and rapidly ejecting northeast.  This is known as a lead-shortwave trough as it “leads” the main shortwave trough.  As the lead shortwave trough races through the flow of the longer wavelength trough, it helps to increase 500 milibar wind speeds in northern New Mexico.

If we think about the main shortwave trough, we would expect to find stronger winds located downstream (east) of the trough axis.  The same can be said of the lead-shortwave trough.  We’d expect to find an increase in wind speed downstream from the trough axis.  When the expected area of increased wind speed from both troughs overlap, we get a cumulative effect where the winds increase to more than what they would without interaction of the two troughs.

If a meteorologist had simply examined the noisy vorticity plot above, the presence of the lead shortwave could have been missed.  The same if a forecaster had only examined the forecast wind speeds.  However, by examining the two together, and understanding that it is possible to indirectly link increases in vorticity to increases in wind speed, we can determine the presence of a lead-shortwave trough.  Why is this important? Downstream from a shortwave trough, rising motion is often found.  When a “cap” (discussed more in upcoming blog posts) is present, the rising motion associated with a shortwave trough will often help thunderstorms initiate, when otherwise thunderstorms wouldn’t.  This often results in what chasers refer to as “the day before the day” events.  That is, thunderstorms and tornadoes developed on the day before the main trough moves through, which most people consider to be “the day”.

Now, I’m not saying that Thursday will be “the day before the day”, however, if moisture return is a little better than forecast, and a lead shortwave trough moves through during the afternoon hours, I wouldn’t be surprised if an isolated severe thunderstorm or two developed out in the Texas panhandle.  I also wouldn’t be surprised if nothing happened, either.

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…