Ramblings of a Graduate Student

A very active weather pattern is shaping up for the next 10 days or so beginning with a shortwave tough (upper-low) moving through the southern United States.  I debated whether to talk about this upper-low as it currently is moving through the south central US or discuss what will happen (partly as a result of this upper-low) along the east coast starting tomorrow into the weekend (another major snow storm for the mid-Atlantic states!).  I figured there would be a lot to talk about tomorrow with respect to the mid-Atlantic snow so I decided to talk about the southern plains tonight.

Below is a radar image taken tonight from the National Weather Service radar in southwest Oklahoma.  At the time this image was taken, southwest Oklahoma was experiencing light to occasionally moderate intensity rain – at the surface.  Why do I make this distinction?  Because above 6000 feet above the ground it was actually snowing!

The radar image above has what is known to meteorologists as a “bright band” signature.  The bright band is the dark green and yellow pixels that appear to make a circle around the radar (the cyan dot with KFDR label).   What is happening is that high in the cloud, the precipitation starts out as snowflakes in the cold air aloft.  As the snow falls and gets closer to the surface it encounters air that is above 0C (32F) that causes the snowflake to begin to melt.  This partially melted snowflake shows up on a radar image much more easily than a snowflake or a raindrop itself would show up.  This causes the level with the most partially melted snowflakes to show up as a bright band or bright circle around the radar site.  (Without getting in the math and physics of it all, everywhere along the the dark green and yellow circle is essentially at the same height.   It has to due with the curvature of the earth’s surface.)

Because the image below has a fairly easily identifiable bright band, I can say with a lot of confidence that it is snowing higher up in the clouds.  Now, the million dollar question, will any of those snowflakes make it to the ground?

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…

As was discussed in yesterday’s post, an upper-level low has begun to affect the southern portion of Texas.  As this upper-level low continues to move slowly east, heavy rains will occur along its path.  The image below is a current radar mosaic for approximately 04 UTC (10:00 PM CST) this evening.  The area of precipitation is moving very slowly to the north, northeast and is continuing to to expand in intensity and coverage.  Upwards of 4-6 inches may fall along portions of the southeast Texas coast over the next 24 – 36 hours.

After nearly a month of strong storm systems moving across the United States, tonight is relatively quiet in terms of precipitation.  There is a weak mid-level low moving across the upper midwest (possibly an Alberta Clipper) resulting in some light snow across the western Great Lakes.  There is heavier precipitation falling along the coast of the Pacific Northwest, associated with a very strong storm system in the eastern Pacific Ocean.  Elsewhere, the main story is the continued cold.

(A current image can be found here: National Mosaic – Rull Resolution.)

Note:  To any of my current and former students who read this far, before you fine me a dollar for using the word “system”, understand that I’m referencing the collection of a surface low pressure center and the associated low height centers aloft.  In this context the term “system” is acceptable because I’m using it in the context of a “system of lows”.

Well, the arctic surge is certainly on its way southward.  The radar image below  captures the first cold front as it moved into the Oklahoma City metro.  The higher reflectivity values (brighter colors; in this case the blues and green) are associated with the density gradient across the cold front.  In other words, these brighter colors are the result of the cold, dense air to the north of the front being advected southward, into the northward advecting warm, (relatively) buoyant air south of the front.

Below is the same image as above, except I’ve “drawn” the front in blue, along with the wind directions north and south of the front.

Note:  I would post a more in-depth explanation of the radar image, however, we lost power about 40 minutes ago and I’m working via a laptop and cell-phone internet.  Losing power when the temperature is supposed to go down to single digits doesn’t seem like a good thing…