Mar 212010
 

Wednesday, 31 March Update

4/2/10 (Friday):  The setup still looks like a convective mess over Texas and Oklahoma, with mainly a hail and damaging wind threat.  But with some luck, a clear slot and good daytime heating, some supercells may be possible across central/north Texas.  Although it’s not a good setup, generally speaking.

4/5/10 (Monday):  Finally, I see a light at the end of the tunnel.  Models show the last shortwave in this pattern ejecting into the Plains early next week.  According to the GFS (operational and ensemble), a deep surface low will form over the central high plains on Monday with a strong 850mb jet out of the south.  This looks very interesting.  It has the potential to be a classic setup for supercells, some tornadic, firing off the dryline from Texas to Nebraska.  We’ll see how the models work out the details in terms of timing, but this day has my attention.

4/6/10 (Tuesday):  How quickly the shortwave ejects eastward will determine if this day holds potential for severe storms across the Plains.  We’ll see…

Beyond 4/7/10:  With the MJO weakening and the El Nino pattern taking hold again, the favorable pattern for chase setups across the Plains will end.  We will have to rely on climo for chase setups as we move into mid April, but the synoptic pattern looks less than ideal.

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Monday, 29 March Update:

4/2/2010: Looks messy across Texas/Oklahoma

Friday, April 2 is not looking very impressive to say the least.  Models continue to show mostly southerly 850mb and 500 mb winds across Texas and Oklahoma.  This will cause storms that do manage to form off the dryline Friday afternoon to move parallel to it, which is not conducive for isolated supercells or tornadoes.  Before any of that, however, we have to deal with the first big issue — low level cloud cover and drizzle.

Intense warm air advection will be in place early Friday morning as a result of a 50 knot 850mb southerly jet.  This will help transport low level moisture over very wet Texas soil.  That will cause a thick low level cloud deck to form accompanied likely by drizzle which may linger into the afternoon.  For the first half of the day,  instability will be quite low, causing much of Texas and Oklahoma to be covered in clouds and drizzle.

As the intense upper trough ejects into western/central Texas Friday afternoon, it will cause all kinds of convective bands to develop.  Some will develop well ahead of the dryline across central/northern Texas from warm air advection, and some may develop along the dryline, extending into central Texas and Oklahoma in the evening.

But even in the absence of solar radiation, modest CAPE will develop.  Based on the latest models, deep layer shear will be mostly supportive of linear storms moving rapidly to the north. Should a clear slot develop in the afternoon across central or western north Texas, it would allow for much greater instability to form, and increase the potential for severe storms.

Overall the setup looks like a  big mess right now.  But given the 50 knot 850mb southerly jet, a 70 knot 500mb jet max, and the ‘hope’ for a clear slot in the afternoon, this setup shouldn’t be completely ignored.   In order for this setup to be more than just a big convective mess, the wind shear needs to improve (more southwesterly 500mb winds or more backed 850mb winds) and the low level cloud deck needs to find a way to mix out by midday so more instability can develop.

Next potential setup April 6/7?

Models are showing another intense shortwave ejecting into the Plains next Tue/Wed (April 6/7).  There is no need to discuss the details now, but that is the next ‘setup’ to keep your eyes on across the southern Plains.  After that it appears the MJO may fade again.  If so the El Nino pattern will become the main driver and, unfortunately, bring back a less than ideal pattern for severe storm setups across the Plains as we head into mid April.

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Saturday, 27 March Update

Friday, April 2 continues to look interesting for potential severe storms.  But nothing about the setup looks great yet.  According to both the European and GFS operational models, west Texas to western north Texas would be the area for severe storms/supercells to occur Friday afternoon.  The setup would feature a thick, low level cloud deck and drizzle for the first half of the day.  But, a pronounced dryline mixing east across west Texas would provide an opportunity for the cloud cover to erode.  Thus, enough instability may develop late in the afternoon across west Texas ahead of the dryline to support severe storms.  Although, how quickly the low level clouds erodes (if at all) is a major concern.

According to the GFS, another major concern will be storm motion.  Given the deep layer shear profile suggested by the GFS, storms would form off the dryline and move quickly to the north-northeast.  This motion would promote storms moving nearly parallel to the dryline and is not very conducive for sustained sueprcells and tornadoes.  However, the early stages of the storm’s life would provide some tornado potential given a strong southeasterly 850 mb jet.

There is still plenty of time for the details to change as the models continue to get a better handle on this system.  But, Friday remains something to keep an eye on across Texas.

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Thursday, 25 March Update

Next Friday through Sunday (April 2 – 4) continues to look interesting.  Friday the 2nd looks the most interesting right now, considering it will probably be the first day when enough quality low level moisture will be in place across western north Texas, the Texas panhandle, and parts of western Oklahoma to support severe storms.  Also, the ensembles show a deep trough approaching the southern plains that day.  So depending on how quickly that trough ejects eastward and how quickly moisture can return, Friday may end up being the first decent chase day of a series of chase days.

I do expect low level cloud cover issues along with drizzle/light rain across Texas next Friday.  This is because sea surface temperatures in the Gulf remain quite cool (2 to 3 degrees below normal, actually).  After two or three days of moisture advection from the warm Caribbean, across the very cool Gulf waters and over the very wet Texas soil, a thick low level cloud deck along with drizzle will develop across the southern plains.   But the approaching strong upper trough should allow for a pronounced dryline to develop, which would be the area to look for severe storms.  Hopefully enough dry air will mix near the dryline to erode most of the cloud cover by late in the afternoon.

00z March 25 GFS Ensemble 500mb Height Anomalies valid Friday, April 2

00z March 25 GFS Ensemble 500mb Height Anomalies valid Friday, April 2

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Tuesday, 23 March Update

The ensembles/longer range models are coming into good agreement with the MJO, GWO and other global signals.  For that reason it’s becoming evident that there will be chase setups during the first week of April across the Plains.  In fact, based on the ensembles, April 2-4 is becoming very interesting concerning severe storm potential.  But how quickly quality moisture can return north remains the biggest concern.   Based on what I am seeing, there should be at least one good setup with quality low level moisture available across the southern Plains in the April 2-4 period.   However, the entire first week of April will feature a weather pattern favorable for severe storm setups.

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I’ve been following the progression of the MJO over the last week or so.  Based on satellite trends and consistent MJO model forecasts it’s becoming more clear that the increasing convection across the Indian Ocean into the Indonesia region (Maritime Continent) is the MJO moving east.  Furthermore, based on the latest Ensemble forecasts across North America for the next 15 days, it’s becoming evident the MJO will play a key role in the weather pattern as we head into early April.  This is significant since, as of late, El Nino and the Arctic Oscillation have been the more significant factors.

As the MJO reaches phase 4 (Indonesia region) in late March/very early April, deep troughs should slam into the west coast of the U.S.  There should be at least a five day period with a mean trough centered over the southwestern U.S. and a mean ridge over the East.  Furthermore, the GWO is in sync with the MJO at this time, responding to low Global Atmospheric Angular Momentum.  This will enhance the deep troughing over the southwestern U.S. and help to provide a bit of ridging over the Southeastern U.S. –although the center of the ridge will be over the Great Lakes/Northeast.   For several days this eastern ridge will hopefully provide time for low level moisture from the Gulf to return northward into the southern Plains.

With all that being said, the synoptic scale weather pattern will become much more favorable for severe storm/chase setups across the southern to central Plains April 1 – 7.   The one key negative factor continues to be a source for quality low level moisture in the Gulf of Mexico.  For this reason, Texas and Oklahoma may again be the hot spots, but we’ll see how the Gulf recovers and how quickly moisture can return.  Soil moisture values remain very high over Texas, so that’s a good thing.

Below is the latest UKMET MJO model forecast.  It brings the MJO into phase 4 by March 25, but it’s unclear exactly how quickly the atmospheric response will impact the U.S.

UKMET MJO model forecast initialized 21 March

UKMET MJO model forecast initialized 21 March

Below is the 12z March 21 GFS Ensemble forecast for April 1.  Notice how it shows a deep trough over the southwestern U.S. ejecting east into the Plains on April 1.  This type of pattern should continue for several more days into April, providing potential chase setups across the Plains.  That is, assuming enough quality low level moisture can come into play over Texas and Oklahoma.

12z 21 March GFS Ensemble 500 mb height anomalies valid April 1

12z 21 March GFS Ensemble 500 mb height anomalies valid April 1

- Jim

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