Severe weather analysis - March 7, 2012
Severe weather/flood threat through the weekend
A closed mid- to upper low in the vicinity of the Arizona/New Mexico border is dragging a cold front southward. As the trough/associated cold front swings eastward toward the lower Mississippi valley, a strengthening low-level jet will promote continued moisture return from the relatively warm Gulf of Mexico waters. While this should effectively promote plenty of low-level moisture, it appears that isentropic lift will be too limited to generate much in the way of severe convection with this system, except for perhaps across portions of the Arklatex, where they will be more collocated with the most favorable dynamics associated with the front. The biggest impediment to a large scale severe weather outbreak are the relatively unimpressive lapse rates; the capping inversion I mentioned yesterday is still likely to limit vertical instability south and east of the Arklatex, curbing severe potential in spite of otherwise favorable atmospheric parameters. The primary concerns in regards to severe weather with this system will be large hail and damaging winds. The tornado threat appears much to conditional and/or marginal to warrant mentioning. Any severe weather associated with this system will likely occur on Thursday.
However, I should emphasize that the greatest threat with this system doesn't appear to be the severe weather, but the potential for flooding. The Hydrometeorological Prediction Center (HPC) in Cape Springs, Maryland, is forecasting a large swath of 3 to 6 inch rains across the lower Mississippi valley and Arklatex region through the five day period valid 00z March 08 through 00z March 13.
Figure 1. Latest 5-day Quantitative Precipitation Forecast (QPF) from the Hydrometeorological Prediction Center (HPC) in Cape Springs, Maryland. Notice the large amount of rain that is forecast to fall in that area though this period, with lesser (but still significant) amounts downstream.
All of this is forecast to occur as the aforementioned upper low, particularly well-defined at the moment, becomes quasi-stationary over the western Gulf Coast area, ultimately opening up into an elongated trough. The prolonged fetch of southerly flow should allow for a several day period of heavy rainfall downstream from the trough axis. I would not be surprised to see flood watches go up for portions of this area by tomorrow. However, any flood potential in this area will be largely mitigated by the extremely dry ground in this region, which bore the brunt of last year's significant drought.
Beyond the weekend
Beyond Saturday, a portion of the energy associated with the four corners upper low is forecast to detach from the decaying trough/cold front and move slowly northward. The models then forecast this impulse to become reinvigorated by a series of fast-moving vorticity maximums riding the jet stream. This appears poised to promote another trough to move eastward across the central plains and lower Mississippi valley, but the severe threat appears much too conditional and uncertain to name any specific risk areas at the moment.
Updated: 3:41 AM GMT on March 08, 2012
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Severe weather analysis - March 7, 2012
Thursday March 08, 2012
Not much is going on across the lower 48 after Friday's destructive tornado outbreak. However, a faint potential exists for some severe weather across portions of the Arklatex as well as perhaps northeastern Texas and extreme northwest Louisiana. Should this occur, it would be on Thursday. This is reflected in the thoughts of the Day 2 Convective Outlook recently issued by the Storm Prediction Center (SPC) in Norman, Oklahoma:
Figure 1. Latest Day 2 Convective Outlook issued by the Storm Prediction Center (SPC) in Norman, Oklahoma. Forecast valid from 1200Z Thursday March 08 to 1200Z March 09, 2012. For translational purposes, 1200Z would be 8:00 AM central standard time (CST).
The catalyst for this weather will be a developing mid- to upper-level trough now seen on water vapor imagery approaching west Texas. This trough is forecast by the reliable global and regional models to become positively-tilted as it gradually progresses eastward. There is already evidence of this happening on the aforementioned imagery. Although warm air advection (WAA) ahead of this system will be fairly large, with at least lower to mid 60s (F) dew points possible across the Arklatex and lower Mississippi Valley on Thursdasy, upper air soundings within the slight risk area indicates that a substantial mid-level capping inversion exists directly above this layer of abundant moisture in the lower troposphere. This cap is not forecast to break significantly during the day on Thursday. This, along with the likelihood of a pre-frontal linear band of convection/weak squall line, suggests the tornado threat is minimal. The primary threats instead appear to be damaging wind gusts and/or large hail.
In comparison to other severe events, and certainly compared to Friday's historic event, the severe weather threat with this system remains marginal at best.
Farther south, the inversion will be even greater, curbing the potential for significant destabilization within the pre-frontal warm sector. However, this trough is forecast to effectively stall over the western Gulf beyond Thursday/Friday, which could lead to a several day period of heavy rainfall. This could help to alleviate the ongoing drought across Texas:
Figure 2. Latest Texas drought map (February 28, 2012) as provided by the US Drought Monitor.
By Sunday/Monday, another significant perturbation within the belt of mid-latitude westerlies is forecast to move eastward and then east-northeast across the lower to middle Mississippi valleys. Not only does predictability remain expectantly low for this next system, it is likely that the several day period of cloud cover/heavy rains associated with the aforementioned system will preclude appreciable destabilization/vertical instability ahead of the cold front.
In short, no significant severe weather events appear likely for the next week or so, the isolated event on Thursday notwithstanding.
Updated: 9:05 AM GMT on March 07, 2012
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