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.