Major Severe Weather Outbreak Possible This Week

[steve2470]

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2952

The atmospheric ingredients are aligning for what could be intense severe weather this week, especially on Wednesday and Thursday. A powerful upper-level low is expected to bring the storminess to the Plains, Midwest, and South as it slowly makes its way eastward. The low is now pushing into California, where its power is being put to good use: providing much-needed snow in the Sierras and rain at lower elevations. From 4” to 8” of snow was observed above 3500 feet in the Sierra over the weekend, and winter storm warnings are in effect for 8-16” of higher-elevation snow late Monday into Tuesday, with local two-foot amounts possible on the highest peaks. The San Francisco Bay area could get anywhere from 0.5” to 2” of rain. While this won’t come close to breaking the severe multi-year drought across California, it’ll at least add a few drops to the bucket and give residents a psychological boost. The cold upper-level air may even lead to severe thunderstorms over central California on Tuesday, with a tornado or two possible.



Figure 1. Dew points as of 9:00 am CDT Monday, April 6, had already risen above 55°F across Texas and Oklahoma, with 65°F values moving north from the Gulf of Mexico. Higher dew points indicate richer moisture near the surface; most severe weather occurs with dew points of at least 55°F. Image credit: NCAR/RAL Real-Time Weather Data.



Ahead of the upper low, high-level southwesterly winds will overspread much of the country over the next several days. This flow over the Rockies will help maintain surface low pressure over the high plains of Colorado and Kansas, and the circulation will pull in plenty of moisture. As shown in Figure 1 above, dew point temperatures (the temperature to which the air needs to be cooled in order for the relative humidity to reach 100%) are already near 70°F along the Texas coast--not too far from typical summertime values. This rich moisture will surge north through the week, with mild, humid air possibly making it as far north as Chicago and Cleveland by Thursday. The juxtaposition of muggy low-level air and the cold upper-level storm will produce strong instability over a wide area. However, a very warm, dry layer sandwiched between the two—an atmospheric “cap”—should keep storms from becoming widespread on Monday and Tuesday. Any storms that do manage to break through the cap could quickly become severe, especially along a dry line from Texas to Missouri. Late Monday and again late Tuesday, overnight storms could produce severe wind or hail across parts of northeast Kansas and Missouri.