Severe Storms Looming for Early April

[steve2470]

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2947

The first ten days of April could produce more severe weather than the modest amount racked up so far across the U.S. in 2015. The same upper-level pattern that kept the West warm and the East cold through most of the winter has also kept severe weather to a minimum, as northwest flow and a series of cold fronts pushed warm, unstable air off the U.S. mainland. This year through March 30, we’ve seen a preliminary total of a mere 38 tornadoes, compared to a January-March average over the preceding three years of 163. As of Tuesday morning, NOAA’s Storm Prediction Center had issued just four tornado watches and four severe thunderstorm watches for the year thus far. This compares to a long-term Jan.-Mar. average (1970 – 2014) of around 39 tornado and 24 severe thunderstorm watches, according to SPC’s Greg Carbin.



Figure 1. Severe-weather outlooks issued by NOAA’s Storm Prediction Center on Tuesday morning, March 31, valid for Tuesday (top), Wednesday (center), and Thursday (bottom). Image credit: NOAA/SPC.


The 2015 numbers look destined to rise over the next few days, as a major circulation change takes place over North America. The long-prevailing northwest flow is being replaced this week by more zonal (west-to-east) flow, with an embedded upper-level impulse reaching the Midwest on Wednesday and another targeting the Southern Plains on Thursday. A moist low-level air mass is already in place, with dew points close to 60°F from Tulsa, OK, to Birmingham, AL. Surface low pressure generated by the upper-level impulses should continue to pull the moisture pool northward and generate unstable conditions. SPC placed a broad belt from Oklahoma to Georgia under a slight risk of severe weather for Tuesday afternoon and evening, with large hail possible as the moisture continue to flow north.

A large swath of the Great Plains is under an slight risk for Wednesday, from northwest Oklahoma to southern Minnesota, with an slight risk in place for Thursday across part of the Southern Plains and mid-Mississippi Valley. The predominantly west-to-east upper flow combined with southerly low-level flow will enhance vertical wind shear, a key ingredient in the formation of supercell thunderstorms. The main threats appear to be high winds and large hail, although tornadoes can’t be ruled out. As was the case last Wednesday, when F2 tornadoes struck the Tulsa suburb of Sand Springs and the Oklahoma City suburb of Moore, the corridor from Interstate 44 into east central Oklahoma could be a particular focal point for supercell formation by late Thursday. Severe weather may continue into the lower Mississippi Valley on Friday before the associated cold front moves into the Gulf of Mexico.

Forecast models indicate a strong upper-level trough will settle across the Great Basin by early next week, setting the stage for what could be a more extensive multiday round of severe weather beginning as soon as Sunday. A strong surface low should develop over the High Plains by late in the weekend, pulling unstable air back northward across a large area beneath west-southwest upper flow. Strong thunderstorms could spread across the Midwest and South by early next week, with several days of focused severe weather possible.