Wednesday, 23 April 2014

Severe weather chances in the USA increasing

The central USA has had a very quiet start to the severe weather season. Indeed, since 'modern' tornado records started (in 1950) the previous latest date that the first tornado fatality occurred was April 21st, which was in 2002. In 2004, it was April 20th - that streak ended abruptly when 8 people were killed in Illinois.

Through today, no deaths have occurred thus far this year, which is excellent news, and marks this year as a record in that regard. However, it seems that a multi-day, multi-faceted severe weather episode is about to unfold. As the first image below shows that an upper trough will move into the central USA today, followed by a much more substantial one this weekend/early next week, as depicted by the ECMWF model, and courtesy of MeteoGroup.

Moisture return has commenced from the Gulf of Mexico, although not yet that robust. By mid-afternoon, dewpoints in the western Plains will be around 15C or so, which, beneath steep mid-level lapse rates, should yield 1000-1500 or so J/Kg of CAPE just east of the dryline, and with reasonable shear, high-based supercells capable of large hail and damaging winds will be the prime threat from Texas to Nebraska.

Stronger moisture return will commence on Saturday as the major upper trough approaches - dewpoints east of the dryline in Texas, Oklahoma, and Kansas should be in the high teens by peak heating on Saturday, with 2000-3000J/Kg, or so, of CAPE amidst increasing deep-layer shear. Supercells capable of very large hail, strong winds, and a few tornadoes seem possible.

By Sunday, strong moisture return through the lower Mississippi Valley, and perhaps eastern KS/OK/TX (depending on the eastward speed of the system) should yield 1500-3500J/Kg of CAPE, or so, amidst strongly veering wind profiles. Supercells may grow upscale into a severe squall line - initial activity poses the risk of strong, long-track tornadoes (dependant on how much convection develops within the return flow).

By Monday, the risk area may have shifted eastwards towards Alabama, etc - both ECMWF and GFS's overall pattern is fairly similar, and one which would support a continuation of strong moisture return (dewpoints 18-21C) amidst strong deep layer shear, and strong low-level shear. Another day of supercells with a risk of strong tornadoes is possible. The chart below shows ECMWF's 500 hPa heights (solid lines), surface dewpoints (colours), and  850 hPa wind barbs for late afternoon on Monday.

In summary, several days of severe weather are likely in the central and southern USA starting today, but Saturday to Monday looks like being potentially significant multi-day severe weather event.