Severe threat in the Plains Friday-Saturday

Sitting in the 40's in Oklahoma, and poised to drop into the mid 30's tonight, it may be difficult to imagine a return to warmth and regular thunderstorm threats. This blast of arctic air will have one last night with us, before giving way to a warm southerly wind upstream of the departing high pressure. By Tuesday, the southern Plains will be returning to temperatures closer to normal for mid October. 

As I discussed in the last post, short wave disturbances rounding the base of the Canadian trough will gradually lift the trough to the northeast through the week. As a result, the Midwest, Great Lakes, and New England will stay on the cold side while the rest of the nation returns to normal temperatures.

Meanwhile, as the block over the eastern Pacific breaks down, a closed low near the California coast will move onshore through the Southwest on Thursday, and approach the lee of the Rockies on Friday. Classic lee-cyclogenesis begins in eastern Colorado, with increasing southeasterly winds downstream across the southern Plains converging toward the surface trough.

12z GFS forecast for 7pm Friday evening

Aloft, the closed low is projected to stay sharp, with strong downstream upper level divergence. Combined with increasing low level moisture, this forcing should be enough to trigger widespread showers and thunderstorms. By Friday afternoon, shear profiles are looking very impressive. The limiting factor for severe weather will be marginal lapse rates, with lackluster CAPE.

The upper level low will continue northeastward on Saturday, with intensifying low pressure at the surface over the central Plains states. Once again, CAPE is limited, but shear is impressive. Any thunderstorms that develop strong updrafts will easily become rotating.

12z GFS forecast for 7pm Saturday evening

As the upper level low moves northward, it will interact with the strong baroclinic zone over the Great Lakes and Northeast, under the upstream side of the Canadian trough. Low pressure at the surface will strengthen in this environment. As it runs into cold high pressure to the north, it will also enhance frontogenesis from across the northern Great Lakes into New England. This could lead to a band of heavy rain impacting the aforementioned areas next weekend.

So we can expect some eventful weather from this system by the end of the week. There are still several details to be ironed out in order to get the full picture of the specific threats we may be facing. The picture will get a little clearer once the storm comes onshore and is fully sampled by sounding data, etc.