Convective Addiction

Significant Severe Weather Outbreak Expected – FCST: 04/10/11

Posted on 10. Apr, 2011 by Jesse in Uncategorized

A significant severe weather outbreak appears poised to take aim on the Upper Mississippi Valley/Superior Uplands region of the Upper Midwest on Sunday afternoon into the evening hours. This potentially significant SVR weather event could feature all modes of SVR weather, including large hail, damaging winds, and strong tornadoes from long-lived, discrete supercells.

A potent trough will continue gyrating eastward, promulgating a 120kt upper-level jet core right towards the risk area by early tomorrow afternoon as trough completes its amplification phase. This stalwart jet stream level wind maxima will be ejecting NE on the lee (east) side of the trough, while at the mid-levels (30,000 ft), a closed low will gyrate eastward across the Dakotas and into MN, while an associated mid-level trough is complemented by a seasonably impressive 90-100kt H5 (500 mb) jet core that takes aim at the I-35 corridor by late afternoon, placing parts of IA/WI/IL/MO/MI in the favored exit regions of the jet streak where quasi-geostrophic forcing for ascent would induce upward vertical motions.  Cold air aloft of -10 to -15 C will serve to steeper lapse rates across the region as the main energy approaches.

A layer of warm air aloft (CINH) should preclude an extremely early initiation, though as the better forcing arrives and instability builds, the CINH should erode by later afternoon.  Strong low-level flow will enhance moisture influx behind the warm front, with surface dewpoints well into the lower 60s prevalent across the warm sector by early afternoon, with 850 mb flow AOA 40 kts.  By 00z the H85 low-level flow could approach 60 kts, creating a potentially serious situation as low-level shear is enhanced, enlarging hodographs and contributing to a marked increase in the tornado threat should supercells maintain a broken, discrete status ahead of the main surface forcing.  Should this occur, a favored environment for long-lived supercells with destructive tornadoes would be plausible given backing surface winds of 15-25 kts.  Ahead of the main cold front, as surface winds orient slightly more parallel to the front, the threat for damaging winds will increase as well, and of course lapse rates appear favorable for large hail > 1.5.

JLR