Valid 180100Z – 181200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE NORTHERN HIGH PLAINS THIS EVENING AND ACROSS PARTS OF THE UPPER
MIDWEST OVERNIGHT…
…SUMMARY…
A severe storm or two remains possible across the northern High
Plains for a few more hours this evening, and overnight in parts of
the Upper Midwest.
…Northern High Plains…
Thunderstorms ongoing across eastern MT into western SD/ND have
produced several reports of 50 mph wind gusts, with two severe
gusts measuring 67 and 72 mph across western SD in the past couple
of hours. This threat will continue to shift northeast across
eastern MT and western ND over the next few hours before storms
cross the international border.
…Upper Midwest Vicinity…
Thunderstorms have developed across central SD in weak low level
convergence along the surface dryline/effective cold front.
Convection is expected to continue northeast and increase in
coverage toward eastern SD into parts of MN and northwest IA
overnight as a low level jet increases and modest forcing associated
with the shortwave trough over MT/WY glances the region. Most of
this activity will likely be elevated given convection is not
expected until after 03z, but modest lapse rates in a favorable
shear environment could produce some isolated, marginal severe hail
with the strongest cells. There is a small chance that an isolated
cell could become nearly surface-based along the immediate frontal
boundary from central MN into northwest IA. If this occurs, strong
gusts be possible. Furthermore, while very low, a brief tornado
cannot be ruled out given higher-quality moisture with eastward
extent and backed low level flow enhancing 0-1 km SRH.
…Upper TX Coast to Southwest LA coast…
Heavy rain is the main concern with Tropical Depression Imelda
overnight. While backed low level flow in a moist and weakly
unstable tropical airmass will result in effective SRH approaching
100-150 m2/s2 over the next 6 hours or so, stronger cells from
Chambers County TX to Cameron Parish LA have only shown very weak
cyclonic shear. Shear will become less favorable after around 06-09z
and the threat for any tropical cyclone-related tornadoes appears
negligible.
..Leitman.. 09/18/2019
$$