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Source: US National Oceanic and Atmospheric Administration

Day 4-8 Severe Weather Outlook Issued on Aug 16, 2019
Updated: Fri Aug 16 08:50:02 UTC 2019


Mon, Aug 19, 2019 – Tue, Aug 20, 2019
Thu, Aug 22, 2019 – Fri, Aug 23, 2019
Tue, Aug 20, 2019 – Wed, Aug 21, 2019
Fri, Aug 23, 2019 – Sat, Aug 24, 2019
Wed, Aug 21, 2019 – Thu, Aug 22, 2019
(All days are valid from 12 UTC – 12 UTC the following day)

Note: A severe weather area depicted in the Day 4-8 period indicates 15%, 30% or higher probability for severe thunderstorms within 25 miles of any point.
PREDICTABILITY TOO LOW is used to indicate severe storms may be possible based on some model scenarios. However, the location or occurrence of severe storms are in doubt due to: 1) large differences in the deterministic model solutions, 2) large spread in the ensemble guidance, and/or 3) minimal run-to-run continuity.
POTENTIAL TOO LOW means the threat for a regional area of organized severe storms appears unlikely (i.e., less than 15%) for the forecast day.

 Forecast Discussion

ACUS48 KWNS 160848
SPC AC 160848

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0348 AM CDT Fri Aug 16 2019

Valid 191200Z – 241200Z

Height rises will occur initially over the CONUS on Mon/D4 as an
upper trough ejects northeast across Quebec. However, a second minor
amplification is expected beginning late D4/Mon night over the
Dakotas and Minnesota as subtle height falls develop out of Canada.
While the ECWMF is more amplified with this trough than the MREF
members, especially into Tue/D5, all models suggest up to 50 kt
northwest 500 mb winds impinging upon a large reservoir of
instability over the northern Plains and Midwest, with impressive
midlevel lapse rates for this time of year (8.0-8.5 C/km). Further,
there is good model overlap with the convective signal which shows
storms initiating D4/Mon evening over SD due to strong heating and
increasing nocturnal warm advection. The result may be a severe MCS
Monday night into Tuesday, from SD into southern MN, IA, and
northern IL, with potentially damaging winds.

Given inherently low predictability for multi-day MCS tracks, will
begin this scenario with a Slight Risk for Mon/D4 from SD into MN,
with the possibility of further upgrades on Tue/D5 into IL should
further model runs show good run-to-run consistency.

By Wed/D6, the previously large pool of instability over the
northern Plains/Midwest will rapidly diminish as the air mass
overturns from the D4-D5 storms, and temperatures aloft warm behind
the upper trough as it continues east across Ontario and Quebec.

..Jewell.. 08/16/2019