(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
ZCZC SPCSWOD48 ALL
ACUS48 KWNS 210849
SPC AC 210849
Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0349 AM CDT Fri Sep 21 2018
Valid 241200Z - 291200Z
...DISCUSSION...
Latest runs of the medium-range models appear to be in good
agreement trough day 7 (Thursday 9-27), with respect to large-scale
pattern evolution. On days 4-5 (Monday and Tuesday), an upper
trough is forecast to move gradually eastward across the central
U.S., while a second trough -- on the western fringe of larger-scale
cyclonic flow -- begins to dig southeastward across western Canada.
By day 6 (Wednesday), as the initial trough shears quickly
east-northeastward across eastern Canada, the digging western trough
is forecast to ride southeastward into the long-wave trough over the
central U.S. -- eventually rotating eastward across the Upper Great
Lakes and ito eastern Canada through day 7 (Thursday).
At the surface, a cold front -- associated with the initial upper
trough -- is forecast to sweep east across the northern and central
Plains U.S. day 4, the Great Lakes/Midwest day 5, and then the
Northeast day 6. Meanwhile, a second front -- associated with the
subsequent/digging trough -- is forecast to cross the northern and
central Plains day 6, and then the Great Lakes/Midwest Day 7 in a
similar manner as the initial front. While both of these fronts
will likely be accompanied by an increase in convective risk, the
second front appears at this time to be following too
closely/quickly the initial boundary, so as to preclude ample
moisture return/destabilization potential, and thus severe risk with
the second front may remain limited.
However, more appreciable severe weather potential appears to exist
with the initial front, particularly day 4 -- from the central
Plains to western portions of the Upper Great Lakes, and day 5 from
the Ozarks to the Great Lakes. Some risk may continue Day 6 into
the Northeast, but risk appears lesser at this time. Meanwhile,
limited risk may redevelop over the central Plains day 6 ahead of
the next front, but -- as mentioned earlier -- questions regarding
moisture return ahead of that second front preclude the introduction
of a day 6 outlook area over the central Plains at this time.
..Goss.. 09/21/2018
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