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ACUS48 KWNS 070937
SWOD48
SPC AC 070936
Day 4-8 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
0436 AM CDT Thu Oct 07 2021
Valid 101200Z - 151200Z
CORRECTED FOR WORDING THIRD PARAGRAPH
...DISCUSSION...
...Sunday/Day 4 to Wednesday/Day 7...
The medium-range models are in relatively good agreement on Sunday,
moving an upper-level trough quickly eastward into the southern High
Plains. Ahead of the approaching trough, low-level moisture will
advect northward across east Texas, eastern Oklahoma and western
Arkansas. As is forecast by the ECMWF, a corridor of moderate
instability is expected to develop from northeast Texas into eastern
Oklahoma by early to mid evening. Thunderstorms should first develop
along a dryline across west-central Oklahoma, with convection
quickly growing upscale and moving eastward across eastern parts of
the southern Plains. Model forecasts suggest a focused band of
large-scale ascent and strong deep-layer shear will overspread the
southern Plains Sunday evening. This should support severe storm
development, with supercells likely. Supercells with large hail and
wind damage will be possible. A tornado threat will also be possible
as the low-level jet ramps up quickly during the early evening.
However, uncertainties still exist concerning the amount of
instability that will develop and type of storms that will form. If
a squall-line organizes quickly, then the favored severe threat
would be wind damage and large hail. At this time, confidence in a
severe event is great enough to maintain the threat area and add a
30 percent contour in for parts of southeastern Oklahoma.
On Monday, the upper-level trough and an associated cold front are
forecast to move into the mid Mississippi Valley. Thunderstorms will
be likely along and ahead of some portions of the front from the
morning into the afternoon. Although instability is forecast to
remain weak in most areas ahead of the front, strong deep-layer
shear will still support severe storm development. The greatest
potential for severe storms would be across parts of Arkansas and
southern Missouri during the morning and early afternoon as an MCS
moves across the region. However, there are uncertainties concerning
how fast the front will move eastward and the positioning of
convection at the start of the period. If a linear MCS is ongoing at
the start of the period, then that would affect the east-to-west
position of the severe threat area.
On Tuesday, an upper-level low is forecast to develop across the
central Rockies as an upper-level trough moves across the
southwestern U.S. A powerful 80 to 100 knot jet is forecast by the
GFS to eject northeastward across New Mexico and into the southern
High Plains. Ahead of the system, low-level moisture and instability
will rapidly increase according to the GFS. Under this scenario,
moderate instability would be expected to develop by afternoon in
much of central/east Texas, Oklahoma and central/ eastern Kansas.
This would involve numerous thunderstorms developing just to the
east of a southern and central Plains dryline. Storms would quickly
obtain a severe threat and move eastward across the region during
the late afternoon and early to mid evening. Surface dewpoints in
the lower to mid 60s F along with moderate instability and strong
deep-layer shear would support supercell development. Supercells
would be capable of producing large hail, wind damage and tornadoes.
However, the ECMWF is much slower with the upper-level system. This
would mean that the event would be delayed until either the
overnight period on Tuesday or even into Wednesday across the
southern and central Plains. Given this uncertainty, will maintain a
15 percent area for Tuesday. If the trough is faster then the threat
area could need to be upgraded. But if the trough is slower, then a
threat area would need to be added for Wednesday.
...Wednesday Night/Day 7 and Thursday/Day 8...
From Wednesday night into Thursday, the models move the upper-level
trough northeastward across the north-central states as a cold front
advances rapidly eastward across the Mississippi, Ohio and Tennessee
Valleys. Thunderstorms will be likely along portions of the front
both Wednesday and Thursday afternoon and evening. Although a severe
threat will be possible on both days, uncertainty is substantial at
this range in the forecast.
..Broyles.. 10/07/2021
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