SPC AC 210535
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1235 AM CDT Sat Sep 21 2019
Valid 211200Z - 221200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE CENTRAL PLAINS INTO THE LOWER TO MID MISSOURI VALLEY AND ACROSS
PARTS OF THE UPPER MISSISSIPPI VALLEY...
Severe storms are possible across parts of the Plains this afternoon
and evening, most concentrated from Kansas into Iowa. Severe storms
are also expected across parts of the Upper Mississippi Valley.
...Central Plains and Lower to Mid Missouri Valley...
Southwest mid-level flow will be in place today from the Great
Plains eastward into the Mississippi Valley. At the surface, a cold
front will advance eastward across the central and northern Plains.
Ahead of the front, surface dewpoints in the upper 60s and lower 70s
F will contribute to the development of a corridor of moderate
instability by afternoon. Thunderstorms are first expected to
develop along the eastern edge of the instability corridor from
eastern Kansas into northern Missouri around midday. Further to the
west, strong surface heating and enhanced low-level convergence
along the front should result in scattered thunderstorm development
during the mid to late afternoon from southwest Kansas northeastward
into southeast Nebraska. MCS formation will be possible across the
area during the early to mid evening.
Concerning the severe weather potential, RAP forecast soundings
along the axis of strongest instability from Wichita, Kansas
northeastward to south of Omaha, Nebraska at 00Z/Sunday show MLCAPE
values peaking in the 2500 to 3500 J/kg range. Wind profiles are
forecast to support supercells with 0-6 km shear in the 35 to 45 kt
range with substantial directional shear in the lowest 2 km AGL.
Steep mid-level lapse rates and strong speed shear in the mid-levels
should result in a threat for large hail. Hailstones greater than 2
inches in diameter will be possible with the more dominant
supercells along the instability axis. An isolated tornado and
wind-damage threat should accompany supercells. As cell coverage
markedly increases along and ahead of the front during the early to
mid evening, the wind-damage threat should become more dominant. If
an MCS can organize, then the wind damage and hail would be possible
into the late evening from south-central Kansas northeastward into
...Upper Mississippi Valley...
An upper-level trough will move quickly northeastward across the
northern Plains today as a cold front advances eastward across the
eastern Dakotas. Ahead of the front, surface dewpoints should reach
the 65 to 70 F range, contributing to the development of moderate
instability by afternoon. Scattered thunderstorms appear likely to
develop along and ahead of the front during the afternoon across
central and northern Minnesota. Another area of convective
development could take place in the post-frontal airmass across
northeastern North Dakota. RAP forecast soundings ahead of the front
across central and northern Minnesota this afternoon show MLCAPE
values in the 2000 to 2500 J/kg range and have strong deep-layer
shear of 50 to 55 kt. This combined with speed shear in the
mid-levels and steep lapse rates should support an isolated large
hail threat, especially if supercells can form. Supercells should
also be accompanied with an isolated tornado and wind-damage threat.
Storms should move quickly east-northeastward across central and
northern Minnesota during the late afternoon and early evening.
Southwest mid-level flow will be in place today across the southern
Plains as an upper-level trough moves across the Desert Southwest. A
lead shortwave trough is forecast to be present in the southern High
Plains this afternoon. Ahead of this feature, scattered thunderstorm
development should take place from west Texas northward into the
Texas Panhandle this afternoon. The airmass is forecast to remain
weakly unstable. In spite of this, moderate deep-layer shear and
steep low-level lapse rates should be enough for a marginal
wind-damage threat by late afternoon. Hail will also be possible
with the stronger updrafts.
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