Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point. Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
Des Moines, IA...St. Joseph, MO...Waterloo, IA...Ames, IA...West Des Moines, IA...
St. Paul, MN...Madison, WI...Rockford, IL...Cedar Rapids, IA...Topeka, KS...
SPC AC 011955
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0255 PM CDT Tue May 01 2018
Valid 012000Z - 021200Z
...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
Numerous severe storms are expected over the central Great Plains to
the Mid-Missouri Valley. The most likely corridor is across parts of
central Kansas between 5 to 9 PM CDT. Tornadoes, a couple of which
could be strong, very large hail, and damaging winds will be
The only appreciable change from the previous outlook involved the
inclusion of parts of northwest OK into the categorical 2 (Slight
Risk). Recent runs of the HRRR suggest convective initiation near
the OK/TX border and subsequent supercell-like storms evolving from
this simulated convection.
.PREV DISCUSSION... /ISSUED 1144 AM CDT Tue May 01 2018/
...Central Great Plains to the Upper MS Valley...
12Z soundings across central OK and north-central TX sampled mean
mixing ratios near 12 g/kg within a plume of 63-64 F surface dew
points. Richer moisture with upper 60s and lower 70s dew points
remains confined to south TX this morning. This suggests
boundary-layer moisture return is probably one day too early from
being optimal for a regional severe weather/tornado outbreak. Even
so, confidence is relatively large in storm initiation and probable
evolution in the late afternoon and evening, rendering increased
large hail probabilities.
The dryline is expected to mix across the eastern TX Panhandle and
western KS as surface temperatures warm into the 80s, creating a
T-intersection with a quasi-stationary front arcing across
west-central KS into eastern NE. A broad plume of MLCAPE from
2000-2500 J/kg is anticipated ahead in the warm/moist sector.
Guidance is consistent in ejecting a mid-level speed max ejects from
the southern High Plains into KS by 00Z. This should aid in ascent
for probable storm development in the 20-22Z time frame along the KS
portion of the dryline. Decreasing certainty exists farther south
along the dryline where weak mid-level height rises are anticipated
to occur this evening as the broader trough over southern CA digs a
bit farther south.
50-kt effective shear, nearly perpendicular deep-layer winds to the
dryline, and rather steep mid-level lapse rates will favor discrete
supercells producing very large hail. A few of these supercells may
become long-tracked across central KS and the threat for tornadoes
will increase between 22-02Z as storms encounter richer
boundary-layer moisture (toward the I-135 corridor) and as low-level
shear increases rapidly in response to a strengthening low-level
jet. The synoptic pattern is largely consistent with past Great
Plains cases of the evening low-level jet strengthening and aiding
in mid/late evening significant tornadoes. However, low-level
moisture may remain on the lower margins of what is typical of the
high-end events (with similar temperature profiles and hodograph
structures). The slightly drier boundary layer will encourage a
little quicker increase in convective inhibition, coincident with
the strengthening low-level shear. Thus, it is not clear whether or
not this scenario will evolve into a long-lived significant tornado
threat near sunset, or if the inhibition increase will tend to limit
the window of opportunity. Otherwise, clusters of storms (with
embedded supercells) should persist well into the overnight hours
with an attendant threat for large hail/damaging gusts and a couple
tornadoes, from the triple point northeastward along the
quasi-stationary front into the Upper MS Valley.
An upslope flow regime will help maintain boundary-layer dewpoints
in the 40s across northeast CO through the afternoon, north of a
stalled front. Surface heating in cloud breaks will yield weak
surface-based buoyancy, while deep-layer vertical shear will be
favorable for supercells. A couple supercells may form this
afternoon in conjunction with the northern periphery of the
mid-level speed max ejecting into KS. These storms should move east
before weakening by mid-evening. Isolated large hail and strong
wind gusts will be possible.
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NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z