Severe Storm Outlook Narrative (AC)
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194
ACUS02 KWNS 250727
SWODY2
SPC AC 250727

Day 2 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
0227 AM CDT Mon Sep 25 2017

Valid 261200Z - 271200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

CORRECTED FOR DAY REFERENCE

...SUMMARY...
A very low threat for severe storms will exist in portions of
eastern Wisconsin Tuesday afternoon and from southeastern Arizona
into southwestern New Mexico from Tuesday evening onward.

...Synopsis...
A longwave trough in the West will finally break down as a
substantial, yet shorter-length mid-level trough ejects east toward
the Upper Mississippi Valley.  Farther southwest, models suggest
that the remaining portion of the trough over the Great Basin will
evolve into a closed low, while migrating southward slowly toward
the Lower Colorado River Valley.  Height rises will be noted
downstream of this closed low across the southern Plains as a ridge
strengthens some over the Lower Mississippi River Valley - and
modest strengthening of mid-level flow should occur across western
New Mexico and vicinity.

At the surface, a cold front will continue to migrate southward
across the southern Plains and extend from near the Big Bend of
Texas northeastward to central Missouri and on to another weak low
located near the Upper Peninsula of Michigan early Tuesday evening.
A modified maritime airmass will remain in place ahead of the front
and foster isolated to scattered convective development throughout
the day, with peak coverage occurring during the afternoon and early
evening.

...Eastern Wisconsin and vicinity...
Forcing for ascent - in the form of an approaching mid-level
shortwave and convergence along an approaching cold front from the
west - will foster isolated convective development for a few hours
in the early evening.  Instability should be relatively weak, with
only 500-1000 J/kg MUCAPE expected within the pre-frontal,
pre-convective airmass.  Wind fields should also be unidirectional
due to veered surface flow, although substantial strengthening in
the vertical (to around 50 kts at 6km AGL) will foster organization
with any storms that do develop.  A lower-end risk for gusty winds
may exist with this convection, and Marginal (5%) severe
probabilities may need to be introduced in later outlooks -
especially if buoyancy ends up stronger that currently anticipated.

...Southeastern Arizona into western New Mexico...
Low-level easterly flow will develop across the region throughout
the day Tuesday in response to surface ridging across the High
Plains.  As this occurs, modest moist advection will foster
increasing (40s to 50s F) dewpoints beneath 30-45 kt southerly
mid-level flow and moderately steep (7-7.5C/km) lapse rates near the
closed low.  This environment will foster isolated storms with a low
risk for hail and damaging wind - perhaps as early as 00Z, but
especially between 06Z-12Z.  At this time, the risk appears to be
too low for Marginal (5%) severe probabilities, especially given the
unfavorable time of day, although convective trends will need to be
monitored.

..Cook.. 09/25/2017

$$



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