Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 210604
SWODY2
SPC AC 210603

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
0103 AM CDT Fri Jul 21 2017

Valid 221200Z - 231200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE MIDWEST
AND EXTENDING EAST ACROSS THE CENTRAL APPALACHIANS...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER THE UPPER
MISSISSIPPI VALLEY AREA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
NORTH CENTRAL AND NORTHEAST U.S. SLIGHT RISK AREAS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
SOUTHERN ARIZONA...

...SUMMARY...
A broad area of severe weather risk is apparent from portions of the
northern and central Plains east across the Midwest to the mid
Atlantic region.  A few severe storms may also occur across portions
of southern Arizona.

...Synopsis...
Moderately strong west-northwesterly flow aloft is forecast to
persist across the northern tier of the U.S. this period, with
numerous small-scale disturbances embedded therein.  A stronger
short-wave trough/low is forecast to move out of south-central
Canada and across the north-central U.S. with time.

At the surface, a migratory low is forecast to move along a
pre-existing front across the Midwest states, eventually reaching
the mid-Atlantic/southern New England coast.  A weaker low/frontal
system is expected to shift across the upper Mississippi Valley
region.

...Midwest to the mid Atlantic...
A large area of convection is forecast to be ongoing over the
southern Great Lakes area and vicinity early in the period, which
should shift east-southeast with time.  While some severe risk may
persist with the ongoing convection, new storm development is
forecast on the southern and western flank of this convection, as
afternoon heating of a very moist boundary layer supports
mixed-layer CAPE values in the 2500 to 3500 J/kg range across much
of the Midwest and into the central Appalachians.  Multiple
bands/clusters of storms will likely traverse the area through the
afternoon and evening, before convection diminishes in coverage and
intensity late in the period.  In the mean time, risk for damaging
winds and hail will exist with stronger storms/storm clusters moving
quickly east-southeast across the region.

...Upper Mississippi Valley vicinity...
In the wake of overnight/early-period convection, some
heating/destabilization is forecast across the upper Mississippi
Valley area in advance of the compact upper low/short-wave trough
progged to be shifting east-southeast out of the Canadian Prairies.
Ascent -- focused near a weak surface frontal system -- should
support scattered storm development during the afternoon and
evening, spreading east toward the western upper Great Lakes area
with time.  Enhanced mid-level westerly flow -- sufficient for
updraft organization -- suggests potential for locally damaging
winds and hail with stronger cells, lingering well into the evening
hours.

...Southern Arizona...
Daytime heating of a moist boundary layer progged to reside over
southern Arizona will support airmass destabilization, with ample
high-based CAPE to support fairly widespread storm development over
the higher terrain.  With mid-level ridging to gradually strengthen
over the Great Basin, some enhancement of northeast mid-level flow
could eventually allow storms to organize/congeal, and spread off
the rim into the lower deserts.  As such, evaporatively enhanced
downdrafts could result in locally damaging winds from late
afternoon through the evening, and possibly into the overnight hours
before convection slowly diminishes.

..Goss.. 07/21/2017

$$


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