Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 210627
SWODY2
SPC AC 210626

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1226 AM CST Sun Jan 21 2018

Valid 221200Z - 231200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF THE SOUTHEAST STATES...

...SUMMARY...
Isolated severe thunderstorms are possible as a cold front moves
through portions of the Southeast on Monday.

...Synopsis...
Mature cyclone initially centered over the central Plains will eject
northeastward through the mid MS Valley and into the upper Great
Lakes region as the overall upper pattern trends more progressive.
Strong low- to mid-level flow will accompany this system, with a
streak of 100+ kt at 500 mb arching from the base of the system
through its eastern periphery and winds over 50 kt at 850 mb
spreading over much of the eastern CONUS.

At the surface, occluded low will track into the Upper Great Lakes
while a cold front sweeps across the Southeast and TN/OH Valleys.
This cold front is expected to extend from the western WV Panhandle
southwestward into the central FL Panhandle at 12Z Tuesday. Showers
and thunderstorms are anticipated along this front but the
increasing displacement between the better low-level moisture
(across the Southeast) and the stronger forcing for ascent (across
the mid MS and OH Valleys) will likely mitigate the overall severe
potential.

...Southeast...
Thunderstorms will likely be ongoing across the lower MS Valley at
the beginning of the period. These storms will likely be in the
midst of a weakening trend as instability wanes and surface
temperatures reach their diurnal minimums. Some reintensification is
possible during the afternoon, particularly across central AL, as
the cold front continues eastward and modest daytime heating and
moisture return occur within the downstream airmass. Instability
will likely remain modest (i.e. MLCAPE below 500 J/kg), limiting
updraft strength and tempering the overall severe threat, but a few
storms capable of damaging wind gusts are possible.

...Central IL...
A shortwave trough is expected to rotate through the parent system
and into the mid MS valley during the afternoon. Forcing for ascent
attendant to this shortwave as well as the resulting surface trough
will provide the impetus for convection within the modestly unstable
environment cultivated by cooling mid-level temperatures. Most of
the storms within the resultant band of convection will be shallow
but updrafts may occasionally be deep enough to support lightning
production as well as small hail.

...VA Piedmont...
Rapid airmass modification is anticipated ahead of the approaching
upper system and cold front. Mid-level temperatures should be cool
enough to support lightning but guidance varies on whether or not
enough modification occurs to support surface-based storms. Given
the strength of the flow across the region (cloud-layer mean wind is
over 60 kt), any surface-based storms would pose a risk for damaging
wind gusts. However, confidence in surface-based convection is too
low to introduce any severe probabilities with this forecast.

..Mosier.. 01/21/2018

$$



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