Severe Storm Outlook Narrative (AC)
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000
ACUS03 KWNS 220831
SWODY3
SPC AC 220829

Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0229 AM CST Wed Feb 22 2017

Valid 241200Z - 251200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FOR CENTRAL AND
NORTHERN INDIANA INTO NORTHWESTERN OHIO AND FAR SOUTHERN LOWER
MICHIGAN...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF
SOUTHERN LOWER MICHIGAN AND MUCH OF THE OHIO VALLEY...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK...

...SUMMARY...
One or more bands of strong to severe thunderstorms are forecast
across the southern Great Lakes and Ohio Valley on Friday and Friday
night.

...Synopsis...
A mid-level trough/low is forecast to move from the middle MO Valley
eastward to the western Great Lakes and OH Valley and assume a
neutral to slightly negative tilt.  As this occurs, a belt of strong
and cyclonically curved 500-mb flow is forecast to intensify in
excess of 100-kt over the OH Valley late Friday night.  A surface
low near the MO/IA border will develop northeastward into northern
Lower Michigan as a warm front to its east advances northward.  A
cold front near the MS River around midday Friday will sweep
eastward across the OH and TN Valleys by daybreak Saturday.

...southern Great Lakes and Ohio Valley...
Low-level moisture is forecast to slowly increase on strong
southerly flow with boundary-layer dewpoints forecast to range 52-58
degrees F.  Although cloud cover will retard strong surface heating,
cooling mid-level temperatures to around -19 degrees C will
contribute to weak buoyancy (ranging from 250-1000 J/kg MUCAPE)
within the northward expanding warm sector during the day.  As
strong forcing for ascent (DCVA) approaches and overspreads the
western parts of the area, a band of thunderstorms will likely
develop and intensify.  Strong effective shear around 50 kt will act
to organize updrafts and strengthening 700-mb flow to the 55-60 kt
range will contribute to cold pool`s organization and upscale
growth. Downward momentum transport via damaging winds are the
predominant severe risk.  However, some forecast soundings show
relatively moist low levels with strong 0-1 km shear in excess of
25-30 kt.  A tornado risk may develop with the maturing squall line
and/or pre-frontal supercell(s) that eventually merges with the
line.  A gradual weakening in buoyancy by the early to mid evening
into the overnight will likely lead to a lessening in the
damaging-wind risk as storms rapidly move east and northeastward
after dark.

...TN and southern Appalachians...
Storm development over this region will likely be inhibited by an
initially capped warm sector until stronger 500-mb height falls
arrive towards evening.  Model guidance suggests a squall line will
zipper southward into parts of TN as shear profiles strengthen.
Damaging winds will be the primary threat.

..Smith.. 02/22/2017

$$



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