Severe Storm Outlook Narrative (AC)
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210
ACUS01 KWNS 200508
SWODY1
SPC AC 200507

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1107 PM CST Sun Nov 19 2017

Valid 201200Z - 211200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
The risk for severe thunderstorms appears negligible across the U.S.
today through tonight.

...Discussion...
Models indicate considerable amplification within the westerlies
across the eastern Pacific into western North America during this
period.  By late tonight, this appears likely to include sharpening
large-scale ridging centered near or just inland of the Pacific
Coast.  Downstream of this ridging, the initially split belts of
westerlies may remain more distinct, but several digging short wave
troughs will contribute to a tendency for increased troughiness east
of the Rockies, after a short-lived deamplification of the upper
flow in the wake of large-scale troughing now rapidly shifting
across and east of the Atlantic Seaboard.

Perhaps the most prominent perturbation, a short wave within the
northern stream, likely will be accompanied by continued significant
surface cyclogenesis from the Canadian Prairies through northwestern
Ontario and adjacent portions of the upper Great Lakes region.
Despite associated strengthening southwesterly low-level flow across
parts of the central Plains into the Great Lakes region, no
appreciable moisture return is expected in the wake of the prior
frontal passage, which has reached southern Florida and southern
portions of the Gulf of Mexico.

At least some boundary layer modification is possible across the
northern Gulf of Mexico, as southern branch troughing digs southeast
of the Texas Gulf Coast into the Gulf of Mexico, but large-scale
ascent and destabilization near or inland of coastal areas is not
expected to become supportive of an appreciable risk for
thunderstorms.  Ahead of the upper troughing, low-level flow may
gradually turn from northeasterly/easterly to easterly/southeasterly
across the Florida Peninsula.  By late tonight (toward 12Z Tuesday),
this may be accompanied by sufficient moistening to contribute to
weak to moderate destabilization near southeast coastal areas, where
pockets of enhanced low-level convergence may support a risk for
isolated to widely scattered thunderstorm activity.

Otherwise, just in advance of the building upper ridge, a mid-level
cold pocket (associated with an increasingly deformed mid-level
impulse migrating inland of the Pacific coast) may potentially
become supportive of weak destabilization across the Puget Sound
vicinity this morning into the mountains of northern Idaho/northwest
Montana this afternoon.  It might not be out of the question that
this could be accompanied by some convection capable of generating
lightning, but any such activity is generally expected to be
short-lived, and too sparse in coverage (less than 10 percent
probability) for a categorical thunder outlook.

..Kerr.. 11/20/2017

$$



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