Severe Storm Outlook Narrative (AC)
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000
ACUS01 KWNS 270544
SWODY1
SPC AC 270542

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1242 AM CDT Mon Mar 27 2017

Valid 271200Z - 281200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FOR PARTS OF
WESTERN KENTUCKY INTO WESTERN AND MIDDLE TENNESSEE AND NORTHERN
MISSISSIPPI...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
THE LOWER MISSISSIPPI VALLEY INTO PARTS OF THE TENNESSEE AND LOWER
OHIO VALLEYS...

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

...SUMMARY...
Severe thunderstorms will be possible from parts of the lower
Mississippi and Tennessee valleys northward to the Ohio Valley today
into this evening.  Hail and damaging winds will be the primary
severe hazards with the stronger storms.

...Synopsis...
A vigorous shortwave trough Sunday night on water-vapor imagery over
the central Great Plains will move eastward into the lower OH Valley
during the period.  An associated strong belt of cyclonically curved
mid-level flow will overspread the lower MS Valley into the TN and
OH Valleys.  In the low levels, a northward advancing warm front
will move through the TN Valley.  A weak surface low will develop
east-northeast across the MS Valley into the OH Valley as a cold
front moves east across parts of the Mid South before stalling over
the lower MS Valley.

...portions of the OH-TN-MS Valleys...
Low-level moisture will return north into the Mid South and OH
Valley within a WAA regime associated with a 35-kt LLJ during the
morning.  Showers and isolated thunderstorms will likely be ongoing
near the MS river at daybreak and move downstream into TN and the
lower OH Valley.  There is some uncertainty regarding the magnitude
of destabilization farther north near the OH River where clouds and
early-day convection may limit destabilization.  Nonetheless, models
show a plume of upper 50s dewpoints near the OH River to the low-mid
60s in the MS Delta.  The increasing moisture coupled with diurnal
destabilization south and west of early activity will probably
result in 1000-1500 J/kg MLCAPE developing by mid afternoon with
weak buoyancy forecast farther north near the OH River.  Forcing for
ascent provided by the mid-level wave in conjunction with
convergence along the surface boundary over eastern AR northward to
the OH-MS River confluence will likely serve as a focus for storm
development during the afternoon.  Deep-layer shear is forecast to
be strongest from TN southward into northern MS/AL with effective
shear magnitudes 35-50 kt.  Cellular clusters of supercells and
organized multicells are forecast with hail (perhaps very large with
the most intense storm or two) and damaging winds the main threats.

Farther southeast in the free warm sector over MS and AL, weak
(30-m) 500-mb height falls during the day coupled with heating and
weakening convective inhibition may result in storms developing
within confluence bands in absence of any well-defined boundary.
Convection-allowing models suggest isolated to widely scattered
storms and a risk for hail and wind damage will probably accompany
the stronger storms.  This activity over the Deep South and TN/OH
Valleys will likely weaken during the evening as it moves towards
the Appalachians owing to the loss of heating and moving east of the
instability axis.

..Smith/Marsh.. 03/27/2017

$$



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