Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 271742
SWODY2
SPC AC 271741

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1241 PM CDT Mon Mar 27 2017

Valid 281200Z - 291200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS TUESDAY
AFTERNOON AND EVENING OVER PARTS OF WEST-CENTRAL TEXAS AND SOUTHWEST
OKLAHOMA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS
WEST-CENTRAL AND NORTH-CENTRAL TEXAS INTO MUCH OF OKLAHOMA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER MUCH OF
TEXAS AND OKLAHOMA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS MUCH OF
THE MID-ATLANTIC REGION...

...SUMMARY...
Severe storms are forecast on Tuesday into Tuesday night over parts
of west-central and north-central Texas, and much of Oklahoma.  A
few strong to severe storms are also possible over parts of the
Mid-Atlantic region.

...Synopsis...
The southern extent of a progressive Pacific trough, currently
moving through OR and northern CA, will amplify today as it digs to
the southeast, reaching the Four Corners region by the start of Day
2.  This trough should move a little more slowly eastward on Tuesday
with a closed midlevel cyclone forming across NM, as the synoptic
trough approaches western portions of the central and southern
Plains.  Farther east, the shortwave trough, currently located
across MO/AR, is expected to be weakening as it moves east across
the Mid-Atlantic region and the Carolinas through Tuesday afternoon.

...TX/OK...
Strengthening southerly low-level wind fields over TX, beginning
late Day 1, will result in the rapid transport of Gulf moisture
northward, with dewpoints in the 60s ahead of a dryline in west TX
Tuesday afternoon.  This moisture combined with strong low-level
warm air advection (along the low-level jet) and forcing for ascent
with the lead shortwave trough suggests early period convection will
be likely across the northern extent of the southern High Plains.
This early day convection (mostly elevated) should include the OK/TX
Panhandles into western OK and southward across west-central TX
(perhaps toward MAF).  Moderate elevated instability and effective
bulk shear of 30-35 kt should allow for a few severe storms within
this Tuesday morning convection, resulting in a little westward
expansion of the marginal severe risk.

The progress of the lead trough across west TX Tuesday morning and
expected early period convection should disrupt the development of
stronger destabilization with northward extent, especially across
the TX Panhandle and parts of western OK for the severe-weather
threat into the afternoon or evening.  And, these Tuesday morning
thunderstorms and cloudiness may tend to retard the northward
movement of the warm front into OK.  Despite these factors
potentially limiting the extent of the greater severe probabilities
into parts of OK Tuesday afternoon and evening, the fact that the
upper trough has just entered the North American upper air
observations, this issuance will not make any changes to the Slight
and Enhanced risk areas.

Thus, the previous discussion remains valid for Day 2.  The slow
movement of the upper trough/closed low on Tuesday suggests the 00Z
ECMWF has a better forecast for the placement of the dryline in
west-central TX Tuesday afternoon. The area near the triple-point of
the aforementioned boundaries is likely to be the focus for
supercell thunderstorm development.  Forecast soundings in the warm
sector continue to indicate ample low level moisture beneath steep
mid level lapse rates, yielding afternoon MLCAPE values around 2000
J/kg.  Substantial low-level and deep-layer vertical shear are
expected, supportive of supercells capable of very large hail,
damaging winds, and tornadoes.

Storms will progress east-northeastward into south-central OK and
north-central TX during the evening, with a continued enhanced-risk
of severe weather.

...Mid-Atlantic region...
No changes are needed with this outlook issuance for Day 2.  The
timing of the MO/AR shortwave on Tuesday with a favorable area of
forcing for ascent moving across eastern VA/NC during the afternoon
(peak heating), and should be offshore around 29/00Z.  Despite a
warm and relatively moist low-levels, with dewpoints around 60 F,
afternoon MLCAPE values close to 1000 J/kg will be possible.
Scattered thunderstorms are forecast to develop ahead of the trough
over parts of PA/MD/VA/NC and track eastward toward the coast by
evening.  The strongest cells in this area could produce hail and
locally damaging wind gusts, with this potential being greatest with
southern extent into NC where deep-layer shear will be stronger.
The primary limiting factors precluding the introduction of higher
severe probabilities for NC, at this time, include the timing of the
shortwave trough and warming 500-mb temperatures tempering midlevel
lapse rates to support sustained updrafts.

..Peters.. 03/27/2017

$$



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