Severe Storm Outlook Narrative (AC)
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000
ACUS01 KWNS 231239
SWODY1
SPC AC 231238

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0638 AM CST Tue Jan 23 2018

Valid 231300Z - 241200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE TIDEWATER AND COASTAL MID-ATLANTIC REGIONS...

...SUMMARY...
Strong thunderstorms may impact parts of the Mid Atlantic Coast
region early today, accompanied by some risk for severe weather.

...Synopsis...
Within a progressive upper-air pattern, the most pertinent feature
for this forecast will be a mid/upper-level cyclone, currently
centered near the southern terminus of Lake Michigan, with shortwave
trough southwestward across the Mid-South region.  As an initially
separate/northern-stream shortwave trough digs southeastward over
the James Bay region, the Great Lakes cyclone will accelerate
east-northeastward and devolve into a strong open-wave trough.  By
00Z, the remnant 500-mb low should be located over easternmost areas
of ON with trough southwestward over the central/southern
Appalachians.  The trough should weaken and exit the Northeast
coastline thereafter.

At the surface, the 11Z analysis shows an occluded surface low over
west-central Lake Huron, and related occluded/cold front arching
across western PA, west-central VA, northwestern SC, southern GA,
and the east-central/south-central Gulf.  The front is forecast to
sweep eastward today and move off most of the Atlantic coast by 00Z,
with the possible exception of eastern New England and southern FL.

...Tidewater/Mid-Atlantic...
An ongoing band of convection was evident in radar composites near
an axis from ILM to RWI then diffusely northeastward across the
Chesapeake Bay area.  This activity mostly has been producing no
lightning, but with forecast/modified soundings suggesting the top
of the buoyant layer close to the lower reaches of suitable
mixed-phase zones, isolated/intermittent thunderstorms are possible
the remainder of this morning as the band continues moving
northeastward.  Deep/speed shear will remain favorable beneath
90-100 kt 500-mb southwesterlies.  Mean flow aligned nearly parallel
to the primary convective band indicates continued quasi-linear
mode.  Modified 12Z WAL/MHX RAOBs and forecast soundings indicate
instability will remain very limited, especially with northward
extent; however, isolated damaging gusts may penetrate to the
surface before this activity moves offshore late morning into early
afternoon.

Limited diurnal destabilization should occur behind the initial
convective band, based on cloud breaks evident in IR imagery and
continued low-level warm advection immediately preceding the cold
front.  However, considerable doubt exists on the development/
coverage of any additional sustained convection in that
northeastward-shifting corridor -- given currently observed and
expected veering of prefrontal surface winds, and resultant negative
effect on boundary-layer convergence near the front.

..Edwards.. 01/23/2018

$$



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