Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 232001
SWODY1
SPC AC 231959

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0259 PM CDT Tue May 23 2017

Valid 232000Z - 241200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR SOUTHEASTERN
GEORGIA AND THE SOUTH CAROLINA COASTAL PLAIN...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR MUCH OF THE
TEXAS COASTAL PLAIN INTO SOUTH-CENTRAL TEXAS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM CENTRAL
TEXAS EASTWARD ALONG THE CENTRAL GULF COAST INTO COASTAL NORTH
CAROLINA...

...SUMMARY...
Isolated severe thunderstorms are possible over southeastern Georgia
and the South Carolina coastal plain through the early evening.  A
few tornadoes are possible along with damaging gusts.  Storms may
also impact parts of south-central Texas into Texas Gulf coastal
areas with a risk for severe wind and hail.

...Discussion...
The only substantial changes to the convective outlook include:
1) reducing severe probabilities across parts of AL-GA-SC to the
west and northwest of a cluster of strong to severe thunderstorms
this afternoon.  A tornado or two will remain possible across the
coastal plain of SC and GA through the early evening within a
moist/sufficiently sheared environment.
2) Removing low-severe probabilities across parts of IL.  Weak
instability and the lack of charge separation both indicate an
appreciable risk for marginally severe storms will be low enough to
not warrant a probability delineation.

..Smith.. 05/23/2017

.PREV DISCUSSION... /ISSUED 1123 AM CDT Tue May 23 2017/

...Synopsis...
Large-scale mid/upper troughing now encompasses much of the interior
U.S. from the Rockies to the Appalachians, and its positively tilted
axis may take on at least a bit more of a neutral tilt while
pivoting from the Plains into the Mississippi Valley through the
remainder of the period.  As it does, models indicate that an
embedded lower/mid tropospheric cyclone will continue to gradually
redevelop south southeastward from the upper into middle Mississippi
Valley, perhaps supporting some consolidation and deepening of
surface low pressure along an associated front near/north of the
lower Ohio Valley by 12Z Tuesday.

A preceding frontal zone has already reached much of the Gulf coast
into south Atlantic coast region, with the boundary currently
stalled to the lee of the southern Appalachians, ahead of a weak
frontal wave now evolving across southern Alabama.  This wave
accompanies one significant impulse embedded within the broader
scale cyclonic mid/upper flow field, which is forecast to continue
migrating east northeast of the lower Mississippi Valley, through
the southern Appalachians and southern/mid Atlantic coast region by
12Z Tuesday.

Upstream, an additional significant perturbation or two appear
likely to dig southeast of the south central high Plains before
turning eastward across the western Gulf coast/lower Mississippi
Valley region.  Associated with these features, a reinforcing cold
front is already in the process of surging into central and
southwest Texas, and is expected to advance through the lower Rio
Grande Valley and Texas Gulf coast by the end of the period.

Aided by destabilization beneath cold mid-level air, the environment
along and ahead of the reinforcing front, from parts of the Upper
Midwest into south Texas, probably will become conducive to
scattered thunderstorm development today into tonight.  However,
seasonably high moisture content air near/south of the lead frontal
zone appears to provide the focus for the most numerous thunderstorm
activity, across parts of the south Atlantic and eastern Gulf coast
states into the northern Gulf of Mexico.

...Eastern Gulf into south Atlantic coast states...
Lower/mid tropospheric lapse rates appear generally weak, but a
seasonably moist environment (surface dew points near 70F)
along/south of the stalled weak frontal zone across the region is
expected to become at least modestly unstable with daytime heating.
Coupled with enlarging low-level hodographs, associated with a 40-50
kt southwesterly 850 mb jet evolving near the frontal wave, the
environment near the northeastward advancing frontal wave appears
conducive to strong/severe storms with a risk for tornadoes and
potentially damaging surface gusts.  The primary threat area now
appears to be evolving across southwestern Georgia, and is expected
to shift east northeastward through portions of the piedmont and
coastal plain of South Carolina by this evening.

...South central Texas into Texas Gulf coastal areas...
Residual modest boundary layer moisture, coupled with steepening
lapse rates associated with pre-frontal surface heating beneath
relatively cold air aloft, probably will contribute to sufficient
instability to support a risk for strong/severe storm development
late this afternoon and evening.  Deep layer shear will be strong
enough beneath 40-50+ kt (and strengthening) cyclonic northwesterly
500 mb flow.  Aided by forcing for ascent associated with an initial
perturbation approaching the region, one or two organizing storm
clusters appears possible, generally focused near the initially slow
moving/stalled frontal zone across mid (and perhaps portions of
upper) Texas coastal areas by this evening.

...North central Illinois...
There are at least some indications in latest model guidance that an
area of enhanced forcing for ascent will become focused across the
region by late afternoon, ahead of the digging mid-level cold core.
Depending on the degree of insolation and destabilization that can
take place by late afternoon, it may not be out of the question that
the environment could become conducive of convection with at least
marginal severe weather potential.

$$



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