Severe Storm Outlook Narrative (AC)
Issued by NWS
ACUS01 KWNS 200551
SPC AC 200550
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1150 PM CST Sun Feb 19 2017
Valid 201200Z - 211200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS EARLY TODAY IN
ACROSS PARTS OF EASTERN TEXAS INTO ADJACENT PORTIONS OF SOUTHEASTERN
OKLAHOMA...SOUTHWESTERN ARKANSAS AND WESTERN LOUISIANA...
Strong thunderstorms could impact a corridor from upper Texas
coastal areas into the Ark-La-Tex region early today, accompanied by
some risk for severe weather.
Much of North America remains under the influence of split branches
of westerlies emanating from the mid-latitude Pacific. Within
perhaps the most prominent branch, models indicate that large-scale
troughing will continue to gradually approach the Pacific coast. On
the southeastern periphery of this feature, and the northwestern
through northern periphery of downstream large-scale ridging
building across the southern Great Basin and Rockies, an intense
mid/upper jet is forecast to continue nosing inland across northern
California through the northern Great Basin. As this occurs,
increasingly sheared downstream troughing is forecast to progress
through Manitoba/Ontario, the Mississippi Valley, the southern U.S.
Plains and the lower Rio Grande Valley, while sharp ridging shifts
east of the Mississippi Valley toward the Atlantic Seaboard.
Associated with a vigorous impulse within the northern portion of
the mid continent troughing, a significant surface cyclone is
forecast to continue deepening while migrating from the eastern
Dakotas through Manitoba/northwestern Ontario today. A trailing
occluding surface front is expected to advance across the Upper
Midwest into the Great Lakes region, while weakening and eventually
shifting northeastward across parts of the northern and central
Plains. Meanwhile, in association with a distinct remnant impulse
within the southern portion of the elongating upper troughing,
surface troughing appears likely to shift from the southern Plains
through the lower Mississippi Valley.
Within a plume of higher moisture content returning northward ahead
of this latter feature, the remnants of an extensive band of
convection may be ongoing across parts of upper Texas coastal areas
through southeastern portions of the Plains at 12z this morning.
Guidance is suggestive than an associated conglomerate surface cold
pool may tend to cut-off, or at least slow, richer boundary layer
moisture return from the western Gulf of Mexico. As this occurs,
potential for vigorous convective development seems likely to wane
inland of the northwestern Gulf coast region, with little potential
for strong/severe storm development evident north of this region
through the period.
...Eastern Texas into southwestern Arkansas/western Louisiana...
Models, including the latest Rapid Refresh, suggest that southerly
850 mb flow will remain on the order of 30-50 kt within a corridor
southward across the Ark-La-Tex into upper Texas coastal areas
through the 12-16Z time frame. This may contribute to an
environment at least marginally conducive to strong surface gusts
near any lingering vigorous thunderstorm activity. Thereafter, as
this belt of stronger flow continues to weaken and/or shift north of
the region, even with modest boundary layer recover by late
afternoon, potential for severe storm development seems minimal.