Severe Storm Outlook Narrative (AC)
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000
ACUS01 KWNS 181300
SWODY1
SPC AC 181259

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0659 AM CST Wed Jan 18 2017

Valid 181300Z - 191200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS SOUTHEAST
TEXAS AND SOUTHWEST LOUISIANA...

...SUMMARY...
Isolated damaging wind gusts and a brief tornado cannot be ruled out
with thunderstorms this morning across southeast Texas, and again
tonight across portions of southwestern Louisiana and southeastern
Texas.

...Southeast TX and southwest LA...
An apparent midlevel impulse may be providing focused ascent for an
ongoing cluster of thunderstorms located in vicinity of the Houston
metro area and northeast along and north of a boundary that extended
southern LA to southwest MS and central AL.  Effective bulk shear of
30-40 kt and a rather moist environment (surface dew points near 70
F along the TX coast to far southwest LA) combined with surface
vorticity near the aforementioned boundary into southeast TX suggest
storms should persist this morning.  And, low-level rotation is
possible as observed with a couple of earlier storms near and
southwest of the Houston metro area.  However, weak lapse rates and
weak winds in the 2-3 km agl layer per LCH 12Z sounding suggest
sustained rotation and storm organization will tend to be hampered.
Thus, low tornado probabilities will not be introduced with this
outlook issuance.

While convection should persist much of this forecast period, an
increase in thunderstorm coverage is expected this evening and
tonight from the TX coast through the lower MS Valley and the Mid
South, with a renewed potential for a marginal severe-weather threat
across southeast TX and southwest LA.  This will occur as greater
height falls spread into this region ahead of an upper trough moving
into the southern Plains.  A jet streak along its eastern flank is
forecast to track eastward across portions of the south-central
States, resulting in a weak wave developing northeastward along a
surface front draped from south Texas to the lower MS Valley region.
Sufficient deep shear will exist around the jet streak to support
weakly organized convective clusters, perhaps supporting localized
damaging wind gusts.  Weak lapse rates will continue to marginalize
the severe threat.

...Central TX to southeast OK and southwest MO...
Trends in lightning data and radar imagery showed a separate cluster
of elevated thunderstorms continuing to develop and track to the
northeast across central TX early this morning.  This convection is
expected to remain well northwest to north of the TX coastal
boundary, with the primary forcing mechanism being an apparent
transient northeastward-moving midlevel impulse.  Downstream
forecast soundings into north-central TX and southeast OK today
suggest elevated thunderstorms rooted in the 850-700 mb layer will
be possible.  Given these factors, the western portion of the
general thunderstorm area has been expanded some from central TX to
southwest MO.  Weak instability should preclude a severe weather
threat with these elevated storms.

..Peters.. 01/18/2017

$$



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