Severe Storm Outlook Narrative (AC)
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
000
ACUS01 KWNS 031259
SWODY1
SPC AC 031257

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0657 AM CST SAT DEC 03 2016

VALID 031300Z - 041200Z

...THERE IS A MRGL RISK OF SVR TSTMS FROM THE LOWER RIO GRANDE
VALLEY TO SOUTHWESTERN LOUISIANA...

...SUMMARY...
THUNDERSTORMS MAY PRODUCE MARGINALLY SEVERE HAIL AND STRONG TO
LOCALLY DAMAGING WINDS ACROSS PARTS OF SOUTH TEXAS...THE UPPER TEXAS
COASTAL PLAIN AND SOUTHWESTERN LOUISIANA.  A TORNADO CANNOT BE RULED
OUT...BUT THIS THREAT IS MARGINAL AND ISOLATED.

...SYNOPSIS...
A HIGHLY AMPLIFIED UPPER-AIR PATTERN FEATURES A STRONG
NORTHERN-STREAM SHORTWAVE TROUGH FROM THE DAKOTAS SOUTHWARD ACROSS
THE CENTRAL PLAINS...AND A CLOSED...NEARLY CUT-OFF SOUTHERN-STREAM
CYCLONE OVER NORTHWESTERN MEXICO...THE GULF OF CALIFORNIA AND BAJA.
THAT CYCLONE IS FORECAST TO PIVOT SOUTHEASTWARD THROUGH MUCH OF THE
PERIOD...THEN EASTWARD TOWARD NORTH-CENTRAL MEXICO.

AT THE SURFACE...AN 11Z ANALYSIS SHOWED A WAVY FRONTAL ZONE FROM
ABOUT 100 NM SOUTH OF THE CENTRAL LOUISIANA COAST TO JUST OFFSHORE
FROM BRAZORIA AND GALVESTON COUNTIES...TX...TO NEAR BAFFIN BAY...TO A
WEAK LOW OVER ZAPATA COUNTY...THEN SOUTHWARD OVER NORTHEASTERN
MEXICO.  THE GULF SEGMENT IS A WARM FRONT DRIFTING ERRATICALLY
NORTHWARD...THE REST QUASISTATIONARY.  THE LOW SHOULD REMAIN NEAR ITS
PRESENT LOCATION...ON EITHER SIDE OF THE RIO GRANDE...UNTIL IT BEGINS
TO SHIFT SOUTHEAST OR EAST LATE TONIGHT.  THE FRONT WILL LIKEWISE
START TO MOVE SOUTHEAST TONIGHT OVER SOUTH TEXAS AND ADJACENT GULF...
PERHAPS ASSISTED BY CONVECTIVE OUTFLOW...WHILE BEING NEARLY
STATIONARY OR DRIFTING VERY SLOWLY NORTHWARD ELSEWHERE.

...SOUTH TEXAS TO SOUTHWESTERN LOUISIANA...
ALTHOUGH ISOLATED STRONG STORMS CANNOT BE RULED OUT IN THE TIME AND
SPACE BETWEEN THEM...TWO REGIMES WILL BE RESPONSIBLE FOR THE BULK OF
THE MARGINAL SEVERE RISK THROUGH THE PERIOD:

1. NOW THROUGH MOST OF THE PERIOD...SOUTHEAST TEXAS AND SOUTHWESTERN
LOUISIANA:
SCATTERED THUNDERSTORMS SHOULD CONTINUE MAINLY ALONG AND NORTH OF
THE FRONT...WITH WIDELY SCATTERED CONVECTION ALONG AN INTERSECTING
WARM-SECTOR CONVERGENCE BOUNDARY NOW EVIDENT FROM WESTERN GALVESTON
ISLAND SOUTH-SOUTHWESTWARD ACROSS THE OPEN GULF.  THE BULK OF
CONVECTION OVER THIS REGION SHOULD ORIGINATE IN THE WARM-SECTOR
CONVERGENCE ZONE AND ITS ELEVATED EXTENSION NORTH OF THE SURFACE
FRONT.  THE SURFACE WARM SECTOR MAY EXTEND SLIGHTLY INLAND OVER THE
UPPER TEXAS COAST AND SOUTHWESTERN LOUISIANA...BUT ONLY IN NARROW
CORRIDOR...GIVEN:
* PERSISTENT CONVECTIVE PRECIP EXPECTED NORTH OF THE FRONT...WHOSE
OUTFLOW AND RELATED REINFORCEMENT OF STATIC STABILITY WILL IMPEDE
THE POLEWARD PROGRESS OF THE BAROCLINIC ZONE;
* THE PRESENCE OF MID/UPPER RIDGING AND ASSOCIATED ANTICYCLONIC FLOW
ALOFT...NEARLY PARALLEL TO THE FRONT; AND
* RELATED LACK OF MEANINGFUL LARGE-SCALE SUPPORT ASIDE FROM WARM
ADVECTION...WHICH WILL CONTRIBUTE OFFSETTING FACTORS IN THE FORM OF
DESTABILIZATION NEAR THE FRONT BUT PARCELS LIFTED TO AN LFC FOR
MAINTENANCE OF STORMS TO ITS NORTH.

STILL...ANY SUPERCELLS THAT CROSS THE WARM FRONT WHILE MATURE MAY
POSE A TORNADO RISK...ALONG WITH DAMAGING GUSTS AND MARGINALLY SEVERE
HAIL.  THAT ENVIRONMENT SHOULD REMAIN CHARACTERIZED BY RELATIVELY
MAXIMIZED LOW-LEVEL VORTICITY/SRH...SURFACE DEW POINTS UPPER 60S TO
LOW 70S F AND WEAK LAPSE RATES ALOFT...WITH 500-800 J/KG MLCAPE.

2.  OVERNIGHT...SOUTH TEXAS:
HEIGHT FALLS ASSOCIATED WITH THE APPROACHING MEXICAN MID/UPPER-LEVEL
LOW WILL BEGIN TO SPREAD ACROSS DEEP SOUTH TEXAS AND THE LOWER RIO
GRANDE VALLEY REGION OVERNIGHT...CONTERMINOUS WITH THE SOUTHEASTWARD
ADVANCE OF THE FRONT.  AS LOW-LEVEL MASS RESPONSE OCCURS BOTH AT AND
ABOVE THE SURFACE...THUNDERSTORM COVERAGE SHOULD INCREASE OVER DEEP
SOUTH TEXAS AND ADJOINING MEXICO AFTER 00Z...SPREADING NORTHEASTWARD
AND BACKBUILDING AS WELL.  STORMS SHOULD UNDERGO A NET EASTWARD
SHIFT ACROSS THE LOWER VALLEY LATE TONIGHT.  ACTIVITY SHOULD BE
CLUSTERED TO QUASI-LINEAR IN MODE...GIVEN THE GEOMETRY OF THE
DEEP-LAYER FLOW...AND WILL POSE A RISK OF ISOLATED DAMAGING GUSTS AND
SEVERE HAIL...AND A TORNADO CANNOT BE RULED OUT WITH ANY QLCS
CIRCULATIONS OR EMBEDDED SUPERCELLS ACCESSING SURFACE-BASED INFLOW
IN THE WARM SECTOR BEFORE MOVING OFFSHORE.

..EDWARDS/LEITMAN.. 12/03/2016

$$



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.