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 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
ACUS01 KWNS 041242
SWODY1
SPC AC 041241

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0641 AM CST SUN DEC 04 2016

VALID 041300Z - 051200Z

...THERE IS A MRGL RISK OF SVR TSTMS FROM COASTAL SOUTHEASTERN
LOUISIANA TO PARTS OF THE WESTERN AND CENTRAL FLORIDA PANHANDLE...

...SUMMARY...
THUNDERSTORMS WITH A MARGINAL WIND DAMAGE AND/OR TORNADO THREAT WILL
BE POSSIBLE ACROSS PARTS OF THE CENTRAL GULF COAST TODAY.

...SYNOPSIS...
IN MUD/UPPER LEVELS...MEAN TROUGHING WILL PERSIST OVER THE ROCKY
MOUNTAIN REGION AS THREE STRONG SHORTWAVE PERTURBATIONS PROGRESS
THROUGH ASSOCIATED CYCLONIC FLOW:
1.  SOUTHERN-STREAM CYCLONE OVER WEST-CENTRAL/NORTHWESTERN MEXICO...
PART OF A HIGH-AMPLITUDE TROUGH FROM SOUTHEASTERN NEW MEXICO TO THE
PACIFIC SOUTH OF BAJA.  THE ASSOCIATED 500-MB LOW SHOULD DEEPEN
SOMEWHAT AS IT PIVOTS EASTWARD THEN NORTHEASTWARD ACROSS NORTHERN
MEXICO...REACHING THE SERRANIAS DEL BURRO RANGE OF NORTHERN COAHUILA
BY 12Z.  AS THIS OCCURS...HEIGHT FALLS WILL SPREAD ACROSS THE WESTERN
HALF OF THE GULF OF MEXICO FROM WEST TO EAST.
2.  NORTHERN-STREAM TROUGH -- CURRENTLY OVER THE DAKOTAS AND CENTRAL
PLAINS AND LOOSELY IN PHASE WITH THE MEXICAN PERTURBATION VIA A
CONNECTIVE VORTICITY BANNER OVER THE SOUTHERN HIGH PLAINS.  THIS
TROUGH AND ITS ASSOCIATED VORTICITY FIELD IS MUCH MORE PROGRESSIVE...
HOWEVER...AND WILL SEPARATE...DEAMPLIFY AND EJECT NORTHEASTWARD ACROSS
THE UPPER GREAT LAKES THROUGH THE PERIOD.
3.  INITIALLY LOW-AMPLITUDE PERTURBATION AND SPEED MAX -- NOW
EVIDENT IN MOISTURE-CHANNEL IMAGERY OVER THE PACIFIC NORTHWEST AND
SOUTHWESTERN CANADA -- WILL DIG EAST-SOUTHEASTWARD ACROSS THE
NORTHERN ROCKIES...HELPING TO MAINTAIN THE MEAN TROUGH THROUGH THE
DAY-2 PERIOD.

AT THE SURFACE...THE 11Z ANALYSIS SHOWED A COLD FRONT...ILL-DEFINED
WELL INLAND...FROM SOUTHWESTERN MISSISSIPPI SOUTHWESTWARD ACROSS
SOUTHWESTERN LOUISIANA AND JUST OFFSHORE FROM THE TEXAS COAST.  A
WARM FRONT EXTENDED FROM A CONVECTIVE COMPLEX SOUTH OF THE
MISSISSIPPI COAST EAST-SOUTHEASTWARD ACROSS THE NORTHEASTERN GULF.
A WEAK FRONTAL-WAVE LOW ANALYZED INITIALLY EAST OF PADRE ISLAND MAY
DEEPEN AND MOVE NORTHEASTWARD TOWARD THE MID/UPPER TEXAS COAST NEAR
THE END OF THE PERIOD.  THE COLD FRONT SHOULD DRIFT EASTWARD AND BE
A DIRECT PLAYER IN CONVECTIVE POTENTIAL ONLY OVER THE GULF.  THE
WARM FRONT WILL APPROACH THE WESTERN/CENTRAL COASTAL FLORIDA
PANHANDLE AND MAY MOVE SLIGHTLY INLAND BEFORE BEING OVERTAKEN BY
CONVECTION.  RAIN TO ITS NORTH WILL CONTRIBUTE TO STABILIZATION THAT
WILL IMPEDED THE NORTHWARD PROGRESS OF THE FRONTAL ZONE ON MESO-BETA
TO LOCAL SCALES.

...CENTRAL GULF COASTAL REGION...
SCATTERED THUNDERSTORMS CURRENTLY ARE EVIDENT IN TWO PRIMARY
CONVERGENCE BANDS:

1.  PREFRONTAL...FROM NEAR LIVINGSTON PARISH...LA...SOUTHWESTWARD OVER
THE NORTHWESTERN GULF.  THIS ACTIVITY WILL SHIFT EASTWARD THROUGH
MUCH OF TODAY...WITH CONDITIONAL POTENTIAL THAT ITS ASSOCIATED
LOW-LEVEL CONVERGENCE ZONE JUST AHEAD OF THE FRONT CAN ENCOUNTER
WARM-SECTOR AIR FARTHER EAST.  MOST...IF NOT ALL...OF THE INLAND AREAS
OF LOUISIANA AHEAD OF THIS ACTIVITY WILL REMAIN ON THE COLD SIDE OF
THE WARM FRONT AND/OR OUTFLOW FROM ACTIVITY FARTHER EAST.  EVENTUAL
SURFACE-BASED INFLOW INTO ANY OF THIS CONVECTIVE PLUME...INLAND FROM
THE IMMEDIATE COAST...WILL BE VERY DEPENDENT ON MESOSCALE PROCESSES
CONTRIBUTING TO DESTABILIZATION BEHIND THE OTHER BAND.

2.  A SHORTER...BROKEN LINE -- GENERATING IN THE SURFACE WARM SECTOR
AND CROSSING THE WARM FRONT -- FROM COASTAL MOBILE COUNTY...AL...
SOUTHWESTWARD ACROSS PLAQUEMINES PARISH...LA.  ALTHOUGH THE WARM
FRONT MAY REMAIN ALONG OR JUST SOUTH OF THE ALABAMA AND FLORIDA
PANHANDLE COAST PRIOR TO PASSAGE OF THESE THUNDERSTORMS...
SURFACE-BASED EFFECTIVE-INFLOW PARCELS EXTEND INTO THE BAROCLINIC
ZONE...AND SOME SEVERE THREAT CANNOT BE RULED OUT.  REFER TO SPC
MESOSCALE DISCUSSION 1866 FOR NEAR-TERM GUIDANCE ON THE MARGINAL
WIND/TORNADO THREAT WITH THIS ACTIVITY.

ADDITIONAL DEVELOPMENT IS POSSIBLE NEAR THE SOUTHERN PORTION OF THE
EASTERN AREA OF LOW-LEVEL CONVERGENCE...WHETHER BEFORE OR WHEN THE
ZONE OF LIFT RELATED TO THE WESTERN BAND CATCHES UP.  COASTAL
COUNTIES FROM ABOUT THE MOBILE BAY AREA EASTWARD OVER PARTS OF THE
FLORIDA PANHANDLE MAY DESTABILIZE THIS AFTERNOON FROM A COMBINATION
OF WARM ADVECTION AND WEAK DIABATIC HEATING...WITH ENLARGED
HODOGRAPHS IN THE WARM FRONTAL ZONE SUPPORTING POTENTIAL FOR
SUPERCELL OR BOWING STRUCTURES.  WEAK DEEP-LAYER LAPSE RATES WILL
STUNT BUOYANCY DESPITE THE RICH WARM-SECTOR MOISTURE...KEEPING SBCAPE
GENERALLY UNDER 800 J/KG OVER THE COAST...AND VERY WEAK AT BEST MORE
THAN ABOUT 30 NM INLAND.

..EDWARDS/LEITMAN.. 12/04/2016

$$


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