AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JACKSON MS
304 PM CST TUE DEC 28 2010
...SEVERE WEATHER EVENT POSSIBLE NEW YEARS EVE...
.DISCUSSION...
THE FIRST SECTION OF THIS DISCUSSION WILL COVER THE SEVERE WEATHER
EVENT POTENTIAL FOR LATE WEEK...AND WILL BE FOLLOWED BY A DISCUSSION
COVERING THE REMAINDER OF THE FORECAST PERIOD.
NEW YEARS EVE...
SYNOPTIC OVERVIEW: LATER THIS WEEK...A LARGE...DEEP-LAYER CYCLONE
WILL BE BARRELING EAST OF THE ROCKIES AS A DEEP-LAYER RIDGE AMPLIFIES
OVER THE EAST COAST AND WRN ATLC. THE TROUGH WILL HAVE GAINED
CONSIDERABLE AMPLIFICATION ON THURSDAY FROM STRONG UPPER-LEVEL JET
ENERGY DIVING INTO THE BACKSIDE OF THE TROUGH. A SW-NE-ORIENTED JET
MAX / 115 KT AT H2 / WILL BE PUSHING THROUGH THE TROUGH AXIS OVER THE
SOUTHERN PLAINS ON FRIDAY. THIS WILL OCCUR AS A MID-LEVEL CYCLONE
CUTS OFF IN THE BASE OF THE TROUGH AND RACES NORTH-NORTHEASTWARD OVER
THE CENTRAL PLAINS...DEEPENING TO A NEAR-518-DECAMETER H5 LOW BY 00Z
ON JANUARY 1 2011. THE MOST INTENSE MID/UPPER-LEVEL FORCING AND DEEP-
LAYER Q-VECTOR CONVERGENCE ASSOCIATED WITH THESE FEATURES IS FORECAST
TO REMAIN NW OF THE CWA AS THE UPPER TROUGH LIFTS NEWD. HOWEVER...
TRAILING MID/UPPER-ASCENT EMBEDDED WITHIN A LONG FETCH OF
DIFFERENTIAL POSITIVE VORTICITY ADVECTION IN SOUTHWESTERLY FLOW ALOFT
IS FORECAST TO SUPPORT THE INITIATION OF CONVECTION ON
FRIDAY...PARTICULARLY AFTER NOON...COMPLEMENTED BY LOW-LEVEL
CONVERGENCE INVOF A POWERFUL COLD FRONT PROGGED TO CROSS THE REGION
NEW YEARS EVE.
MOISTURE OVERVIEW: AN ELONGATED SYNOPTIC FRONT OVER THE WRN AND
CNTRL ATLC CURRENTLY EXTENDS FROM NEAR NOVA SCOTIA TO NEAR 30N53W TO
JUST EAST OF PUERTO RICO. THE FRONT EXTENDS FURTHER SWWD ACROSS THE
CNTRL CARIBBEAN SEA TO THE COAST OF COLOMBIA AS A STATIONARY FRONT.
A PLUME OF DEEP TROPICAL MOISTURE HAS BEEN STEADILY BUILDING DURING
THE LAST FEW DAYS INVOF THE FRONT AND SURFACE LOW NEAR THE COAST OF
COLOMBIA AS A SERIES OF WEAK TROPICAL IMPULSES HAVE DRIFTED WWD AND
BECOME ABSORBED INTO THE FRONT. AMSU/SSMI TOTAL PRECIPITABLE WATER
PRODUCT DEPICTS A NEARLY 180-MILE WIDE ZONE OF 2.0-2.2-INCH PWAT
VALUES POOLING INVOF THE FRONT OVER THE CARIBBEAN...MARKING THE DEEP
MOISTURE BREWING WELL TO THE SOUTH...DESPITE THE COLD/DRY AIRMASS
ENTRENCHED ACROSS THE ARKLAMISS.
THIS MOISTURE WILL BE A KEY PLAYER IN THE EVENTUAL EVOLUTION OF
SEVERE CONVECTION POSSIBLE LATER THIS WEEK. THIS WILL OCCUR AS A
DEEP-LAYER RIDGE TRANSLATES FROM THE SERN CONUS AND GULF OF MEXICO
INTO THE SW NORTH ATLC. SRLY RETURN FLOW WEST OF RIDGE AXIS IS
FORECAST TO PUMP COPIOUS AMOUNTS OF MOISTURE...WITH TROPICAL
ORIGINS...INTO THE CWA FOLLOWING THE INITIAL MID-WEEK MOISTURE
SURGE. MULTI-MODEL CONSENSUS IS FOR SURFACE DEWPOINTS TO CLIMB INTO
THE MIDDLE 60S ACROSS THE FORECAST AREA ON FRIDAY AHEAD OF A
POWERFUL COLD FRONT PROGGED TO CROSS THE REGION FRIDAY EVENING INTO
FRIDAY NIGHT...WITH DEWPOINTS IN THE 64-66F RANGE NOSING UP TO THE
INTERSTATE 20 CORRIDOR. PREFRONTAL PWAT VALUES OF 1.30-1.35 INCHES
ARE PROGGED TO BE RUNNING UP TO 200 PERCENT OF NORMAL...WITH H85
THETA-E VALUES REACHING THE 325-327K RANGE IN RESPONSE TO STRONG AND
PROLONGED LLVL SOUTHERLY MOISTURE TRANSPORT AT 30-40 KT DURING THE
PRECEDING COUPLE OF DAYS. THESE MOISTURE PARAMETERS ARE WELL ABOVE
NORMAL THIS DEEP INTO THE COLD SEASON.
MESOSCALE PARAMETERS: NWP CONSENSUS SUGGESTS THAT THE COMBINATION OF
THE AFOREMENTIONED MOISTURE PARAMETERS AND MID-LEVEL LAPSE RATES OF
6.3-6.5 C/KM WILL SUPPORT A RELATIVELY WIDE ZONE OF UPWARDLY BUOYANT
PARCELS...WITH AN AXIS OF SBCAPE/MLCAPE VALUES OF 600-1100 J/KG
AHEAD OF THE FRONT. WITH SREF MEDIAN EFFECTIVE BULK SHEAR MAGNITUDES
OF 40-50 KT...THE JUXTAPOSITION OF THE AFOREMENTIONED INSTABILITY
AND MODERATE/STRONG DEEP-LAYER SHEAR WILL SUPPORT ORGANIZED
CONVECTION. THE CURRENT THINKING IS THAT MULTIPLE CONVECTIVE
MORPHOLOGIES COULD BE SUPPORTED. GIVEN THE ANTICIPATED DISPLACEMENT
OF THE STRONGEST DEEP ASCENT NORTHWEST OF THE CWA...THERE WILL BE
THE POTENTIAL FOR DISCRETE SUPERCELLS TO EVOLVE AHEAD OF THE FRONT.
STRONG LINEAR FORCING ALONG THE FRONT COULD ALLOW FOR CONVECTION TO
EVOLVE TOWARD A MORE QUASILINEAR CONVECTIVE SYSTEM MODE LATER IN THE
EVENT AS THE FRONT CROSSES THE REGION. ULTIMATELY...IT IS TOO EARLY
TO PIN DOWN THE EXACT DETAILS ON CONVECTIVE MORPHOLOGY...THOUGH
ORGANIZED CONVECTION IS LOOKING INCREASINGLY LIKELY.
CONVECTIVE HAZARDS: VERY STRONG LOW-LEVEL FLOW / SOUTHWESTERLY H85
WINDS OF 40-45 KT / WILL CERTAINLY RESULT IN STRONG LLVL SHEAR.
WHERE SURFACE WINDS ARE RELATIVELY BACKED AHEAD OF THE FRONT...
STRONG DIRECTIONAL SHEAR ASSOCIATED WITH STRONGLY CURVED LLVL
HODOGRAPHS WILL SUPPORT 0-1 KM STORM RELATIVE HELICITY VALUES OF
150-300 M2/S2...RESULTING IN THE POTENTIAL FOR TORNADOES. THE
QUANTITY OF LLVL MOISTURE AND LLVL BUOYANCY WILL FURTHER ENHANCE THE
POTENTIAL FOR TORNADOES. MODELS PROJECT THE SIGNIFICANT TORNADO
PARAMETER TO REACH 2-5 UNITS RANGE ACROSS A LARGE PART OF THE
CWA...WHICH STUDIES SUGGEST COULD BE SUPPORTIVE OF SIGNIFICANT
TORNADOES. AN ANALOG TO THIS EVENT...THE 26 NOVEMBER 1988
EVENT...WITH SIMILAR SYNOPTIC AND MOISTURE GEOMETRIES...SUPPORTED
MULTIPLE TORNADOES INCLUDING STRONG TORNADOES ACROSS PORTIONS OF THE
REGION. THE UPCOMING NEW YEARS EVENT IS FORECAST TO FEATURE A MORE
INTENSE CYCLONE OVER THE UPPER MIDWEST WITH MINIMUM SURFACE PRESSURES
DOWN TO NEAR 990 MB...SUGGESTING THE POTENTIAL FOR GREATER MOISTURE
TRANSPORT INTO THE CWA. DAMAGING WINDS WILL ALSO BE A CONCERN WITH
THE CONVECTION...WITH THE POTENTIAL FOR A WIDESPREAD... SIGNIFICANT
DAMAGING WIND EVENT IF A QLCS EVOLVES. GIVEN THE DEGREE OF
INSTABILITY AND DEEP-LAYER SHEAR...WITH H5 TEMPS FALLING TO THE -13
TO -15C RANGE...LARGE HAIL WILL ALSO BE A CONCERN. HEAVY RAINFALL
COULD ALSO BECOME A CONCERN...ESPECIALLY IF THE SYSTEM SLOWS DOWN AS
EARLIER RUNS OF THE ECMWF HAVE SHOWN.
FORECAST CHANGES: GIVEN THE POTENTIAL FOR A HIGHER-END SEVERE
WEATHER THREAT...ALONG WITH THE POTENTIAL IMPACTS GIVEN ITS
OCCURRENCE ON NEW YEARS EVE...THE NATIONAL WEATHER SERVICE WILL BE
RAMPING UP THE SEVERE POTENTIAL IN OFFICIAL PRODUCTS. A SPECIAL
WEATHER STATEMENT IS BEING ISSUED HIGHLIGHTING THE POTENTIAL FOR A
SEVERE WEATHER EVENT ON NEW YEARS EVE...AND ESPECIALLY THE CONCERN
FOR TORNADOES. THIS SYSTEM WILL HAVE THE POTENTIAL TO HAVE A
SIGNIFICANT IMPACT ON THE ARKLAMISS REGION...AND EVERYONE SHOULD
CLOSELY MONITOR FUTURE FORECASTS AND OUTLOOKS ISSUED BY THE NATIONAL
WEATHER SERVICE IN JACKSON FOR UPDATES ON THIS SITUATION. /COHEN/
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