Area Forecast Discussion
Issued by NWS Huntsville, AL

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000
FXUS64 KHUN 011211
AFDHUN

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
611 AM CST SUN MAR 1 2015

.UPDATE...
FOR 12Z TAFS

&&

.DISCUSSION...
/ISSUED 608 AM CST SUN MAR 1 2015/
OVERALL THERE HAS BEEN VERY LITTLE CHANGE IN FORECAST REASONING FOR
TODAY AND TONIGHT...WITH MID-LEVEL WEST-SOUTHWESTERLY FLOW FORECAST
TO VEER TO THE WEST AS A SUBTROPICAL RIDGE BECOMES CENTERED ACROSS
THE NORTHWEST CARIBBEAN SEA. WEAK UPWARD VERTICAL MOTION ALONG THE
290-300K SURFACES HAS ALREADY RESULTED IN THE DEVELOPMENT OF A THICK
DECK OF STRATUS CLOUDS...AND EXPECT THIS PATTERN TO CONTINUE TODAY.
FORECAST SOUNDINGS FROM VARIOUS MODELS INDICATE THAT ASCENT COUPLED
WITH THE NORTHEASTWARD ADVECTION OF RICH MOISTURE /PWATS OF 1-1.2
INCHES/ WILL LEAD TO A SATURATED PROFILE BELOW 8-10 KFT BY 00Z THIS
EVENING...AND HAVE INCREASED POPS ACCORDINGLY AS STRONG WARM
ADVECTION OCCURRING IN THIS LAYER WILL YIELD A FAIRLY WIDESPREAD
COVERAGE OF LIGHT RAIN. DEEP-LAYER ASCENT MAY ALSO BE ENHANCED LATE
THIS AFTERNOON/EARLY THIS EVENING BY THE ARRIVAL OF A WEAK 500-MB
VORTICITY MAXIMA CURRENTLY CROSSING THE BIG BEND REGION OF WEST
TEXAS.

LATER TONIGHT...A NORTHERN STREAM TROUGH SPREADING EASTWARD THROUGH
THE GREAT LAKES AND ITS ASSOCIATED SURFACE CYCLONE WILL SEND A
STRONG COLD FRONT SOUTHWARD INTO THE REGION BETWEEN 06-12Z MONDAY.
WESTERLY FLOW OF 20-25 KNOTS ATOP THE SHALLOW COLD AIRMASS WILL
GENERATE A FAVORABLE LOW-LEVEL VEERING/WARM ADVECTION REGIME FOR THE
SUPPORT OF WIDESPREAD LIGHT POSTFRONTAL RAINFALL. MODELS SUGGEST THAT
LOW/MID- LEVEL SUBSIDENCE WILL GRADUALLY WORK SOUTHWARD INTO THE AREA
TOMORROW MORNING...WITH HIGHEST MORNING POPS FOR LIGHT RAIN GENERALLY
ALONG AND SOUTH OF THE TENNESSEE RIVER. ALTHOUGH A NOTICEABLE
DECREASE IN THE COVERAGE OF LIGHT RAIN IS EXPECTED DURING THE
AFTERNOON AS 850 MB FLOW WEAKENS AND VEERS TO WEST-NORTHWEST...
LINGERING CLOUD COVER AND LIGHT NORTHEAST WINDS WILL ONLY ALLOW HIGH
TEMPERATURES TO RISE INTO THE LOWER/MID 40S AT BEST.

IT APPEARS AS IF STRONGER FORCING FOR ASCENT WILL ARRIVE IN THE
REGION ONCE AGAIN MONDAY EVENING...AS ANOTHER SHORTWAVE TROUGH
EJECTS RAPIDLY NORTHEASTWARD DOWNSTREAM FROM A POTENT MID-LEVEL
TROUGH EJECTING NORTHEASTWARD OVER THE FOUR CORNERS/CENTRAL ROCKIES.
THE LATTER FEATURE WILL INDUCE A DEEPENING SURFACE LOW ACROSS
NEBRASKA/KANSAS...WITH A WARM FRONT FORECAST TO SHIFT RAPIDLY
NORTHWARD BETWEEN 06-12Z TUESDAY. FORECAST SOUNDINGS CONTINUE TO
SUGGEST AN ENVIRONMENT THAT WILL BE QUITE FAVORABLE FOR THE
DEVELOPMENT OF NOCTURNAL ELEVATED CONVECTION AS WESTERLY FLOW IN THE
700-500 MB ADVECTS A PLUME OF STEEP LAPSE RATES OVER THE AREA. HAVE
INCREASED POPS INTO THE LIKELY CATEGORY FOR THIS PERIOD...WITH THIS
ACTIVITY EXPECTED TO BE SHIFTING RAPIDLY NORTHEASTWARD INTO TENNESSEE
DURING THE MORNING HOURS. ALTHOUGH WARM SECTOR SHOWERS AND A FEW
STORMS WILL BE POSSIBLE TUESDAY AFTERNOON...COVERAGE WILL NOT
INCREASE SUBSTANTIALLY ONCE AGAIN UNTIL AFTER DARK WHEN THE
SOUTHWESTERLY LOW-LEVEL JET INCREASES TO 50-60 KNOTS. FORECASTED
VALUES OF LOW-LEVEL AND DEEP-LAYER WIND SHEAR INDICATE THAT
CONVECTIVE ACTIVITY ON TUESDAY NIGHT/EARLY WEDNESDAY MORNING WILL
NEED TO BE MONITORED CLOSELY FOR THE DEVELOPMENT OF LOW-LEVEL
MESOCYCLONES AND THREAT FOR SEVERE -- DESPITE SB CAPE OF ONLY 250-500
J/KG. OUTSIDE OF CONVECTION...MAGNITUDE OF SOUTHWEST FLOW WILL LIKELY
WARRANT A WIND ADVISORY FOR HIGHER TERRAIN...WITH WINDY CONDITIONS TO
PREVAIL THROUGH WEDNESDAY.

IN TRUE FASHION FOR EARLY MARCH...A MARGINAL RISK FOR STRONG/SEVERE
CONVECTION WILL BE FOLLOWED BY THE PASSAGE OF POWERFUL ARCTIC COLD
FRONT WEDNESDAY MORNING. GLOBAL MODELS HAVE COME INTO BETTER
AGREEMENT WITH FRONTAL PASSAGE EXPECTED TO OCCUR BETWEEN 12-18Z FOR
MOST OF THE AREA. HOURLY TEMPERATURE GRIDS WERE ADJUSTED TO REFLECT
EARLY MORNING HIGHS...WITH TEMPERATURES EXPECTED TO FALL INTO THE
UPPER 20S/LOWER 30S FOR ALL BUT THE EXTREME SOUTHEASTERN COUNTIES BY
LATE AFTERNOON. WIDESPREAD POSTFRONTAL PRECIPITATION WILL CONTINUE IN
THE WAKE OF THE FRONT...WITH STRONG SOUTHWEST FLOW ALOFT TO THE EAST
OF THE BROADER CENTRAL PLAINS LONGWAVE PROVIDING SEVERAL
OPPORTUNITIES FOR THE ENHANCEMENT OF UVM AND PRECIP FROM WEDNESDAY
INTO WEDNESDAY NIGHT. CONSENSUS OF GFS/ECMWF FORECAST SOUNDINGS FROM
POINTS AROUND THE REGION SUGGEST THAT RAIN WILL RAPIDLY CHANGE TO
FREEZING RAIN WEDNESDAY AFTERNOON...AND GRADUALLY BECOME MIXED WITH
SLEET FROM NW- TO-SE AT SOME POINT WEDNESDAY EVENING/EARLY THURSDAY
MORNING -- ENDING AS LIGHT SNOW OR FLURRIES ON THURSDAY MORNING.
POINTS TO THE NW STAND THE HIGHEST CHANCE OF SEEING SNOW...AND POINTS
TO THE SE HAVE THE HIGHEST CHANCE OF A LONGER FREEZING RAIN EVENT.
EITHER WAY...THIS SYSTEM WILL HAVE SIGNIFICANT IMPACTS ON THE ENTIRE
REGION -- LIKELY HIGHER THAN ANY WINTER PRECIP EVENT WE HAVE HAD THUS
FAR. OF PARTICULAR CONCERN IS THE POTENTIAL FOR NORTH WINDS OF 10-15
MPH TO OCCUR DURING THE PERIOD OF GREATEST FREEZING RAIN AND SLEET.
THE THREAT FOR A SIGNIFICANT WINTER STORM HAS BEEN RAISED IN THE
HWO...AND THIS EVENT WILL CONTINUE TO BE WATCHED VERY CLOSELY. QUIET
AND COLD CONDITIONS ARE EXPECTED BEYOND THURSDAY...AS ARCTIC SURFACE
RIDGE SHIFTS EASTWARD OVER THE OHIO VALLEY/CENTRAL APPALACHIANS.

70/DD

&&

.AVIATION...
FOR 12Z TAFS...

BIT OF A PESSIMISTIC FORECAST OVER THE NEXT 24 HOURS AND
BEYOND...WITH MFVR CONDITIONS EXPECTED TO PREVAIL THROUGH AT LEAST
04Z TONIGHT. LOW LEVEL FLOW WILL CONTINUE TO PUSH MOISTURE INTO THE
REGION...WITH MVFR CEILINGS AND DRIZZLE/LIGHT RAIN POSSIBLE.
VISIBILITIES WILL LIKELY DROP INTO THE MVFR CATEGORY AS WELL BY THE
AFTERNOON. IFR CEILINGS WILL BEGIN TO MOVE INTO THE REGION LATE THIS
EVENING AS LOW LEVEL MOISTURE AND LIFT CONTINUE TO INCREASE. THERE IS
A CHANCE FOR LIFR CONDITIONS EARLY MONDAY MORNING...HOWEVER
CONFIDENCE IS LOW AND WILL THEREFORE LEAVE MENTION OF OCCURRENCE OUT
OF THE TAFS.

73

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

FOR MORE INFORMATION PLEASE VISIT OUR WEBSITE
AT WEATHER.GOV/HUNTSVILLE.


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