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000
FXUS64 KHUN 171541 AAB
AFDHUN

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
1041 AM CDT FRI MAY 17 2013

.UPDATE...
FOR ADJUSTMENTS TO THE POP/WX/DEWPOINT/QPF FORECAST THIS MORNING.

&&

.DISCUSSION...
A SLOW MOVING WEAK MID-LEVEL SHORTWAVE TROUGH IS ENHANCING MOISTURE
TRANSPORT AND LIFT ACROSS THE TN VALLEY. THIS IS RESULTING IN
NUMEROUS SHOWERS AND ISOLATED THUNDERSTORMS DEVELOPING AND MOVING
EAST ACROSS THE NORTH MS INTO NW AL. SFC AND 850 MB ANALYSIS
INDICATES A NARROW AXIS OF LOW-LEVEL MOISTURE STRETCHING FROM THE LA
COASTLINE NORTHWARD INTO THE MID MS RIVER VALLEY. MOST OF THE
ACTIVITY IS ORIGINATING WITH PARCELS LIFTING OVER THE MID-LEVELS OF
THE VERTICAL COLUMN (090-120 KFT) AT THIS POINT. SATURATION AND
DIMINISHING DEWPOINT DEPRESSIONS AT THE LOW LEVELS HAVE COMBINED TO
PRECIP MAKING IT TO THE GROUND. BASED ON RADAR/SAT TRENDS, HAVE
INCREASED POPS FOR THIS MORNING INTO THE EARLY AFTERNOON WITH
ISOLATED THUNDERSTORMS THIS MORNING.

VIS SATELLITE IS INDICATING POCKETS OF CLEARING WITH THINNER CLOUDS
JUST TO THE WEST/SW OF CONVECTION SPARKED BY THE SHORTWAVE OVER
SOUTH CENTRAL MS AND THE MS RIVER. THIS COULD BE INDICATIVE OF A DRY
SLOT WHICH COULD DESTABILIZE THE TN VALLEY BRIEFLY AND RESULT IN SOME
DAYTIME HEATING (ENOUGH TO WARM TEMPERATURES INTO THE UPPER 70S TO
LOW 80S). IF THIS OCCURS, A NARROW WINDOW OF OPPORTUNITY FOR STRONGER
STORMS WILL BE POSSIBLE MAINLY OVER NW AL (AROUND THE SOUTHERN
PERIPHERY OF THE SHORTWAVE TROUGH/DEFORMATION ZONE) WHERE GREATER
WIND SHEAR (30-35 KTS) AND LIFT WILL EXIST. THUS, WILL KEEP ONGOING
TEMP FORECAST FOR NOW, BUT FURTHER ADJUSTMENTS MAY BE NEEDED IF THE
BREAKS IN THE CLOUDS DO NOT MATERIALIZE AND INSTABILITY REMAINS
ELEVATED.

DAYTIME HIGHS STILL LOOK ON TRACK WITH COOLER HIGHS OVER NW AL AND
WARMER HIGHS OVER THE REST OF THE AREA. NOW, THIS TEMP FORECAST IS
BASED ON THE ASSUMPTION THAT THE CLOUD COVER BREAKS DO INDEED
MATERIALIZE. DID MAKE SOME SLIGHT ADJUSTMENTS TO THE DEWPOINT AND QPF
FORECAST BASED ON THE CURRENT TRENDS. OTHERWISE, THE REST OF THE
FORECAST LOOKS FINE.

SL.77

&&

.AVIATION...
/ISSUED 653 AM CDT FRI MAY 17 2013/
FOR 12Z TAFS...ALTHOUGH RECENT REPORTS FROM MSL/HSV CONFIRM VFR
CONDS...EXPECT CIGS TO DETERIORATE TO MVFR LEVELS SHORTLY AS RAFL
BEGINS TO SATURATE THE LOWER-LEVELS. RA/TSRA WILL NOT OCCUR AT EACH
TERMINAL FOR THE ENTIRE TAF PERIOD...BUT FEEL CONFIDENT ENOUGH TO
INCLUDE -RA WITH VCTS/CB AS PREVAILING WX COND. CIGS WILL LIKELY LIFT
TO AROUND 3500 FT FOR A BRIEF PERIOD THIS AFTN...WITH A DESCENDING
TREND EXPECTED ONCE AGAIN BY LATE THIS EVENING. SFC WINDS TODAY WILL
GENERALLY BE FROM SSE AT 8-12 KNOTS...WITH FLOW VEERING TO SSW
OVERNIGHT.

70/DD

&&

.PREV DISCUSSION...
/ISSUED 345 AM CDT FRI MAY 17 2013/
NOT A GREAT DEAL HAS CHANGED SINCE THE LAST FCST PACKAGE YESTERDAY
AFTERNOON...ALTHOUGH CONFIDENCE HAS INCREASED WITH PRECIP/TSTM
CHANCES FOR TODAY/TONIGHT. STILL UNSURE ABOUT OVERALL IMPACTS FOR
THIS WEEKEND...PARTICULARLY ON SUNDAY. MODELS APPEAR TOO ROBUST WITH
PRECIP ESP FOR SUNDAY AND APPEAR TO BE SUFFERING FROM CONVECTIVE
FEEDBACK ISSUES. WILL KEEP GENERALLY LOW POPS FOR THAT TIMEFRAME.
WEATHER CONDITIONS FOR MON/TUES STILL LOOK RELATIVELY WARM AND
DRY...WHILE PRECIP CHANCES HAVE BEEN RETAINED FOR WED DUE TO AN
IMPENDING FRONT. ALTHOUGH...THIS FRONT MAY STALL INITIALLY. DRIER
AIR STILL APPEARS POISED TO ENTER THE REGION BY LATE NEXT WEEK...BUT
A BUILDING RIDGE AND HIGH AMPLITUDE PATTERN SUGGESTS STAGNATION
AND VERY WARM AIR PERHAPS NEXT WEEKEND INTO THE NEXT WEEK.

EARLY MORNING ANALYSIS FEATURES A BROAD BUT LOW AMPLITUDE RIDGE
ACROSS THE CENTRAL/EASTERN CONUS...WHILE A TROUGH HAS PUSHED INTO
THE WEST COAST. THE POLAR WESTERLIES WERE LOCATED WELL NORTH OF OUR
AREA STRETCHING FROM THE INTERMOUNTAIN WEST TO GENERALLY ALONG THE
US/CANADIAN BORDER. A SUBTROPICAL JET WAS ALIGNED TO OUR
SOUTH...FROM NORTHERN MEXICO ACROSS THE GULF AND FL PENINSULA. A
CLOSED UPR LOW CONTINUED TO SPIN ACROSS THE MID MISS
VALLEY...CENTERED OVER THE OZARK REGION AT ABOUT 2 AM CDT. MOST OF
THE ASSOCIATED DEEP CONVECTION WAS LOCATED ON THE SOUTHERN FLANK OF
THE SYSTEM...IN THE AREA CONTAINING THE BEST SYNOPTIC SCALE LIFT AND
INSTABILITY. MULTI-SPECTRAL SATL IMAGERY SHOWED DEEP CONVECTIVE
CLOUD TOPS AS A FAIRLY PERSISTENT FEATURE ARND THE BASE OF THE UPR
LOW. REGIONAL RADARS SHOWED  DEEP CONVECTION CENTERED MAINLY OVER SE
ARKANSAS AND FAR WESTERN MISS...WITH A FEW LIGHTNING STRIKES NOTED
IN THE NLDN. OUTSIDE OF THE DEEP CONVECTION...BANDS OF MOSTLY
LGT/MDT SHOWERS WERE OBSERVED...INSTIGATED BY SHEARED VORT AND
DEFORMATION BANDING IN ASSOC/W THE PARENT LOW.

ONE NARROW BAND OF -SHRA WAS PUSHING ACROSS CENTRAL PORTIONS OF THE
HUN FCST AREA...AND WILL CONTINUE AN EASTWARD MOVEMENT THIS MORNING.
OTHER BANDS WERE NOTED UPSTREAM IN NORTHERN MISS AND TOGETHER THESE
WARRANT CHANCE POPS FOR THE MORNING TIMEFRAME. PRECIP AMOUNTS WILL
BE GENERALLY LIGHT THOUGH...AT ABOUT 0.10" OR LESS. ALTHOUGH CLOUDS
ARE LIKELY TO BE MORE ABUNDANT TODAY...BOTH IN DEPTH AND
COVERAGE...SUFFICIENT DESTABILIZATION IN ASSOC/W THE UPR LOW AND
BROADER SYNOPTIC SCALE ASCENT WILL LIKELY LEAD TO GREATER
SHOWER/TSTM COVERAGE LATER THIS AFTERNOON. SB AND ML CAPE VALUES
ARND 1-1.5 KJ/KG APPEAR POSSIBLE DESPITE THE POTENTIAL FOR THICKER
CLOUD COVER LATE THIS AFTERNOON. AS A ~30KT LOW-LVL JET ENTERS NW
ALABAMA LATER THIS AFTERNOON...ENHANCED STORM ORGANIZATION IS
POSSIBLE. WHILE BETTER ORGANIZATION OF STORMS IS MORE LIKELY JUST TO
OUR WEST LATER TODAY...A FEW STRONG TO SEVERE STORMS ARE POSSIBLE BY
THE LATE AFTERNOON INTO THE EVENING. SHEAR AND INSTABILITY COULD BE
SUPPORTIVE OF MARGINAL SEVERE HAIL AND WIND GUSTS IN A FEW STORMS IN
THE REGION. WHILE THE THREAT APPEARS TO BE SMALL...A TORNADO CANNOT
BE RULED OUT. THE GREATEST THREAT FOR ANY STRONG/SEVERE THUNDERSTORM
ACTIVITY APPEARS BEST IN NW ALABAMA WHERE LOW/MID LEVEL FLOW AND
RESULTING SHEAR WILL BE GREATEST. DUE TO THE INCREASED CONFIDENCE
FOR SHOWER/TSTM ACTIVITY TODAY/TONIGHT...POPS WERE INCREASED TO
LIKELY/CATEGORICAL FOR MOST AREAS.

ALTHOUGH SHEAR MAY INCREASE A LITTLE TONIGHT...THE LOSS OF DAYTIME
HEATING AND SFC-BASED INSTABILITY WILL LIMIT THE SEVERE THREAT BY
LATE IN THE EVENING. NEVERTHELESS...ELEVATED INSTABILITY WILL KEEP
THE THREAT FOR TSTMS CONTINUING.

THE UPR LOW WILL LIKELY BE TO OUR NORTH ON SAT AND MAY MERGE WITH
ANOTHER SHORT WAVE CURRENTLY MOVING THROUGH THE MIDWEST. BOTH OF
THESE FEATURES WILL BEGIN THE PROCESS OF MOVING AWAY FROM THE REGION
DURING THE WEEKEND...HOWEVER THEIR EFFECTS WILL LINGER ON SATURDAY
WITH CONTINUED INSTABILITY AND BROAD SCALE LIFT...ESPECIALLY IN OUR
EASTERN AREAS. POPS WERE INCREASED FOR MOST AREAS AND WERE TAPERED
FROM EAST TO WEST. FCST SOUNDINGS INDICATE MORE OF A STRONG WIND OR
SEVERE HAIL THREAT FOR SATURDAY AFTERNOON...MAINLY IN THE EASTERN
HALF OF THE AREA.

SHOWER/TSTM CHANCES WERE LEFT FOR SAT NIGHT...MAINLY TO ACCOUNT FOR
THE LIKELIHOOD OF ONGOING CONVECTION FROM SAT AFTERNOON. POPS WERE
RETAINED FOR SUN FOR MOST AREAS BUT THE WEST...TO ACCOUNT FOR
UNCERTAINTY WITH THE PACE OF THE DEPARTING UPR LOW. THE BEST CHANCES
FOR ANY SHOWERS OR TSTMS WILL PROBABLY BE IN THE MORNING...SINCE
STRONG UPR RIDGING APPEARS INCREASINGLY LIKELY FOR THE AFTERNOON.
HIGHS ON SUNDAY...DEPENDING ON THE AMOUNT OF EVENTUAL CLOUD
COVER...MAY CLIMB WELL INTO THE 80S.

WEATHER CONDITIONS FOR THE MON-TUES TIMEFRAME STILL LOOK DRY AND
VERY WARM. HIGHS AROUND 90 DEGREES LOOK POSSIBLE...PARTICULARLY FOR
MORE URBAN LOCATIONS.

THE TROUGH CURRENTLY MAKING INROADS INTO THE WESTERN CONUS WILL
LIKELY BE SITUATED IN THE CENTRAL/NORTHERN PLAINS BY MIDWEEK. GLOBAL
MODELS HAVE SHOWN SOME CONSISTENCY OVER RECENT RUNS IN BRINGING A
SFC FRONT INTO THE TN VALLEY REGION BY LATE WED. SHOWERS/TSTMS
ANTICIPATED AHEAD OF THIS FRONT WERE THUS RETAINED IN THE FCST FOR
NOW. THE EXACT EXTENT OF THE PUSH OF THIS FRONT INTO OUR AREA
REMAINS LARGELY UNCERTAIN THOUGH...AND THERE IS SOME INDICATION THAT
THE FRONT MAY INITIALLY STALL. AS THE UPR TROUGH MERGES WITH ANOTHER
DIGGING TROUGH IN THE NERN CONUS HOWEVER...A SECONDARY FRONT MAY
CROSS THE REGION. SHOWER/TSTM CHANCES WERE RETAINED THROUGH THURS
NIGHT.

KDW

&&

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

$$

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AT WEATHER.GOV/HUNTSVILLE.