Area Forecast Discussion
Issued by NWS Huntsville, AL

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000
FXUS64 KHUN 202342
AFDHUN

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
542 PM CST THU NOV 20 2014

.UPDATE...
FOR 00Z TAFS

&&

.DISCUSSION...
/ISSUED 204 PM CST THU NOV 20 2014/
A BROAD AREA OF HIGH PRESSURE OVER THE EASTERN HALF OF THE UNITED
STATES ALONG WITH ZONAL FLOW ALOFT WILL KEEP MILD CONDITIONS ACROSS
THE AREA THROUGH FRIDAY. A COLD FRONT WILL DROP SOUTH OF THE AREA
OVERNIGHT WITH JUST A SLIGHT WIND SHIFT BEHIND IT. AS THE HIGH
ROTATES TO THE EAST...A PLUME OF LOW-MID LEVEL MOISTURE SURGES NORTH
OVERNIGHT TONIGHT LENDING TO PARTLY CLOUDY SKIES. IF LESS CLOUD
COVER MATERIALIZES...TEMPS MAY NEED TO BE ADJUSTED DOWN. CONDITIONS
ON FRIDAY WILL BE SIMILAR TO THURSDAY WITH HIGHS IN THE MID TO UPPER
50S. AS THE NEXT SYSTEM APPROACHES...CLOUDS WILL INCREASE BUT WITH
STRONG HIGH PRESSURE TO THE EAST...IT MAY KEEP THE EASTERN HALF OF
THE AREA WITH LESS CLOUDS. AS THE HIGH BREAKS DOWN AND MOVES
EAST...CLOUDS OVERSPREAD THE AREA ON SATURDAY. WINDS SHIFT FROM THE
SW AND HIGH TEMPS AROUND NORMAL VALUES WILL FINALLY BE REACHED.

THE NEXT SYSTEM STARTS TO TAKE SHAPE AT THE END OF THE WEEK WITH A
DEEP UPPER TROUGH DIGGING INTO THE SOUTHWESTERN STATES. SURFACE LOW
PRESSURE QUICKLY DEVELOPS IN EASTERN TEXAS AND DEEPENS RAPIDLY AS IT
MOVES TO THE NORTHEAST THROUGH MONDAY. THERE ARE QUITE A FEW
FEATURES OF THIS SYSTEM THAT ARE WORTH NOTING.

DYNAMICS ARE STRONG WITH THE LOW RAPIDLY DEEPENING AS IT MOVES OFF
TO THE NORTHEAST. A 50-60KT LOW LEVEL JET WILL DEVELOP AHEAD OF THE
UPPER DYNAMICS BETWEEN 6-18Z LEADING TO VERY HIGH SHEAR VALUES. 0-
6KM BULK SHEAR IS 60-70KTS WITH 0-1 KM SHEAR 30-40 KT. EVEN ABOVE
THE LLJ...WINDS WILL BE 70-80KT THROUGH THE PROFILE. BECAUSE OF
THIS...HELICITY VALUES WILL BE WELL INTO THE 400-500
RANGE...POSSIBLY HIGHER AT TIMES.

IN TERMS OF MOISTURE...PLENTY OF IT WILL BE PRESENT IN THIS SYSTEM
WITH A STRONG SOUTHERLY FLOW BEHIND A WEAK WARM FRONT THAT MOVES
NORTH THROUGH THE AREA SUNDAY MORNING.

THE INSTABILITY WILL BE VERY LOW THROUGH THIS EVENT...EVEN WITH
TEMPS FORECAST TO GET INTO THE UPPER 60S TO AROUND 70. BUT BECAUSE
THE SYSTEM IS EXPECTED TO MOVE THROUGH BEFORE 18Z...THESE WARMER
TEMPS WILL NOT BE REALIZED. BOTH THE NAM AND GFS ARE SHOWING A
STRONGLY CAPPED ATMOSPHERE AND STRONG INVERSION AT THE LOW LEVELS.
SURFACE BASED AND MUCAPE IS ONLY A COUPLE HUNDRED J/KG BRIEFLY
BETWEEN 12-18Z. LOW LEVEL LAPSE RATES ARE ALSO VERY LOW LIKELY DUE
TO THE STRONG CAPPING INVERSION. THIS WILL SEVERELY LIMIT THE
SUPERCELL POTENTIAL FOR SUNDAY.

THE TIMING OF ALL OF THE FEATURES IS WHAT WILL DETERMINE THE
ULTIMATE OUTCOME. THE MAIN ISSUE WITH TIMING IS THAT THE PRECIP AND
THE LOW LEVEL JET COME BEFORE 18Z WHEN INSTABILITY IS LACKING.
THEN...MID LEVEL DRY AIR QUICKLY ROTATES IN BEHIND THE DEEPENING
LOW. THIS WILL SHUT OFF THE PRECIP AND THUNDERSTORM POTENTIAL FAIRLY
RAPIDLY AFTER 18Z. HOWEVER...IF THE TIMING CAN HOLD OFF A FEW HOURS
TO ALLOW THE ATMOSPHERE TO WARM AND DESTABILIZE A BIT MORE ALONG WITH
GETTING ENOUGH MID LEVEL DRY AIR TO COINCIDE WITH
PRECIP...THUNDERSTORMS WITH DAMAGING WINDS ARE MUCH MORE LIKELY.

AS FOR THE MAIN THREATS...A PERIOD OF MODERATE TO HEAVY RAINFALL IS
LIKELY BETWEEN ABOUT 6-18Z SUNDAY. AS WELL...WITH SUCH STRONG WINDS
ALOFT...CANNOT RULE OUT THESE WINDS BEING DRAGGED DOWN IN ANY
CONVECTION...EVEN LOP TOPPED...CREATING DAMAGING WIND POTENTIAL.
THEN...IF PRECIP DOES HOLD ON WHEN THE MID LEVEL DRY AIR MOVES
IN...WE COULD AGAIN SEE SOME STRONG DOWNDRAFTS AND DAMAGING WIND.
WITH INSTABILITY LACKING...IT WILL BE DIFFICULT FOR THE ATMOSPHERE
TO REALIZE THE SHEAR AND HELCITY. EVEN SO...THE CHANCE OF ROTATION
WILL STILL NEED TO BE MONITORED. A CHANCE OF THUNDERSTORMS WILL
REMAIN IN THE FORECAST BUT IF THE TIMING COMES TOGETHER IN A MORE
FAVORABLE MANNER THE CHANCE MAY INCREASE. IN TERMS OF WIND...WITH
THE DEEP INVERSION...THE HIGHEST WINDS WILL LIKELY ONLY MIX DOWN IN
CONVECTION SO DO NOT EXPECT SYNOPTIC SCALE STRONG WINDS.

BEHIND THIS DEEP UPPER LOW...A SECONDARY COLD FRONT MOVES THROUGH ON
MONDAY WITH WITH COOLER AIR AND A NW WIND FILTERING IN. TEMPS TUE-
THUS WILL BE BACK TO WELL BELOW NORMAL VALUES.

LN

&&

.AVIATION...
FOR 00Z TAFS...
LATEST IR SATELLITE AND SURFACE OBS INDICATE A DECK OF SCT TO BKN
CLOUDS BETWEEN 5 AND 7KFT LOCATED OVER NORTH CENTRAL MS THIS
EVENING. THESE CLOUDS WILL GRADUALLY MOVE INTO NORTHERN ALABAMA
OVERNIGHT. HIGH PRESSURE CURRENTLY OVERHEAD WILL MOVE EAST ON FRIDAY
AND SHIFT WINDS TO THE EAST WITH SPEEDS REMAINING BELOW 10KTS. A
ROBUST LOW PRESSURE SYSTEM WILL INFLUENCE THE WEATHER PATTERN THIS
WEEKEND. MOIST SOUTHERLY FLOW IN THE MID LEVELS AHEAD OF THIS SYSTEM
SHOULD BE ENOUGH TO KEEP THESE MID LEVEL CLOUDS AROUND THROUGH THE
WEEKEND.

STUMPF

&&

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

$$

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



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