Area Forecast Discussion
Issued by NWS Huntsville, AL

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000
FXUS64 KHUN 272341
AFDHUN

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
541 PM CST FRI FEB 27 2015

.UPDATE...
FOR 00Z TAFS

&&

.SYNOPSIS...
/ISSUED 239 PM CST FRI FEB 27 2015/

COLD CONDITIONS EXIST ACROSS MUCH OF THE CONUS AS THE POLAR JET
REMAINS ACROSS THE SOUTHERN TIER OF THE CONUS. A JET STREAK CURRENTLY
EXTENDS FROM WEST TEXAS TO THE NORTHERN ATLANTIC COAST OF NOVA
SCOTIA, WHILE ANOTHER JET STREAK EXISTS OFF THE WEST COAST.

THE UPPER LEVEL PATTERN IS PROGNOSTICATED BY THE MED RANGE MODELS TO
CHANGE TO A MORE AMPLIFIED PATTERN LATER IN THE FORECAST. THIS WILL
BRING QUITE A SWING IN TEMPERATURES AND IMPACTS ESPECIALLY TOWARD THE
MIDDLE PART OF NEXT WEEK WHICH IS TO BE EXPECTED AS WE DRAW NEARER
TO SPRING.

SL.77

&&

.DISCUSSION...
/ISSUED 239 PM CST FRI FEB 27 2015/

COLD AND DRY CONDITIONS SHOULD PREVAIL TONIGHT AS A SFC HIGH
PRESSURE SYSTEM CENTERED OVER MO SHIFTS EASTWARD WHILE LOW-MID S-SW
FLOW DEVELOPS ALOFT. THERE IS THE POSSIBILITY THAT ENOUGH LIFT MAY BE
PRESENT TO CAUSE SNOW FLURRIES TO DEVELOP IN A SCT LOW-LEVEL CLOUD
LAYER. FURTHERMORE, VIS SAT DOES SHOW MID-HIGH LEVEL CLOUDS MOVING
ACROSS THE MS RIVER VALLEY FROM THE SOUTHERN PLAINS. EXPECTING THESE
CLOUDS TO OVERSPREAD THEN TN VALLEY TONIGHT WHICH MAY INHIBIT TEMPS
FALLING DRAMATICALLY. HOWEVER, WITH SNOW COVER STILL PRESENT ACROSS
PORTIONS OF THE TN VALLEY (AND SOURCE REGION OF COLD ARCTIC AIRMASS)
STILL EXPECTING TEMPS TO DROP BELOW FREEZING.

THEN, THE UPPER LEVEL PATTERN BEGINS TO SHIFT WITH THE SHEARED
TROUGH AXIS (JET STREAK) OVER THE EASTERN PART OF THE COUNTRY MOVING
OFF TO THE NORTHEAST WHILE THE WEST COAST JET STREAK BEGINS TO
AMPLIFY INTO A TROUGH AND GAINS A POSITIVE TILT. MEANWHILE, AT THE
SFC ACROSS THE LEEWARD SIDE OF THE APPALACHIANS, CAD IS EXPECTED TO
DEVELOP AS THE SFC HIGH PRESSURE SYSTEM MOVES OFF THE ATLANTIC COAST
ON SATURDAY INTO SUNDAY AS MID-HIGH LEVEL CLOUDS REMAIN ACROSS THE TN
VALLEY. ISENTROPIC UPGLIDE (290-295K LEVELS) IS ALSO NOTED FROM THE
NAM/GFS MODELS WHICH MAY CAUSE ISOLATED-SCATTERED SHOWERS TO DEVELOP
AHEAD OF THE WEDGE FRONT CROSSING N GA ON SATURDAY NIGHT INTO SUNDAY
WHILE A WAA PROFILE DEVELOPS. THE SOUTHERLY FLOW WILL TRANSLATE TO
WARMER DAYTIME HIGHS/OVERNIGHT LOWS ON BOTH SATURDAY AND SUNDAY THAN
HAS BEEN OBSERVED OVER THE PREVIOUS WEEK. HAVE MADE ADJUSTMENTS TO
THE POP/WX GRIDS TO REFLECT THAT AND EFFECTS FROM CAD, ALONG WITH
COOLER TEMPERATURES/DEWPOINTS OVER NE AL. CHANCES FOR SHOWERS WILL
CONTINUE THROUGH THE WEEKEND AT TIMES DUE TO ISENTROPIC LIFT AND LIFT
ASSOCIATED WITH THE WEDGE FRONT.

A MID-LEVEL SHORTWAVE TROUGH TRAVELING ACROSS THE GREAT LAKES OVER
THE OH RIVER VALLEY WILL PUSH THE ARCTIC FRONT BACK TOWARDS THE TN
VALLEY ON MONDAY. MEANWHILE, THE PREVIOUSLY MENTIONED TROUGH OFF THE
WEST COAST APPEARS TO CUTOFF OFF FROM THE PREVAILING WESTERLIES JUST
OFFSHORE OF CENTRAL CA. BY MONDAY/MONDAY NIGHT THIS ANOTHER UPSTREAM
MID-LEVEL SHORTWAVE TROUGH ACTS TO AMPLIFY THIS CUTOFF LOW INTO A
LONGWAVE TROUGH. IT IS THE TIMING OF THE TROUGH THAT CONTINUES TO
INTRODUCE CHAOS INTO THE MODEL SOLUTIONS.

DOWNSTREAM RIDGING FROM THIS AMPLIFYING TROUGH WILL BRING SYNOPTIC
RIDGING ACROSS THE SOUTHERN TIER STATES (AND NORTH OVER OH RIVER
VALLEY) AND WARMER/HUMID CONDITIONS BEGINNING MONDAY BUT ESPECIALLY
NOTICEABLE ON TUESDAY. AS THE LONGWAVE TROUGH APPROACHES, SENSIBLE
IMPACTS BECOME WIDELY DIVERGENT IN TIME WITH THE MED RANGE MODELS. IT
DOES APPEAR THAT THUNDERSTORMS WILL BE POSSIBLE AS MID-LEVEL LAPSE
RATES INCREASE AHEAD OF A COLD FRONT MOVING AHEAD OF THE TROUGH.
TEMPERATURES TUESDAY COULD BE IN THE 70S IF SOME BREAKS IN THE CLOUDS
ARE REALIZED WITH OVERNIGHT LOWS IN THE 50S BUT HAVE LEFT OFFICIAL
FORECAST AS MID 60S FOR TUESDAY. THE ECMWF IS THE SLOWEST SOLUTION SO
FAR WITH THE PROGRESSION OF THE TROUGH WHICH COULD ACTUALLY BE THE
CASE GIVEN THE TYPICAL MODEL PROGRESSION OF A SLOW MOVING/AMPLIFYING
TROUGH. DESPITE THIS, HAVE LEFT THUNDERSTORMS IN FORECAST FOR TUESDAY
AFTERNOON (RATHER THAN REMOVE) ESPECIALLY WITH THE POSSIBILITY OF UVM
WITHIN THE WARM SECTOR. MODEL OUTPUT DOES SUGGEST A POTENTIAL QLCS
EVENT ALONG/AHEAD OF THE COLD FRONT WITH WEAK INSTABILITY AND HIGH
SHEAR (ESPECIALLY AT 0-3 KM). SO, DEPENDING ON THE TIMING COULD BE
OUR FIRST AT LEAST STRONG THUNDERSTORM EVENT EITHER ON TUESDAY OR
WEDNESDAY. COOLER WEATHER IS THEN EXPECTED BEHIND THE FRONT AND WITH
5-6 STANDARD DEVIATIONS PRESENT WITH TEMPERATURE FORECAST HAVE
TRENDED DOWNWARD RATHER THAN INCORPORATE A SINGLE BLEND/MODEL
FORECAST.

SL.77

&&

.AVIATION...
FOR 00Z TAFS...
VFR CONDITIONS AND WINDS UNDER 10KTS ARE EXPECTED THROUGH A MAJORITY
OF THE TAF PERIOD. N/NE WINDS OVERNIGHT SHIFT TO E/SE SATURDAY
MORNING. THIS WIND SHIFT WILL START TO SPREAD LOW LEVEL MOISTURE
BACK INTO THE AREA. CIGS LOWER TO AROUND 5KFT LATE SATURDAY
AFTERNOON THEN LOWER TO AROUND 2KFT IN THE EVENING/OVERNIGHT AS LIGHT
RAIN MOVES IN.

STUMPF

&&

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

$$

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



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