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Area Forecast Discussion

752
FXUS64 KHUN 210330 AAB
AFDHUN

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
930 PM CST SAT DEC 20 2014

.UPDATE...
LOWERED LOW TEMPERATURES IN NE AL FOR THE OVERNIGHT. ALSO ADDED
PATCHY FOG FOR THE EAST LATE TONIGHT.

&&

.DISCUSSION...
AFTER A LONG BOUT OF CLOUDY SKIES FOR ALMOST THE PAST WEEK...MOST OF
THE TENNESSEE VALLEY GOT A WELCOMED CHANGED AS THE SUN MADE AN
APPEARANCE FOR THE LAST DAY OF AUTUMN (WITH THE WINTER SOLSTICE AT
503 PM SUN AFTERNOON). HOWEVER...PESKY STRATUS HELD FIRM OVER OUR
EASTERN AREAS...WITH PORTIONS OF FAR NE ALABAMA (EASTERN JACKSON/
MARSHALL...AND MOST OF DEKALB COUNTIES STAYING IN THE CLOUDS - DURING
THE DAYLIGHT HOURS). AS OF THIS WRITING...THE STRATUS HAS MOVED EAST
OF NE AL...WITH THE STRATUS HANGING TOUGH ALONG THE NE AL/NW GA
BORDER.

THE CLOUD DECK NOW EAST OF OUR NE GROUP HAS RESULTED IN TEMPERATURES
RAPIDLY COOLING...NOW INTO THE LOWER 30S IN FT PAYNE AND SCOTTSBORO.
THIS OF COURSE REQUIRED UPDATES FOR THE OVERNIGHT PERIOD...MAINLY TO
LOWER LOW TEMPERATURES FOR OUR EASTERN AREAS. ALSO MADE OTHER MINOR
TEMP/DEWPOINT ADJUSTMENTS ELSEWHERE. CLEARED SKIES FOR THE EASTERN
AREAS TO FOR THE OVERNIGHT. GIVEN THE EARLIER CLOUD COVERAGE HAS
HELPED THOSE AREAS RETAIN LOWER LEVEL MOISTURE VALUES...INTRODUCED
PATCHY FOG LATE TONIGHT FOR THE WIND SHELTERED AND LARGE BODY OF
WATER AREAS OF NE AL. THOSE UPDATES IN THE AFM/PFM AND GRIDS HAVE
BEEN SENT.

RSB

&&

.AVIATION...
/ISSUED 533 PM CST SAT DEC 20 2014/
FOR 00Z TAFS...
AN AREA OF VFR ALTITUDE STRATUS WAS SLOWLY THINNING ACROSS PARTS OF
FAR NE AL. CLEAR SKIES OTHERWISE PREVAILED THIS EVENING. VFR WEATHER
SHOULD CONTINUE INTO THE LATE NIGHT FOR THE AREA. RESIDUAL MOISTURE
AND NIGHTTIME COOLING COULD RESULT IN MVFR MIST AROUND DAYBREAK SUN.
A SYSTEM MOVING ACROSS THE NORTHERN GULF WILL APPROACH THE SOUTHERN
GA APPALACHIANS DURING SUN...SPREADING CLOUDS FROM SW-NE. THE LOWER
ALTITUDE CLOUDS THIS GO AROUND WILL BE MORE PRESENT ACROSS NE AL
RATHER THAN THE NW PART OF THE FORECAST AREA LATE IN THE PERIOD.

RSB

&&

.PREV DISCUSSION...
/ISSUED 111 PM CST SAT DEC 20 2014/
VIS IMAGERY TELLS THE STORY OF TWO FORECASTS. ONE IS CLOUDY SE OF
BRIDGEPORT-GRANT-CULLMAN, AND SUNNY ELSEWHERE. TEMPS WILL VARY A BIT
THIS AFTERNOON WITH WARMER HIGHS NW AND COLDER SE. TONIGHT, THAT WILL
REVERSE WITH A RAPID COOL DOWN NW AND RATHER STEADY TEMPS SE. SHORT
RANGE MODELS SUGGEST THAT THE CLOUD LINE WILL HOLD VERY CLOSE TO ITS
CURRENT POSITION UNTIL VERY LATE TONIGHT OR EARLY SUNDAY AS
STREAMFLOW VEERS TO THE E AND SE. A WEAK UPPER IMPULSE IN TX WITHIN
THE SWLY FLOW ALOFT WILL ARRIVE IN SRN AL SUNDAY MORNING. THIS WILL
LOWER PRESSURES ACROSS THE SE AND INDUCE LOW LEVEL S-SELY FLOW WHICH
SHOULD ADVECT LOW CLOUDS INTO THE AREA. ISENTROPIC ASCENT MAY BE
ENOUGH TO GENERATE PATCHY DZ OR -RA, ESPECIALLY SUNDAY NIGHT INTO
MONDAY WHEN A STRONGER IMPULSE ARRIVES.

WHOLESALE WARM AND MOIST ADVECTION WILL PICK UP IN EARNEST MONDAY
NIGHT INTO TUESDAY AS THE NEXT UPPER WAVE AT THE SOUTHERN BASE OF THE
TROF PIVOTS INTO THE DEEP SOUTH. THE AXIS OF THE LLJ ON TUESDAY WILL
BE ANCHORED WELL AHEAD OF THE TROF AND COLD FRONT ACROSS THE FL
PANHANDLE INTO WESTERN AND NRN GA. THIS WILL LIKELY KEEP A STEADY
CONVEYER BELT OF SHRA AND PERHAPS A FEW TS, WHILE A SECOND AXIS OF
PRECIP WILL BE MORE CO-LOCATED WITH COLD FRONT IN THE MS VALLEY
TUESDAY EVENING AS APPROACHING HEIGHT FALLS/COLD AIR ALOFT AND
SHORTWAVE ARRIVE. THE PROGRESS OF THIS SYSTEM APPEARS A BIT SLOWER
THAN PREVIOUS RUNS.  LOW LEVEL LAPSE RATES WILL BE A BIG NEGATIVE
FACTOR FOR GENERATING SFC-BASED INSTABILITY. THE STRONGER OMEGA
FIELDS AND BULK SHEAR VALUES MAY OVERCOME THIS, BUT A CAD WILL ALSO
INFLUENCE OUR AREA THRU MUCH OF TUESDAY, ESPECIALLY IN OUR EASTERN
COUNTIES. NEVERTHELESS, THE MODELS CONTINUE TO INDICATE AN INFLUX OF
LOW LEVEL MOISTURE AND DEW POINTS INTO THE 50S BY TUESDAY EVENING. WE
WILL CONTINUE A LIKELIHOOD OF TSRA IN THE FORECAST, AND MAY MENTION A
FEW STRONG STORMS IN THE HWO. INITIAL SHERBE VALUES IN OUR AREA LOOK
MARGINAL ATTM, AND BETTER TO OUR SOUTH.

THE 00Z AND 12Z ECWMF CONTINUES ITS TREND OF FORECASTING
BOMBOGENESIS WITH THIS SFC LOW (00Z RUN WITH A 29MB/24H RATE AND
12Z WITH A 24-25MB/24H RATE). THE GFS IS CONSIDERABLY WEAKER AND
FURTHER W (AS IT WAS YESTERDAY), BUT STILL A SIGNIFICANT CYCLONE THAT
WILL GENERATE QUITE A BIT OF GRADIENT WIND ACROSS THE GREAT LAKES, OH
VALLEY AND HERE IN THE TN VALLEY FOR A PERIOD LATE TUESDAY NIGHT INTO
WEDNESDAY BEHIND THE COLD FRONT. A PRONOUNCED DRY SLOT AND SUBSIDENCE
FIELD ARE INDICATED ON WEDNESDAY IN THE RH AND DIVQ FIELDS. ANY POST-
FRONTAL PRECIP MAY BE OF SHORT DURATION, BUT HAVE HELD THE MIX OF
SHRA/SHSN ON WEDNESDAY. I HAVE LOW CONFIDENCE IN THIS PRECIP DUE TO
THE SUBSIDENCE AND SHORTAGE OF MOISTURE IN WAKE OF THIS FAST
DEPARTING SYSTEM. THE GREATEST IMPACT FROM THIS SYSTEM MAY BE THE
GRADIENT WINDS THAT DEVELOP BEHIND THE COLD FRONT, AND WE MAY NEED
WIND HEADLINES IF THIS VERIFIES.

AK

&&

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

$$

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



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