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Area Forecast Discussion

829
FXUS64 KHUN 201139 AAA
AFDHUN

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
539 AM CST SAT DEC 20 2014

.UPDATE...
FOR 12Z TAFS

&&

.DISCUSSION...
/ISSUED 534 AM CST SAT DEC 20 2014/
WATER VAPOR IMAGERY SUGGESTS A PREDOMINATELY ZONAL FLOW ALOFT ACROSS
THE SOUTHERN CONUS THIS MORNING. A WEAK 500-MB SHORTWAVE TROUGH IS
FORECAST REASONABLY WELL BY SHORT TERM MODELS TO SHIFT EASTWARD
THROUGH THE TENNESSEE VALLEY THIS MORNING...BEFORE WEAKENING AND
EJECTING NORTHEASTWARD OFF THE MID-ATLANTIC COAST BY THIS EVENING.
ALTHOUGH THE THREAT FOR LIGHT PRECIPITATION HAS ENDED...MID-LEVEL
CLOUDS WILL BE SLOW TO CLEAR THE REGION AND COMBINE WITH LIGHT
NORTHERLY FLOW TO RESULT IN ANOTHER COOL DAY WITH HIGHS IN THE UPPER
40S/LOWER 50S.

MEANWHILE...ALL GLOBAL MODELS ARE IN AGREEMENT IN FORECASTING A
GRADUAL AMPLIFICATION OF THE MID/UPPER-LEVEL FLOW PATTERN ACROSS
NORTH AMERICA -- BEGINNING LATER TODAY AS A BROAD SUBTROPICAL RIDGE
STRENGTHENS OVER THE EASTERN PACIFIC. THIS AMPLIFICATION PROCESS WILL
BE FURTHER ENHANCED BY THE PHASING OF TWO PRONOUNCED VORTICES ACROSS
THE NORTHERN PLAINS/SOUTHERN CANADA. THE RESULT OF THIS PATTERN
TRANSITION FOR THE TENNESSEE VALLEY WILL BE AN INCREASE IN SOUTHWEST
FLOW ALOFT BY LATE TONIGHT. MOSTLY CLEAR SKIES AND LIGHT NORTHEAST
FLOW THIS EVENING WILL ALLOW TEMPERATURES TO FALL INTO THE UPPER
20S/LOWER 30S BEFORE HIGH-LEVEL CLOUDS -- ASSOCIATED WITH A
DISTURBANCE EJECTING NORTHEASTWARD FROM NORTHERN MEXICO -- SPREAD
ACROSS THE REGION.

GRADUAL AMPLIFICATION OF THE CENTRAL PLAINS LONGWAVE TROUGH IS
FORECAST TO CONTINUE FROM SUNDAY THROUGH MONDAY...AS HEIGHTS RISE
WITHIN THE EASTERN PACIFIC SUBTROPICAL ANTICYCLONE AND A STRONG VORT
MAX BEGINS TO CLOSE OFF ACROSS THE NORTHERN PLAINS/UPPER MISSISSIPPI
VALLEY. SEVERAL MID-LEVEL WAVES WILL SPREAD NORTHEASTWARD ACROSS THE
LOCAL AREA DURING THIS PERIOD...THE FIRST OF WHICH WILL ARRIVE SUNDAY
AFTERNOON AND INTERACT WITH SHALLOW ISENTROPIC ASCENT ABOVE THE
RETREATING COLD AIRMASS TO GENERATE THICKENING CLOUDS AND PERHAPS
SOME LIGHT DRIZZLE DURING THE AFTERNOON. CHANCES FOR MEASURABLE LIGHT
RAIN WILL INCREASE -- ESPECIALLY FOR THE SOUTHEASTERN HALF OF THE
FORECAST AREA -- ON SUNDAY NIGHT AND MONDAY AS THE SSWLY LOW- LEVEL
JET INCREASES TO 25-35 KNOTS AND ENHANCES LARGE SCALE ASCENT FROM
WARM ADVECTION.

ATTENTION IN THE EXTENDED FORECAST WILL BE FOCUSED ON AN INTENSE VORT
MAX AND ASSOCIATED 140-160 KNOT UPPER JET WHICH IS PROGGED TO DIG
SOUTHEASTWARD FROM THE CENTRAL ROCKIES INTO THE SOUTHERN HIGH PLAINS
MONDAY NIGHT. THE ARRIVAL OF THIS FEATURE WILL FORCE EVEN MORE
SIGNIFICANT AMPLIFICATION OF THE CENTRAL PLAINS LONGWAVE TROUGH ON
TUESDAY...WITH THE MID-LEVEL TROUGH AXIS EXPECTED TO LIFT
NORTHEASTWARD OVER THE REGION ON TUESDAY NIGHT/WEDNESDAY. AT THE
SURFACE...A CYCLONE WILL INITIALLY DEVELOP ACROSS SOUTH TEXAS TUESDAY
MORNING BEFORE LIFTING NORTHEASTWARD INTO CENTRAL MISSISSIPPI BY
TUESDAY EVENING -- AND DEEPENING RAPIDLY AS IT LIFTS INTO CENTRAL
KENTUCKY BY 12Z WEDNESDAY. SHOWER ACTIVITY WILL INCREASE FROM SW-TO-
NE ACROSS THE REGION EARLY TUESDAY MORNING AS THE LOW-LEVEL JET
INCREASES TO 30-40 KNOTS...BUT SHOULD BECOME MOST WIDESPREAD DURING
THE DAY ON TUESDAY AS PWAT VALUES RISE INTO THE 1-1.25 INCH RANGE.
FORECAST SOUNDINGS SUGGEST THAT PERSISTENT NORTHWARD ADVECTION OF
DEWPOINTS IN THE UPPER 50S/LOWER 60S AND STEEPENING MID-LEVEL LAPSE
RATES WILL LEAD TO SB CAPE VALUES OF 500-1000 J/KG...AND HAVE
INCREASED THE PROBABILITIES FOR THUNDERSTORMS. A FINAL ROUND OF
SHOWERS/STORMS WILL SWEEP EASTWARD ACROSS THE REGION TUESDAY EVENING
AS THE SURFACE LOW LIFTS NORTHEASTWARD AND THE TRAILING COLD FRONT
PUSHES RAPIDLY THROUGH THE REGION. OVERALL STRENGTH OF DEEP-LAYER
SHEAR AND MAGNITUDE OF LOW-LEVEL JET CERTAINLY POINT TO THE
POSSIBILITY OF STRONG/SEVERE THUNDERSTORMS FROM TUESDAY THROUGH
TUESDAY EVENING. HOWEVER...IMPACTS OF EARLY MORNING CLOUDS/RAINFALL
MAY LIMIT DESTABILIZATION -- ESPECIALLY THIS FAR INLAND FROM THE GULF
COAST. REGARDLESS A FEW STRONG STORMS ARE POSSIBLE WITH GUSTY WINDS
THE MAIN POTENTIAL HAZARD.

IN THE WAKE OF THE FRONTAL PASSAGE...A MUCH COLDER CANADIAN AIRMASS
WILL BE ADVECTED INTO THE REGION ON 15-25 KNOT WEST-NORTHWEST FLOW
AROUND THE DEEPENING CYCLONE. WITH THE POTENTIAL FOR A SUBSTANTIAL
AMOUNT OF WRAP-AROUND PRECIPITATION ON THE SOUTHWESTERN FLANK OF THE
SURFACE LOW...AT LEAST LIGHT PRECIPITATION IS EXPECTED TO CONTINUE
ON WEDNESDAY MORNING AS THERMAL PROFILES DROP BELOW FREEZING. A BRIEF
PERIOD OF VERY LIGHT SNOW IS POSSIBLE -- MAINLY FOR THE NORTHERN
HALF OF THE AREA...WITH STRONG UPSLOPE FLOW PERHAPS LEADING TO A
CONTINUATION OF SNOW FLURRIES IN THE SOUTHERN TENNESSEE/NORTHEAST
ALABAMA COUNTIES WEDNESDAY AFTERNOON/EVENING. THE COLD TEMPERATURES
WILL BE BRIEF...AS LOW-LEVEL FLOW IS FORECAST TO INCREASE FROM THE
SOUTHWEST ONCE AGAIN ON THURSDAY. ALTHOUGH THE GFS IS FAR LESS
AMPLIFIED WITH THE FLOW PATTERN LATE NEXT WEEK...THE TRADITIONALLY
MORE RELIABLE GEM/ECMWF AT THIS TIME RANGE SUGGEST THAT ANOTHER COLD
FRONT MAY PUSH ACROSS THE REGION LATE FRIDAY NIGHT/SATURDAY MORNING.
CHANCES FOR SHOWERS WERE INCREASED DURING THIS PERIOD...AND
THUNDERSTORMS MAY ALSO HAVE TO BE INCLUDED IF FORECASTED INSTABILITY
PARAMETERS INCREASE FURTHER.

70/DD

&&

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

VFR CONDITIONS WILL PREVAIL THROUGH THE TAF PERIOD. SKIES WILL BEGIN
CLEAR THROUGH THE DAY...WITH SCATTERED MID LEVEL CLOUDS LINGERING
THROUGH 00Z. LIGHT WINDS WILL BE VARIABLE AFTER MIDNIGHT...WITH SKIES
CLEARING FURTHER. WITH MOSTLY CLEAR SKIES AND LIGHT WINDS...THERE
COULD BE SOME MVFR VISIBILITIES AT BOTH TERMINALS ON SUNDAY
MORNING...HOWEVER CONFIDENCE IS LOW DUE TO THE DRY AIR IN PLACE AT
THE SURFACE. THEREFORE...WILL LEAVE MENTION OUT FOR NOW.

73

&&

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

$$

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AT WEATHER.GOV/HUNTSVILLE.



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