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Area Forecast Discussion

722
FXUS64 KHUN 211152 AAA
AFDHUN

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
552 AM CST SUN DEC 21 2014

.UPDATE...
FOR 12Z TAFS

&&

.DISCUSSION...
/ISSUED 543 AM CST SUN DEC 21 2014/
CURRENT CONDITIONS ACROSS THE CENTRAL TENNESSEE VALLEY FEATURE MOSTLY
CLEAR SKIES AND LIGHT WINDS...WHICH HAVE ALLOWED FOR STRONG
RADIATIONAL COOLING AND PATCHY FREEZING FOG DEVELOPMENT ACROSS THE
REGION -- WITH TEMPS IN THE UPPER 20S/LOWER 30S. HOWEVER...LOW
STRATUS CLOUDS AND THICKER FOG ARE POISED ACROSS CENTRAL
ALABAMA/NORTH GEORGIA -- WITH THIS AIRMASS ALREADY BEGINNING TO
ENCROACH ON THE SOUTHEASTERN ZONES.

SHORT TERM MODEL GUIDANCE HAS CHANGED LITTLE SINCE YESTERDAY...WITH A
TRANSITION TO SOUTHWEST FLOW ALOFT CURRENTLY IN PLACE ACROSS THE
SOUTHEASTERN CONUS. AN INITIAL/WEAK MID-LEVEL VORT MAX WILL CONTINUE
TO SHIFT NORTHEASTWARD FROM SOUTHERN MISSISSIPPI INTO NORTHERN
GEORGIA THIS MORNING...WITH A STRONGER WAVE EXPECTED TO SHIFT
EASTWARD ACROSS THE CENTRAL PLAINS LATER TODAY WITHIN AN AMPLIFYING
LONGWAVE TROUGH. AS THIS OCCURS...SOUTH-SOUTHWESTERLY FLOW IN THE
850-700 MB LAYER WILL STRENGTHEN TO 20-30 KNOTS BY LATE AFTERNOON...
RESULTING IN FAVORABLE ISENTROPIC ASCENT AROUND 290K. THUS...EXPECT
THE DECK OF LOW CLOUDS CURRENTLY SOUTHEAST OF THE REGION TO REDEVELOP
NORTHWESTWARD THROUGH THE COURSE OF THE DAY...WITH PATCHY LIGHT
DRIZZLE POSSIBLE FOR MANY LOCATIONS THIS AFTERNOON. UPWARD VERTICAL
MOTIONS ALONG THE 290K SURFACE WILL STRENGTHEN FURTHER THIS EVENING
AND COINCIDE WITH VERY LOW CPD VALUES TO BRING A CHANCE OF
MEASURABLE LIGHT RAIN TO MOST OF THE REGION -- WITH HIGHEST
PROBABILITIES IN THE EAST. HIGH TEMPERATURES TODAY WILL BE
CHALLENGING DUE TO DIFFICULTIES IN TIMING ARRIVAL OF OVERCAST
CEILINGS FROM THE SOUTHEAST. HAVE TAKEN A BLEND OF MAV/MET/ECE MOS
GUIDANCE FOR MAXES...WITH LOWS TONIGHT EXPECTED TO ONLY FALL INTO THE
LOWER 40S BENEATH A THICK BLANKET OF LOW CLOUDS.

MODELS REMAIN IN GOOD AGREEMENT THAT A STRONG VORT MAX DIGGING INTO
THE NORTHERN HIGH PLAINS LATE TODAY WILL CLOSE OFF ACROSS THE
DAKOTAS/NEBRASKA AND DEEPEN FURTHER DURING THE DAY ON MONDAY. THIS
WILL RESULT IN A SUBTLE INCREASE IN SOUTH-SOUTHWEST FLOW ALOFT /35-45
KNOTS/ ACROSS THE REGION...WITH CLOUDS AND LIGHT DRIZZLE CONTINUING
-- ALTHOUGH STRENGTHENING WARM/MOIST ADVECTION MAY ULTIMATELY YIELD A
GREATER AREAL COVERAGE OF LIGHT RAIN BY LATE MONDAY AFTERNOON AS PWAT
VALUES RISE TO AROUND 0.75 INCH. A STRONGER VORT MAX ROTATING AROUND
THE NORTHERN PLAINS CLOSED LOW IS FORECAST TO BECOME NEGATIVELY-
TILTED MONDAY NIGHT AS IT EJECTS INTO THE WESTERN GREAT LAKES
REGION...AND THIS WILL SUPPORT THE EASTWARD MOVEMENT OF A COLD FRONT
INTO WESTERN TENNESSEE/MISSISSIPPI BY EARLY TUESDAY MORNING. ALTHOUGH
THE COVERAGE OF SHOWERS WILL BEGIN TO INCREASE SUBSTANTIALLY FOR ALL
AREAS LATE MONDAY NIGHT/TUESDAY MORNING AS THE SSWLY LOW- LEVEL JET
PEAKS AT 40-50 KNOTS...A POTENTIALLY WIDESPREAD COVERAGE OF PRECIP IS
POSSIBLE FOR THE WESTERN HALF OF THE AREA AS THE COLD FRONT NUDGES
TOWARD THE REGION. FORECAST SOUNDINGS SUGGEST A FAVORABLE SETUP FOR
ELEVATED CONVECTION AS 800-500 MB LAPSE RATES STEEPEN...RESULTING IN
500-1000 J/KG CAPE AVAILABLE FOR A PARCEL LIFTED ABOVE THE SURFACE-
BASED INVERSION. GIVEN STRENGTH OF INSTABILITY AND SHEAR...ROTATING
UPDRAFTS CAPABLE OF PRODUCING AT LEAST SMALL HAIL WILL BE POSSIBLE
MONDAY NIGHT INTO TUESDAY MORNING.

BY LATE MONDAY NIGHT AND TUESDAY...A MORE INTENSE MID-LEVEL VORT MAX
AND UPPER JET STREAK IS FORECAST TO DIG INTO SOUTHWEST TEXAS...
AMPLIFYING THE BROADER LONGWAVE TROUGH EVEN FURTHER AND INITIATING
THE DEVELOPMENT OF A SURFACE LOW ACROSS THE NORTHWEST GULF COAST.
THUS...EXPECT NOCTURNAL CONVECTION ON MONDAY NIGHT TO CONTINUE INTO
THE DAY ON TUESDAY...AND GRADUALLY BECOME SURFACE-BASED AS THE
NORTHWARD ADVECTION OF UPPER 50/LOWER 60 DEWPOINTS GRADUALLY ERODES
THE INVERSION. STRONG WIND GUSTS WILL BECOME AN INCREASING THREAT
FROM TUESDAY AFTERNOON THROUGH TUESDAY EVENING AS THIS OCCURS. THE
SURFACE LOW IS FORECAST TO GRADUALLY DEEPEN AS IT LIFTS NORTH-
NORTHEASTWARD OVER THE REGION EARLY WEDNESDAY MORNING...WITH WIND
SHIFT TO THE WEST MARKING THE END OF THE THREAT FOR CONVECTION JUST
BEFORE SUNRISE. HOWEVER...SUBSTANTIAL WRAP-AROUND PRECIPITATION IS
EXPECTED AROUND THE DEEPENING LOW...AND WITH MID-LEVEL TROUGH AXIS
NOT EXPECTED TO CLEAR THE REGION UNTIL LATE WEDNESDAY...RAIN WILL
LIKELY PERSIST THROUGH THE MORNING. FORECAST SOUNDINGS STILL SUGGEST
THAT A TRANSITION TO A LIGHT RAIN/SNOW MIX IS POSSIBLE JUST BEFORE
PRECIP ENDS...WITH A FEW FLURRIES POSSIBLE IN SOUTHERN TENNESSEE/NE
ALABAMA THROUGH WEDNESDAY EVENING. WINDY CONDITIONS ARE ALSO EXPECTED
REGION-WIDE WEDNESDAY MORNING...AND A WIND ADVISORY WILL LIKELY BE
REQUIRED FOR AT LEAST ELEVATED TERRAIN AREAS.

QUIET CONDITIONS ARE EXPECTED THURSDAY/THURSDAY NIGHT AS FLOW ALOFT
BRIEFLY TRANSITIONS TO A ZONAL ORIENTATION. HOWEVER...SOUTHWEST
WINDS WILL INCREASE ONCE AGAIN BY FRIDAY AS ANOTHER MID-LEVEL TROUGH
BEGINS TO EJECT NORTHEASTWARD OVER THE CENTRAL PLAINS. THIS FEATURE
IS FORECAST TO SHEAR APART...WITH THE TRAILING COLD FRTON PUSHING
SOUTHWARD INTO THE REGION LATE FRIDAY NIGHT/SATURDAY MORNING. DEEP-
LAYER MOISTURE RETURN APPEARS TO BE QUITE LIMITED...AND WILL ONLY
INCLUDE A LOW CHANCE OF SHOWERS AT THIS TIME. FINALLY -- ALTHOUGH IT
IS JUST BEYOND THIS FORECAST PERIOD...MOST GLOBAL MODELS SUGGEST A
SIGNIFICANT INTRUSION OF VERY COLD ARCTIC AIR INTO THE REGION ABOUT
ONE WEEK FROM TODAY. TEMPERATURES WERE COOLED NEXT SUNDAY TO REFLECT
THIS...BUT MAY NEED MAJOR DOWNWARD ADJUSTMENTS IF THIS TREND
CONTINUES.

70/DD

&&

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

BIT OF A PESSIMISTIC AND LOW CONFIDENCE FORECAST DURING THE NEXT 18
TO 24 HOURS. DRY CONDITIONS AT THE SURFACE WILL QUICKLY BE OVERCOME
WITH MOISTURE...AS LOW LEVEL FLOW BECOMES SOUTHERLY. MVFR CEILINGS
WILL APPROACH THE KHSV TERMINAL AT 20Z AND THEN AT 00Z AT THE KMSL
TERMINAL. ALTHOUGH CONFIDENCE IS LOW...THERE IS A POSSIBILITY THAT
LOW CLOUDS WILL ENTER THE TERMINALS SOONER THAN FORECAST...BEGINNING
AS EARLY AS 15Z AT THE KHSV TERMINAL. CONDITIONS WILL NOT IMPROVE ANY
THROUGH THE REMAINDER OF THE FORECAST PERIOD...WITH LIGHT RAIN
POSSIBLE AFTER 06Z. CEILINGS BELOW 1000 FT WILL ALSO BE POSSIBLE
EARLY MONDAY...HOWEVER AGAIN...CONFIDENCE IS NOT HIGH ENOUGH AT THE
MOMENT TO MENTION ANYTHING BEYOND A SCATTERED DECK IN THE TAFS.

73

&&

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

$$

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



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