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.
Posted on: Sun, 21 Dec 2014 11:53:37 +0000
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