000
FXUS64 KHUN 222334 AAA
AFDHUN

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
634 PM CDT WED MAY 22 2013

.UPDATE...
FOR 00Z TAFS

&&

.DISCUSSION...
/ISSUED 259 PM CDT WED MAY 22 2013/
AN UPPER LOW CURRENTLY CENTERED ACROSS NORTHERN IOWA/SOUTHERN
MINNESOTA IS FORECAST TO WEAKEN AND OPEN INTO A WAVE LATER THIS
AFTERNOON...BEFORE BECOMING ABSORBED IN THE BROADER CIRCULATION
AROUND A STRONGER UPPER LOW SHIFTING SOUTHWARD ACROSS EASTERN HUDSON
BAY/NORTHERN QUEBEC EARLY TONIGHT. ONLY VERY WEAK LARGE SCALE FORCING
FOR ASCENT IS EVIDENT IN ALL NWP GUIDANCE...WITH THE POSITIVELY-
TILTED ORIENTATION OF THE TROUGH AXIS FORCING EMBEDDED SHORTWAVE
FEATURES TO TRANSLATE NORTHEASTWARD FROM THE MID-SOUTH REGION ACROSS
THE OHIO VALLEY. HOWEVER...LATEST GUIDANCE DOES SUGGEST THAT THE
FORECAST AREA WILL REMAIN WITHIN THE WARM SECTOR OF THE SURFACE LOW
OVERNIGHT...AND WILL MAINTAIN A LOW CHANCE POP FOR EASTERN AREAS THIS
EVENING IN THE EVENT THAT ACTIVITY CURRENTLY DEVELOPING WITHIN A
DIFFUSE PREFRONTAL CONVERGENCE AXIS LINGERS JUST BEYOND 23/00Z BEFORE
DISSIPATING.

SHORT TERM MODELS INDICATE THAT THE REMNANT MID-LEVEL VORTICITY
LOBE...ASSOCIATED WITH THE WEAK UPPER LOW CURRENTLY OVER THE
NORTHERN PLAINS...WILL DIG EAST-SOUTHEASTWARD THROUGH THE OHIO AND
UPPER TENNESSEE VALLEYS TOMORROW AND REACH THE CENTRAL APPALACHIANS
BY TOMORROW EVENING. AS THIS OCCURS...THE FIRST AND WEAKER OF TWO
SURFACE COLD FRONTS WILL PUSH SOUTHWARD THROUGH THE REGION TOMORROW
MORNING. A GUSTY WEST-NORTHWEST WIND WILL DEVELOP IN THE WAKE OF THE
FRONTAL PASSAGE...WITH A SLIGHTLY DRIER AIRMASS SLOWLY FILTERING INTO
THE REGION DURING THE AFTERNOON. A FEW SHOWERS/THUNDERSTORMS WILL BE
POSSIBLE WITH FRONTAL PASSAGE IN THE MORNING...BUT WEAK LOW LEVEL
CONVERGENCE COUPLED WITH GENERALLY WEAK DEEP-LAYER FORCING AND PWATS
ONLY AROUND 1 INCH SHOULD LIMIT THE COVERAGE OF PRECIPITATION ALONG
THE FRONT. A FEW SHOWERS WILL ALSO BE POSSIBLE THROUGH THE
AFTERNOON...MAINLY ACROSS SOUTHERN MIDDLE TENNESSEE AND NORTHEAST
ALABAMA WHERE A STRENGTHENING UPSLOPE COMPONENT TO FLOW WILL AID LOW-
LEVEL ASCENT. HOWEVER...THE GREATEST COVERAGE OF RAIN TOMORROW WILL
EXTEND FROM THE OHIO VALLEY SOUTHEASTWARD INTO THE SOUTHERN
APPALACHIANS. MEANWHILE...A STRONGER 500-MB VORT MAX IS PROGGED TO
DIG SOUTHEASTWARD THROUGH THE BASE OF THE LONGWAVE TROUGH...REACHING
THE CENTRAL APPALACHIANS THURSDAY NIGHT AND THE MID-ATLANTIC REGION
ON FRIDAY. THIS FEATURE WILL SEND A SECONDARY AND MUCH STRONGER COLD
FRONT SOUTHWARD THROUGH THE CENTRAL TENNESSEE VALLEY BETWEEN
24/06-12Z. NORTHERLY WINDS WILL STRENGTHEN ONCE AGAIN AS A HIGHLY
MODIFIED ARCTIC AIRMASS MOVES INTO THE REGION...WITH DEWPOINT VALUES
DROPPING TO UNSEASONABLY LOW VALUES IN THE MID/UPPER 40S. AS WAS
MENTIONED BY PREVIOUS FORECAST SHIFTS...HIGHS ON FRIDAY WILL STRUGGLE
TO REACH THE LOWER/MID 70S AS GUSTY NORTHERLY WINDS AND LOW-LEVEL
COLD ADVECTION OFFSET THE LATE MAY DIURNAL WARMING CYCLE. LOWS ON
FRIDAY NIGHT WILL FALL INTO THE MID 40S COURTESY OF STRONG
RADIATIONAL COOLING.

MEDIUM RANGE SOLUTIONS FROM THE GFS/ECMWF CONTINUE TO INDICATE THAT
THE MID-LEVEL TROUGH NEAR OR JUST OFFSHORE THE MID-ATLANTIC REGION
AT THE END OF THE SHORT TERM PERIOD WILL LIFT SLOWLY NORTHWARD ACROSS
COASTAL NEW ENGLAND BEFORE PHASING WITH THE CANADIAN TROUGH ON
SATURDAY NIGHT/SUNDAY. THE EASTWARD PROGRESSION OF THIS FEATURE WILL
BE BLOCKED THROUGH AT LEAST TUESDAY BY A STOUT MID-LEVEL RIDGE OVER
THE WESTERN NORTH ATLANTIC. THIS WILL INHERENTLY RESULT IN A
NORTHWEST FLOW REGIME ACROSS THE LOCAL AREA...AS A HIGH-AMPLITUDE
RIDGE IS FORECAST TO EXTEND NORTHWARD FROM THE CENTRAL PLAINS INTO
THE NORTHWEST TERRITORIES. THE FIRST OF SEVERAL LOW-AMPLITUDE MID-
LEVEL DISTURBANCES PROGGED TO SHIFT SOUTHEASTWARD IN THIS REGIME MAY
IMPACT THE REGION AS EARLY AS LATE SATURDAY AFTERNOON. HOWEVER...
LIMITED LOW-LEVEL MOISTURE RETURN WILL LIKELY PRECLUDE A SUBSTANTIAL
THREAT FOR SHOWERS/THUNDERSTORMS UNTIL LATE SUNDAY/EARLY MONDAY...
WHEN A DIFFUSE WARM FRONT MAY LIFT NORTHWARD ALLOWING GULF MOISTURE
TO RETURN TO THE FORECAST AREA. THE ECMWF IS FAR MORE AGGRESSIVE THAN
THE GFS WITH THE NORTHWARD RETURN OF A MOIST/UNSTABLE AIRMASS...AND
WILL LEAVE POPS IN THE SLIGHT CHANCE CATEGORY UNTIL THESE
UNCERTAINTIES ARE RESOLVED. THERE IS SOME INDICATION THAT THE
ATLANTIC SUBTROPICAL ANTICYCLONE MAY BEGIN TO BUILD WESTWARD ACROSS
THE SOUTHEASTERN CONUS BY THE MIDDLE OF NEXT WEEK...WHICH WOULD LEAD
TO LOWER RAIN CHANCES BUT VERY WARM/MUGGY CONDITIONS TO END THE MONTH
OF MAY.

70/DD

&&

.AVIATION...
FOR 00Z TAFS...
VFR CONDITIONS ARE EXPECTED TO PREVAIL ACROSS N AL AND S MIDDLE TN
FOR THE MOST NEXT 24 HOURS. ISOL -TSRA ARE POSSIBLE MAINLY BTWN
00-02Z ACROSS KHSV. THE MAIN EXCEPTION IS THE POSSIBILITY OF FOG
FORMATION ACROSS THE AREA WITH MVFR VIS POSSIBLE BTWN 08-12Z. FURTHER
DROPS IN VIS ARE POSSIBLE (ESPECIALLY AT KMSL), BUT NOT ENOUGH
CONFIDENCE TO INCLUDE AT THIS TIME. VFR CONDITIONS SHOULD THEN
PREVAIL AFTER 14Z. WIND SPEEDS WILL INCREASE TO 13 KTS GUSTING TO 20
KTS IN THE WAKE OF A COLD FRONT PASSING OVER THE REGION TOMORROW
MORNING.

SL.77

&&

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

$$

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