National Weather Service Weather Forecast Office

Area Forecast Discussion

288
FXUS64 KHUN 192331 AAA
AFDHUN

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
631 PM CDT WED JUN 19 2013

.UPDATE...
FOR 00Z TAFS

&&

.DISCUSSION...
/ISSUED 325 PM CDT WED JUN 19 2013/
A PREVAILING WEST-NORTHWEST FLOW REGIME ALOFT CONTINUES ACROSS THE
CENTRAL TENNESSEE VALLEY THIS AFTERNOON...WITH ONE MID-LEVEL
SHORTWAVE TROUGH LOCATED ACROSS CENTRAL ALABAMA MOVING AWAY FROM THE
REGION...WHILE ANOTHER WEAKER WAVE IS PROGGED TO ARRIVE LATE THIS
AFTERNOON/EARLY THIS EVENING. ISOLATED TO WIDELY SCATTERED SHOWERS
AND THUNDERSTORMS HAVE DEVELOPED THIS AFTERNOON...IN RESPONSE TO
FORCED LOW-LEVEL ASCENT ALONG A CONVERGENCE BOUNDARY SINKING
SOUTHWARD INTO THE REGION AND ADEQUATE DEEP-LAYER MOISTURE. WE WILL
CARRY A VERY LOW EARLY EVENING POP FOR SOUTHERN MIDDLE TENNESSEE AND
NORTHEAST ALABAMA TO ACCOUNT FOR LINGERING ACTIVITY...WITH DRY
CONDITIONS ANTICIPATED AFTER 20/03Z AS LOW TEMPERATURES FALL INTO THE
MID 60S.

SHORT TERM MODELS INDICATE THAT A SLIGHTLY STRONGER SHEAR AXIS --
CURRENTLY LOCATED ACROSS THE NORTHERN/CENTRAL PLAINS -- WILL
PROGRESS EAST- SOUTHEASTWARD TO THE MID/UPPER-MISSISSIPPI VALLEY
OVERNIGHT AND ARRIVE IN THE LOCAL AREA TOMORROW. WEAK FORCING
ASSOCIATED WITH THIS FEATURE MAY TEND TO ENHANCE THE COVERAGE OF
DIURNAL CONVECTION TOMORROW...BOTH ALONG A WEAK/DIFFUSE FRONT THAT
WILL BE IN THE VICINITY OF THE AREA AND ACROSS THE HIGHER TERRAIN OF
THE CUMBERLAND PLATEAU. HOWEVER...DEEPENING THERMALS WILL BE FIGHTING
THE EFFECTS OF MID-LEVEL HEIGHT RISES...AS THE PACIFIC SUBTROPICAL
RIDGE BUILDS EASTWARD INTO THE SOUTHERN PLAINS DOWNSTREAM FROM A
NEARLY STATIONARY UPPER LOW SITUATED ACROSS THE PACIFIC NORTHWEST.
WITH THIS IN MIND...WILL KEEP POPS ON THE LOW SIDE AT 20 PERCENT --
ESPECIALLY AS PWAT VALUES WILL REMAIN GENERALLY IN THE 1.3-1.5 INCH
RANGE AND A LIGHT STEERING FLOW WILL RESULT IN WEAK/ERRATIC STORM
MOTIONS. WEAK FORCING FOR UVM WILL PERSIST ON THURSDAY
NIGHT/FRIDAY...AS THE REMNANT SHEAR AXIS SHIFTS SOUTHWARD AND
EVENTUALLY SOUTHWESTWARD AROUND THE STRENGTHENING MID-LEVEL HIGH
BUILDING NORTHEASTWARD INTO THE MID-MISSISSIPPI VALLEY. LOW POPS
WILL BE MAINTAINED THROUGH THE PERIOD...BOTH TO ACCOUNT FOR LINGERING
AFTERNOON ACTIVITY...AS WELL AS FOR REASONS MENTIONED ABOVE. HIGH
TEMPERATURES WILL GENERALLY BE IN THE UPPER 80S/LOWER 90S...WITH LOWS
IN THE MID 60S.

BASED ON LATEST GUIDANCE FROM THE GFS/ECMWF/GEM...IT STILL APPEARS AS
IF THE REMNANT SHEAR AXIS THAT IS FORECAST TO DRIFT SOUTHWARD
ACROSS THE LOCAL AREA THROUGH THE END OF THE WEEK WILL GRADUALLY
EVOLVE INTO AN INVERTED MID-LEVEL TROUGH CENTERED ACROSS SOUTH
ALABAMA OR THE WESTERN FLORIDA PANHANDLE BY FRIDAY NIGHT OR SATURDAY.
THIS FEATURE IS THEN EXPECTED TO DRIFT VERY SLOWLY NORTHWESTWARD FROM
SATURDAY NIGHT THROUGH SUNDAY NIGHT WITHIN A BROAD WEAKNESS IN THE
SUBTROPICAL RIDGE INDUCED BY A SERIES OF MID-LEVEL VORTICES EJECTING
NORTHEASTWARD FROM THE CENTRAL PLAINS INTO THE GREAT LAKES REGION. A
DEEPENING/STRENGTHENING SOUTHEASTERLY FLOW REGIME WILL DEVELOP AS
THIS OCCURS...WITH PWAT VALUES INCREASING INTO THE 1.6-1.8 INCH RANGE
EXPECTED TO SUPPORT AN INCREASE IN THE COVERAGE OF SHOWERS AND STORMS
BY SUNDAY. BEYOND THIS POINT...THERE IS CONSIDERABLE UNCERTAINTY IN
THE FORECAST REGARDING WHETHER OR NOT THE WEAKNESS IN THE
SUBTROPICAL RIDGE WILL REMAIN ACROSS THE TENNESSEE VALLEY OR DRIFT
EASTWARD -- ALLOWING A STRONGER HIGH TO BUILD INTO THE AREA FROM THE
WEST. AT THIS POINT...WILL MAINTAIN SLIGHT CHANCE POPS FOR MAINLY
DIURNAL CONVECTION DURING THE MIDDLE OF NEXT WEEK AND HOLD
TEMPERATURES CLOSE TO CLIMO VALUES OF HIGHS IN THE UPPER 80S-AROUND
90 AND LOWS IN THE MID/UPPER 60S.

70/DD

&&

.AVIATION...
FOR 00Z TAFS...VFR CONDITIONS ARE EXPECTED THROUGH THE PERIOD EXCEPT
FOR A SHORT PERIOD BTWN 10-12Z TOMORROW MORNING WHEN PATCHY MVFR FOG
MAY DEVELOP. A SCT/BKN MID LEVEL DECK OF CLOUDS WILL DEVELOP DURING
THE AFTN TOMORROW AND A FEW SHRA/TSRA MAY POP UP DURING PEAK
AFTERNOON HEATING. COVERAGE WILL BE LIMITED AND THEREFORE NO LONG
DURATION IMPACTS TO FLIGHT OPERATIONS ARE ANTICIPATED.

CCC

&&

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

$$

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