Area Forecast Discussion
Issued by NWS Huntsville, AL

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000
FXUS64 KHUN 101125 AAA
AFDHUN

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
625 AM CDT THU JUL 10 2014

.UPDATE...
FOR 12Z TAFS

&&

.DISCUSSION...
/ISSUED 538 AM CDT THU JUL 10 2014/
WEAK CYCLONIC FLOW ALOFT PREVAILS ACROSS MUCH OF EASTERN NORTH
AMERICA THIS MORNING...WITH ANALYSIS OF WATER VAPOR SATELLITE AND
RAOB DATA SUGGESTING THE PRESENCE OF A BROAD LONGWAVE TROUGH SITUATED
BETWEEN MID-LEVEL SUBTROPICAL ANTICYCLONES OVER THE WESTERN ATLANTIC
AND SOUTHERN ROCKIES. AT THE SURFACE...A WEAK COLD FRONT REMAINS
POSITIONED ACROSS MIDDLE TENNESSEE...WITH PERSISTENT SHOWER AND
THUNDERSTORM ACTIVITY OCCURRING ACROSS THE LOCAL AREA -- POSSIBLY
BEING ENHANCED BY THE SOUTHERN END OF A WEAK MID-LEVEL WAVE MOVING
EASTWARD ACROSS KENTUCKY/NORTHERN TENNESSEE. THE FINAL MID-LEVEL
VORTICITY LOBE EMBEDDED WITHIN THE LONGWAVE TROUGH IS CURRENTLY
DIGGING SOUTHEASTWARD ACROSS THE MID-MISSISSIPPI VALLEY AND SHOULD
WEAKEN AS IT PUSHES SOUTHEASTWARD INTO KENTUCKY/TENNESSEE LATER
TODAY. NONETHELESS...SYNOPTIC SCALE ASCENT ASSOCIATED WITH THIS
FEATURE WILL LIKELY BE SUFFICIENT TO GENERATE ISOLATED/WIDELY
SCATTERED CONVECTION ALONG THE COLD FRONT AS IT DRIFTS SOUTHWARD INTO
THE REGION THIS MORNING.

SHORT TERM MODELS AGREE THAT THE EASTERN NORTH AMERICA LONGWAVE
TROUGH AXIS WILL SLOWLY SHIFT EASTWARD AND OFFSHORE OF THE EASTERN
SEABOARD DURING THE PERIOD FROM TONIGHT-SATURDAY AS THE ATLANTIC
SUBTROPICAL HIGH WEAKENS SUBSTANTIALLY. MEANWHILE...500 MB HEIGHTS
ARE FORECAST TO RISE CONSIDERABLY ALONG AN AXIS EXTENDING FROM THE
CENTRAL PLAINS SOUTHEASTWARD INTO THE LOWER MISSISSIPPI VALLEY...AND
THIS WILL EFFECTIVELY PLACE THE LOCAL FORECAST AREA BENEATH A
LIGHT/MODERATE NORTHERLY FLOW ALOFT. A NOTICEABLY DRIER AIRMASS WILL
FINALLY BEGIN TO ADVECT SOUTHWARD INTO THE REGION TONIGHT...WITH
DEWPOINTS THROUGH THE REMAINDER OF THE SHORT TERM FORECAST PERIOD
EXPECTED TO BE IN THE LOWER/MID 60S. DIURNAL MIXING OF THE BOUNDARY
LAYER MAY ALSO ALLOW DEWPOINTS TO FALL INTO THE MID 50S BOTH
FRIDAY/SATURDAY AFTERNOONS. WITH NO EVIDENCE OF ANY FORCING FOR
ASCENT...DRY CONDITIONS ARE EXPECTED TO PREVAIL -- ALTHOUGH A FEW
AFTERNOON SHOWERS/STORMS MAY RETURN TO EASTERN AREAS ON SATURDAY.
HIGHS WILL BE WARM GIVEN LACK OF ANY APPRECIABLE CLOUD COVER...AND
GENERALLY IN THE LOWER/MID 90S. ASSUMING THAT THE BOUNDARY LAYER
DECOUPLES BOTH TONIGHT/FRIDAY NIGHT...LOWS WILL FALL TO PLEASANT
VALUES IN THE MID 60S. PATCHY FOG WILL BE POSSIBLE IN THIS
REGIME...BUT MAINLY IN VALLEYS AND NEAR BODIES OF WATER.

THE MAIN FEATURE OF INTEREST IN THE EXTENDED PERIOD IS A STRONG UPPER
TROUGH...THAT IS WELL FORECAST BY ALL OF THE GLOBAL MODELS TO DROP
SOUTHEASTWARD ACROSS THE NORTHWEST TERRITORIES LATER THIS WEEK AND
PHASE WITH A PREEXISTING TROUGH TO FORM A LARGE/COLD UPPER LOW OVER
NORTHERN MANITOBA BY SATURDAY EVENING. THIS CYCLONE IS FORECAST TO
DROP SOUTHWARD ACROSS MANITOBA/ONTARIO -- EVENTUALLY REACHING THE
WESTERN GREAT LAKES BY MONDAY EVENING. THIS FIRST INDICATIONS OF THE
APPROACH OF THIS FEATURE WILL BE AN INCREASE IN LOW-LEVEL MOISTURE BY
SATURDAY NIGHT/SUNDAY MORNING...AS LOW-LEVEL FLOW BEGINS TO INCREASE
FROM THE SOUTHWEST. THIS WILL TRANSLATE TO A SLIGHTLY HIGHER
PROBABILITY OF AFTERNOON SHOWERS/STORMS ON SUNDAY. HOWEVER...PRECIP
CHANCES WILL INCREASE MOST SUBSTANTIALLY ON MONDAY...AS THE
SUBTROPICAL RIDGE AXIS BECOMES SUPPRESSED FURTHER TO THE SOUTH AND
LOWER/MID-TROPOSPHERIC WINDS BEGIN TO INCREASE FROM THE WEST-
NORTHWEST. THE DEEPENING SURFACE LOW ASSOCIATED WITH THE GREAT LAKES
CYCLONE WILL SEND A RATHER SIGNIFICANT MID-SUMMER COLD FRONT
SOUTHWARD...WITH THE FRONT LIKELY TO EXTEND FROM THE OHIO VALLEY
SOUTHWESTWARD INTO SOUTHERN OKLAHOMA BY 12Z TUESDAY. IN SIMILAR
FASHION TO WHAT WE JUST EXPERIENCED...SEVERAL WAVES OF SHOWERS/STORMS
WILL BE POSSIBLE FROM LATE MONDAY THROUGH LATE TUESDAY...AS THE FRONT
PUSHES SOUTHWARD ACROSS THE REGION. STRONGER WINDS ALOFT WILL
ACCOMPANY THIS SYSTEM...WHICH MAY ULTIMATELY LEAD TO A SLIGHTLY
GREATER RISK OF SEVERE THUNDERSTORMS. OTHERWISE...THREATS SHOULD BE
THE SAME -- WITH LOCALLY HEAVY RAINFALL A DISTINCT CONCERN. COOLER
AND DRIER AIR WILL SPREAD INTO THE AREA BY THE END OF THE FORECAST
PERIOD...BUT DUE TO UNCERTAINTY ON THE SOUTHWARD PROGRESSION OF THE
FRONT WILL LEAVE A SLIGHT CHANCE POP IN PLACE THROUGH WEDNESDAY.

70/DD

&&

.AVIATION...
FOR 12Z TAFS...
SCATTERED SHOWERS AND STORMS CONTINUE TO DEVELOP ALONG A BOUNDARY
PUSHING TO THE SOUTHEAST TOWARD THE KHSV TERMINAL AT THIS TIME. VFR
CIGS ARE IN PLACE AT KHSV...BUT THE -TSRA APPROACHING ALONG THE
BOUNDARY COULD LOWER VSBYS OR CIGS INTO THE MVFR REALM THROUGH
10/15Z. THE CIGS AT 4000 FEET HAVE SCATTERED OUT AT KMSL...BUT
THROUGH 10/14Z VSBYS COULD DROP DUE TO FOG DEVELOPMENT. ALSO A
STRATUS DECK /LIFR AND IFR/ BEHIND THE BOUNDARY COULD ADVECT OVER
THE TERMINAL. OTHERWISE EXPECT VFR CONDITIONS AT BOTH TERMINALS...AS
THE BOUNDARY PUSHES WELL SOUTH OF THESE LOCATIONS AFTER 10/16Z.

KTW

&&

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

$$

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



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