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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
639 AM CDT SUN JUL 5 2015

.UPDATE...
FOR 12Z TAFS 

&& 

.DISCUSSION...
/ISSUED 550 AM CDT SUN JUL 5 2015/
LATEST WATER VAPOR SATELLITE IMAGERY SUGGESTS THAT THE MID-LEVEL LOW
RESPONSIBLE FOR FAIRLY WIDESPREAD LIGHT/MODERATE RAINFALL ACROSS
THE TENNESSEE VALLEY OVERNIGHT CONTINUES TO DRIFT EAST-NORTHEASTWARD
OVER MIDDLE TENNESSEE. HOWEVER...THIS FEATURE IS EXPECTED TO BEGIN
LIFTING NORTHEASTWARD LATER TODAY AS AN UPSTREAM SHORTWAVE TROUGH
BEGINS TO EJECT EASTWARD ACROSS THE NORTHERN ROCKIES. BASED ON
CURRENT TRENDS IN DOPPLER RADAR DATA...LIGHT RAINFALL WILL BE ONGOING
AT 12Z ACROSS THE NORTHEASTERN QUADRANT OF THE FORECAST AREA AND
MORNING POPS HAVE BEEN ADJUSTED TO REFLECT A SHARP SW-TO-NE GRADIENT
IN RAIN CHANCES. REDEVELOPMENT OF CONVECTION LATER THIS AFTERNOON AND
EARLY THIS EVENING IS STILL A POSSIBILITY...AS ALL MODELS AGREE TO
SOME EXTENT THAT WEAK MID-LEVEL FORCING FOR ASCENT WILL BE PRESENT AS
A DISTURBANCE TRACKS SOUTHEASTWARD FROM THE MID-MISSISSIPPI VALLEY
REGION IN NORTHWEST FLOW ALOFT. THIS ACTIVITY MAY ACTUALLY DEVELOP TO
THE SOUTHWEST OF THE REGION WHERE LESS RESIDUAL CLOUD COVER WILL
ALLOW FOR STRONGER DESTABILIZATION THIS AFTERNOON...BUT STORMS SHOULD
TRACK NORTHEASTWARD INTO OUR AREA DURING THE EARLY EVENING HOURS.
ANOTHER MID-LEVEL SHORTWAVE DISTURBANCE OF SIMILAR MAGNITUDE MAY
INITIATE A SEPARATE EPISODE OF NOCTURNAL CONVECTION TO THE SOUTHWEST
OF THE FORECAST AREA EARLY MONDAY MORNING. THIS ACTIVITY MAY TEND TO
SPREAD NORTHEASTWARD ACROSS OUR REGION DURING THE LATE MORNING HOURS
AS WINDS ALOFT BACK TO THE WEST IN THE WAKE OF SHEARING MID-LEVEL
CYCLONE ACROSS THE CENTRAL APPALACHIANS. LOW/MID-LEVEL WINDS
THROUGHOUT THIS PERIOD WILL REMAIN AOB 20 KNOTS...BUT THE PRESENCE OF
DRY AIR ALOFT SUGGESTS THAT ANY STRONGER STORMS MAY BE CAPABLE OF
PRODUCING GUSTY WINDS AND PERHAPS SOME SMALL HAIL. LOCALLY HEAVY
RAINFALL MAY ALSO RESULT IN SOME MINOR FLOODING...ALTHOUGH WE HAVE
LET THE FLASH FLOOD WATCH EXPIRE DUE TO A MUCH LOWER THREAT FOR
WIDESPREAD HEAVY RAINFALL AND FLOODING.

DURING THE PERIOD FROM MONDAY NIGHT-TUESDAY...FLOW ALOFT WILL BACK
FURTHER TO THE WEST-SOUTHWEST AS A VORT MAX RIDING ALONG THE
U.S.-CANADIAN BORDER BEGINS TO LIFT NORTHEASTWARD AND THE ATLANTIC
SUBTROPICAL RIDGE STRENGTHENS AND BUILDS WESTWARD INTO THE
SOUTHEASTERN ATLANTIC/GULF COAST REGIONS. THIS PATTERN TRANSITION
WILL ALLOW PWAT VALUES -- WHICH MAY FALL INTO THE 1.1-1.3 RANGE EARLY
IN THE FORECAST PERIOD -- TO RECOVER INTO THE 1.3-1.5 INCH RANGE BY
TUESDAY. A VERY WEAK MID-LEVEL WAVE IS EVIDENT IN SOLUTIONS FROM THE
GFS/ECMWF AND THE PASSAGE OF THIS FEATURE MAY LEAD TO AN INCREASE IN
CONVECTION ONCE AGAIN DURING THE MORNING HOURS ON TUESDAY. DUE TO
INCREASING SUBSIDENCE ASSOCIATED WITH BUILDING MID/UPPER-LEVEL RIDGE
TO THE EAST...AFTERNOON SHOWERS/STORMS ON TUESDAY WILL LIKELY BE
CONFINED TO THE HIGHER TERRAIN OF NORTHEAST ALABAMA AND SOUTHERN
TENNESSEE.

LATEST MEDIUM RANGE GUIDANCE HAS TRANSITIONED TO A MUCH WARMER AND
DRIER SCENARIO FOR THE CENTRAL TENNESSEE VALLEY DURING THE EXTENDED
FORECAST PERIOD...AS THE CENTER OF A STRENGTHENING SUBTROPICAL RIDGE
WILL RETROGRADE SLOWLY ACROSS THE SOUTHERN CONUS FROM TUESDAY NIGHT
THROUGH THE END OF THE WORK WEEK. IT NOW APPEARS THAT A BELT OF
STRONGER SOUTHWEST FLOW ALOFT WILL REMAIN TO THE NORTH OF THE
REGION...WITH WEAK FLOW AND STRONG SUBSIDENCE LEADING TO LITTLE IF
ANY THREAT FOR PRECIPITATION. WE HAVE MAINTAINED A SLIGHT CHANCE POP
EACH DAY FROM WEDNESDAY-FRIDAY...DUE TO ABUNDANT LOW-LEVEL MOISTURE
THAT WILL LIKELY BE PRESENT IN THE WAKE OF RECENT RAINFALL, STRONG
BOUNDARY LAYER HEATING EXPECTED EACH AFTERNOON, AND WEAK FLOW THROUGH
THE COLUMN -- WHICH WILL BE FAVORABLE FOR DEVELOPMENT OF AIRMASS
CONVECTION EACH AFTERNOON. HIGH TEMPERATURES WILL WARM INTO THE
LOWER/MID 90S FOR ALL BUT THE HIGHER TERRAIN LOCATIONS OF THE
NORTHEAST...WITH LOWS LIKEWISE WARMING INTO THE UPPER 60S/LOWER 70S.
EFFICIENT EVAPORATION OF SOIL MOISTURE WILL MAINTAIN HIGH
DEWPOINTS...WITH HEAT INDICES FORECAST TO REACH THE 98-102 DEGREE
RANGE FOR MUCH OF THE REGION FROM WEDNESDAY-SATURDAY. MODELS FORECAST
THE UPPER RIDGE TO RETROGRADE INTO THE SOUTHERN HIGH PLAINS REGION BY
NEXT WEEKEND...WHICH WILL PLACE NORTHERN ALABAMA/SOUTHERN TENNESSEE
IN AN INCREASINGLY STORMY NORTHWEST FLOW ALOFT BY SATURDAY/SUNDAY AND
POPS HAVE BEEN RAISED ACCORDINGLY.

70/DD

&&

.AVIATION...
FOR 12Z TAFS...UPPER LEVEL SYSTEM IS NOW LOCATED NE OF THE NASHVILLE
REGION AND SHOULD CONTINUE TO SLOWLY MOVE EAST THROUGHOUT THE DAY.
THIS SYSTEM HELPED TO KEEP SHOWERS AND MAINLY MID-LEVEL CLOUDS OVER
THE REGION OVERNIGHT. TO THE WEST...WHERE SKIES REMAINED MOSTLY CLEAR
OVERNIGHT LOW CLOUDS AND FOG HAVE DEVELOPED AND SOME OF THIS MAY
FILTER INTO THE KMSL TERMINAL THROUGH 14-15Z THIS MORNING.
HOWEVER...CONFIDENCE IS LOW IN THIS AND ANY LOW CLOUDS THAT DO MOVE
IN SHOULD BE LIMITED. MAINLY VFR CONDITIONS ARE EXPECTED AT THE
TERMINALS THROUGH THE TAF PERIOD. THUNDERSTORMS ARE POSSIBLE AGAIN
THIS AFTERNOON BUT NOT CONFIDENT IN TIMING TO INCLUDE IN THIS TAF
PACKAGE.

STUMPF

&&

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

$$ 

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