000
FXUS64 KHUN 160209 AAB
AFDHUN

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
909 PM CDT WED MAY 15 2013

.UPDATE...
EVENING FORECAST UPDATE

&&

.DISCUSSION...
WHILE THE MAIN WEATHER STORY THIS EVENING ACROSS THE CONUS REMAINS
WITH THE CONVECTION ACROSS THE SOUTHERN PLAINS...THE TENNESSEE VALLEY
FORECAST FOR TONIGHT REMAINS PREDOMINANTLY A TEMPERATURE FORECAST.
CLOUD COVER WILL CONTINUE TO INCREASE AHEAD OF THIS NEXT
SYSTEM...SLOWING THE DIURNAL DROP IN TEMPERATURES ACROSS THE FORECAST
AREA.

HIGHS THIS AFTERNOON TOPPED OUT IN THE LOWER TO MID 80S...NEAR OR
JUST A COUPLE OF DEGREES WARMER THAN VALUES SEEN YESTERDAY. BY
8PM...CONDITIONS HAD COOLED SIGNIFICANTLY THOUGH...WITH KMSL THE
WARMEST SITE AT 74 DEGREES WHILE CLEARER SKIES OVER NE AL HAVE
ALLOWED FT PAYNE AND CULLMAN TO DROP INTO THE 60S. THIS IS SIMILAR TO
TEMPS SEEN AT THE SAME TIME LAST NIGHT THOUGH. A BROAD SWATH OF
MID/UPPER LEVEL MOISTURE AHEAD OF THE MID LEVEL WAVE IS PRODUCING THE
CLOUD COVER SEEN ON REGIONAL SATELLITE (HANDLED WELL BY THE GFS
ISENTROPIC OUTPUT AT THE 325K LVL...ARND 450MB/20KFT...AND ALSO AT
LOWER LEVELS) AND THIS WILL CONTINUE TO TRACK NE OVERNIGHT. THESE
CLOUDS SHOULD CONTINUE TO SLOW THE COOLING TREND ACROSS THE CWA BUT
LEAVE A W-E GRADIENT IN LOWS SIMILAR TO THAT SEEN LAST NIGHT. COOLEST
TEMPS SEEN EARLY THIS MORNING RANGED FROM 65 (KMSL) - 59 (KHSV) - 50
(4A9) SO HAVE INCREASED THE GOING FORECAST IN CENTRAL/WESTERN
PORTIONS OF THE CWA TO REMAIN CLOSE TO LOWS FROM LAST NIGHT. HAVE
KEPT THE FCST OF THE MID 50S IN NE AL SINCE MORE CLOUDS TONIGHT
SHOULD PREVENT THE SAME DROP.

THE ONLY OTHER CHANGE MADE TO THE GOING FORECAST WAS TO INCREASE THE
DEWPOINTS THROUGH THE OVERNIGHT PERIOD. CURRENT VALUES ARE IN THE
MID/UPPER 50S AND WEAK MOISTURE ADVECTION FROM LOCATIONS TO OUR SW
(WHERE TD VALUES ARE IN THE UPPER 50S/LOWER 60S) COMBINED WITH THE
REDUCED MIXING SHOULD KEEP TDS IN THE 50S THROUGH THE MORNING. EVEN
WITH THIS REMAINING MOISTURE...THE VIRGA SEEN ON REGIONAL RADAR
MOSAIC OVER N/CENTRAL MS WILL BE BATTLING WITH A RELATIVELY DRIER
NEAR SFC LAYER AS IT APPROACHES THE TN VALLEY. THIS SHOULD LEAD TO
ANY PRECIP HOLDING OFF UNTIL LATER IN THE MORNING TOMORROW...AT THE
EARLIEST.

CCC

&&

.AVIATION...
/ISSUED 626 PM CDT WED MAY 15 2013/
FOR 00Z TAFS...VFR CONDITIONS WILL PERSIST THROUGH THE PERIOD. WINDS
WILL SLACKEN OVERNIGHT AND THEN SHIFT TO BECOME MORE SOUTHERLY BY
THE DAYLIGHT HOURS TOMORRROW...REMAINING LARGELY BELOW 10KTS. SCT -RA
WILL DEVELOP AFTER 14Z ACROSS AL/TN AHEAD OF THE NEXT SYSTEM...BUT
CIGS/VIS ARE EXPECTED TO REMAIN ABV MVFR LIMITS.

CCC

&&

.PREV DISCUSSION...
/ISSUED 247 PM CDT WED MAY 15 2013/
A PREVAILING SPLIT JET ZONAL UPPER FLOW PATTERN ACROSS CENTRAL NORTH
AMERICA WILL TRANSITION TO A MORE AMPLIFIED PATTERN BY THE MIDDLE OF
THE FORECAST. CURRENTLY, A CUTOFF LOW IS MEANDERING OVER THE
SOUTHERN PLAINS WHICH IS PROVIDING ENOUGH LIFT AND MOISTURE
TRANSPORT TO INCREASE INSTABILITY TO CAUSE STRONG TO SEVERE STORMS
OVER TX/OK. MEANWHILE, A BROAD TROUGH IS SLOWLY PROGRESSING EASTWARD
OVER THE WEST COAST.

AS THE CUTOFF LOW MOVES EASTWARD, THE MED-RANGE MODELS INDICATE A
WEAKENING TREND AND ACTUALLY TRANSITIONS IT TO AN OPEN WAVE.
MOISTURE ADVECTION AND SOMEWHAT STEEPER LAPSE RATES ALOFT WILL
CONTRIBUTE TO WEAK INSTABILITY OVER THE TN VALLEY ON THURSDAY WITH
THE APPROACH OF THIS WAVE. IN ADDITION, WEAK POSITIVE VORTICITY
ADVECTION WILL MEAN THE POSSIBILITY OF ISOLATED AFTERNOON
THUNDERSTORMS. THOUGH SYNOPTIC SCALE WIND SHEAR IS RATHER WEAK,
THERE COULD BE SOME LOCALIZED ENHANCEMENT WITH ANY VORTICITY MAXIMA
THAT MOVE ACROSS THE REGION. THESE STORMS MAY CONTINUE INTO THE
EVENING AND OVERNIGHT HOURS, BUT SHOULD BE VERY ISOLATED IN NATURE.
WITH A GREATER ENHANCEMENT OF LIFT (IN A RELATIVE SENSE) ON FRIDAY
AS THE WAVE AXIS MOVES ACROSS THE TN/OH RIVER VALLEYS, PREVAILING
WARM/MOIST AIRMASS, AND APPROACH OF A MOISTURE CONVERGENCE ZONE,
EXPECTING THAT THUNDERSTORMS WILL INCREASE IN COVERAGE ON FRIDAY
AFTERNOON. THUS, HAVE RAISED POPS SLIGHTLY.

RIDGE BUILDING THEN BEGINS ACROSS EASTERN NORTH AMERICA, FROM THE
GULF OF MEXICO NORTHWARD, ON FRIDAY EVENING INTO THE REST OF THE
FORECAST. THE TN/OH VALLEYS DO REMAIN IN NW FLOW ON FRIDAY NIGHT
INTO SATURDAY IN THE WAKE OF THE EXITING WEAK SHORTWAVE. BEGINNING
ON FRIDAY NIGHT AND CONTINUING THROUGH SATURDAY NIGHT, IN THE MIDST
OF THE RIDGE BUILDING, THE MED RANGE MODELS ARE SHOWING THE
POSSIBILITY OF A COUPLE OF MCS` CROSSING THE REGION (MORE
REMINISCENT OF SUMMER TIME PATTERN). THE FIRST ONE LOOKS TO DEVELOP
OVER THE EASTERN CORN BELT REGION AND RAPIDLY SHIFT SOUTHEASTWARD
WHICH DEPENDING ON ITS ACTUAL TRAJECTORY LOOKS TO CLIP THE TN VALLEY
ON FRIDAY NIGHT INTO SATURDAY MORNING. THUS, HAVE INCORPORATED A NE
TO SW POP GRADIENT DURING THE OVERNIGHT HOURS THROUGH SATURDAY
MORNING. THERE MAY BE ADDITIONAL THUNDERSTORMS ON SATURDAY AFTERNOON
WITH ANY REMAINING OUTFLOW BOUNDARIES FROM THE EARLIER MCS, AND HAVE
KEPT THE THREAT FOR THUNDERSTORMS ONGOING THROUGH THE DAY DUE TO
THIS THREAT.

YET ANOTHER MCS FEATURE LOOKS TO DEVELOP SLIGHTLY FURTHER EAST (OVER
THE EASTERN CORN BELT), WHICH WOULD TRANSLATE TO A LESSER IMPACT
OVER THE TN VALLEY ON SATURDAY NIGHT AS THE RIDGE BUILDS JUST TO THE
WEST. DUE TO SOME UNCERTAINTY WITH THIS MCS, HAVE ONLY INCLUDED SCHC
POPS ESPECIALLY OVER NE AL (CLOSER TO ITS PROBABLE PATH).

AS ALLUDED TO EARLIER, A MORE AMPLIFIED PATTERN IS EXPECTED TO
DEVELOP AS A BROAD TROUGH DIGGING OVER THE WEST COAST MOVES EASTWARD
OVER THE CENTRAL ROCKIES ON SATURDAY INTO SUNDAY. THIS WILL LIKELY
LEAD TO A FEW ROUNDS OF SEVERE WEATHER OVER THE CENTRAL PLAINS AS
SEVERAL EMBEDDED SHORTWAVE TROUGHS ROUND THE BASE OF THE LONGWAVE
TROUGH DURING THE REST OF THE EXTENDED. DUE TO THE ONGOING
PROGRESSION AND MODEL DISCREPANCIES WITH THE STRENGTH OF THE RIDGE
OVER EASTERN NORTH AMERICA, THE MODEL HAS BEEN DEPICTING A SLOWER
MOVEMENT IN THE TROUGH EASTWARD TOWARDS THE MS/TN RIVER VALLEYS.
THUS, HAVE KEPT SCHC POPS IN THE FORECAST FOR BOTH MONDAY AND
TUESDAY, MAINLY FOR IMPACTS RELATED TO INCREASED LLJ AND A COUPLE OF
WEAK SHORTWAVES IN THE SW FLOW CAUSING ISOLATED THUNDERSTORMS. IN
ADDITION, HAVE RETAINED THE OVERALL INCREASE IN STORM ACTIVITY FOR
TUESDAY NIGHT, BUT FURTHER ADJUSTMENTS MAY BE NECESSARY AS THE
PROGRESSION OF THIS TROUGH AND ACCOMPANYING SFC COLD FRONT APPEAR TO
BE SLOWING.

DAYTIME HIGH TEMPERATURES ARE EXPECTED TO WARM CLOSE TO 90 DEGREES
BY SUNDAY AND NEAR NORMAL TEMPERATURES (IF NOT SLIGHTLY ABOVE) IN
THE 80S FOR THE REST OF THE FORECAST AS DEEP SW FLOW RE-DEVELOPS
OVER THE REGION. OVERNIGHT LOWS SHOULD BE IN THE 60S FOR MOST OF THE
TIME PERIOD, WHILE SOME AREAS MAY EXPERIENCE UPPER 50S WHERE
CONVECTION HAS OCCURRED.

SL.77

&&

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

$$

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