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000
FXUS64 KHUN 170319 AAB
AFDHUN

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
1019 PM CDT THU MAY 16 2013

.UPDATE...
EVENING FORECAST UPDATE

&&

.DISCUSSION...
EVEN WITH DECENT UPPER LEVEL CLOUD COVER ACROSS THE TN
VALLEY...TEMPERATURES HAVE LARGELY FALLEN INTO THE 60S BY 10PM. THIS
IS AFTER AFTERNOON HIGHS THAT TOPPED OUT IN THE LOWER AND MID
80S...JUST SLIGHTLY ABOVE SEASONABLE NORMS FOR MID MAY. TEMPERATURES
SHOULD CONTINUE TO DROP GENERALLY INTO THE LOWER 60S...WITH A W-E
GRADIENT AS WAS SEEN LAST NIGHT ACROSS THE AREA (I.E. COOLER IN THE
E).

TO OUR WEST A MCS CONTINUES TO SLOWLY TRACK ACROSS THE ARK-LA-
MISS...WITH THE MAIN CORE OF CONVECTION CURRENTLY SITUATED OVER S ARK
BUT ADDITIONAL PRECIP EXTENDING ACROSS NW MS IN THE DOWNWIND ANVIL
SHIELD. MOVEMENT OF THIS MCS IS THE MAIN CONCERN FOR OUR PRECIP
FORECAST TONIGHT...WITH BULK SHEAR VECTORS DIRECTED TOWARD THE NE AS
THE COMPLEX ROUNDS THE BASE AND COMES UP THE LEE OF THE MID LEVEL
TROF. MUCH OF THE MODEL OUTPUT HAS INDICATED THAT THE MCS ASSOCIATED
WITH THIS SYSTEM SHOULD TRACK NW OF OUR CWA DURING THE OVERNIGHT
HOURS. BUT THE CURRENT PLACEMENT OF THE PRECIP SHIELD IS FAR ENOUGH
EAST THAT EVEN WITH THE EXPECTED SHIFT TO A MORE NORTHERLY DIRECTED BULK
SHEAR...LOCATIONS WEST OF I-65 WILL HAVE DECENT CHANCE OF SEEING SOME
PRECIP AFTER MIDNIGHT. HAVE INCLUDED A CHC RAIN BUT ONLY A SLT CHC
THUNDER ATTM SINCE THE BETTER CONVECTION CORE SHOULD STILL REMAIN
FURTHER WEST OF THE CWA. AS THIS AREA OF SHRA/TSRA MOVES CLOSER TO
THE CWA...MOISTURE SHOULD CONTINUE TO POOL N INTO THE CWA AND KEEP TD
VALUES IN THE UPPER 50S AND SLOW THE TEMP DROP OFF SLIGHTLY.

CCC

&&

.AVIATION...
/ISSUED 621 PM CDT THU MAY 16 2013/
FOR 00Z TAFS...WHILE VFR CONDS ARE EXPECTED TO CONTINUE THRU THE
EVENING...CHANCES FOR LOWER CEILINGS LATE TONIGHT ARE LOOKING MORE
LIKELY. EXPECT LOW/MID LVL CIGS TO INCREASE AFTER 06Z WITH MVFR CIGS
POSSIBLE AS AN AREA OF PRECIP APPROACHES ARND 12Z (BUT AS EARLY AS
09Z). CIGS COULD FALL BELOW 2KFT IF ANY PRECIP DIRECTLY IMPACTS THE
TERMINALS...WILL CONTINUE TO MONITOR EVOLUTION TO OUR W BEFORE
INCLUDING IT IN THE TAFS. LOWEST CIGS SHOULD REMAIN N/W OF THE
TERMINALS THOUGH. CIGS SHOULD THEN IMPROVE SLIGHTLY DURING THE AFTN
AS ANOTHER WAVE OF SCT TSRA APPROACH N AL.

CCC

&&

.PREV DISCUSSION...
/ISSUED 303 PM CDT THU MAY 16 2013/
THE POLAR WESTERLIES REMAIN TO THE NORTH, OVER THE CANADA/US BORDER,
WHILE AN OPEN SHORTWAVE TROUGH (FORMERLY CUTOFF LOW) MOVES ACROSS THE
OZARKS INTO THE MS RIVER VALLEY. A COMPLEX SCENARIO WILL UNFOLD AS
THIS OPEN WAVE SHIFTS EASTWARD, AND IT WILL BE THE MAIN PLAYER FOR
IMPACTS IN THE SHORT TERM. MEANWHILE, AN APPROACHING LONGWAVE UPPER
TROUGH JUST OFF THE WEST COAST WILL BECOME IMPORTANT BY THE MIDDLE
PART OF NEXT WEEK. THEN, WELL TO THE SOUTH IS A WEAK SUBTROPICAL JET
STRETCHING ACROSS NORTHERN MEXICO.

AS THE OPEN SHORTWAVE TROUGH EDGES CLOSER ON FRIDAY INTO THE OH/MS
RIVER VALLEYS, ADDITIONAL SHOWER AND THUNDERSTORM DEVELOPMENT IS
EXPECTED INTO THE MORNING. ELEVATED INSTABILITY WILL ACTUALLY BE
SOMEWHAT HIGHER WITH SOMEWHAT STEEPER MID-LEVEL LAPSE RATES. FREEZING
LEVELS ALSO LOOK TO DROP CLOSE TO 10-12 KFT BY FRIDAY MORNING. THUS,
HAVE INCREASED POPS TO LIKELY MAINLY OVER NW AL (CLOSER TO THE MID-
LEVEL SHORTWAVE TROUGH AXIS). THERE WILL LIKELY BE A BREAK DURING THE
MIDDLE OF THE DAY BEFORE A WEAK VORT MAX ROTATES AROUND THE WAVE AS
IT MOVES ACROSS S IL ON FRIDAY AFTERNOON/EVENING POSSIBLY SPARKING
OFF ADDITIONAL SHOWERS/STORMS. HAVE KEPT THUNDERSTORMS IN THE
FORECAST ON FRIDAY NIGHT TO ACCOUNT FOR ANY LINGERING CONVECTION AND
DUE TO THE POSSIBILITY OF COLD POOL INDUCED CONVECTION. DID KEEP
DAYTIME HIGHS MAINLY IN THE UPPER 70S TO LOW 80S ON FRIDAY DESPITE
CONVECTION AND CLOUD COVER. WITH THE PREVAILING NW FLOW OVER THE TN
VALLEY, AN ADDITIONAL VORT MAX ROTATING AROUND THE SHORTWAVE TROUGH
MAY SPARK OFF AN MCS OVER THE EASTERN CORN BELT THAT MAY DROP OVER
THE TN VALLEY LATE FRIDAY NIGHT INTO SATURDAY MORNING. THOUGH MODELS
HAVE DIFFICULTY DEPICTING THESE FEATURES THIS FAR IN ADVANCE, WILL
KEEP CHC POPS. AS THE SHORTWAVE OR REMNANT WEAK LIFT EXITS THE REGION
ON SATURDAY, THERE MAY BE ADDITIONAL DEVELOPMENT MAINLY OVER
NORTHEAST AL/S MIDDLE TN THROUGH SATURDAY EVENING.

MEANWHILE, MODEL DEPICTIONS OF THE PREVIOUSLY MENTIONED LONGWAVE
TROUGH OVER THE WEST COAST SHOW IT MOVING INTO THE ROCKIES BY SUNDAY
WHILE RIDGE BUILDING BEGINS OVER EASTERN NORTH AMERICA. THIS WILL
TRANSLATE TO EVEN WARMER CONDITIONS ON SUNDAY THROUGH TUESDAY OVER
THE TN VALLEY WITH DAYTIME HIGHS IN THE LOW 90S, AND DEEP SW WARM
FLOW. AS THE UPPER TROUGH MOVES EASTWARD, EACH MODEL HAS A SLIGHTLY
DIFFERENT PROGRESSION OF THE UPPER TROUGH. HOWEVER, IT DOES APPEAR
THAT NEGATIVELY TILTED SHORTWAVE TROUGHS EMBEDDED AND ROTATING
WITHIN THE PRIMARY BROAD UPPER TROUGH WILL ENHANCE A LLJ (INCREASING
MOISTURE), STEEPEN LAPSE RATES, AND INCREASE WIND SHEAR, BRINGING
THE POSSIBILITY OF SEVERE WEATHER OVER THE CENTRAL AND SOUTHERN
PLAINS ON SUNDAY NIGHT THROUGH MONDAY NIGHT. AN ACCOMPANYING COLD
FRONT WILL LIKELY TRANSLATE TO A LINE OF SHOWERS AND THUNDERSTORMS
SHIFTING SOUTHEAST AND SLOWING (AS IT APPROACHES THE MS RIVER) AS
THE UPPER TROUGH LIFTS NORTHEAST ATOP THE BUILDING RIDGE. THE MODEL
DEPICTIONS OF THE PROGRESSION OF THIS TROUGH IS DIFFERENT WITH ONE
PARTICULAR MODEL RUN (ECMWF 16/12Z) SHOWING THE TROUGH CLOSING OFF
INTO A LOW AND ROTATING OVER THE NORTHERN PLAINS FOR A COUPLE OF
DAYS. THIS COULD MEAN THAT THE SHOWERS AND STORMS MAY NOT IMPACT THE
TN VALLEY UNTIL LATER ON WEDNESDAY. HOWEVER, WILL KEEP THE CURRENT
TIMING FOR NOW WITH FURTHER ADJUSTMENTS LIKELY, ESPECIALLY WITH
TYPICALLY SLOWER MOVEMENT OF TROUGHS THIS TIME OF YEAR AS RIDGES
BEGIN TO STRENGTHEN.

SL.77

&&

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

$$

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