000
FXUS64 KHUN 152326 AAA
AFDHUN

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

.UPDATE...
FOR 00Z TAFS

&&

.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.

CIRRUS BLOW OFF FROM CONVECTION OVER E TX/OK/LA/AR WILL ADVECT
ACROSS THE TN VALLEY OVER THE COURSE OF THE NIGHT TONIGHT.
OTHERWISE, A QUIET EVENING IS IN STORE WITH TEMPERATURES FALLING
INTO THE UPPER 50S TO LOW 60S. HAVE INCLUDED A SLIGHT GRADIENT WITH
HIGHER TEMPS EXPECTED OVER NW AL WHERE MORE CLOUD COVER WILL DAMPEN
RADIATIONAL COOLING.

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

&&

.AVIATION...
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

&&

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

$$

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