Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 251612 AAB
AFDHUN

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
1112 AM CDT MON MAY 25 2015

.UPDATE...
MORNING UPDATE.

&&

.DISCUSSION...
A LINE OF SHOWERS WITH EMBEDDED THUNDERSTORMS ARE CURRENTLY MOVING
NNE ACROSS NW AL. THIS LINE WAS SEVERE WHEN IT WAS STILL IN MS WHERE
GREATER SHEAR AND HELICITY WAS PRESENT.

CAPE VALUES HAVE NOT BEEN HIGH THIS MORNING AND DO NOT LOOK TO BE
THROUGHOUT TODAY EITHER...WITH VALUES UP TO 1000 J/KG. WITH GOOD LOW
LEVEL LAPSE RATES NEAR THE SURFACE...ALREADY STARTING TO MIX DOWN A
FEW GUSTS 10-15KTS AROUND THE AREA. A SMALL CAP AROUND 900MB SEEMS TO
HOLD IN THE SOUNDINGS THROUGHOUT THE DAY KEEPING SFC CAPE LOW. LOOKING
AT THETA E AS WELL...800-700MB IS WHERE WE GET MOST OF OUR LIFT
TODAY. THROUGH TODAY...THERE ARE BULK SHEAR VALUES 25-30KTS LOWERING
AS TIMES GETS CLOSER TO 00Z.

A STRATUS DECK IS STILL PRESENT THIS MORNING WITH LOCATIONS RANGING
FROM 2500FT IN NE AL TO 1000FT IN NW AL. THIS WILL PLAY A ROLE IN
OUR TEMPS TODAY. SLIGHTLY ADJUSTED THE CLOUD COVER FOR TODAY BUT
LOOKS LIKE MOST OF US WILL SEE MOSTLY CLOUDY TO OVERCAST SKIES FOR
TODAY.

WITH MOISTURE CONVERGENCE OVER THE AREA...PWATS WILL RISE OVER 1.5
INCHES TODAY. WITH TRAINING OF SHOWERS/THUNDERSTORMS ALREADY THIS
MORNING ALONG WITH RAINFALL FROM YESTERDAY IN NW AL...WILL MONITOR
FOR ANY FLOODING CONCERNS DUE TO ADDITIONAL RAINFALL TODAY AND
TOMORROW. WIND FIELD FLOW TODAY IS NEARLY PARALLEL SO THE PROGRESSION
OF THIS LINE HAS BEEN SLOW TO MOVE EAST VERY QUICKLY.

THE LINE WILL CONTINUE TO MOVE NNE ACROSS THE HUN CWA TODAY. DAYTIME
HEATING WILL BE LIMITED TO ASSIST IN DEVELOPMENT BUT SHOWERS AND
THUNDERSTORMS WILL BE SEEN BY MOST AT SOME POINT TODAY. PRIMARY
HAZARDS WITH THE STRONGEST STORMS WOULD BE GUSTY WINDS AND HEAVY
RAIN.

JMS

&&

.AVIATION...
/ISSUED 654 AM CDT MON MAY 25 2015/
FOR 12Z TAFS...BAND OF MOD/HVY SHRA AND EMBEDDED TSRA CONTINUES TO
IMPACT MAINLY NW ALABAMA EARLY THIS MORNING...AND IS EXPECTED TO
SHIFT VERY SLOWLY EWD THRU REMAINDER OF THE DAY. HAVE INCLUDED
SHRA/VCTS AS PREVAILING WX COND FOR MSL THRU 25/17Z AND HSV BTWN
15-19Z. STRATUS WILL BECOME ESTABLISHED IN THE 2500-4000 FT LAYER
WITHIN BAND OF PCPN...AND ALTHOUGH SOME MVFR/IFR VSBY WILL ALSO BE
PSBL IN HEAVIER SHRA/TSRA THIS IS TOO DIFFICULT TO PINPOINT ATTM. LOW
CLOUDS WILL TEMPORARILY SCATTER LATE THIS AFTN AS PCPN BAND SHIFTS
EWD INTO NE ALABAMA. HOWEVER...MVFR STRATUS WILL REDEVELOP BY 26/02Z
IN VERY MOIST AIRMASS. ANOTHER PASSING UPPER DISTURBANCE WILL BRING
RENEWED CHANCES FOR TSRA BTWN 08-12Z...AND VCTS/CB WAS INCLUDED FOR
THIS PERIOD. OUTSIDE OF CONVECTION...PREVAILING SFC FLOW WILL REMAIN
FROM THE SSE AT 5-10 KTS.

70/DD

&&

.PREV DISCUSSION...
/ISSUED 344 AM CDT MON MAY 25 2015/

SYNOPSIS...
THE QUASI-PERMA LOW OVER THE CENTRAL ROCKIES THAT HAS BROUGHT
MULTIPLE EPISODES OF SEVERE WEATHER AND SIGNIFICANT FLASH FLOODING
ACROSS PORTIONS OF THE SOUTHERN PLAINS IS FINALLY BEGINNING TO SHIFT
EAST. AS A RESULT, A WET PATTERN HAS RETURNED TO THE TN VALLEY FOR
THE FORECAST PERIOD. THE DRY WEEKEND CAME TO AN ABRUPT END AS
EVIDENCED BY THE SHOWERS AND THUNDERSTORMS THAT TRAVERSED THE REGION
SUNDAY EVENING.

SINCE THE TN VALLEY HAS BEEN DRY FOR SEVERAL DAYS PRIOR TO THIS
PATTERN CHANGE, AND BECAUSE THE PREVAILING MODELS SHOW A BREAK IN THE
CONVECTIVE ACTIVITY (WITHIN A SERIES OF SEVERAL SHORTWAVE TROUGHS
TRAVERSING THE REGION), NOT EXPECTING A LARGE SCALE FLASH FLOODING
EVENT. HOWEVER, THIS POTENTIAL WILL BE MONITORED OVER THE NEXT FEW
DAYS.

DISCUSSION...
SHOWERS AND THUNDERSTORMS THAT ARE ALREADY PRESENT ACROSS MS WILL
GRADUALLY SPREAD EAST AS A MOISTURE CONVERGENCE AXIS AND ACCOMPANYING
SHORTWAVE TROUGH (PRESENT BTWN 850-500 HPA) GRADUALLY MOVES NORTHEAST
ACROSS THE OH RIVER VALLEY THIS MORNING. THERE HAVE BEEN DISCRETE
CELLS AND MULTI-CELL CLUSTERS FORMING WITHIN THE SOUTHERLY FLOW AHEAD
OF THE MORE WIDESPREAD SHOWERS/T-STORMS WITHIN THE CONVERGENCE AXIS.
WITH WEAK INSTABILITY AND WIND FIELDS (I.E. LOW WIND SHEAR) ONLY
EXPECTING BENIGN THUNDERSTORMS IN ANY STRONGER CONVECTIVE CELLS.
ADDITIONAL SHOWERS AND THUNDERSTORMS ARE ANTICIPATED LATE THIS
MORNING INTO EARLY AFTERNOON AS A WEAK MESOSCALE VORTEX CROSSES THE
TN VALLEY. THE SHOWERS AND THUNDERSTORMS SHOULD THEN DIMINISH IN
COVERAGE FOR THIS EVENING AND TONIGHT BUT WITH SOME VERY WEAK
UNDULATIONS WITHIN THE PREVAILING SATURATED SOUTHERLY FLOW, CHANCE
POPS ARE STILL WARRANTED.

FOR TUESDAY, A MID-LEVEL SHORTWAVE TROUGH CURRENTLY TRAVERSING THE
DESERT SOUTHWEST AROUND THE PRIMARY BROAD TROUGH WILL MOVE ACROSS THE
SOUTHERN PLAINS ON MONDAY AFTERNOON EXACERBATING THE FLOODING PROBLEM
THERE. THIS TROUGH WILL ACT TO SEND WHAT APPEARS TO BE AN MCS
NORTHWARD TOWARDS THE MS RIVER VALLEY POSSIBLY IMPACTING THE TN
VALLEY EARLY TUESDAY MORNING AND DISSIPATING. DEPENDING ON ITS
TRAJECTORY THE TN VALLEY MAY BE STABILIZED BY ITS DEBRIS CLOUDS OR
ANY THUNDERSTORM ACTIVITY THAT ENCROACHES OVER FAR NW AL. HOWEVER,
MODEL GUIDANCE HAS BEEN QUITE VARIANT WITH THE TRAJECTORY AND
POSSIBLE DESTABILIZATION DURING TUESDAY AFTERNOON. ADDITIONALLY, MID-
LEVEL WIND FIELD SOLUTIONS ARE VARIANT AS WELL WHICH WOULD ENHANCE OR
WEAKEN WIND SHEAR. THOUGH THE GFS APPEARS TO BE THE STRONGEST WITH
WIND FIELD AND AFTERNOON DESTABILIZATION, THE OTHER MODELS PRESENT
DIFFERING REPRESENTATIONS OF THESE FIELDS (MAINLY WEAKER) AND MCS
TRAJECTORY. MODEL CONSENSUS DOES SHOW SHOWERS AND THUNDERSTORMS TO BE
MORE WIDESPREAD AND WILL KEEP HIGH POPS FOR ALL OF TUESDAY. HOWEVER,
WITH ALL OF THE UNCERTAINTIES ASSOCIATED WITH THE SCENARIO THAT IS
PAINTED MAINLY BY THE GFS SOLUTION, HAVE LITTLE CONFIDENCE IN
SUPPORTING THE THREAT FOR SEVERE WEATHER.

ANOTHER MID-LEVEL SHORTWAVE TROUGH MAY IMPACT THE TN VALLEY ON
WEDNESDAY AND HAVE KEPT CHANCE POPS THAT DAY AND AGAIN ON THURSDAY
FOR THE NEXT SHORTWAVE TROUGH. HOWEVER, IT SHOULD BE NOTED THAT
SEVERAL ADJUSTMENTS AND SOMETIMES WIDE VARIATIONS IN POPS ARE LIKELY
GIVEN THE MODELS` STRUGGLE WITH TIMING OF MID-LEVEL SHORTWAVE TROUGHS
WITHIN THIS REGIME. THEN, SHORTWAVE RIDGING OCCURS ON WEDNESDAY AND
THURSDAY OVER THE SOUTHERN PLAINS BEFORE THIS RIDGE SHIFTS EAST
ACROSS THE SOUTHEAST ON FRIDAY INTO SATURDAY. THIS WILL ACT TO
DIMINISH THUNDERSTORM CHANCES AND INCREASED TEMPERATURES ON FRIDAY
AND SATURDAY. GIVEN WHAT LOOKS LIKE A REPEAT IN THE UPPER FLOW
PATTERN, ANOTHER BROAD TROUGH APPEARS TO SETUP OVER THE S PLAINS BY
THURSDAY AFTERNOON/FRIDAY. THIS FEATURE MAY BRING THE NEXT
OPPORTUNITY FOR SHOWERS AND THUNDERSTORMS LATE IN THE EXTENDED.

SL.77

&&

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

$$

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


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