000
FXUS64 KHUN 211142 AAA
AFDHUN

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
642 AM CDT TUE MAY 21 2013

.UPDATE...
FOR 12Z TAFS

&&

.SYNOPSIS...
/ISSUED 330 AM CDT TUE MAY 21 2013/
LATEST WATER VAPOR SATL IMAGERY SHOWING A LARGE UPPER LEVEL VORTEX
SPINNING OVER THE NORTHERN PLAINS (DAKOTAS) WITH A SMALLER UPR LVL
CIRCULATION MOVING ACROSS NEW MEXICO...ALONG THE BASE OF THE MAIN
VORTEX. FARTHER EAST...A NARROW UPR LVL RIDGE OF HIGH PRESSURE
RESIDES OVER THE TN/OHIO VALLEYS BUT IS GETTING SQUEEZED BETWEEN THE
BROAD UPR LOW OVER THE NRN PLAINS AND SEVERAL WEAKER UPR LVL
DISTURBANCES ALONG THE EAST COAST...BETWEEN FLORIDA AND VIRGINIA.

THIS PRIMARY UPR LVL VORTEX WHICH HAS BEEN RESPONSIBLE FOR THE
DEVASTATING TORNADOES ACROSS OK/TX THE PAST FEW DAYS...WILL
GRADUALLY MIGRATE EAST OVER THE NEXT 24-48 HOURS...LOSING MUCH OF
ITS PUNCH IN THE PROCESS BUT STILL SETTING THE STAGE FOR A FEW ROUNDS
OF SHOWERS/THUNDERSTORMS ACROSS THE TN VALLEY LATE TONIGHT THROUGH
THURSDAY. LATEST REGIONAL RADAR MOSAIC SHOWING WELL-DEFINED LINE OF
THUNDERSTORMS JUST CROSSING THE MS RIVER INTO WRN TN/NW MS. GENERAL
MOVEMENT OF THE STORMS IS N/NE...WITH A FEW DISCRETE STORMS SEEN
DEVELOPING AHEAD OF THE LINE OVER THE PAST COUPLE HOURS.

DJN.83

&&

.DISCUSSION...
/ISSUED 330 AM CDT TUE MAY 21 2013/
WARM AND MAINLY CONDITIONS ARE EXPECTED TODAY AS THE NARROW UPR
RIDGE HOLDS ONE MORE DAY ACROSS THE REGION. BROAD AREA OF CONVECTION
EXTENDING FROM EAST TEXAS TO THE UPR MIDWEST/GREAT LAKES WILL
CONTINUE TO DRIFT EAST THROUGH THE MORNING. AFOREMENTIONED LINE OF
STORMS PUSHING INTO WRN TN/NE MS HAS CREATED A FAIRLY PRONOUNCED
RAIN-COOLED OUTFLOW BOUNDARY WHICH SHOULD END UP AFFECTING PORTIONS
OF NW ALABAMA LATER THIS MORNING. THIS BOUNDARY COULD INITIATE NEW
CONVECTION AS AIRMASS HEATS UP BY MIDDAY. HOW FAR EAST THIS POTENTIAL
CONVECTION GOES BEFORE INTERACTING WITH UPR RIDGE WILL BE A CHALLENGE
TODAY.

ON THE FRONT SIDE OF THE UPR RIDGE...WEAK ENERGY ROTATING COUNTER-
CLOCKWISE AROUND EAST COAST DISTURBANCES COULD TOUCH OFF A FEW
SHRA/TSRA ACROSS THE SRN APPALACHIANS BY EARLY AFTN. DISCOUNTING THE
OVERLY AGGRESSIVE 00Z/21 NAM FOR TODAY. OVERALL...BELIEVE MUCH OF THE
AREA WILL REMAIN DRY TODAY...HOWEVER WILL MENTION THREAT OF ISOLD
AFTN (10-20% POP) SHRA/TSRA ACROSS NW AL AND NE AL/SRN MIDDLE TN. IF
ANYTHING DEVELOPS...NOT EXPECTING CONVECTION TO LAST LONG BENEATH UPR
RIDGE.

TONIGHT INTO WEDNESDAY...DEEPER MOISTURE AND ENERGY ARRIVE AS
LONGWAVE UPR TROUGH FINALLY APPROACHES. CONVECTION WILL LIKELY
SUSTAIN ITSELF ALONG LEADING PRE-FRONTAL/850 MB BOUNDARY WITH MAXIMUM
LOW LVL CONVG AND LOW LVL WIND ENERGY EXISTING ACROSS NW ALABAMA BY
12Z/WED. POPS OVERNIGHT WILL BE HIGHEST OVER NW AL...WITH LESSER
CHANCES TO THE SOUTH AND EAST UNTIL DURING THE DAY WEDNESDAY WHERE
INCREASED LOW LVL FORCING /ALBEIT WEAK/ ALONG WITH A THETA-E MAXIMUM
WILL SUPPORT SCT-NUMRS SHRA AND A FEW TSRA ACROSS THE FORECAST AREA.
MARGINAL LOW LVL SHEAR/INSTABILITY AND RELATIVELY WEAK MID LVL LAPSE
RATES SUGGEST A LOW POTENTIAL FOR SVR WEATHER WEDNESDAY AT THIS TIME.
HOWEVER...A FEW HEAVIER SHOWERS/STORMS EMBEDDED IN PERHAPS A WEAK
LEWP COULD PRODUCE BRIEF STRONG WIND GUSTS LATE TONIGHT THROUGH
WEDNESDAY AFTERNOON.

WEDNESDAY NIGHT/THURSDAY...SHRA/TSRA ARE EXPECTED TO DIMINISH
WEDNESDAY NIGHT BUT MOIST/UNSTABLE AIRMASS COUPLED WITH PASSING UPR
LVL WAVES COULD KEEP A FEW -SHRA GOING...ESPECIALLY ACROSS NE AL.
SFC FRONT/LEADING EDGE OF MUCH DRIER AIR WILL EXTEND ACROSS NRN
ARKANSAS/NW TN/WRN KY BY 15Z/THURSDAY. THIS BOUNDARY WILL ACCELEATE
SOUTHEAST AND CROSS THE FORECAST AREA THURSDAY NIGHT. BEFORE THIS
HAPPENS...STILL INFLUENCED BY BROAD LONGWAVE UPR TROUGH...ADDITIONAL
UPR LVL ENERGY WITHIN NW FLOW ALOFT WILL MARCH ACROSS THE AREA
THURSDAY. THIS COUPLED WITH THE APPROACHING SFC BOUNDARY COULD
SUPPORT A BRIEF ROUND OF SHRA/TSRA THURS AFTN. THE 00Z/21 NAM IS THE
MOST ROBUST WITH THIS POTENTIAL SOLN BUT EVEN THE MEDIUM RANGE MODELS
SHOW SOMETHING COMING TOGETHER.

.EXTENDED /FRIDAY THROUGH MONDAY/...
MUCH DRIER AND COOLER CONDITIONS WILL PREVAIL FRIDAY INTO SATURDAY
AS HIGH PRESSURE BUILDS ACROSS THE EASTERN CONUS IN THE WAKE OF A
DEPARTING LONGWAVE TROUGH. AFTER A COOL START FRIDAY MORNING /LOWS IN
THE MID 50S/...TEMPS SHOULD REBOUND TO THE MID TO UPR 70S BENEATH
PLENTY OF SUNSHINE. THE COLDEST MORNING IS EXPECTED TO BE SATURDAY AS
LOWS COULD FALL TO THE UPR 40S IN SOME VALLEY LOCATIONS. A "DIRTY"
RIDGE OF HIGH PRESSURE OVER THE CENTRAL CONUS WILL GRADUALLY MOVE
ACROSS THE REGION BY THE END OF THE WEEKEND INTO EARLY NEXT WEEK.
WITH THE AREA IN A NORTHWEST FLOW ALOFT...THE 00Z/21 ECMWF IS
SLIGHTLY WETTER AND MORE AGGRESSIVE WITH UPR WAVES TRAVERSING THE
DOWN THE FRONTSIDE OF THE UPR RIDGE. THIS IS A NEW SOLN AND RELATIVE
OUTLIER WITH THE OTHER OPER MODEL/ENSEMBLES...SO WILL LEAN TOWARD A
DRIER SOLN SUNDAY/MONDAY BUT SHOW INCREASING CLOUDS. TEMPS WILL
REBOUND SEVERAL DEGREES EACH DAY SUN/MON UNDER THE INFLUENCE OF UPR
RIDGE.

DJN.83

&&

.AVIATION...
FOR 12Z TAFS...
MORNING MVFR HAZE OVER KMSL WILL FADE WITHIN A FEW HOURS...WITH VFR
WEATHER GENERALLY PREVAILING FOR THE REMAINDER OF THE PERIOD.
ISOLATED CONVECTION REMAINS POSSIBLE OVER PARTS OF NW ALABAMA
TODAY...AS CONVECTION NEARS FROM THE WEST. THAT CONVECTION WILL
SLOWLY OVERSPREAD THE REGION FROM WEST TO EAST DURING LATE TUE
NIGHT/WED MORNING. LIGHT WINDS THIS MORNING WILL BECOME S-SW IN THE
5-15 KT RANGE. VARIABLE/GUSTY WINDS POSSIBLE IN/NEAR THUNDERSTORMS.

RSB

&&

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

$$

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AT WEATHER.GOV/HUNTSVILLE.