Area Forecast Discussion
Issued by NWS Birmingham, AL

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000
FXUS64 KBMX 181158
AFDBMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BIRMINGHAM AL
658 AM CDT SAT APR 18 2015

.UPDATE...
FOR 12Z AVIATION.

&&

.DISCUSSION...

A BREAK IN THE RAIN THIS MORNING FOR CENTRAL ALABAMA WILL BE
SHORTLIVED. THE CLOSED LOW ROTATING OVER THE SOUTHWESTERN CONUS
FOR THE LAST SEVERAL DAYS...AND ONE OF THE REASONS FOR THIS MOIST
PATTERN...WILL PUSH INTO THE SOUTHERN PLAINS LATER TODAY. AHEAD OF
THIS CLOSED LOW...A STRONG SHORTWAVE WILL ROUND THE BASE OF THE
TROUGH AND EJECT NORTHEASTWARD. STRONG DIVERGENCE ALOFT WILL
OVERCOME THE RELATIVELY DRY AIR CURRENTLY IN PLACE AND RAINFALL
WILL SPREAD BACK INTO THE AREA BEGINNING LATE THIS AFTERNOON.
ALREADY SEEING A LARGE RAIN SHIELD MOVING OUT OF LA THIS MORNING.

ANOTHER RIPPLE IN MID LEVEL FLOW AND ASSOCIATED SPEED MAX WILL
HELP PUSH A WARM FRONT NORTH OF THE AREA THIS EVENING. THERE REMAINS
SOME CONCERNS AND QUESTIONS REGARDING INSTABILITY TONIGHT IN THE
MOIST WARM SECTOR. WITH WIDESPREAD RAINFALL OCCURRING...PROFILES
ARE SATURATED WITH PW VALUES OF 1.7-1.8 INCHES...NEAR THE TOP FOR
OBSERVED VALUES FOR THIS TIME OF YEAR. RAINFALL BOTH ACROSS THE
AREA AND TO OUR SOUTH SHOULD DISRUPT LOW LEVEL FLOW NORTHWARD.
HOWEVER...A STRENGTHENING SURFACE LOW WILL MOVE NORTHWARD ALONG
THE MISSISSIPPI RIVER...AND LOW LEVEL HELICITY VALUES INCREASE
SOUTH OF THE WARM FRONT AS 0-6KM SHEAR INCREASES TO AROUND 40KTS
AFTER 09Z. A WEAK UPPER LEVEL JET OF 80KTS ALSO NOSES NORTHWARD
OVERNIGHT. THE AIRMASS SHOULD BE FAIRLY WORKED OVER BY THIS
TIME...LIMITING OR EVEN PREVENTING ANY STRONG TO SEVERE
CONVECTION. WILL HAVE TO WATCH FOR DEVELOPMENT THOUGH IN THE WARM
SECTOR...SOUTH OF THE EXITING RAIN SHIELD LATE TONIGHT AND EARLY
SUNDAY MORNING. STRONGER STORMS COULD POSE A HAIL AND DAMAGING
WIND THREAT...AND LIKELY WILL SHOW SIGNS OF ROTATION DUE TO THE
INCREASED WIND PARAMETERS...SO A BRIEF TORNADO CAN NOT BE RULED
OUT EITHER.

A LULL IN ACTIVITY IS EXPECTED SUNDAY MID DAY...AS THAT INITIAL
SURFACE LOW MOVES NORTHWARD INTO THE OHIO RIVER VALLEY AND WIND
FIELDS DIMINISH BRIEFLY BETWEEN SYNOPTIC FEATURES. INSTABILITY
FIELDS DO INCREASE THROUGH THE AFTERNOON...AS SHEAR VALUES INCREASE
WITH THE APPROACH OF THE NEXT SHORTWAVE AND LAPSE RATES INCREASE
TO OVER 7C/KM. A FEW ISOLATED STORMS WILL BE POSSIBLE BY LATE
AFTERNOON/EARLY EVENING. GIVEN THE INCREASINGLY SHEARED
ENVIRONMENT...STRONGER STORMS WILL HAVE TO BE WATCHED FOR
ROTATION.

SUNDAY EVENING...AHEAD OF AN APPROACHING COLD FRONT AND THE
PREVIOUSLY MENTIONED CLOSED LOW...WE WILL BE LOOKING TO OUR WEST
AS THAT AIRMASS WILL BE SLIGHTLY MORE ROBUST AND SUPPORTIVE OF
SEVERE STORMS. NOT TO DISCOUNT THE SHEAR VALUES LOCALLY. THEY
WILL BE STRONGER THAN WHAT IS EXPECTED TONIGHT...AND CAPE VALUES
REMAIN AROUND 1500-2000J/KG AFTER SUNSET. WITH TIME...LOW LEVEL
WINDS BECOME SOUTHWESTERLY AND STORMS BECOME MORE LINEAR AS THEY
PUSH EASTWARD INTO CENTRAL ALABAMA...POSING MORE OF A WIND THREAT
THAN TORNADO THREAT. SYNOPTIC FORCING WEAKENS AS THE CLOSED LOW
OPENS AND PULLS AWAY FROM THE AREA...BUT A WEAKENING LINE OF
STRONG TO SEVERE STORMS IS EXPECTED TO PUSH INTO WEST CENTRAL
ALABAMA SUNDAY NIGHT.

GOOD NEWS...ONCE MOISTURE AND LINGERING SHOWERS CLEAR THE AREA
MONDAY MORNING...WE EXPECT AT LEAST A COUPLE DAYS OF DRY
CONDITIONS NEXT WEEK BEFORE RAIN CHANCES RETURN MID WEEK.

14


&&

.AVIATION...
12Z TAF DISCUSSION.

CEILING IMPROVEMENT WILL BE A SLOW PROCESS FOR MOST FOLKS TODAY...
ALTHOUGH IT ALSO WOULDN`T SURPRISE ME IF ONE OR TWO SPOTS SHOT UP
TO MVFR (OR BETTER) BEFORE 15Z. ASIDE FROM DEALING WITH CEILING
LEVELS THROUGH THE AFTERNOON...LOOKS LIKE THE MAIN CONCERN OVER
THE NEXT 24 HOURS WILL BE THE ARRIVAL OF ANOTHER BATCH OF SHOWERS
AND THUNDERSTORMS TONIGHT. I`VE INTRODUCED THUNDERSTORMS TO THE
FORECAST TONIGHT STARTING AROUND 00Z OR 01Z...WHICH IS ASSOCIATED
WITH A WARM FRONT THAT WILL BE PUSHING NORTHWARD. WHILE
CONIFDENCE IS HIGH THAT THERE WILL BE SOME RAIN TONIGHT IN MOST
(IF NOT ALL) AREAS...THERE IS STILL LOW CONFIDENCE ON THE TIMING
OF ARRIVAL AS WELL AS CEILINGS/VISIBILITY ASSOCIATED WITH THE RAIN
ONCE IT DOES ARRIVE.

/61/

&&

.BMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$



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