Severe Thunderstorm Warning 408, NWS Birmingham, AL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BIRMINGHAM AL
645 PM CDT WED JUL 29 2009

.UPDATE...AVIATION DISCUSSION.

&&

.DISCUSSION...

A RATHER CHAOTIC WEATHER PATTERN FOR MID SUMMER WITH AN ACTIVE
WESTERLY UPPER LEVEL FLOW WITH DAILY IMPULSES MOVING ACROSS THE
GULF COAST STATES. CONVECTION HAS EVEN BEEN ACTIVE DURING THE
OVERNIGHT PERIODS. THE MODELS ARE SHOWING NO SIGNIFICANT CHANGE IN
THE OVERALL PATTERN THROUGH THE WEEKEND WITH A BROAD UPPER TROF
OVER THE EASTERN UNITED STATES.

FOR TONIGHT...MOST OF THE ACTION SHOULD BE CONFINED TO AREAS NORTH
OF I-20 AS A MID LEVEL AXIS OF LIFT PUSHES NORTH ACROSS
MISSISSIPPI AND INTO NW ALABAMA. THE CONVECTION HAS BEEN WEAKENING
AS IT MOVES CLOSER TO ALABAMA...BUT WITH TEMPERATURES IN THE UPPER
80S IN THIS AREA...EXPECT CONVECTION TO MAINTAIN INTENSITY AS IT
SPREADS EASTWARD. ON THURSDAY...THERE DOES NOT APPEAR TO BE ANY
SIGNIFICANT UPPER LEVEL FEATURE THAT WILL IMPACT CENTRAL ALABAMA...
AND CONVECTION SHOULD BE MORE DIURNAL IN NATURE...BUT WITH
PRECIPITABLE WATER STAYING NEAR 2 INCHES AND FORECAST HIGHS NEAR
90...THERE WILL LIKELY BE GOOD AREAL COVERAGE OF RAINFALL ONCE
AGAIN.

STRONG SHORT WAVE TROF DROPPING SOUTHWARD OUT OF THE PLAINS STATES
WILL INCREASE LIFT AND PRODUCE STRONG CONVECTION WEST OF THE
MISSISSIPPI RIVER ON THURSDAY. THIS FEATURE WILL PUSH EASTWARD
THURSDAY NIGHT AND LIKELY PUSH A LINE OF THUNDERSTORMS INTO WEST
ALABAMA LATE THURSDAY NIGHT. FORECAST SOUNDINGS INDICATE
INCREASING LEVELS OF SHEAR AND HELICITY OVER WEST ALABAMA THURSDAY
NIGHT...SO THIS WILL HAVE TO BE MONITORED CLOSELY. MODELS ALSO
SHOW A SURFACE LOW DEVELOPING WITH THIS SYSTEM WHICH COULD ENHANCE
LOW LEVEL CONVERGENCE.

THE SHORT WAVE TROF WILL LIFT EAST OF THE AREA ON FRIDAY BUT
LEAVE A SURFACE BOUNDARY IN ITS WAKE...SO GOOD CHANCES FOR RAIN
AGAIN ON FRIDAY, YET ANOTHER SHORT WAVE TROF WILL PUSH TOWARDS THE
AREA ON SATURDAY...SO THE WET AND STORMY PATTERN WILL LIKELY LAST
INTO THE WEEKEND. THERE MAY BE A SMALL BUT NOTICEABLE BREAK IN
THE WET PATTERN NEXT WEEK IF UPPER RIDGE OVER TEXAS SHIFTS EASTWARD
AS FORECAST BY THE GFS MODEL.

58/ROSE

&&

.AVIATION...00Z TAF DISCUSSION.

AN UPPER LEVEL TROUGH REMAINS ACROSS MUCH OF THE EASTERN UNITED
STATES. THIS PATTERN WILL KEEP THE WEATHER CHAOTIC ACROSS CENTRAL
ALABAMA THIS FORECAST PERIOD. SURFACE WINDS GENERALLY REMAIN OUT OF
THE SOUTHWEST OUTSIDE OF RAIN AREAS AND UNDER 10 KTS. MOST TERMINALS
START OUT VFR BUT WILL DROP CEILINGS TO MVFR FOR NORTHERN SITES BY
06Z AND ADD VCSH AND CB. SEVERAL IMPULSES IN THE MID LEVEL FLOW WILL
KEEP THE SHOWERS AND THUNDERSTORMS OVER MISSISSIPPI MOVING EASTWARD
OVERNIGHT AND INTO THURSDAY. CEILINGS SHOULD RISE INTO THE VFR RANGE
BY 13-15Z. WITH THE MOISTURE CONTENT IN THE AIR...LOCALLY HEAVY RAIN
IS POSSIBLE WHICH MAY LOCALLY DECREASE CEILINGS AND VIS THROUGH THE
PERIOD.

75/LINHARES

&&

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

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