Mike Wilhelm's Alabama Weather Blog

---

Bamawx.com

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BIRMINGHAM AL
658 AM CDT FRI APR 23 2010

...DANGEROUS SEVERE WEATHER OUTBREAK EXPECTED ON SATURDAY...

.DISCUSSION...

LARGE SCALE UPPER AND SURFACE LOW CURRENTLY MOVING THROUGH THE
CENTRAL PLAINS IS ALREADY CAUSING HAVOC ACROSS MUCH OF THE
NATION'S HEARTLAND...AND AS IT TRANSITIONS INTO THE MS VALLEY MORE
DANGEROUS WEATHER IS EXPECTED LATER TODAY. THIS WILL BE THE FIRST
OF A SIGNIFICANT ONE-TWO PUNCH THAT IS EXPECTED ACROSS CENTRAL
ALABAMA OVER THE NEXT TWO DAYS.

MY FIRST CONCERN IS LATER TONIGHT AS THE LEFTOVERS APPROACH OUR
WESTERN COUNTIES...PROBABLY NEAR SUNSET. TONIGHT'S FORECAST
REMAINS CONFUSING...TO SAY THE LEAST...AS THERE IS A LOT OF
UNCERTAINTY TO WHETHER UPSTREAM CONVECTION WILL AFFECT THE WESTERN
HALF OF THE CWA. TO MAKE IT EVEN MORE COMPLEX...THE NUMBER TWO PUNCH
WILL BE GETTING ITSELF RAMPED UP SHORTLY AFTER MIDNIGHT AS A
DECENT WARM FRONT WILL BEGIN SETTING UP ACROSS AREAS OF CENTRAL
MS...EASTWARD INTO WESTERN AND CENTRAL ALABAMA. ALTHOUGH THE
INITIAL THREAT THIS EVENING AND TONIGHT LOOKS TO BE RATHER
MINIMAL...A FEW SURFACE BASED STORMS ARE POSSIBLE AS THEY ROLL
THROUGH CENTRAL AND SOUTHERN MS LATER TODAY...AND WILL BE ON OUR
DOORSTEP...LIKELY AFTER 6 PM OR SO. THEN LATER AS THE WARM FRONTAL
BOUNDARY BEGINS TO GET ESTABLISHED AFTER MIDNIGHT...ELEVATED
STORMS COULD THREATEN A GOOD PORTION OF THE WEST-CENTRAL AREA WITH
HAIL AND SOME DAMAGING WINDS. IT IS ALSO POSSIBLE...BUT NOT LIKELY
THAT A FEW OF THESE STORMS COULD GET SURFACE ROOTED...BUT WILL
REALLY HAVE TO WATCH THE MESOSCALE CLOSELY AS THIS PRELIMINARY
EVENT BEGINS TO UNFOLD.

THIS LEADS RIGHT INTO PUNCH NUMBER TWO...WHICH IS SLATED FOR MUCH
OF THE DAY SATURDAY AS A SHORTWAVE VORT MAX ROLLS AROUND THE
SOUTHERN EDGE OF THE MAIN UPPER LOW. THIS WILL CREATE A MORE THAN
IMPRESSIVE SURFACE LOW JUST TO OUR WEST THAT WILL MOVE AND DEEPEN
RAPIDLY ACROSS THE MID MS VALLEY AND EVENTUAL OHIO VALLEY. THIS
WILL CREATE AN ENVIRONMENT ACROSS CENTRAL ALABAMA THAT WILL BE
NOTHING SHORT OF A CLASSIC SEVERE WEATHER OUTBREAK. EXPECT OUR
OLD WARM FRONT TO LIFT RAPIDLY NORTHWARD THROUGH THE MORNING HOURS
ON SATURDAY...LEAVING THE AREA IN A HIGHLY UNSTABLE AND FAVORABLY
SHEARED AIRMASS FOR MUCH OF SATURDAY AFTERNOON....EVENING...AND AT
LEAST THE FIRST HALF OF THE OVERNIGHT HOURS. WHILE THE INSTABILITY
PARAMETERS REMAIN SOMEWHAT MARGINAL...BUT SIGNIFICANT ENOUGH TO
SUPPORT LONG-LIVED SUPERCELLS...THE BACKING SURFACE WINDS AND MID
AND UPPER JET WILL CERTAINLY BE CONDUCIVE FOR TORNADOES...SOME OF
WHICH WILL BE STRONG AND LONG-LIVED. NOT TO MENTION THE THREAT OF
HEAVY RAIN...LARGE HAIL...AND DAMAGING SURFACE WINDS WHICH WILL
ACCOMPANY THESE CELLS. THE DANGER WILL CONTINUE INTO THE EVENING
HOURS AS THE STEEP UPPER LAPSE RATES AND SOME MID LEVEL DRY AIR WILL
KEEP THE THREAT GOING UNTIL IT EXITS NORTH AND EAST THROUGH THE
OVERNIGHT HOURS.

THIS IS A HIGHLY DANGEROUS SITUATION THAT LOOMS FOR
SATURDAY...WHERE THE GREATEST THREAT WILL LIKELY REMAIN FROM AROUND
NOON UNTIL MIDNIGHT. PLEASE MAKE PREPARATIONS AND PLANS NOW IN THE
EVENT THAT SEVERE WEATHER WILL AFFECT YOU AT SOME POINT ON
SATURDAY. TAKE ALL NECESSARY PRECAUTIONS TO PROTECT YOUR LIFE AND
PROPERTY!

17/KLAWS

-----

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HUNTSVILLE AL
652 AM CDT FRI APR 23 2010


.DISCUSSION... /ISSUED 437 AM CDT FRI APR 23 2010/

..SEVERE WEATHER OUTBREAK EXPECTED ACROSS THE TENNESSEE VALLEY...

LATEST WATER VAPOR IMAGERY AND H5 RUC ANALYSIS CONTINUE TO SHOW
AGGRESSIVE UPPER LEVEL CYCLONE OVER THE FOUR CORNERS REGION...WITH
BOTH THE MID AND UPPER LEVEL SPEED MAX ROUNDING THE BASE OF THE
TROUGH ATTM. AT THE SURFACE...A SQUALL LINE STRETCHES SOUTH INTO
TEXAS FROM THE CENTER OF THE LOW IN EAST COLORADO...WITH A WARM
FRONT EXTENDING INTO WEST TENNESSEE. CLOSER TO HOME...WINDS ARE
GENERALLY LIGHT AND SOUTHERLY WITH A FEW CLOUDS BRUSHING OVER THE
AREA. DEWPOINTS HAVE STEADILY BEEN CLIMBING...AND ARE NOW IN THE LOW
50S.

ATTENTION FOR THE FORECAST CONTINUES TO FOCUS UPON THE CYCLONE TO
THE WEST...AS THIS SYSTEM WILL BRING MULTIPLE ROUNDS OF SEVERE
WEATHER TO THE GREATER SOUTHEAST TODAY AND SATURDAY.

MUCH OF TODAY WILL BE QUIET AS WEAK UPPER LEVEL RIDGING REMAINS IN
PLACE THROUGH MIDDAY. HOWEVER AS TEMPERATURES CLIMB THIS AFTERNOON
INTO THE 80S /AND DEWPOINTS INCREASE INTO THE UPPER 50S/...SURFACE
BASED ISOLATED CONVECTION WILL DEVELOP THIS AFTERNOON IN THE WEST.
THESE STORMS WILL BE CAPABLE OF GUSTY WINDS...HAIL...AND LOCALLY
HEAVY RAINFALL. WHILE THE GFS SUGGESTS WE WILL REMAIN CAPPED THROUGH
THE AFTERNOON...DO NOT BUY INTO THAT SOLUTION ATTM GIVEN THE
EXPECTED DEGREE OF WARMING AT THE SURFACE.

HOWEVER THIS EVENING AS INSOLATION WANES AND WE DECOUPLE...ANY
LINGERING EVENING STORMS SHOULD BECOME MORE ELEVATED IN NATURE.
ADDITIONAL THUNDERSTORMS /ASSOCIATED WITH THE CURRENT SURFACE LOW/
SHOULD BREAK OUT ACROSS THE AREA...AND THESE WILL NEED TO BE
MONITORED FOR THEIR SEVERE POTENTIAL OF PRODUCING DAMAGING WINDS AND
HAIL. MEANWHILE TO OUR SOUTH...A SECONDARY SURFACE LOW WILL FORM
OVER SE TEXAS...AND RAPIDLY SHIFT TOWARD NORTHERN LOUISIANA BY 12Z
SATURDAY. LOCALLY RUN WRF INDICATES A BREAK IN THE THUNDERSTORMS
SATURDAY MORNING /AROUND 15Z/ FOR A FEW HOURS AS THIS SECOND SURFACE
LOW RAPIDLY SHIFTS TOWARD THE NORTHEAST.

MUCH OF THE SATURDAY FORECAST HINGES ON CONDITIONS EARLY SATURDAY
MORNING. FOR EXAMPLE...FRIDAY NIGHT THUNDERSTORMS MAY WORK OVER THE
ATMOSPHERE AND PRECLUDE CONVECTION INITIATION OVER THE CWFA UNTIL AT
LEAST MIDDAY SATURDAY. ON A CONTRARY...IF THERE ARE ANY BREAKS IN
THE CLOUDS ON SATURDAY MORNING...INSOLATION WILL RAPIDLY DESTABILIZE
THE ATMOSPHERE. FOR NOW...WILL HEDGE THE FORECAST TOWARD THE LOCALLY
RUN WRF...WHICH INITIALIZES DISCREET SUPERCELLS SOUTHWEST OF THE CWFA
SATURDAY MORNING...AND TRACK THEM ACROSS THE MS/AL BORDER AROUND
19Z SATURDAY. WHILE THIS HAS BEEN MENTIONED FOR SEVERAL FORECAST
DISCUSSIONS NOW IT BEARS REPEATING: ANY STORMS THAT DEVELOP ON
SATURDAY WILL BE CAPABLE OF PRODUCING TORNADOES...LARGE
HAIL...DAMAGING WINDS AND LOCALLY HEAVY RAINFALL. HODOGRAPHS AND LOW
LEVEL HELICITY CONTINUE TO BE IMPRESSIVE /WITH 0-3 SRH AROUND 7-800
M2/S2/ INDICATIVE OF THE VERY HIGH MID AND LOW LEVEL BULK SHEAR
VALUES PRESENT ACROSS THE AREA. THE LATEST OUTLOOK FROM THE STORM
PREDICTION CENTER /SPC/ PLACES THE TENNESSEE VALLEY IN A MODERATE
RISK FOR THE DAY...AND HIGHLIGHTS THE POTENTIALLY SERIOUS NATURE OF
THIS EVENT. 

TOWARD DUSK ON SATURDAY...EXPECT A TRANSITION TOWARD A MORE LINEAR
TYPE OF STORM SYSTEM AHEAD OF THE MAIN COLD FRONT. LOCALLY RUN WRF
SUGGESTS THIS LINE WILL FORM IN THE WEST AROUND 00Z SUNDAY...AND
PUSH THROUGH THE CWFA DURING THE EVENING HOURS...EXITING NORTHEAST
ALABAMA AROUND MIDNIGHT. NAM INDICATES AN INSTABILITY AXIS
RESURGENCE ALONG THE LINE AND SHEAR SHOULD BECOME MORE
UNIDIRECTIONAL...INDICATING MOSTLY A DAMAGING WIND THREAT /ALONG
WITH THE CONTINUED THREAT OF TORNADOES WITHIN ANY LINE BREAK/.

STARTING TONIGHT THROUGH SATURDAY NIGHT...LOCALLY HEAVY RAINFALL IS
POSSIBLE GIVEN THE MOIST ENVIRONMENT /AS INDICATED BY THETAE AXIS
AND PWATS AOA 1.50IN/ AND THE MULTIPLE ROUNDS OF THUNDERSTORMS.
STORM TOTAL PRECIPITATION COULD BE AS HIGH AS 2 OR 3 INCHES IN SOME
LOCATIONS. AS AN ADDITIONAL HAZARD...GRADIENT WINDS ON SATURDAY WILL
BE VERY GUSTY GIVEN THE STRONG LOW LEVEL JET /35-45KT AT 2K
FT/...AND A WIND ADVISORY MAY BE NEEDED FOR PORTIONS OF THE AREA.

IF YOU HAVE NOT ALREADY DONE SO...TAKE THE TIME TODAY TO PREPARE FOR
SEVERE WEATHER...THIS INCLUDES KNOWING WHERE YOU WILL GO IN THE
EVENT A TORNADO WARNING IS ISSUED FOR YOUR AREA. OUR OFFICE WILL
CONTINUE TO HIGHLIGHT THIS SEVERE WEATHER THREAT IN OUR SPECIAL
WEATHER STATEMENT /SPS/ AND OUR HAZARDOUS WEATHER OUTLOOK
/HWO/...AND STAY IN CONSTANT CONTACT WITH OUR LOCAL PARTNERS AS WE
GEAR UP FOR THIS WEEKEND EVENT.

THE FORECAST PAST SUNDAY REMAINS LARGELY UNCHANGED...AS UNRULY
NORTHWEST FLOW ON THE BACKSIDE OF THE DEPARTING UPPER TROUGH SENDS
MULTIPLE SHORTWAVES ACROSS THE AREA. THE CHANCE FOR LIGHT RAIN
SHOWERS WILL PERSIST THROUGH TUESDAY ACROSS THE AREA BEFORE SURFACE
HIGH PRESSURE BUILDS INTO THE AREA ON WEDNESDAY. AS THE LOW LEVEL
FLOW BECOMES SOUTHERLY BY THURSDAY AND FRIDAY...TEMPERATURES AND
DEWPOINTS WILL WARM AHEAD OF A SYSTEM APPROACHING FROM THE WEST.

&&

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

$$

DISCUSSION...JLL