628 FXUS64 KBMX 180406 AFDBMX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BIRMINGHAM AL 1006 PM CST FRI FEB 17 2012 .UPDATE... DISCUSSION. && .DISCUSSION... TEMPERATURES IN THE 40S AND 50S ACROSS CENTRAL ALABAMA THIS EVENING. CLOUD COVER HAS BEEN VARIABLE AND MAINLY HIGH CLOUDINESS SO FAR...WITH AN OVERALL INCREASE EXPECTED OVERNIGHT AHEAD OF THE DEVELOPING LOW TO OUR WEST. OVERNIGHT LOWS WILL BE TRICKY AS MANY PLACES ALREADY A BIT LOWER THAN PREVIOUSLY EXPECTED...BUT SOME DECREASE IN THE FALL ANTICIPATED LATE. MADE SOME ADJUSTMENTS TO THE HOURLIES AND LOWS BASED ON THESE TRENDS. MOST CURRENT MODEL GUIDANCE HAS THE RAIN HOLDING OFF UNTIL AFTER SUNRISE SATURDAY AND HAVE REMOVED MENTION OF RAIN SOUTHWEST AND WEST OVERNIGHT. THE 00Z RAOB IS DRY AND WE HAVE A LITTLE WAYS TO GO BEFORE ANYTHING HITS THE GROUND. ADDITIONALLY...THE MAIN TREND OF THE MODEL SUITE HAS BEEN TO SLIGHTLY SLOW THINGS DOWN ON SATURDAY. SO ADJUSTED THE RAIN TRENDS EARLY SATURDAY...BUT THERE SHOULD BE A RAPID RAIN COVERAGE SPREAD INTO THE EARLY AFTERNOON. WHAT I'VE SEEN OF THE LATEST MODEL RUNS...STILL A THREAT OF SOME SEVERE STORMS SATURDAY EVENING AND OVERNIGHT SOUTH. WILL HAVE TO MONITOR THE TRENDS OF THE WARM FRONT AND HOW FAR INLAND IT CAN MOVE BY EVENING. WITH ALL THE RAIN EXPECTED NORTH OF THE WARM FRONT SATURDAY...IT MAY HOLD THE FRONT FARTHER SOUTH. THIS MAY LIMIT THE THREAT TO FAR SOUTHEASTERN AREAS...BUT SOMETHING TO KEEP IN MIND. MORE WILL DISCUSSED ON THIS DEVELOPING SITUATION ON THE MORNING PACKAGE ISSUANCE. 75 && .AVIATION... 00Z TAF DISCUSSION. VFR CONDITIONS ARE EXPECTED ONCE AGAIN TONIGHT AND THROUGH THE MORNING ON SATURDAY...WITH ONLY BROKEN HIGH CIRRUS EXPECTED AND LIGHT NORTHERLY TO NORTHEASTERLY WINDS SHIFTING TO THE EAST DURING THE OVERNIGHT HOURS. AS THE LOW PRESSURE SYSTEM APPROACHES TOMORROW AFTERNOON AND EVENING...LOWER CLOUDS AND RAINFALL WILL BEGIN TO SPREAD INTO THE STATE FROM SOUTHWEST TO NORTHEAST. THE LATEST TRENDS ARE FOR THE LOWER MVFR CIGS TO BE DELAYED A BIT...REACHING KTCL BY 18Z...AND KANB AND KTOI BY 21Z. FOR PLANNING PURPOSES...WENT AHEAD AND ADDED IN -RA AT KTCL BY 21Z WITH LOW END MVFR TO IFR CIGS ALREADY IMPACTING THE TERMINAL. WILL WAIT FOR THE NEXT TAF PERIOD TO INCLUDE -RA IN THE TAF FOR THE REMAINING TERMINALS AND LOWER CLOUDS AS WELL. 56/GDG && .PREV DISCUSSION... 318 PM CST FRI FEB 17 2012 DISCUSSION... SPLIT FLOW PATTEN ACROSS THE CONUS TODAY... WITH CUTOFF LOW OVER NORTHERN MEXICO BEGINNING TO EJECT EASTWARD IN RESPONSE TO SHORTWAVE TROUGH DIGGING SOUTHWARD FROM THE DAKOTAS. THE CUTOFF LOW SHOULD OPEN INTO ITS OWN SHORTWAVE TROUGH SATURDAY ACROSS TEXAS... WHICH WILL RESULT IN A SURFACE LOW FORMING AND THEN MOVING NORTHEASTWARD ACROSS THE NORTHERN GULF COAST SATURDAY AFTERNOON AND EVENING. THIS SURFACE LOW SHOULD ALSO DEEPEN WITH TIME. THIS SURFACE LOW AND ITS ASSOCIATED SHORTWAVE TROUGH WILL BRING HEAVY RAIN AND A CHANCE FOR SEVERE THUNDERSTORMS... INCLUDING DAMAGING WINDS AND TORNADOES... AS THEY PASS ACROSS CENTRAL ALABAMA. LOOKS LIKE A HIGH SHEAR/LOW INSTABILITY ENVIRONMENT... AND THE AREA IN CENTRAL ALABAMA THAT WILL BE MOST LIKELY TO EXPERIENCE SEVERE WEATHER WILL BE HEAVILY DEPENDENT ON THE TRACK OF THE SURFACE LOW AND HOW FAR NORTH THAT LOW CAN BRING GULF MOISTURE NEAR THE SURFACE. BULK SHEAR VALUES START INCREASING QUICKLY AFTER 6 PM SATURDAY... AND MAX OUT NEAR 90-100 KTS AROUND MIDNIGHT FOR THE SOUTHERN HALF OF OUR AREA AS A LOW LEVEL JET KICKS IN. 0-1 KM AND 0-3 KM HELICITIES ARE ALSO VERY FAVORABLE FOR ROTATING STORMS OVERNIGHT. THE ONLY LIMITING FACTOR WILL BE INSTABILITY. SURFACE BASED CAPES ARE MINIMAL... BUT MOST UNSTABLE MIXED LAYER CAPES SUGGEST THAT BARELY SUFFICIENT INSTABILITY WILL BE PRESENT TO SUPPORT UPDRAFTS ALONG AND SOUTH OF THE WARM FRONT. CURRENTLY THINK THAT THE WARM FRONT WILL STRUGGLE TO MOVE MUCH FARTHER NORTH THAN I-20/I-59... SO LIMITED THE SEVERE THREAT TO SOUTH OF THERE. STORM MODE WILL LIKELY BE A SQUALL LINE COMING OUT OF LOUISIANA AND MISSISSIPPI... WITH MAYBE A STORM JUST AHEAD OF THE LINE. ONE THING THAN CONCERNS ME IS THE SURFACE LOW DEEPENING AS IT MOVES OVER ALABAMA. THIS MAY BRING THE WARM FRONT FARTHER NORTH THAN ANTICIPATED... AND THEREFORE THE SEVERE RISK TOO. A DEEPENING LOW WOULD ALSO INCREASE LOW LEVEL SHEAR AS SURFACE WINDS BACK TO THE SOUTHEAST OR EAST IN RESPONSE. AS FAR AS TIMING GOES... BOTH THE GFS AND ECMWF HAVE BEEN TRENDING SLOWER WITH THE ARRIVAL OF THE SURFACE LOW. THINK THAT WESTERN PORTIONS OF CENTRAL ALABAMA WILL BEGIN TO BE AFFECTED AFTER 6 PM SATURDAY... AND THE SEVERE THREAT SHOULD CONTINUE OVERNIGHT ALONG AND SOUTH OF THE WARM FRONT UNTIL SUNDAY MORNING. HOW QUICKLY THE CUTOFF LOW IN NORTHERN MEXICO EJECTS WILL INFLUENCE THE ARRIVAL OF THE SEVERE WEATHER THREAT... AND WILL BE CLOSELY MONITORED. LATER IN THE PERIOD... ANOTHER SYSTEM LOOKS LIKE IT MAY BRING MORE RAIN TO CENTRAL ALABAMA AROUND MIDWEEK. TEMPERATURES SHOULD MODERATE AFTER THE COLD FRONT PASSAGE SUNDAY. 77/GLEASON HYDROLOGY... RAINFALL AMOUNTS OF GENERALLY 1 TO 3 INCHES CAN BE EXPECTED OVER THE SOUTHERN HALF OF THE AREA SATURDAY AFTERNOON THROUGH SUNDAY MORNING... WITH THE HIGHEST TOTALS... POSSIBLY UP TO 4 INCHES... NEAR THE I-85 CORRIDOR SOUTHWARD. DUE TO DROUGHT CONDITIONS IN THESE AREAS... RIVERS AND STREAM LEVELS SHOULD BE ABLE TO ABSORB THESE HIGHER TOTALS. HOWEVER... PROLONGED HIGHER RAINFALL RATES FROM TRAINING SHOWERS AND THUNDERSTORMS ALONG THE WARM FRONT COULD LEAD TO QUICK RUNOFF AND FLASH FLOODING IN AND AROUND URBAN AREAS. A FLASH FLOOD WATCH IS IN EFFECT FROM 3 PM SATURDAY AFTERNOON THROUGH 6 AM SUNDAY MORNING ACROSS OUR SOUTHERN COUNTIES. 77/GLEASON && .PRELIMINARY POINT TEMPS/POPS... GADSDEN 42 59 49 51 29 / 10 100 100 40 10 ANNISTON 45 62 50 54 30 / 10 100 100 40 10 BIRMINGHAM 47 61 51 52 30 / 10 100 100 30 10 TUSCALOOSA 47 60 54 53 30 / 10 100 90 30 10 CALERA 49 60 52 53 30 / 10 100 100 30 10 AUBURN 50 62 52 58 32 / 10 100 100 30 10 MONTGOMERY 48 63 56 58 32 / 10 100 100 20 10 TROY 47 65 59 60 33 / 10 100 100 20 10 && .BMX WATCHES/WARNINGS/ADVISORIES... FLASH FLOOD WATCH FROM SATURDAY AFTERNOON THROUGH LATE SATURDAY NIGHT FOR THE FOLLOWING COUNTIES: AUTAUGA...BARBOUR...BULLOCK... DALLAS...ELMORE...GREENE...HALE...LEE...LOWNDES...MACON... MARENGO...MONTGOMERY...PERRY...PIKE...RUSSELL...SUMTER. && $$ 75/77/56