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Southeast Discussion

By: NWS
By: NWS

.DISCUSSION...01 UTC SURFACE ANALYSIS SHOWS A 998 MB SURFACE LOW ABOUT NEAR DES MOINES WITH A COLD FRONT EXTENDING DOWN THE MID AND SOUTHERN MISSISSIPPI RIVER VALLEY. DEWPOINT ANALYSIS SHOWS THE 60 DEGREE LINE JUST NORTH OF OUR FORECAST AREA WITH DEWPOINTS GENERALLY IN THE LOWER 60S ACROSS NORTH FLORIDA WITH MID 60S JUST OFFSHORE. SURFACE DATA SHOW VERY WEAK ONSHORE FLOW AHEAD OF THE COLD FRONT ACROSS LOUISIANA. UPPER AIR DATA FROM 00 UTC SHOWS AN IMPRESSIVE 175KT 250 MB JET MOVING ACROSS THE NORTHWESTERN GULF OF MEXICO. EVEN THE SOUNDING FROM KTAE AT 250 MB SHOWED A WIND OF 153 KT WITH THE PEAK WIND IN THE SOUNDING (164 KT) REPORTED AT 200 MB. VAPOR IMAGERY AND 500 MB DATA SHOW A LOW RIGHT ON TOP OF THE SURFACE FEATURE ACROSS IOWA WITH THE SECONDARY SOUTHERN STREAM IMPULSE MOVING INTO THE ARKLATEX. IT IS THIS SECONDARY IMPULSE THAT SHOULD GET THINGS GOING ACROSS THE WESTERN GULF COAST STATES TONIGHT. THE KTAE SOUNDING SHOWS A RELATIVELY DRY LAYER FROM 900 MB ON UP TO 600 MB. LOW LEVEL WARM ADVECTION OVER THE LAST 24 HOURS HAS INDUCED A CAPPING INVERSION AT 900 MB. LAPSE RATES IN THE 700-500 MB LAYER ARE ACTUALLY PRETTY GOOD AND WITH SOME DYNAMICAL COOLING EXPECTED AS THE UPPER IMPULSE APPROACHES...THIS LAYER WILL BECOME INCREASINGLY MORE FAVORABLE FOR CONVECTION. FOR THE IMMEDIATE SHORT TERM OF THE FORECAST...VERY LITTLE IS EXPECTED TO HAPPEN TONIGHT. LOWER AND MID LEVEL DRY AIR WILL HELP LIMIT THE OVERALL COVERAGE OF PRECIPITATION AND KEEP IT ON THE LIGHT SIDE. OVER TIME...AS THE LOW LEVELS MOISTEN...PRECIPITATION SHOULD BECOME MORE WIDESPREAD...BUT MODEL SOUNDINGS TEND TO AGREE THAT THIS WON`T HAPPEN UNTIL CLOSER TO DAYBREAK. AFTER SUNRISE THERE IS SOME CONCERN AS TO HOW THE CONVECTION WILL UNFOLD. THE 11/09Z WRF WAS SHOWING VERY LITTLE ACTIVITY IN THE AFTERNOON...MAINLY JUST CONFINED TO THE FRONTAL BOUNDARY. THE 11/21Z WRF HOWEVER SHOWS MUCH MORE WIDESPREAD COVERAGE WITH EVEN SOME DECENT CONVECTIVE ELEMENTS PRESENT IN THE SIMULATED REFLECTIVITY BY AFTERNOON. THIS ALSO SEEMS TO BE CLOSER TO THE 11/18Z NAM. AS FAR AS SEVERE CHANCES GO...0-6 KM SHEAR VALUES APPROACH 50-55 KTS BY MID MORNING ACROSS THE REGION...ESPECIALLY ACROSS THE NORTHERN HALF OF THE FORECAST AREA. THE SHEAR PROFILE BECOMES MORE UNIDIRECTIONAL AS THE COLD FRONT NEARS...WITH THE BULK OF THE FASTER FLOW IN THE 3-6 KM LAYER. THUS...THE OVERALL TORNADO POTENTIAL SEEMS RELATIVELY LOW. HOWEVER...THERMODYNAMICALLY...THE MODELS ARE LATCHING ON TO SOME BETTER INSTABILITY IN THE LAST COUPLE OF CYCLES. THE GFS HAS BETTER INSTABILITY THAN ANY OF THE MODELS...MUCAPES IN THE 1000-1500 J/KG RANGE PRIMARILY IN SOUTHWEST GEORGIA. SREF DATA DOES SHOW DECENT (30-50 PERCENT) PROBABILITIES OF THESE HIGHER MUCAPE VALUES...SO THERE IS SOME SUPPORT OUTSIDE THE GFS FOR THESE VALUES. THE REMAINING CONCERN WILL BE HOW MUCH OF A STABILIZING IMPACT THE MARINE LAYER WILL HAVE ON CONVECTION. THIS IMPACT IS ILLUSTRATED WELL IN THE LOCALLY RUN SEVERE PROBABILITIES WHICH SHOW 15% OR HIGHER SVR PROBS NORTH OF THE FLORIDA BORDER. GIVEN BETTER INSTABILITY PROFILES (AND LOWER FREEZING LEVELS) PRIMARY THREAT WILL BE LARGE HAIL WITH A DAMAGING WIND THREAT DEVELOPING PARTICULARLY IF ANY CONVECTIVE ELEMENTS CAN ORGANIZE INTO LINEAR SEGMENTS ALONG OR JUST AHEAD OF THE FRONTAL BOUNDARY. AN ISOLATED TORNADO CANNOT BE RULED OUT WITH SUFFICIENT DEEP LAYER SHEAR FOR ROTATING UPDRAFTS...BUT IT APPEARS TO BE MORE OF A SECONDARY THREAT AT THIS TIME. BASED ON THE LAST COUPLE OF MODEL RUNS WILL GO AHEAD AND PLACE SOME T+ WORDING IN THE GRIDS...BUT ONLY IN THE HIGHER CONFIDENCE AREAS OF SOUTH GEORGIA WHERE IT LOOKS LIKE THE COMBINATION OF SHEAR AND BETTER INSTABILITY WILL OVERLAP. WILL KEEP MENTION OF GUSTY WINDS ELSEWHERE. THE FLOOD WATCH WILL BE KEPT IN PLACE FOR NOW ACROSS THE HEAVILY RAINED ON AREAS OF NORTH FLORIDA AS A COUPLE OF MODELS DO SHOW THE POTENTIAL OF ANOTHER QUICK 1-2 INCHES WITH HIGHER AMOUNTS POSSIBLE IN THE HEAVIER STORMS. WILL BE MAKING A FEW OTHER MINOR TWEAKS TO THE CURRENT GRIDS...BUT NO OTHER SIGNIFICANT CHANGES PLANNED AT THIS TIME.


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