594  
FXUS64 KHUN 261127  
AFDHUN  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE HUNTSVILLE AL  
627 AM CDT WED APR 26 2017  
   
UPDATE
 
 
FOR 12Z TAFS.  
 

 
   
NEAR TERM
 
(TODAY)  
ISSUED AT 307 AM CDT WED APR 26 2017  
 
MILD CONDITIONS EXIST OVERNIGHT WITH CLEAR SKIES AND TEMPS IN THE  
MID TO UPPER 50S AS OF 3AM. SYNOPTICALLY, THE AREA SITS UNDER AN  
UPPER RIDGE BETWEEN A DEPARTING UPPER LOW TO THE NORTHEAST AND AN  
APPROACHING TROUGH CURRENTLY DIGGING INTO THE SOUTHWESTERN STATES. A  
SURFACE LOW EXISTS IN NE IOWA WITH A COLD FRONT EXTENDING SOUTH. A  
SQUALL LINE IS ALREADY NOTED AHEAD OF THIS FRONT, STRETCHING THROUGH  
CENTRAL MISSOURI SOUTH TO EASTERN OKLAHOMA. THIS LINE IS EXPECTED TO  
MOVE EAST THROUGH THE DAY AND EVENTUALLY REACH THE LOCAL FORECAST  
AREA. HOWEVER, IT LOOKS LIKE ALL PRECIP WILL HOLD OFF THROUGH THE  
TODAY PERIOD/7PM TONIGHT.  
 
WITH CLEAR SKIES, A DRY SOUNDING AND A SOUTHERLY WIND, WE SHOULD  
WARM FAIRLY QUICKLY TODAY. HAVE CONTINUED THE DAY SHIFTS TREND OF  
KEEPING HIGHS ABOVE GUIDANCE, IN THE MIDDLE 80S. WINDS MAY BRIEFLY  
TURN SOUTHERLY THIS MORNING BUT SHOULD RETURN TO A SOUTHEASTERLY  
FLOW THIS AFTERNOON. AT THE SAME TIME, THE PRESSURE GRADIENT  
TIGHTENS AS HIGH PRESSURE DEPARTS AND THE COLD FRONT APPROACHES. A  
LOW LEVEL JET WILL ALSO START TO STRENGTHEN AS WE APPROACH THE  
TONIGHT PERIOD AIDING TO THE INCREASE OF SURFACE WINDS. RIGHT NOW,  
WE ONLY HAVE SUSTAINED WINDS AROUND 15-20MPH AND GUSTS AROUND 25 MPH  
WHICH IS BELOW WIND ADVISORY CRITERIA.  
 
WITH THESE STRONG AFTERNOON WINDS, DID BUMP DOWN AFTERNOON  
DEW POINTS A DEG OR TWO AS GUIDANCE DOES NOT USUALLY HANDLE THESE  
GOOD AFTERNOON MIXING SET UPS.  
   
SHORT TERM
 
(TONIGHT THROUGH FRIDAY)  
ISSUED AT 307 AM CDT WED APR 26 2017  
 
BY THE START OF THE TONIGHT PERIOD/00Z TONIGHT, THE FRONT SHOULD BE  
IN EASTERN ARKANSAS AND WILL QUICKLY MOVE EAST THANKS TO STRONG MID  
AND UPPER LEVEL FORCING AS WELL AS UPPER JET DYNAMICS. THE SQUALL  
LINE IS FORECAST TO GET TO NW AL AROUND 06Z AND THIS IS FAIRLY WELL  
FORECAST BY THE RANGE OF MODEL GUIDANCE. THUNDERSTORMS ARE LIKELY AS  
THIS SQUALL LINE MOVES EAST BUT THE SEVERITY OF THOSE STORMS ARE  
QUESTIONABLE. ONE QUICK THING TO NOTE- WITH THE SPLIT JET STREAM,  
THE GFS/ECMWF ARE SHOWING THE LOCAL AREA GETTING "SPLIT" WITH THE  
HIGHEST QPF TO THE NORTH AND TO THE SOUTH.  
 
WITH OR WITHOUT THE SPLIT, THERE IS A LOT WORKING AGAINST THE THREAT  
FOR SEVERE WEATHER. FOR ONE, THE SOUTHEAST FLOW THAT BUILDS BACK IN  
THIS AFTERNOON SHOULD REMAIN UNTIL THE FRONTAL PASSAGE WHICH WILL  
MAKE IT DIFFICULT TO GET DEW POINTS MUCH HIGHER THAN THE MID TO UPPER  
50S. GUIDANCE IS SHOWING A SURGE OF MOISTURE AHEAD OF THE FRONT WITH  
THE S-SW LLJ BUT ONLY BRIEFLY. THIS COMBINED WITH STEEP MID LEVEL  
LAPSE RATES ALLOWS SOME INSTABILITY TO BE PRESENT. THERE IS A RANGE  
OF IDEAS FOR HOW MUCH CAPE BUT SREF PLUMES SHOWED AN AVERAGE OF  
600-800 J/KG OF MUCAPE. NOTE- THIS IS ALL ELEVATED INSTABILITY WITH  
LITTLE TO NO SURFACE BASED. THIS SITS WELL WITH THE FACT WE ARE WELL  
CAPPED THROUGH THE NIGHT WITH THE NAM TRYING TO SHOW US BREAKING THE  
CAP FOR AN HOUR OR TWO WHICH WOULD BE POSSIBLE BUT WE WOULD STILL BE  
TOO SATURATED TO GET ANYTHING SURFACE BASED. SPEAKING OF SATURATION,  
WE ARE GOING TO BE VERY DRY TODAY SO IT'S GOING TO TAKE A LOT TO  
OVERCOME THE DRY AIR AS WELL.  
 
SHEAR IS THE ONLY THING WE WOULD NEED TO WATCH AS 0-1KM SHEAR IS  
AROUND 35-40KTS AND 0-6KM SHEAR AROUND 70 KTS. LOW LEVEL HELICITY  
WILL BE IN THE 200-300 M2/S2 RANGE WITH THE FRONT AS IT MOVES  
THROUGH. THIS LEADS ME TO BELIEVE THE SQUALL LINE WILL BE ABLE TO  
SUSTAIN ITSELF AS IT MOVES EAST AND A FEW CELLS COULD BECOME STRONG  
IF THE ELEVATED INSTABILITY IS REALIZED. WET BULB ZERO HEIGHTS ARE  
HIGH BUT GIVEN THE ELEVATED INSTABILITY, SMALL HAIL IS POSSIBLE.  
THINKING STRONG WINDS WOULD BE THE GREATEST THREAT GIVEN THE MID  
LEVEL DRY AIR AND THE ONGOING GUSTY SURFACE WINDS TO AROUND 30MPH.  
SIMILAR TO WHAT THE DAY SHIFT WAS FORECASTING, THERE COULD BE A BRIEF  
WINDOW IN NE AL FROM 9-12Z WHERE WE COULD SEE A LONGER TIME FOR THE  
DEW POINTS TO RISE AS WELL AS THE LLJ PASSING OVER THEN AND THE UPPER  
JET FINALLY NOSING IN. HOWEVER, THE UPPER LIFT PASSES WELL TO THE  
NORTH AT THIS TIME. I DON'T THINK THIS IS A HIGH POSSIBILITY BUT IT'S  
WORTH A MENTION.  
 
THE LINE SHOULD MOVE THROUGH FAIRLY RAPIDLY AND GUIDANCE ISN'T  
SHOWING THE FRONT GETTING HELD UP OVER THE AREA ANYMORE. DRY AIR  
MOVES IN PRETTY QUICKLY SO SKIES SHOULD CLEAR BY THE LATE AFTERNOON.  
THE FRONT WILL BRING A BRIEF RELIEF TO THE WARMER TEMPS WITH HIGHS  
IN THE MID TO UPPER 70S. CLOUDS WILL START TO INCREASE AGAIN THU  
NIGHT AHEAD OF THE NEXT UPPER SYSTEM WITH LOWS IN THE MIDDLE 50S.  
 
A WARM FRONT STARTS TO TRACK NORTH TOWARDS THE AREA ON FRIDAY WITH A  
WARM, MOIST SOUTHERLY FLOW RETURNING, WITH THE HELP OF ANOTHER  
DEVELOPING LLJ. DEW POINTS ARE FORECAST TO RISE RAPIDLY ON FRIDAY  
WITH SOME GUIDANCE, SUCH AS THE GFS, SUGGESTING THEY GET INTO THE  
LOWER 70S BY THE AFTERNOON. THIS SEEMS A BIT AGGRESSIVE AND QUICK TO  
ME SO WILL CONTINUE WITH BLENDED VALUES IN THE MIDDLE 60S.  
INSTABILITY IS QUICK TO INCREASE BUT WITH LITTLE MOISTURE ALOFT AND  
NO UPPER LIFT, JUST HAVE SLIGHT CHANCE FOR THUNDERSTORMS FOR FRIDAY.  
HIGHS WILL BE BACK INTO THE UPPER 80S.  
   
LONG TERM
 
(FRIDAY NIGHT THROUGH TUESDAY)  
ISSUED AT 307 AM CDT WED APR 26 2017  
 
AS THE WARM FRONT FROM FRIDAY AFTERNOON/EVENING CONTINUES TO MOVE  
NORTH, A FEW ISOLATED LINGERING STORMS COULD MEANDER OR DEVELOP OVER  
CENTRAL TN AND BACKBUILD OVER PORTIONS OF SOUTHERN TN/NORTHEAST AL  
OVERNIGHT FRIDAY NIGHT INTO EARLY SATURDAY MORNING. MEANWHILE, THE  
RIDGE OVER THE WESTERN ATLANTIC WILL CONTINUE TO BUILD OVER THE  
SOUTHEAST WHICH WILL ACT TO ADVECT THE MARITIME TROPICAL AIRMASS  
FURTHER NORTH ON SATURDAY. THE RESULT, WILL BE A CONDITIONALLY  
UNSTABLE ENVIRONMENT WITH HOT SFC DAYTIME HIGHS NEAR 90 AND SFC  
DEWPOINTS IN THE MID TO UPPER 60S BY SATURDAY AFTERNOON (HOT/HUMID  
CONDITIONS). MESOSCALE OUTFLOW BOUNDARIES FROM EARLY CONVECTIVE  
ACTIVITY ALONG WITH OROGRAPHIC LIFT ON SATURDAY AFTERNOON MAY CAUSE  
ISOLATED AFTERNOON/EVENING THUNDERSTORMS--ESPECIALLY OVER NORTHEAST  
AL/SOUTHERN MIDDLE TN. CURRENT MODEL RUNS HAVE BETWEEN 1500-2500 J/KG  
OF SBCAPE WHICH COULD LEAD TO A SMALL HAIL THREAT WITH FREEZING  
LEVELS AROUND 11-12KFT. ANY STORM ACTIVITY SHOULD THEN DIMINISH BUT  
MAY NOT COMPLETELY END WITH THE ELEVATED INSTABILITY AND OUTFLOW  
BOUNDARIES MOVING OVER THE AREA. AS A RESULT, HAVE LEFT THUNDERSTORMS  
IN THE FORECAST FOR SATURDAY NIGHT. QPF MAY BE A BIT OVERDONE FOR  
BOTH SATURDAY AND SATURDAY NIGHT, AND FURTHER ADJUSTMENTS MAY BE  
NECESSARY AS WE APPROACH THE DAY OF.  
 
FURTHER TO THE WEST A STRONG LLJ AND DYNAMICAL AMPLIFYING TROUGH  
WILL BE BRINGING SEVERE WEATHER ACROSS THE PLAINS TO THE MS RIVER  
VALLEY ON SATURDAY. THE PROGRESSION OF THIS TROUGH HAS BEEN  
FASCINATING TO WATCH AS THE MODEL DEPICTIONS OF THE DOWNSTREAM RIDGE  
HAS CHANGED AT TIMES. AT THIS POINT, THE LEAD SHORTWAVE TROUGH  
WITHIN THE AMPLIFYING LONGWAVE TROUGH AXIS LOOKS TO BECOME  
NEGATIVELY TILTED OVER THE CORN BELT REGION AS IT MOVES NORTHEAST ON  
THE NW FLANK OF THE RIDGE. AS IT DOES SO, AN EVEN STRONGER LLJ WILL  
BACK TO THE SOUTH WHICH COMBINED WITH SOME COOLING ALOFT WILL CREATE  
A MODESTLY UNSTABLE, BUT HIGH SHEAR ENVIRONMENT. IN TERMS OF THE 850  
MB JET STREAK, THE LATEST MODEL RUNS HAVE BETWEEN 40-55 KTS OVERLAID  
ACROSS THE AREA. IN ADDITION, FORECAST SOUNDINGS DEPICT A SLIGHT  
VEERING PROFILE AND CURVED HODOGRAPHS ON SATURDAY AFTERNOON AS WHAT  
LOOKS LIKE A STRONG TO SEVERE QLCS WITH EMBEDDED SUPERCELLS MOVES  
TOWARDS NW AL AHEAD OF A COLD FRONT. THE ARRIVAL TIMING IS STILL A  
QUESTION MARK, BUT STILL LOOKS TO BE AT LEAST ARRIVING AS A SEVERE  
LINE PERHAPS DURING THE AFTERNOON OVER NW AL. THEN, THE QLCS SHOULD  
CONTINUE TO MOVE EAST. RIGHT NOW, GIVEN THE HIGH AMOUNT OF SHEAR AND  
ENOUGH INSTABILITY, ALL IMPACTS COULD BE POSSIBLE (DAMAGING WINDS,  
LARGE HAIL, AND TORNADOES) ESPECIALLY OVER NW AL. HOWEVER, THIS  
OVERALL SEVERE TREND COULD CHANGE WITH THE MODELS AS THE JET STREAK  
RESPONSIBLE FOR THE FORMATION OF THIS TROUGH IS STILL OVER THE  
PACIFIC. AN ADDITIONAL IMPACT THAT IS MORE RELATED TO THE DYNAMICAL  
NATURE OF THIS SYSTEM IS THAT NON-THUNDERSTORM WINDS COULD GUST UP TO  
25 MPH IN SOME AREAS.  
 
CONVECTIVE ACTIVITY COULD CONTINUE UNTIL AT LEAST THE EARLY MORNING  
HOURS ON MONDAY MORNING. AFTER THE FRONT AND TROUGH AXIS MOVES  
ACROSS ON MONDAY, EXPECTING MODERATED TEMPERATURES WITH HIGHS CLOSE  
TO 70 DEGREES. THIS COOLER WEATHER WILL BE SHORT-LIVED AS WESTERLY  
FLOW ALOFT AND SUNNY CONDITIONS SHOULD MODIFY THE AIRMASS QUICKLY ON  
TUESDAY WITH HIGHS BACK CLOSE TO 80.  
 

 
   
AVIATION
 
(FOR THE 12Z TAFS THROUGH 12Z THURSDAY MORNING)  
ISSUED AT 625 AM CDT WED APR 26 2017  
 
VFR CONDITIONS WILL EXIST THROUGH TODAY WITH WINDS INCREASING. GUSTS  
AROUND 25KT ARE POSSIBLE THIS AFTERNOON WITH OCCASIONAL GUSTS TO 30KT  
POSSIBLE AFTER 00Z. A LINE OF THUNDERSTORMS WILL IMPACT THE TERMINALS  
AFTER 06Z AND HAVE INCLUDED A VCTS AT THIS TIME TO ACCOUNT FOR TIMING  
UNCERTAINTY. CIGS WILL STAY AROUND 2KFT FROM ABOUT 8-12Z WITH WINDS  
SHIFTING TO THE NW BEHIND THE COLD FRONT AND DIMINISHING SLIGHTLY.  
 

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

 
 

 
 
NEAR TERM...LN  
SHORT TERM...LN  
LONG TERM...SL.77  
AVIATION...LN  
 
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