986  
FXUS64 KHUN 212326  
AFDHUN  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE HUNTSVILLE AL  
526 PM CST WED FEB 21 2018  
   
UPDATE
 
 
FOR 00Z TAFS.  
 

 
   
NEAR TERM
 
(TONIGHT)  
ISSUED AT 307 PM CST WED FEB 21 2018  
 
A FAIRLY STRONG COLD FRONT CURRENTLY EXTENDS FROM ITS PARENT LOW OVER  
NE CANADA, SOUTHWESTWARD ALONG THE APPALACHIANS AND THROUGH NORTHERN  
LOUISIANA. AHEAD OF THIS FRONT, STRONG SOUTHERLY FLOW HAS CONTINUED  
TO ADVECT MOISTURE ACROSS MUCH OF THE SOUTHEASTERN CONUS, WHILE  
ABUNDANT SUNSHINE HAS HELPED TEMPERATURES WARM INTO THE UPPER 70S TO  
LOWER 80S. BEHIND THE FRONT, COLD AIR ADVECTION WAS HELPING TO BRING  
TEMPERATURES DOWN INTO THE 50S ACROSS NORTHERN MISSISSIPPI AND  
WESTERN TENNESSEE. THE FRONT HAS MADE ITS WAY THROUGH MUSCLE SHOALS,  
AND ALTHOUGH RECORD BREAKING TEMPS (80 DEGREES) WERE OBSERVED AROUND  
NOON, TEMPS HAVE FALLEN INTO THE MID 60S ACROSS EXTREME NORTHWEST  
ALABAMA. THE FRONT WILL LIKELY TRACK SLIGHTLY FURTHER EASTWARD,  
HOWEVER ALL GUIDANCE HAS IT STALLING BEFORE IT MAKES ITS NORTHERN  
RETREAT LATE TONIGHT.  
 
IN TERMS OF THE SHOWER AND THUNDERSTORM POTENTIAL, AREA RADARS SHOW  
MOST OF THE PRECIP BEHIND THE FRONT, WITH ONLY ISOLATED SHOWERS OUT  
AHEAD OF IT. AS IT STALLS ACROSS NORTHWEST ALABAMA, WE MAY SEE MORE  
WIDESPREAD SHOWERS MOVE IN, BUT THE GRADIENT WILL BE FAIRLY HIGH,  
WITH ONLY SCATTERED SHOWERS EXPECTED FURTHER EAST. THUNDERSTORM  
POTENTIAL STILL SEEMS LOW DESPITE AMPLE INSTABILITY. WE MAY SEE  
ISOLATED THUNDERSTORMS THIS EVENING, BUT ORGANIZED CONVECTION IS NOT  
ANTICIPATED AT THIS TIME. THIS IS DUE IN PART TO WEAKER LAPSE RATES  
ALOFT, AND WEAKENING SHEAR THROUGH THE EVENING HOURS. CAN'T RULE OUT  
A ROUGE STORM CAPABLE OF GUSTY WINDS, BUT GIVEN TRENDS AND  
OBSERVATIONS OVER THE LAST FEW HOURS (AND A LACK OF ANY THUNDER NORTH  
OF I-20), THIS IS NOT A VERY HIGH CONCERN. TEMPERATURES WILL BE  
SOMEWHAT TRICKY TONIGHT AND TOTALLY DEPENDENT ON HOW FAIR EAST THE  
COLD FRONT REACHES, ALONG WITH HOW QUICKLY THE FRONT BEGINS TO LIFT  
NORTHWARD. AT THIS POINT, MOST OF THE AREA WILL SEE TEMPERATURES ONLY  
FALL INTO THE MID TO UPPER 60S, WITH THE EXCEPTION BEING FAR  
NORTHWEST ALABAMA, WHERE SLIGHTLY COOLER TEMPS ARE POSSIBLE THIS  
EVENING.  
   
SHORT TERM
 
(THURSDAY THROUGH FRIDAY NIGHT)  
ISSUED AT 307 PM CST WED FEB 21 2018  
 
LINGERING SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE THROUGH THE  
MORNING HOURS ON THURSDAY, MAINLY ACROSS NORTHWEST ALABAMA AND  
SOUTHERN MIDDLE TENNESSEE AS A COLD FRONT REMAINS STALLED NEAR THE  
AREA. THE SUBTROPICAL RIDGE IS EXPECTED TO STRENGTHEN THROUGH THE  
DAY, WHICH WILL HELP LIFT THE COLD FRONT FURTHER NORTHWARD, ALONG  
WITH REDUCING RAIN CHANCES THROUGH THE LATE MORNING AND AFTERNOON  
HOURS. ALONG WITH REDUCING RAIN CHANCES, THE INCREASING LOW AND MID  
LEVEL HEIGHTS WILL COMBINE WITH CONTINUED SOUTHERLY SURFACE FLOW TO  
ALLOW TEMPERATURES TO WARM INTO THE UPPER 70S ONCE AGAIN. IF WE CAN  
LOSE SOME OF THE CLOUD COVER TOMORROW, A FEW SPOTS MAY BREAK 80  
DEGREES FOR THE 2ND OR THIRD DAY IN A ROW.  
 
ALTHOUGH THE FORECAST AREA SHOULD REMAIN MOSTLY DRY THURSDAY NIGHT,  
AREAS NORTH OF THE TN RIVER MAY SEE SOME ISOLATED SHOWERS IF THE  
AFOREMENTIONED COLD FRONT DOES NOT RETREAT AS FAR NORTH AS CURRENTLY  
FORECAST. ANY LIGHT SHOWERS THAT DO DEVELOP WILL BE SHORT-LIVED,  
WITH FRIDAY EXPECTED TO BE MOSTLY DRY ACROSS THE TN VALLEY. THE ABOVE  
NORMAL TEMPS WILL CONTINUE, WITH LOWS THURSDAY NIGHT IN THE 60S ONCE  
AGAIN, AND HIGHS ON FRIDAY IN THE LOWER TO MID 70S. BY FRIDAY NIGHT,  
A SYSTEM WELL TO OUR WEST WILL BEGIN TO APPROACH THE AREA, AND PUSH  
THE COLD FRONT WELL NORTH OF THE AREA. THE SUBTROPICAL RIDGE WILL  
BEGIN TO BREAKDOWN AS AN UPPER TROUGH MOVES INTO THE PLAINS. WEAK  
SHORTWAVES MAY BE ABLE TO PROPAGATE AROUND THE NORTHERN PERIPHERY OF  
THE RIDGE AND BRING ISOLATED TO SCATTERED SHOWERS ACROSS THE TN  
VALLEY LATE FRIDAY NIGHT.  
   
LONG TERM
 
(FRIDAY NIGHT THROUGH TUESDAY)  
ISSUED AT 307 PM CST WED FEB 21 2018  
 
HIGH PRESSURE OFF THE SOUTHEAST ATLANTIC COAST WILL GRADUALLY  
FLATTEN AND SHIFT FURTHER TO THE SOUTH AND EAST ON SATURDAY, AS A  
DEEPENING UPPER-TROUGH OVER THE CENTRAL PLAINS EJECTS NORTHEASTWARD  
INTO THE GREAT LAKES REGION. IN RESPONSE, A SURFACE COLD FRONT WILL  
MOVE EAST FROM THE SOUTHERN PLAINS INTO THE MID-SOUTH AND LOWER  
MISSISSIPPI VALLEY DURING THE DAY ON SATURDAY, AND EVENTUALLY INTO  
THE TENNESSEE VALLEY SATURDAY NIGHT. THE ECMWF AND GFS DIFFER  
SLIGHTLY WITH THE MAGNITUDE OF THIS FEATURE, WITH THE GFS THE MORE  
AGGRESSIVE SOLUTION IN SHOWING A STRONGER, SHARPER TROUGH (AND THUS  
GREATER WIND SHEAR AND DYNAMICS). ECMWF CONTINUES TO TREND WITH A  
BROADER UPPER-TROUGH AND POTENTIALLY LESS DYNAMICS/LIFT.  
 
AHEAD OF THE FRONT, SOUTHWESTERLY FLOW WILL CONTINUE TO ADVECT A  
WARM, MOIST AIR MASS INTO THE REGION, WITH DEWPOINTS CLIMBING INTO  
THE MID 60S BY SATURDAY AFTERNOON. INSTABILITY WILL SOMEWHAT BE IN  
QUESTION, AS MORNING CLOUD COVER WILL LIKELY LIMIT DAYTIME HEATING  
AND POTENTIAL DESTABILIZATION OF THE ATMOSPHERE. ADDITIONALLY, LAPSE  
RATES, ESPECIALLY LOW-LEVEL APPEAR TO BE FAIRLY WEAK. MODEL  
SOUNDINGS DO SUGGEST SOME SBCAPE VALUES AROUND 1000 J/KG, WITH  
ML/MUCAPE AROUND 700-800 J/KG AROUND 00Z SUNDAY AS THERE WILL BE SOME  
BREAKS IN THE CLOUDS. COINCIDING WITH THIS, A 60+ KTS LLJ WILL RAMP  
UP ALONG THE MISSISSIPPI RIVER BY 00Z AND EVENTUALLY SHIFT INTO  
TENNESSEE VALLEY AROUND 06Z. THIS FEATURE WILL GENERATE MORE ROBUST  
CONVECTION ALONG AND AHEAD OF THE FRONT AS IT PUSHES EAST.  
ADDITIONALLY, 0-1 KM SHEAR VALUES WILL INCREASE TO 40 KTS, WITH 0-3  
AND 0-6 KM VALUES RAMPING UP TO 50-60 KTS -- AND EFFECTIVE INFLOW  
SHEAR ROUGHLY PROGGED TO BE AROUND 45 MPH. THIS WILL HELP TO ORGANIZE  
THESE STORMS INTO A MCS/QLCS FEATURE THAT COULD POTENTIALLY MAINTAIN  
A FEW ROTATING MESOCYCLONES. AS A RESULT, AT LEAST A LIMITED SEVERE  
THUNDERSTORMS THREAT IS POSSIBLE, WITH DAMAGING STRAIGHT-LINE WINDS  
APPEARING TO BE THE PRIMARY IMPACT AT THIS TIME.  
 
THE GOOD NEWS IS THAT THE FRONT AND SQUALL-LINE SHOULD BE MOVING  
INTO A LIKELY LESS FAVORABLE THERMODYNAMIC ENVIRONMENT BY 06Z SUNDAY,  
SO A WEAKENING TREND IS POSSIBLE AS IT MOVES ACROSS NORTHERN ALABAMA  
AND SOUTHERN MIDDLE TENNESSEE DURING THE NIGHTTIME HOURS ON SATURDAY  
NIGHT. THIS FRONTAL BOUNDARY WILL DROP JUST SOUTH OF THE REGION ON  
SUNDAY, BUT WITH CONTINUED WSW FLOW ALOFT AND THE APPEARANCE OF A  
COUPLE OF WEAK SHORTWAVE TROUGHS, CONTINUED OPPORTUNITIES FOR  
ISOLATED TO SCATTERED SHOWERS WILL CONTINUE THROUGH MONDAY. A  
SLIGHTLY COOLER AIR MASS WILL ESTABLISH ITSELF EARLY NEXT WEEK WITH  
HIGHS REMAINING IN THE 60S FOR MOST OF THE NEXT SEVERAL DAYS. RIDGING  
WILL OCCUR LATE IN THE PERIOD AS A UPPER-HIGH OVER THE GULF OF  
MEXICO NOSES ITS WAY INTO THE AREA. THIS WOULD SUPPORT A DRIER DAY  
WITH FAIR WEATHER TO CLOSE OUT THE PERIOD.  
 

 
   
AVIATION
 
(FOR THE 00Z TAFS THROUGH 00Z THURSDAY EVENING)  
ISSUED AT 525 PM CST WED FEB 21 2018  
 
VFR CONDITIONS ARE EXPECTED TO PREVAIL FOR MOST OF THE NEXT 6 HOURS  
MAINLY OVER NORTHEAST ALABAMA AND PORTIONS OF NORTH CENTRAL ALABAMA.  
HOWEVER, A COLD FRONT THAT IS STALLING ACROSS NW AL MAY CAUSE MVFR  
CIGS TO SHIFT EAST DUE TO SHOWER DEVELOPMENT. THIS SHOWER  
DEVELOPMENT WILL BE MORE CONCENTRATED OVER NW AL CLOSER TO THE FRONT  
THROUGHOUT THE PERIOD. THE FRONT IS EXPECTED TO SHIFT BACK TO THE NW  
AFTER 06Z WHICH IS WHY THERE IS A NOTABLE WIND DIRECTION CHANGE FROM  
NE TO SE BETWEEN 00Z-06Z. OSCILLATIONS IN THE CIGS BETWEEN VFR AND  
MVFR ARE LIKELY BETWEEN 04-06Z AT KMSL, BEFORE MVFR AND POSSIBLY IFR  
CONDITIONS ARE POSSIBLE AFTER 09Z AT BOTH TAF SITES. LIFTING OF THESE  
LOW CLOUDS MAY OCCUR AROUND 15Z BEFORE THE CIGS SCATTER AFTER 18Z AT  
HSV AND MAYBE BY 20Z AT MSL. LIGHT SHOWERS AND POSSIBLY ISOLATED  
STORMS WILL BE LIKELY ESPECIALLY AT KMSL THROUGH AT LEAST 14Z.  
 

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

 
 

 
 
NEAR TERM...73  
SHORT TERM...73  
LONG TERM...AMP.24  
AVIATION...SL.77  
 
FOR MORE INFORMATION PLEASE VISIT OUR WEBSITE  
AT WEATHER.GOV/HUNTSVILLE.  
 
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