994  
FXUS64 KHUN 211704  
AFDHUN  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE HUNTSVILLE AL  
1204 PM CDT THU SEP 21 2017  
   
UPDATE
 
 
FOR 18Z TAFS.  
 

 
   
NEAR TERM
 
(REST OF TODAY)  
ISSUED AT 827 AM CDT THU SEP 21 2017  
 
AS OF 13Z THIS MORNING THERE IS STILL FOG IN MANY OF THE VALLEY  
LOCATIONS OF NE AL AND SOUTHERN MIDDLE TN WITH SOME PATCHY DENSE FOG  
STILL POSSIBLE FOR ANOTHER HOUR OR SO. THE ONE REAL BIG CHANGE WITH  
THIS UPDATE IS TO ADJUST POPS AS STORMS HAVE RAPIDLY DEVELOPED NEAR  
TULLAHOMA. BASED ON SATELLITE THE SHORTWAVE/VORT-MAX THAT WAS  
FORECAST TO DROP SOUTH MIGHT BE ARRIVING A BIT EARLIER THAN  
PREVIOUSLY THOUGHT. LOOKING THROUGH HIRES GUIDANCE AND THE HIRESW  
ARW/NMM AND NSSLWRF APPEAR TO BE HANDLING THE QUICKER DEVELOPMENT  
BETTER THAN ANY OTHER MODEL. SO HAVE ADJUSTED POPS BASED ON THOSE  
FORECASTS. THE STEERING FLOW IS PRETTY MUCH DUE SOUTH AND FOR THE  
MOST PART EXPECT STORMS TO DROP SOUTH THROUGH THE MORNING HOURS. WE  
MIGHT ALSO SEE SOME ADDITIONAL DEVELOPMENT ON THE OUTFLOWS FROM THE  
STORMS. MODIFYING THE 12Z OHX SOUNDING FOR WHAT IS EXPECTED THIS  
AFTERNOON RESULTS IN 1,000-1,500 J/KG OF CAPE BUT LESS THAN 20KTS OF  
SHEAR. DRIER AIR ALOFT EVIDENT IN THIS MORNINGS SOUNDINGS COULD ALLOW  
FOR SOME STRONGER GUSTS OF 40-50MPH BUT FOR THE MOST PART EXPECT  
STORMS TO BE ON THE WEAKER SIDE.  
 
THE ONLY QUESTION IS WHAT IF ANY THIS EARLIER DEVELOPMENT OF STORMS  
MIGHT HAVE ON TEMPS THIS AFTERNOON. THINKING THAT ONCE STORMS DROP  
SOUTH WE MAY SEE A BRIEF COOL DOWN BUT THEN REBOUND BACK INTO THE MID  
80S. WILL WATCH TRENDS AND MAY HAVE TO MAKE SOME TWEAKS TO THE HIGHS  
LATER THIS MORNING.  
   
SHORT TERM
 
(TONIGHT THROUGH SATURDAY NIGHT)  
ISSUED AT 441 AM CDT THU SEP 21 2017  
 
THUNDERSTORMS WILL QUICKLY COME TO AN END BY SUNSET, WITH THE LOSS  
OF DAYTIME HEATING AND AS THE MID LEVEL FORCING EXITS THE AREA.  
SKIES WILL CLEAR ONCE AGAIN DURING THE LATE EVENING HOURS AND WINDS  
WILL GENERALLY REMAIN CALM. GIVEN DECENT RADIATIONAL COOLING  
CONDITIONS, TEMPERATURES WILL COOL INTO THE UPPER 60S, CLOSE TO  
FORECAST DEWPOINTS. PATCHY FOG WILL BE POSSIBLE EARLY FRIDAY MORNING,  
AND MAY BE A LITTLE MORE DENSE IN SOME AREAS, DEPENDING ON HOW MUCH  
RAIN OCCURS.  
 
SIMILAR CONDITIONS ARE EXPECTED ON FRIDAY, AS THE UPPER RIDGE TRIES  
TO SLOWLY BUILD EASTWARD ON FRIDAY. MEANWHILE SURFACE HIGH PRESSURE  
ALONG THE SOUTHEAST COAST KEEPS OUR WINDS IN AN EASTERLY DIRECTION.  
WEAKNESSES IN THE UPPER RIDGE WILL MOVE ACROSS THE REGION, AND MAY  
PROVIDE SOME AID TO DIURNALLY DRIVEN CONVECTION DURING THE AFTERNOON  
HOURS. COVERAGE MAY BE SLIGHTLY LOWER ON FRIDAY DUE TO SLIGHTLY LESS  
UPPER FORCING, AND WILL BE MOSTLY DEPENDENT ON MESOSCALE BOUNDARIES.  
TEMPERATURES WILL RISE INTO THE UPPER 80S TO LOWER 90S ONCE AGAIN.  
THUNDERSTORMS WILL DIMINISH BY SUNRISE AND OVERNIGHT LOWS WILL FALL  
INTO THE MID TO UPPER 60S ONCE AGAIN FRIDAY NIGHT.  
 
A WEAK AREA OF LOW PRESSURE WILL DEVELOP ALONG THE NORTHEAST GULF OF  
MEXICO ON SATURDAY AND LIFT NORTH-NORTHEASTWARD THROUGH THE DAY.  
MEANWHILE, WINDS WILL SHIFT TO A MORE SOUTHEASTERLY DIRECTION,  
FILTERING IN SLIGHTLY MORE MOISTURE ACROSS THE TN VALLEY. THIS WILL  
LEAD TO ANOTHER DAY OF ISOLATED SHOWERS AND THUNDERSTORMS, MAINLY  
DURING THE AFTERNOON. ALTHOUGH COVERAGE MAY BE A BIT HIGHER THAN  
CURRENTLY FORECAST, ESPECIALLY SOUTH OF THE TN RIVER, THE INFLUENCE  
OF THE UPPER RIDGE TO OUR NORTH MAY LIMIT THE COVERAGE. THEREFORE  
WILL CAP POPS AT 20 PERCENT. CLOUD COVER MAY BE A BIT HIGHER ON  
SATURDAY, AND WITH HEIGHT FALLS, TEMPERATURES MAY BE A COUPLE OF  
DEGREES COOLER THAN THOSE OBSERVED THE PREVIOUS DAYS. EVEN STILL,  
TEMPS WILL RISE INTO THE MID TO UPPER 80S ONCE AGAIN. OVERNIGHT LOWS  
ON SATURDAY NIGHT WILL FALL INTO THE LOWER TO MID 60S.  
   
LONG TERM
 
(SUNDAY THROUGH WEDNESDAY)  
ISSUED AT 441 AM CDT THU SEP 21 2017  
 
GUIDANCE FROM THE VARIOUS GLOBAL MODELS IS IN AGREEMENT ON THE MID-  
LEVEL HEIGHT PATTERN AT THE BEGINNING OF THE EXTENDED PERIOD, WITH A  
STRONG 500-MB HIGH EXPECTED TO BE CENTERED ACROSS THE LOWER GREAT  
LAKES AND A WEAK INVERTED TROUGH FORECAST TO BE ALONG THE CENTRAL  
GULF COAST. DURING THE PERIOD FROM SUNDAY-MONDAY, THE INVERTED TROUGH  
WILL LIFT SLOWLY NORTH-NORTHWESTWARD AROUND THE WESTERN PERIPHERY OF  
THE WEAKENING RIDGE AS IT TRANSLATES EASTWARD INTO NEW ENGLAND. WITH  
THE TROUGH EXPECTED TO EVENTUALLY BECOME CENTERED ACROSS AL/MS, AN  
ISOLATED SHOWER OR THUNDERSTORM WILL BE POSSIBLE BOTH SUNDAY AND  
MONDAY AFTERNOONS. HOWEVER, BASED ON THE WEAK NATURE OF THE TROUGH  
AND NO EVIDENCE OF SYNOPTIC SCALE LOW-LEVEL CONVERGENCE, THE COVERAGE  
OF THIS ACTIVITY WILL BE QUITE SPARSE. DUE TO THE PERSISTENT NATURE  
OF THIS REGIME, HIGH TEMPS WERE INCREASED AND LOW TEMPS WERE  
DECREASED A BIT ABOVE/BELOW BLENDED GUIDANCE. MOST LOCATIONS WITHIN  
THE FORECAST AREA SHOULD REACH WELL INTO THE M/U 80S DURING THE  
AFTERNOON, AND FALL INTO THE L/M 60S AT NIGHT.  
 
BEGINNING MONDAY NIGHT, THE TROUGH ACROSS OUR REGION IS EXPECTED TO  
WEAKEN AND ACCELERATE SOUTHEASTWARD ACROSS NORTHERN FL AND INTO THE  
ATLANTIC, EVENTUALLY BECOMING ABSORBED IN THE BROADER CIRCULATION  
AROUND TC MARIA. THIS WILL OCCUR AS MID-TROPOSPHERIC RIDGING BUILDS  
FROM THE LOWER MS VALLEY NORTHEASTWARD INTO THE TN VALLEY DOWNSTREAM  
FROM AN AMPLIFYING TROUGH OVER THE ROCKIES. THUS, WE ANTICIPATE DRY  
CONDITIONS FROM MONDAY NIGHT-WEDNESDAY, ALONG WITH EVEN WARMER  
AFTERNOON TEMPS IN THE MID 80S-AROUND 90. FORTUNATELY, DEWPOINTS IN  
THE L/M 60S WILL MITIGATE THE RISK FOR HIGH HEAT INDICES, AND SHOULD  
ALSO ALLOW LOWS TO FALL TO SIMILAR VALUES IN THE L/M 60S EACH NIGHT.  
ALTHOUGH NOT EXPLICITLY MENTIONED IN THE WEATHER GRIDS, AT LEAST  
PATCHY FOG WILL BE POSSIBLE IN RIVER VALLEYS AND NEAR LARGE BODIES OF  
WATER EACH NIGHT DURING THE EXTENDED PERIOD.  
 

 
   
AVIATION
 
(FOR THE 18Z TAFS THROUGH 18Z FRIDAY AFTERNOON)  
ISSUED AT 1202 PM CDT THU SEP 21 2017  
 
ISOLATED TO SCATTERED STORMS ARE FORMING AROUND THE TN VALLEY. HAVE  
ADDED A TEMPO GROUP TO ACCOUNT FOR ANY STORMS THAT MOVE OVER ANY OF  
THE TERMINALS. COULD SEE BRIEF DROPS TO MVFR/IFR VIS WITH ANY OF THE  
STORMS. WINDS REMAIN LESS THAN 5KTS OUT OF THE NORTH BUT GUSTS OF 20  
TO 30 MPH ARE POSSIBLE IF A STORM MOVES OVER EITHER HSV OR MSL THIS  
AFTERNOON.  
 
FOG IS FORECAST TO DEVELOP AGAIN AFTER 08Z TONIGHT WITH SOME DENSE  
FOG POSSIBLE IN AREAS THAT RECEIVE HEAVY RAIN TODAY AND IN THE VALLEY  
LOCATIONS IN NE AL.  
 

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

 
 

 
 
NEAR TERM...STUMPF  
SHORT TERM...73  
LONG TERM...70/DD  
AVIATION...STUMPF  
 
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