117  
FXUS65 KTFX 200522  
AFDTFX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE GREAT FALLS MT  
1120 PM MDT TUE OCT 19 2021  
 
AVIATION SECTION UPDATED.  
   
SYNOPSIS
 
 
FOG IS POSSIBLE IN SOME VALLEYS OF SOUTHWEST AND CENTRAL MONTANA  
TONIGHT, ALONG WITH THE IMMEDIATE PLAINS EAST OF THE ROCKY  
MOUNTAIN FRONT. LOW TEMPERATURES ARE EXPECTED TO FALL INTO THE  
MID-20S TO LOW 30S ACROSS THE REGION, WITH CLEAR TO MOSTLY CLEAR  
SKIES.  
 

 
   
UPDATE
 
 
SKIES CONTINUE TO CLEAR OUT THIS EVENING AS A DISTURBANCE OVER  
WYOMING CONTINUES TO MOVE EASTWARD. BIGGEST CONCERN TONIGHT WILL  
BE THE POTENTIAL FOR FOG TO DEVELOP OVER THE PLAINS, ESPECIALLY  
ALONG THE ROCKY MOUNTAIN FRONT AND OTHER AREAS ACROSS THE PLAINS  
AS EASTERLY WINDS UPSLOPE OVERNIGHT TONIGHT. LUDWIG  
 

 
   
AVIATION
 
 
1120 PM MDT TUE OCT 19 2021 (20/06Z TAF PERIOD)  
 
AS A WEATHER SYSTEM MOVES EAST OVER WYOMING THIS EVENING, EXPECT  
SKIES TO CONTINUE TO REMAIN CLEAR THROUGH THE OVERNIGHT AT MOST  
TERMINALS. THE EXCEPTIONS TO THIS WILL BE IN THE KWYS VICINITY AS  
SOME LOW CLOUDS LINGER, RESULTING IN INTERMITTENT LIFR CONDITIONS,  
AS WELL AS PARTS OF THE NORTH-CENTRAL PLAINS, WHERE SOME FOG OR LOW  
STRATUS WILL BE POSSIBLE, PARTICULARLY IN THE VICINITY OF KCTB.  
OTHERWISE, VFR CONDITIONS ARE EXPECTED, EVEN AS SOME HIGH CLOUDS  
FILTER BACK INTO THE AREA TOMORROW. LUDWIG  
 
REFER TO WEATHER.GOV/ZLC FOR MORE DETAILED REGIONAL AVIATION  
WEATHER AND HAZARD INFORMATION.  
 

 
   
PREV DISCUSSION
 
 
/ISSUED 545 PM MDT TUE OCT 19 2021/  
 
REST OF TODAY THROUGH TONIGHT...PRIMARY CONCERN WILL BE POTENTIAL  
FOR FOG DEVELOPMENT LATE THIS EVENING THROUGH THE MORNING HOURS ON  
WEDNESDAY.  
 
REX BLOCK OVER THE ROCKIES IS SLOWLY BEGINNING TO BREAK DOWN AS A  
H500 LONGWAVE TROUGH OVER THE EASTERN PACIFIC BEGINS TO APPROACH THE  
WESTERN CONUS. LOW LEVEL STRATUS, THANKS IN PART TO LOW LEVEL  
NORTHEAST TO EASTERLY UPSLOPE FLOW, WAS FINALLY ERODING ACROSS  
CENTRAL AND NORTH CENTRAL MONTANA EARLY THIS AFTERNOON. EXPECT CLOUD  
COVER ACROSS THE ENTIRE NORTHERN ROCKIES TO CONTINUE ERODE/SLIDE  
EAST THROUGH THE REMAINDER OF THE DAY, LEADING TO CLEAR TO MOSTLY  
CLEAR SKIES FOR MUCH OF THE OVERNIGHT HOURS. WHILE SURFACE WINDS  
WILL BEGIN TO TURN MORE SOUTHEASTERLY THROUGH THE REMAINDER OF THE  
DAY AND INCREASE OVER THE PLAINS OF CENTRAL AND NORTH CENTRAL  
MONTANA DUE TO A STRENGTHENING LOW LEVEL JET (I.E. H850 WINDS  
INCREASING FROM 5-15KTS TO 15-30KTS), MOST VALLEYS OF SOUTHWEST AND  
CENTRAL MONTANA WILL DECOUPLE THIS EVENING. THESE DECOUPLED VALLEYS  
COMBINED WITH CLEARING SKIES AND ANTECEDENT LOW LEVEL MOISTURE WILL  
CREATE THE POTENTIAL FOR FOG DEVELOPMENT, MOST NOTABLY IN THE  
HELENA, SMITH, GALLATIN, HEBGEN VALLEYS AND ALONG THE ROCKY MOUNTAIN  
FRONT WEST OF I-15 FROM GREAT FALLS TO CUT BANK. WILL NEED TO  
MONITOR THE POTENTIAL NEED FOR DENSE FOG ADVISORIES IN THESE  
LOCATIONS AS SOME MODELS ARE SUGGESTING VISIBILITIES BELOW 1/4MILE,  
BUT FOR NOW HAVE REFRAINED FROM ISSUING ANY HIGHLIGHTS. OTHERWISE,  
EXPECT ANOTHER COOL NIGHT WITH LOWS FALLING INTO THE MID-20S TO LOW  
30S. - MOLDAN  
 
WEDNESDAY THROUGH NEXT TUESDAY... A DEEPENING LOW PRESSURE TROUGH  
OFF THE PACIFIC COAST WILL PUT THE AREA UNDER MORE OF A  
SOUTHWESTERLY FLOW ALOFT ON WEDNESDAY. A SHORTWAVE TROUGH WILL  
EJECT FROM THIS TROUGH AND MOVE OVER THE AREA WEDNESDAY INTO  
WEDNESDAY NIGHT, BRINGING A CHANCE OF PRECIPITATION TO THE  
MOUNTAINS ALONG AND WEST OF THE CONTINENTAL DIVIDE; HOWEVER,  
SOUTHERLY WINDS WILL KEEP MOST OF THE AREA DRY WITH SLIGHTLY  
ABOVE NORMAL TEMPERATURES. A WEAK HIGH PRESSURE RIDGE WILL THEN  
MOVE IN BEHIND THE SHORTWAVE AND AHEAD OF THE LARGE-SCALE TROUGH,  
WHICH WILL KEEP THE AREA DRY WITH TEMPERATURES UP TO 10 DEGREES  
ABOVE NORMAL THURSDAY INTO FRIDAY. THE PACIFIC TROUGH WILL  
BROADEN AND SPREAD EAST INTO THE ROCKIES FRIDAY THROUGH TUESDAY,  
WHICH WILL KEEP THE FORECAST AREA UNDER MORE OF AN UNSETTLED  
PATTERN, BUT WITH SLIGHTLY ABOVE NORMAL TEMPERATURES. AS A  
RESULT, THERE WILL BE A GOOD CHANCE OF MOUNTAIN SNOW AND RAIN EACH  
DAY, WITH SNOW ACCUMULATIONS LIKELY REMAINING ABOVE MOUNTAIN PASS  
LEVEL. HOWEVER, WINDS SHOULD REMAIN MORE SOUTHWESTERLY AND  
DOWNSLOPING OVER THE PLAINS, WHICH WILL LIMIT PRECIPITATION THERE.  
-COULSTON  
 

 
   
PRELIMINARY POINT TEMPS/POPS
 
 
GTF 29 61 38 63 / 0 0 0 0  
CTB 28 56 34 60 / 0 0 10 0  
HLN 32 63 37 65 / 0 10 10 0  
BZN 28 59 33 63 / 0 0 0 0  
WYS 25 48 29 50 / 20 10 0 0  
DLN 32 54 33 59 / 0 0 0 0  
HVR 27 58 33 64 / 0 0 0 0  
LWT 27 58 36 62 / 0 0 0 0  
 

 
   
TFX WATCHES/WARNINGS/ADVISORIES
 
 
NONE.  

 
 

 
 
HTTP://WWW.WEATHER.GOV/GREATFALLS  
 
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