265  
FXUS65 KMSO 212048  
AFDMSO  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MISSOULA MT  
248 PM MDT FRI JUL 21 2017  
   
DISCUSSION
 
WESTERLY FLOW WILL CONTINUE ACROSS THE IDAHO AND  
MONTANA AS BROAD HIGH PRESSURE CENTERED NEAR THE NEVADA/UTAH  
BORDER SLOWLY GROWS STRONGER. THIS WILL LEAD TO A GRADUAL WARMING  
TREND THROUGH THE WEEKEND UNDER AN ABUNDANTLY SUNNY SKY. TYPICAL  
AFTERNOON BREEZINESS IS EXPECTED WITH PERHAPS STRONGER WINDS ON  
SUNDAY DUE TO THE PASSAGE OF A WEAK, DRY CANADIAN COLD FRONT THAT  
CLIPS NW MONTANA. IN FACT, THE BREEZY WINDS FROM THIS FEATURE MAY  
KEEP THE ATMOSPHERE MORE TURBULENT THAN NORMAL SUNDAY NIGHT AND  
LEAD TO LESS COOLING IN THE VALLEYS FOR MONDAY MORNING. AND WHILE  
MOISTURE IS LACKING, SOME CUMULUS CLOUD DEVELOPMENT SEEMS POSSIBLE  
ALONG THE FRONTAL BOUNDARY SUNDAY AFTERNOON WITH THE SLIGHTEST OF  
CHANCES FOR A WEAK THUNDERSTORM.  
 
SOUTHERLY FLOW ALOFT WILL BEGIN TO SLOWLY SEEP MONSOONAL MOISTURE  
NORTHWARD INTO IDAHO ON MONDAY, RESULTING IN A PROGRESSIVELY  
GROWING THREAT FOR CONVECTION BY MONDAY AFTERNOON. THE CHANCES  
MONDAY WILL BE BEST ACROSS CENTRAL IDAHO AND FAR SW MONTANA. BUT,  
STEADILY INCREASING MOISTURE OVERNIGHT WILL SUBSEQUENTLY LEAD TO  
THE THREAT FOR NOCTURNAL THUNDERSTORMS. BY TUESDAY THE MODELS ARE  
SHOWING AN EXTREMELY MOIST AIR MASS ACROSS MUCH OF THE NORTHERN  
ROCKIES WITH PRECIPITABLE WATER VALUES RUNNING IN THE 80TH  
PERCENTILE. THIS ANOMALOUSLY HIGH AMOUNT OF ATMOSPHERIC MOISTURE  
WHEN COMBINED WITH THE ADDED INSTABILITY OF A PASSING WEATHER  
DISTURBANCE WILL LEAD TO WIDESPREAD THUNDERSTORMS BY TUESDAY  
AFTERNOON. IT SEEMS QUITE LIKELY THAT THESE STORMS, AT LEAST  
INITIALLY, WILL HAVE SLOW FORWARD MOTION AND THUS COULD PRESENT  
THE THREAT FOR VERY HEAVY RAIN. THIS THREAT IS LIKELY TO EXTEND  
INTO WEDNESDAY, THOUGH THE STORMS DO APPEAR TO PICK UP A LITTLE  
BIT MORE SPEED. EITHER WAY IT DOES SEEM QUITE LIKELY THAT THE  
NORTHERN ROCKIES REGION WILL HAVE A CHANCE FOR SOME MUCH NEEDED  
RAINFALL. THOUGH WE WILL STILL HAVE TO CONTEND WITH THE THREAT FOR  
GUSTY WINDS AND LIGHTNING WITH THESE SAME STORMS.  
 
THE CHANCE FOR THUNDERSTORMS IS FORECAST TO DECREASE SIGNIFICANTLY  
BY THURSDAY AS DRIER AIR SHIFTS THE MONSOONAL MOISTURE SOUTH AND  
EAST. WARM TEMPERATURES (A LITTLE ABOVE NORMAL) AND BREEZY  
AFTERNOON WINDS REMAIN IN THE FORECAST THROUGH THE REST OF NEXT  
WEEK AND LIKELY INTO THE WEEKEND.  
 

 
   
AVIATION
 
WINDS ARE INCREASING THIS AFTERNOON WITH A TIGHTENING  
PRESSURE GRADIENT. THESE WINDS WILL GENERALLY BE WESTERLY BETWEEN  
10-20 KTS AND GUSTS UP TO 25 KTS THROUGH AROUND SUNSET. LATEST  
SATELLITE IMAGERY SHOW WILD FIRES IN THE REGION BECOMING MORE  
ACTIVE IN RESPONSE TO WARMING TEMPERATURES AND INCREASED WINDS, SO  
WILDFIRE SMOKE MAY CREATE IMPACTS TO VISIBILITY.  
 

 
   
MSO WATCHES/WARNINGS/ADVISORIES
 
 
MT...ID...NONE.  

 
 

 
 
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