216  
FXUS65 KBOI 211613  
AFDBOI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE BOISE ID  
1013 AM MDT THU OCT 21 2021  
   
DISCUSSION
 
CLOUDS WILL RIDE ALONG A BUILDING UPPER RIDGE TODAY  
AS THE AIR MASS CONTINUES TO WARM. HIGH TEMPERATURES WILL RANGE  
10-15 DEGREES ABOVE NORMAL THIS AFTERNOON. AN APPROACHING STORM  
WILL DRAW SOUTH TO EAST WINDS ACROSS OPEN TERRAIN TODAY, THE  
STRONGEST ACROSS EASTERN OREGON. NO MORNING UPDATE AS CURRENT  
FORECAST IS ON TRACK.  
 

 
   
AVIATION
 
MAINLY VFR. INCREASING CLOUD COVER TODAY AHEAD OF THE  
NEXT TROUGH FOR FRIDAY. SURFACE WINDS SW-SE 5-15 KT, WITH GUSTS TO  
20-25 KT THIS AFTERNOON. LOW-LEVEL WIND SHEAR IN HARNEY COUNTY  
THROUGH 17Z/THU. WINDS ALOFT AT 10K FT MSL: SW 20-35 KT.  
 
WEEKEND OUTLOOK...PERIODS OF RAIN AND SNOW FRIDAY THROUGH SUNDAY  
MORNING. THEN, WIDESPREAD RAIN AND SNOW SUNDAY AFTERNOON/EVE INTO  
MONDAY, WITH MODERATE TO HEAVY PRECIPITATION RATES POSSIBLE. SNOW  
LEVELS 6000 FT MSL OR HIGHER. MOUNTAINS WILL BECOME OBSCURED. MVFR  
TO LIFR IN RAIN/SNOW. SURFACE WINDS WILL INCREASE SUNDAY.  
 

 
   
PREV DISCUSSION
 
 
SHORT TERM...TODAY THROUGH SATURDAY NIGHT...SPRINKLES IN WESTERN  
IDAHO WILL END BEFORE MORNING AS A WEAK WARM FRONT EXITS NORTHEAST.  
AN UPPER RIDGE WILL AMPLIFY RAPIDLY OVER OUR AREA TODAY BRINGING  
MOSTLY SUNNY AND WARMER WEATHER WITH HIGH TEMPS IN THE 60S IN THE  
MOUNTAINS AND EASTERN OREGON, TO THE MID 70S IN THE LOWER SOUTHWEST  
IDAHO VALLEYS. THE RIDGE WILL SHIFT EAST TONIGHT AS A PACIFIC TROUGH  
WITH A LOT OF MOISTURE COMES INLAND. RAIN WILL BEGIN IN HARNEY AND  
BAKER COUNTIES TOWARD MORNING. THE FULL SURGE OF MOISTURE ALONG A  
COLD FRONT WILL SPREAD ACROSS EASTERN OREGON FRIDAY AND INTO  
WESTERN IDAHO FRIDAY AFTERNOON. SOUTH-CENTRAL IDAHO WILL STAY DRY  
AND WARM THROUGH FRIDAY. THE COLD FRONT AND MAIN RAIN WILL SHIFT  
EASTWARD ACROSS IDAHO NORTH OF THE SNAKE BASIN FRIDAY EVENING.  
RAIN WILL DECREASE IN VALLEY AND NORTHERN ELMORE COUNTIES OVERNIGHT  
AND GENERALLY END ELSEWHERE. BUT ANOTHER PACIFIC TROUGH WILL  
ALREADY BE SPREADING MORE RAIN INTO HARNEY COUNTY BEFORE DAWN  
SATURDAY, AND THE REST OF EASTERN OREGON SATURDAY MORNING, AND  
THEN WESTERN IDAHO SATURDAY AFTERNOON. AGAIN, SOUTH-CENTRAL IDAHO  
WILL LARGELY MISS OUT ON THIS RAIN. LIGHT RAIN WILL CONTINUE IN  
OTHER AREAS THROUGH SATURDAY NIGHT, BUT WILL INCREASE YET AGAIN  
IN HARNEY COUNTY TOWARD SUNDAY MORNING AS ANOTHER STRONG PACIFIC  
STORM COMES INLAND. HIGH TEMPERATURES WILL COOL TO NORMAL FRIDAY,  
BUT LOWS WILL STAY ABOVE NORMAL. THE SNOW LEVEL WILL BE ABOVE  
8000 FEET THROUGH FRIDAY, THEN LOWER TO 6000-6500 FEET FRIDAY  
NIGHT THROUGH SATURDAY NIGHT. WINDS TODAY WILL BE LIGHT TO MODERATE  
SOUTHEASTERLY IN IDAHO BUT SOUTHWESTERLY 20 TO 30 MPH IN OREGON  
THIS AFTERNOON. THE SAME IS EXPECTED FRIDAY, EXCEPT A LITTLE  
STRONGER IN IDAHO. LIGHT TO MODERATE SOUTHEAST TO SOUTHWEST WINDS  
IN ALL AREAS SATURDAY AND SATURDAY NIGHT.  
 
LONG TERM...SUNDAY THROUGH THURSDAY...A VERY ACTIVE, WET PATTERN  
WILL CONTINUE SUNDAY AND INTO NEXT WEEK. A STRONG ATMOSPHERIC RIVER  
EVENT WILL ARRIVE ON SUNDAY AS A LARGE TROUGH APPROACHES THE PACIFIC  
NORTHWEST. THIS TROUGH WILL INGEST SIGNIFICANT SUBTROPICAL MOISTURE  
THAT WILL SPREAD INTO THE FORECAST AREA SUNDAY AND MONDAY. SNOW  
LEVELS WILL REMAIN HIGH WITH THIS FLOW, KEEPING ACCUMULATIONS  
GENERALLY ABOVE 7000 FEET MSL. LIQUID PRECIPITATION TOTALS HAVE  
THE POTENTIAL TO REACH OVER AN INCH FOR THE MOUNTAINS AND AROUND  
A HALF INCH FOR THE LOWER ELEVATIONS DURING THIS TIME-FRAME.  
THEREFORE, HIGH ELEVATION SNOWFALL RATES AND ACCUMULATIONS COULD  
BE CONSIDERABLE. THE COLDER AIR ASSOCIATED WITH THE TROUGH WILL  
MOVE OVER THE REGION ON TUESDAY AS WEST-NORTHWEST FLOW DEVELOPS  
ALOFT, AND SNOW LEVELS WILL BRIEFLY LOWER TO AROUND 5500 FT MSL  
BEFORE GRADUALLY RISING AGAIN. TEMPERATURES WILL ALSO LOWER TO  
AROUND 10 DEGREES BELOW NORMAL BY TUESDAY. THEREAFTER, THE WEST-  
NORTHWEST FLOW ALOFT WILL BRING A CHANCE FOR ADDITIONAL  
PRECIPITATION WEDNESDAY. A BREAK IN THE ACTIVE PATTERN IS POSSIBLE  
ON THURSDAY AS A WEAK RIDGE BUILDS OVER THE AREA, THOUGH FORECAST  
CONFIDENCE REMAINS LOW.  
 

 
   
BOI WATCHES/WARNINGS/ADVISORIES
 
 
ID...NONE.  
OR...NONE.  
 

 
 

 
 
WWW.WEATHER.GOV/BOISE  
WWW.FACEBOOK.COM/NWSBOISE  
WWW.TWITTER.COM/NWSBOISE  
 
DISCUSSION...DG  
AVIATION.....JS  
PREV SHORT TERM...LC  
PREV LONG TERM....SH  
 
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