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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