585
FXUS65 KBOI 270217
AFDBOI
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOISE ID
817 PM MDT TUE AUG 26 2025
DISCUSSION
FLASH-FLOOD-PRODUCING THUNDERSTORMS BEGAN IN
SOUTHWESTERN IDAHO AROUND 330 PM MDT AND CONTINUE THIS EVENING,
IN ELMORE, BOISE, AND EASTERN ADA COUNTIES. SEVERAL FLASH FLOOD
WARNINGS HAVE ALREADY BEEN ISSUED IN THESE AREAS. IDAHO STATE
HIGHWAY 21 HAS BEEN CLOSED BY A LANDSLIDE BETWEEN GRANDJEAN AND
THE CUSTER COUNTY LINE, BUT ROCK AND MUD SLIDES AND STANDING
WATER ALSO OCCURRED BETWEEN LOWMAN AND GRANDJEAN, SO FLASH FLOOD
WARNINGS WERE ISSUED ALONG HIGHWAY 21 ALL THE WAY FROM LOWMAN
TO THE CUSTER COUNTY LINE. HEAVIEST RAIN FROM ANY STORM
(ESTIMATED NEAR 3.25 INCHES) HAS FALLEN BETWEEN LUCKY PEAK DAM
IN EASTERN ADA COUNTY THROUGH ARROWROCK RESERVOIR IN BOISE
COUNTY SINCE 630 PM MDT AND WAS STILL OCCURRING AT 8 PM MDT.
RADAR SHOWED THESE HEAVY RAIN-PRODUCING STORMS MOVING NORTHWARD
ONLY AROUND 10 MPH. HI-RES MODELS SHOW THEM GETTING INTO
WESTERN VALLEY AND ADAMS COUNTIES, BUT WEAKER OVERNIGHT THROUGH
WEDNESDAY MORNING. RELATIVELY LESS RAIN, BUT STILL MODERATE TO
HEAVY, IS STILL POSSIBLE ANYWHERE IN OUR CWA (EXCEPT HARNEY
COUNTY OR) THROUGH WEDNESDAY EVENING.
AVIATION
NUMEROUS SHOWERS/THUNDERSTORMS, BECOMING SCATTERED
WEDNESDAY LATE AFTERNOON. GREATEST IMPACT ACROSS SW IDAHO. OUTFLOW
GUSTS UP TO 25-35 KT. HEAVY RAIN PRODUCING MVFR TO LIFR CONDITIONS
AND MTN OBSCURATION. SURFACE WINDS: VARIABLE 5-15 KT, THEN SW-NW 5-
15 KT WED AFTERNOON. WINDS ALOFT AT 10KFT MSL: VARIABLE 5-15 KT,
THEN W 15-30 KT AROUND WED/12Z.
KBOI...PERIODS OF MODERATE TO HEAVY RAIN SHOWERS AT KBOI THROUGH
WEDNESDAY MORNING, DECREASING WEDNESDAY AFTERNOON. MVFR/IFR
VISIBILITY IN HEAVY RAIN, WITH FOOTHILLS OBSCURED. SURFACE
WINDS: EASTERLY AROUND 5-10 KT, BECOMING VARIABLE OVERNIGHT.
PREV DISCUSSION
SHORT TERM...TONIGHT THROUGH THURSDAY NIGHT...
A VERY MOIST ATMOSPHERE REMAINS IN PLACE AS SOUTHERLY FLOW
TRANSPORTS MONSOON MOISTURE OVER THE FORECAST AREA. THE MORNING
SOUNDING MEASURED 1.20 INCHES OF PRECIPITABLE WATER WITH LIGHT
SOUTHERLY STEERING FLOW. THIS IS A FAVORABLE SETUP FOR SLOW-
MOVING SHOWERS AND THUNDERSTORMS AND LOCALLY HEAVY RAIN, THUS A
FLASH FLOOD WATCH REMAINS IN EFFECT. SHOWERS AND THUNDERSTORMS
WILL BE ON THE UPTICK LATE THIS AFTERNOON AND EVENING,
CONTINUING OVERNIGHT AND WEDNESDAY. GREATEST COVERAGE IS
EXPECTED ACROSS SOUTHWEST IDAHO AS A WEAK UPPER TROUGH LIFTS
SLOWLY NORTH ACROSS THE AREA TONIGHT AND WEDNESDAY. WESTERLY
FLOW ON THE BACK SIDE OF THE TROUGH IS FORECAST TO BRING DRIER
AIR INTO SE OREGON LATE TONIGHT WHICH WILL DIMINISH THE
PRECIPITATION CHANCES OVER HARNEY COUNTY BY WEDNESDAY MORNING.
THEREFORE THE FLASH FLOOD WATCH WAS ADJUSTED TO END LATE TONIGHT
FOR HARNEY COUNTY BUT CONTINUES ELSEWHERE THROUGH WEDNESDAY
EVENING. WESTERLY FLOW AND DRIER MORE STABLE AIR WILL SPREAD
ACROSS THE AREA THURSDAY AS THE UPPER TROUGH DEPARTS AND RIDGING
STARTS TO BUILD OVER THE AREA. THIS WILL LIMIT THE CHANCE OF
SHOWERS AND THUNDERSTORMS TO PRIMARILY THE HIGHER TERRAIN ACROSS
SW IDAHO THURSDAY WHILE TEMPERATURES TREND UP.
LONG TERM...FRIDAY THROUGH TUESDAY...A BROAD UPPER RIDGE WILL
BUILD INTO THE REGION BY FRIDAY RESULTING IN A WARMING AND
DRYING TREND HEADING INTO THE WEEKEND. HIGH TEMPERATURES IN THE
LOWER VALLEYS ARE EXPECTED TO RETURN TO THE UPPER 80S AND LOWER
90S FRIDAY THROUGH SUNDAY, A FEW DEGREES ABOVE NORMAL. MODEL
GUIDANCE SHOWS AN UPPER LOW ALONG THE PACIFIC NW COAST EARLY
NEXT WEEK. THERE IS CURRENTLY STRONG GUIDANCE DISAGREEMENT ON
WHETHER THE AFOREMENTIONED UPPER LOW WILL EITHER SPLIT OFF OR
REMAIN QUASI-STATIONARY OVER THE NW COAST BY TUESDAY. THIS
WILL BRING A 20% CHANCE OF SHOWERS/THUNDERSTORMS EARLY NEXT
WEEK OVER THE MOUNTAINS OF SW IDAHO.
BOI WATCHES/WARNINGS/ADVISORIES
ID...FLASH FLOOD WATCH THROUGH WEDNESDAY EVENING
IDZ011>016-028>030-033.
OR...FLASH FLOOD WATCH THROUGH WEDNESDAY EVENING ORZ062>064.
FLASH FLOOD WATCH UNTIL 5 AM PDT WEDNESDAY ORZ061.
WWW.WEATHER.GOV/BOISE
INTERACT WITH US VIA SOCIAL MEDIA:
WWW.FACEBOOK.COM/NWSBOISE
WWW.X.COM/NWSBOISE
DISCUSSION...LC
AVIATION.....SH
SHORT TERM...TL
LONG TERM....TL
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