319
FXUS65 KBOI 190018
AFDBOI
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOISE ID
618 PM MDT FRI SEP 18 2026
UPDATE
UPDATED AVIATION DISCUSSION.
KEY MESSAGES
- NUMEROUS SHOWERS, ISOLATED-TO-SCATTERED THUNDERSTORMS, AND
COOLER TEMPERATURES TODAY. STRONGEST THUNDERSTORMS IN CENTRAL
IDAHO WITH GUSTY OUTFLOW WINDS, HAIL, AND BRIEF HEAVY RAIN.
- SHOWERS AND THUNDERSTORMS CONTINUE TONIGHT AND TOMORROW OVER
THE CENTRAL IDAHO MOUNTAINS.
- GRADUAL WARMING AND DRYING TREND BEGINNING SATURDAY AND
CONTINUING THROUGH EARLY NEXT WEEK.
SHORT TERM /THROUGH SUNDAY NIGHT/
ISSUED 217 PM MDT FRI SEP 18 2026
CURRENTLY, THE UPPER-LEVEL LOW PRESSURE SYSTEM IS MOVING
DIRECTLY OVERHEAD AS OF 1400 MDT, CAUSING NUMEROUS SHOWERS AND
ISOLATED- TO- SCATTERED THUNDERSTORMS OVER SOUTHWEST IDAHO.
CONCERN HAS INCREASED THROUGH THE DAY AS WE HAVE BEEN OBSERVING
THESE CELLS, WITH QPE TOTALS (OVER 2+ INCHES SE OF GLENNS FERRY,
SMALL AREA OF 3+ INCHES) INSTIGATED BY TRAINING THUNDERSTORMS
CAUSING THE ISSUANCE OF A FLOOD AND FLASH FLOOD PRODUCTS. THE
WATCH IS OF PARTICULAR CONCERN FOR THE BOISE MOUNTAINS, WITH
EXTRA ATTENTION ON A FEW BURN SCARS IN THE AREA. ISOLATED
THUNDERSTORMS ARE ALSO ONGOING OVER SOUTHERN MALHEUR COUNTY AND
MOVING INTO NORTHWEST BAKER COUNTY. WHILE BURN SCARS OVER SE
OREGON COULD ALSO SEE IMPACTS FROM THESE THUNDERSTORMS,
CONFIDENCE IS NOT AS HIGH AS OF THIS FORECAST ISSUANCE.
THESE AREAS OF SHOWERS AND THUNDERSTORMS ARE EXPECTED TO
CONTINUE THROUGH THE AFTERNOON INTO EVENING FOR BOTH SE OREGON
AND SW IDAHO. THUNDERSTORMS WILL BE CAPABLE OF PRODUCING GUSTY
WINDS UP TO 40 MPH, SMALL HAIL, AND BRIEF HEAVY RAIN. ABUNDANT
LIGHTNING IS ALSO LIKELY, WITH RAINFALL TOTALS VARYING HEAVILY
DEPENDENT ON WHERE THE THUNDERSTORM CORES MOVE OVER. ONCE LATE
EVENING ROLLS AROUND, COVERAGE WILL DECREASE GREATLY COMPARED TO
THE AFTERNOON, BECOMING LARGELY CONFINED TO THE BOISE AND WEST-
CENTRAL ID MOUNTAINS. WHILE OVERALL COVERAGE WANES TONIGHT,
SHOWERS AND THUNDERSTORMS ARE EXPECTED TO CONTINUE THROUGH
SATURDAY AFTERNOON AS MOISTURE WRAPS AROUND THE EASTWARD MOVING
LOW PRESSURE CENTER. CONFIDENCE OF SHOWERS IS HIGH (30-50%
CHANCE), BUT CONFIDENCE IN THUNDERSTORMS IS LOW (10-15% CHANCE).
FOG IS LIKELY IN THE LATE OVERNIGHT TO EARLY MORNING PERIODS
TONIGHT AND SATURDAY NIGHT, ESPECIALLY IN AREAS WHERE THERE WAS
A LOT OF RAINFALL.
BEHIND THE LOW, HIGH PRESSURE RETURNS AS A RIDGE AXIS MOVES
OVER THE REGION, LEADING TO WARMING CONDITIONS LATE SATURDAY AND
THROUGH SUNDAY. TEMPERATURES WARM UP TO ABOUT 5 DEGREES ABOVE
NORMAL BY SUNDAY.
LONG TERM /MONDAY THROUGH FRIDAY/
ISSUED 217 PM MDT FRI SEP 18 2026
AN UPPER LEVEL RIDGE OVER THE INTERMOUNTAIN WEST AND ROCKY MOUNTAINS
WILL REMAIN THE DOMINATE WEATHER FEATURE THROUGH THE EXTENDED
PERIOD. EARLY IN THE WEEK ON MONDAY AND TUESDAY, THE REGION WILL SIT
UNDER A MODEST NORTHWESTERLY FLOW. DYNAMICS WILL FAVOR DEVELOPMENT
OF SHOWERS IN CENTRAL AND EASTERN IDAHO MONDAY AND TUESDAY
AFTERNOONS. THE CHANCE OF PRECIPITATION EACH AFTERNOON IS ONLY 10%.
OTHERWISE, THAT ZONAL FLOW TO THE NORTH MODERATES TEMPERATURES,
HOLDING THEM STEADY AT 5 DEGREES ABOVE NORMAL FOR MOST OF THE LONG
TERM. UPPER LEVEL RIDGE AMPLIFIES TO THE EAST AS A STRONG ALASKAN
LOW PRESSURE SYSTEM APPROACHES THE PACIFIC NORTHWEST. THIS WILL
BRING WELL ABOVE NORMAL TEMPERATURES FOR THURSDAY AND FRIDAY.
TEMPERATURES LOOK TO RUN AT LEAST 5 TO 10 DEGREES ABOVE SEASONAL
NORMALS. THAT WOULD MEAN TEMPERATURES IN THE UPPER 80S TO LOW 90S IN
THE LOWER VALLEYS.
AVIATION /00Z SATURDAY THROUGH SUNDAY/
ISSUED 607 PM MDT FRI SEP 18 2026
MAINLY VFR, WITH ISOLATED SHOWERS/THUNDERSTORMS CONTINUING OVER
EAST-CENTRAL OR AND WEST-CENTRAL ID MOUNTAINS THIS EVENING,
BEFORE TAPERING OFF OVER SOUTHWEST ID THROUGH SATURDAY. LOCAL
MVFR CONDITIONS POSSIBLE IN SHOWERS. PATCHY FOG IN SHELTERED
MOUNTAIN VALLEYS OVERNIGHT THROUGH SATURDAY MORNING. SURFACE
WINDS: N-NW 5-10 KT THIS EVENING THROUGH OVERNIGHT, EXCEPT SW
7-15 KT WITH GUSTS UP TO 20 KT OVER SOUTH-CENTRAL ID. WINDS
ALOFT AT 10KFT MSL: NW-N 5-15 KT.
KBOI...MAINLY VFR. AROUND A 20% CHANCE OF SHOWERS AND A 10-15%
CHANCE OF LIGHTNING AT THE TERMINAL LATE THIS EVENING. CLEARING
EARLY SAT MORNING WITH SCT CLOUDS AND LIGHTER WINDS. SURFACE
WINDS: NW 5-15 KT.
WEEKEND OUTLOOK...BRIEF MVFR AND MOUNTAIN OBSCURATION IN LINGERING
SHOWERS NEAR KTWF-KJER ON SATURDAY, WITH SHOWERS TAPERING OFF BY
MIDDAY. EXPECT VFR CONDITIONS OTHERWISE THROUGH SUNDAY WITH VARIABLE
WINDS LESS THAN 10 KT OVERNIGHT AND IN HIGH TERRAIN AND NW 5-15 KT
IN VALLEYS DURING THE AFTERNOON.
BOI WATCHES/WARNINGS/ADVISORIES
ID...FLASH FLOOD WATCH UNTIL MIDNIGHT MDT TONIGHT FOR IDZ013-014-
016-028.
OR...NONE.
WWW.WEATHER.GOV/BOISE
INTERACT WITH US VIA SOCIAL MEDIA:
WWW.FACEBOOK.COM/NWSBOISE
WWW.X.COM/NWSBOISE
AVIATION.....JY
SHORT TERM...CH
LONG TERM....JDS
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