368
FXUS65 KBOI 281701
AFDBOI
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOISE ID
1001 AM MST SUN DEC 28 2025
DISCUSSION
GETTING OFF TO A COLD START THIS MORNING,
ESPECIALLY WHERE IT CLEARED OUT OVERNIGHT. IF NOT THE COLDEST
MORNING SO FAR THIS WINTER (3 DEGREES SO FAR THIS MORNING IN
MCCALL), THEN CERTAINLY THE COLDEST WE'VE SEEN SINCE THE START
OF THE MONTH. SATELLITE IMAGERY SHOWING SOME AREAS OF CLOUD
COVER THIS MORNING WITH ANY FOG/LOW-STRATUS LIMITED TO MTN
VALLEYS AND PORTIONS OF E-CENTRAL OREGON. WHAT HAS FORMED
OVERNIGHT WILL BURN OFF BY EARLY THIS AFTERNOON. THE CURRENT
FORECAST IS ON TRACK FOR TODAY, WITH THE AIR STAGNATION ADVISORY
CONTINUING.
AVIATION
VFR TODAY. ISOLATED IFR IN MTN VALLEY FOG WILL BURN
OFF BY 18Z TODAY. EXPECT SIMILAR CONDITIONS TO REDEVELOP
TONIGHT. SURFACE WINDS: VARIABLE LESS THAN 10 KT. WINDS ALOFT
AT 10KFT MSL: NLY 10-20KT.
KBOI...VFR. SURFACE WINDS LESS THAN 7 KT.
AIR STAGNATION
AN UPPER LEVEL RIDGE BUILDING OVER THE REGION
MONDAY WILL CREATE STRONG TEMPERATURE INVERSIONS AND LIGHT
WINDS. THIS WILL LEAD TO DETERIORATING AIR QUALITY AND
STAGNATION ISSUES IN THE VALLEYS OF SOUTHWEST IDAHO AND
SOUTHEAST OREGON. MIXING HEIGHTS WILL BE VERY LOW STARTING
MONDAY AND PERSISTING THROUGH AT LEAST THURSDAY. AN AIR
STAGNATION ADVISORY IS IN EFFECT FOR MOST VALLEY LOCATIONS
THROUGH THE WEEK. CONDITIONS MAY IMPROVE LATE THURSDAY OR FRIDAY
AS THE APPROACHING PACIFIC SYSTEM INCREASES MIXING.
PREV DISCUSSION
SHORT TERM...TODAY THROUGH TUESDAY NIGHT...COLD NORTHWESTERLY
FLOW ALOFT CONTINUES TODAY AS AN UPPER LEVEL RIDGE AXIS BUILDS
TO THE WEST. THIS WILL KEEP TEMPERATURES BELOW NORMAL ACROSS THE
AREA FOR ONE MORE DAY. THE UPPER LEVEL RIDGE MOVES OVER THE
INTERMOUNTAIN WEST ON MONDAY AND WILL REMAIN THE DOMINANT
FEATURE THROUGH THE REST OF THE SHORT TERM PERIOD. THIS PATTERN
WILL BRING DRY AND STABLE CONDITIONS TO THE REGION. AS THE RIDGE
STRENGTHENS, STRONG TEMPERATURE INVERSIONS WILL DEVELOP AND
INTENSIFY. THIS WILL RESULT IN STAGNANT AIR IN THE VALLEYS OF
SOUTHWEST IDAHO AND SOUTHEAST OREGON. AN AIR STAGNATION ADVISORY
IS IN EFFECT AND CONTINUES THROUGH THE WEEK DUE TO THE EXPECTED
POOR MIXING HEIGHTS AND LIGHT WINDS. WHILE MOUNTAIN LOCATIONS
WILL EXPERIENCE MOSTLY SUNNY SKIES AND TEMPERATURES WARMING TO 5
TO 10 DEGREES ABOVE NORMAL, THE VALLEYS WILL REMAIN COOLER
UNDER THE INVERSION. PATCHY FOG AND LOW STRATUS ARE LIKELY TO
DEVELOP DURING THE LATE NIGHT AND MORNING HOURS IN THE FAVORED
SHELTERED VALLEYS, POTENTIALLY SUPPRESSING DAYTIME HIGHS IN
THOSE AREAS. WINDS WILL REMAIN GENERALLY LIGHT AND TERRAIN
DRIVEN, THOUGH TYPICAL DRAINAGE FLOWS ARE EXPECTED OVERNIGHT.
LONG TERM...WEDNESDAY THROUGH SUNDAY...THE UPPER LEVEL RIDGE
WILL MAINTAIN ITS INFLUENCE EARLY IN THE LONG TERM PERIOD,
KEEPING CONDITIONS DRY AND STABLE WITH VALLEY INVERSIONS
PERSISTING THROUGH AT LEAST EARLY THURSDAY. BY LATE THURSDAY,
THE RIDGE IS FORECAST TO SHIFT EASTWARD AS A PACIFIC TROUGH
APPROACHES THE WEST COAST. THIS SYSTEM WILL DIRECT A PLUME OF
SUBTROPICAL MOISTURE NORTHWARD INTO OUR AREA, INCREASING
PRECIPITATION CHANCES LATE THURSDAY INTO FRIDAY. MODELS INDICATE
A TRANSITION TO A MORE ACTIVE PATTERN TO CLOSE OUT THE WEEK.
WIDESPREAD PRECIPITATION IS LIKELY FRIDAY AS THE TROUGH AND
ASSOCIATED FRONTAL SYSTEM MOVE INLAND. SNOW LEVELS ARE EXPECTED
TO BE RELATIVELY HIGH INITIALLY, RANGING FROM 4000 TO 6000 FEET,
OWING TO THE MILD SOUTHWEST FLOW AHEAD OF THE TROUGH. THIS WILL
RESULT IN RAIN FOR MOST VALLEY LOCATIONS AND SNOW RESTRICTED TO
THE MOUNTAINS. COOLER AIR WILL ARRIVE BEHIND THE FRONT, LIKELY
LOWERING SNOW LEVELS TO AROUND 3500 FEET BY SATURDAY.
PRECIPITATION CHANCES LOOK TO DECREASE GRADUALLY ON SUNDAY AS
THE MAIN TROUGH AXIS PASSES TO THE EAST, THOUGH SCATTERED
SHOWERS MAY LINGER IN THE MOUNTAINS. TEMPERATURES WILL AVERAGE
ABOVE NORMAL THROUGH THE PERIOD, COOLING CLOSER TO SEASONAL
NORMALS BY THE WEEKEND.
BOI WATCHES/WARNINGS/ADVISORIES
ID...AIR STAGNATION ADVISORY FROM 6 PM THIS EVENING TO 10 AM MST
THURSDAY IDZ012-014-016-028-033.
OR...AIR STAGNATION ADVISORY FROM 6 PM MST /5 PM PST/ THIS EVENING
TO 10 AM MST /9 AM PST/ THURSDAY ORZ061>064.
WWW.WEATHER.GOV/BOISE
INTERACT WITH US VIA SOCIAL MEDIA:
WWW.FACEBOOK.COM/NWSBOISE
WWW.X.COM/NWSBOISE
DISCUSSION...DG
AVIATION.....DG
SHORT TERM...JDS
LONG TERM....JDS
AIR STAGNATION...JDS
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