752
FXUS65 KBOI 210352
AFDBOI
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOISE ID
852 PM MST TUE JAN 20 2026
DISCUSSION
THE SYNOPTIC PATTERN REMAINS LOCKED IN PLACE WITH
A STRONG UPPER LEVEL RIDGE CENTERED OVER THE INTERMOUNTAIN
WEST. THIS FEATURE CONTINUES TO TRAP A COLD, STABLE AIR MASS IN
THE LOWER VALLEYS OF THE SNAKE BASIN AND SOUTHEAST OREGON.
SATELLITE IMAGERY SHOWS NO SIGNIFICANT CHANGE IN THE STRATUS
DECK ENTRENCHED FROM THE MAGIC VALLEY THROUGH THE TREASURE
VALLEY AND INTO THE LOWER VALLEYS OF BAKER AND MALHEUR COUNTIES.
HIGH CLOUDS WILL INCREASE FROM THE SOUTHWEST TONIGHT AS A LOW
PRESSURE SYSTEM APPROACHES THE CALIFORNIA COAST. HOWEVER, THIS
WILL DO LITTLE TO CHANGE THE STRATUS AND FOG PROFILE TONIGHT.
HAVE ADDED A MENTION OF FLURRIES TONIGHT THROUGH WEDNESDAY AS
SEEDING FROM CLOUDS ALOFT OR LOCAL GROUND SOURCES MAY PRODUCE
VERY LIGHT SNOWFALL AT TIMES WITHIN THE STRATUS DECK. OTHERWISE,
THE FORECAST REMAINS ON TRACK WITH TEMPERATURES WELL BELOW
NORMAL IN THE VALLEYS AND MILDER CONDITIONS IN THE MOUNTAINS.
AVIATION
MVFR-IFR BELOW 5K-6K FEET MSL IN THE SNAKE PLAIN AND
SURROUNDING BASINS DUE TO PERSISTENT LOW STRATUS AND FOG. LOWER
MOUNTAINS/FOOTHILLS OBSCURED. SURFACE WINDS: VARIABLE UP TO 8 KT.
WINDS ALOFT AT 10KFT MSL: NW 10-20 KT.
KBOI...MVFR-IFR IN LOW STRATUS. SURFACE WINDS: VARIABLE OR SE AROUND
5 KT.
AIR STAGNATION
A STRONG TEMPERATURE INVERSION COMBINED WITH
LIGHT BOUNDARY LAYER WINDS WILL CONTINUE TO SUPPORT STAGNANT
CONDITIONS ACROSS ALL LOWER VALLEYS OF SOUTHWEST IDAHO AND
SOUTHEAST OREGON. OBSERVATIONS INDICATE MIXING HEIGHTS REMAINING
SUPPRESSED BELOW 2000 FEET AGL WITH TRANSPORT WINDS GENERALLY
UNDER 10 MPH. THESE CONDITIONS ARE FAVORABLE FOR THE BUILDUP OF
POLLUTANTS NEAR THE SURFACE. THE AIR STAGNATION ADVISORY REMAINS
IN EFFECT THROUGH THURSDAY MORNING. WHILE THE APPROACHING WEAK
SYSTEM ON FRIDAY MAY PROVIDE MINOR IMPROVEMENT, IT IS UNCERTAIN
IF IT WILL BE SUFFICIENT TO END THE STAGNATION THREAT, AND THE
ADVISORY MAY NEED TO BE EXTENDED.
PREV DISCUSSION
SHORT TERM...TONIGHT THROUGH THURSDAY NIGHT...FORECAST REMAINS
STRONGLY TIED TO PERSISTENCE, WITH ALL MODEL ELEMENTS AND MOS
VALUES ESSENTIALLY UNCHANGED FROM DAY TO DAY THROUGH THROUGH
THURSDAY NIGHT. WE MERELY CORRECTED THEM TO RECENT OBSERVED
VALUES. THE TOP OF THE VALLEY INVERSION WILL STAY NEAR 4000
FEET MSL IN OREGON, SLOPING TO 4500 FEET MSL IN IDAHO, WITH
CLOUDY, COLD WEATHER AND PATCHY NIGHT/MORNING FOG BELOW THOSE
LEVELS, BUT MOSTLY CLEAR SKIES AND MILD DAYTIME TEMPS ABOVE.
NIGHTS WILL BE COLD, THOUGH, WITH GOOD RADIATIONAL COOLING
UNDER THE MOSTLY CLEAR SKIES. LIGHT WINDS.
LONG TERM...FRIDAY THROUGH TUESDAY...THE BACK END OF A MOSTLY
DRY LONGWAVE TROUGH WILL STEER OUT EAST FRIDAY THROUGH THE
WEEKEND, CONTINUING NORTHWEST FLOW AND DRY CONDITIONS THROUGH
EARLY NEXT WEEK. THE NORTHWEST FLOW WILL BRING COOLER AIR ALOFT,
WHICH MAY HELP MIX OUT VALLEY STRATUS AND FOG. UNCERTAINTY OVER
THIS REMAINS HIGH, AS A LARGE UPPER LEVEL RIDGE WILL BUILD INTO
THE REGION LATE SUNDAY THROUGH TUESDAY. THIS STRONG RIDGING
PATTERN WILL LIKELY RETURN WARM AIR ALOFT AND VALLEY INVERSIONS.
IF THIS PROJECTED PATTERN HOLDS, VALLEYS WOULD AGAIN SEE BELOW
NORMAL TEMPERATURES AND INVERSION STRATUS WHILE MOUNTAINS SEE
ABOVE NORMAL TEMPERATURES THROUGHOUT THE EXTENDED PERIOD. FOR
NOW, AN INVERSION PATTERN REMAINS IN PLACE FOR EARLY NEXT WEEK.
MOST GUIDANCES ALSO SUGGEST THE RIDGE STEERING IN SOME REMNANT
PACIFIC MOISTURE OVER THE CENTRAL MOUNTAINS OF EASTERN OR AND
WESTERN ID, FAVORING SMALL CHANCES OF MOUNTAIN SNOWFALL LATE
MONDAY.
BOI WATCHES/WARNINGS/ADVISORIES
ID...AIR STAGNATION ADVISORY UNTIL 5 AM MST FRIDAY
IDZ011>014-016-028-033.
OR...AIR STAGNATION ADVISORY UNTIL 5 AM MST /4 AM PST/ FRIDAY
ORZ061>064.
WWW.WEATHER.GOV/BOISE
INTERACT WITH US VIA SOCIAL MEDIA:
WWW.FACEBOOK.COM/NWSBOISE
WWW.X.COM/NWSBOISE
DISCUSSION...JDS
AVIATION.....SH
AIR STAGNATION...JDS
SHORT TERM...LC
LONG TERM....JY
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