Area Forecast Discussion
Issued by NWS Boise, ID

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FXUS65 KBOI 080353
AFDBOI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOISE ID
853 PM MST SUN FEB 7 2016

.DISCUSSION...WITH BUILDING UPPER RIDGE AXIS TO THE WEST...SEEING
A CONTINUING FEED OF HIGH CLOUDS IN NORTHERLY FLOW. SIMILAR TO
LAST NIGHT THIS IS INHIBITING THE ONSET OF FOG THIS EVENING. AREAS
OF FOG OR A LOW STRATUS WILL FORM ACROSS LOWER VALLEYS /FOG HAS
ALREADY SET UP IN THE LONG VALLEY/ BY MONDAY MORNING AS NEAR-
SURFACE LAYER IS FAIRLY MOIST /DEWPOINTS IN THE LOW 30S IN THE
SNAKE RIVER PLAIN/ AND WINDS ARE LIGHT. HAVE MADE A FEW UPDATES TO
TIMING OF FOG OVERNIGHT AND MINIMUM TEMPERATURES IN THE WEISER
VALLEY AND WEST-CENTRAL MTNS.

&&

.AVIATION...MAINLY VFR. SURFACE BASED TEMPERATURE INVERSION
DEVELOPING TONIGHT WILL PERSIST AT LEAST THROUGH WEDNESDAY NIGHT
UNDER WARM HIGH PRESSURE ALOFT. EXPECT AREAS OF IFR/LIFR FOG AND LOW
STRATUS THROUGHOUT THE SNAKE RIVER VALLEY TONIGHT THROUGH EARLY
MONDAY AFTERNOON...WITH PATCHY VALLEY FOG IN THE KBNO..KBKE..AND
KMYL AREAS. DAYTIME HEATING SHOULD INDUCE ENOUGH MIXING TO DISSIPATE
THE FOG AND STRATUS BY ABOUT 21Z MONDAY...BUT IT WILL REDEVELOP
MONDAY EVENING. SURFACE WINDS...VARIABLE 10 KTS OR LESS. WINDS
ALOFT...NORTHWEST 10-20KTS THROUGH 10K FT MSL.

&&

.AIR STAGNATION...INVERSION WILL STRENGTHEN OVER THE NEXT 48
HOURS...PEAKING ON TUESDAY. LOWER VALLEYS WILL SEE AFTERNOON
MIXING HEIGHTS DROP TO AROUND 1500 FEET AGL BY TUESDAY AFTERNOON.
EASTERLY WINDS IN THE MIXED LAYER WILL INCREASE ON TUESDAY...
ESPECIALLY OVER THE UPPER TREASURE AND WESTERN MAGIC. INVERSION
WILL HOLD INTO NEXT WEEKEND /IF NOT BEYOND/ THOUGH WITH DRY AIR
ALOFT AND EASTERLY WINDS CONFIDENCE ON FOG/STRATUS EVOLUTION IS
LOW.

&&

.PREV DISCUSSION...
SHORT TERM...TONIGHT THROUGH TUESDAY...
LARGE UPPER RIDGE BUILDING OVER THE AREA TODAY AND MODEL FORECAST
SOUNDINGS SHOWING THE MID LEVELS OF THE ATMOSPHERE CONTINUING TO
WARM OVERNIGHT. SO STRONG INVERSION TO BE IN PLACE STARTING
TONIGHT. THE BIGGEST QUESTION IS THE AREAL EXTENT OF THE FOG AND
WHEN DOES IT BEGIN TO DISSIPATE. CURRENT THINKING IS THAT GIVEN
THAT THE LOW STRATUS AND FOG IS JUST NOW DISSIPATING FROM THE
MAGIC VALLEY WESTWARD TO MOUNTAIN HOME...THIS MAY BE THE PRECURSOR
TO WHATS TO COME. AS THE SUN SETS...LOOK FOR THE FOG TO SPREAD
BACK INTO THE LOWER TREASURE VALLEY ALONG WITH THE BURNS/BAKER AND
MCCALL REGIONS. SOMEWHAT MORE CONFIDENT ON EXTENT OF THE FOG THAN
THAN THE TIMING OF WHEN IT SHOULD DISSIPATE...NO SNOW/MOISTURE ON
THE GROUND WOULD MAY HELP LIMIT THE HOURS OF FOG ACROSS THE AREA.
ANY LOW ELEVATION SURFACE MOISTURE WOULD HAVE TO ADVECT FROM
EASTERN IDAHO IN THE LIGHT EASTERLY WINDS EXPECTED OR DRAINAGE OUT
OF THE MOUNTAINS INTO THE LOWER VALLEYS...THAT WOULD KEEP
FOG/STRATUS IN THE PICTURE. THIS MAKES THE FORECAST TEMPERATURES
DIFFICULT AS IF FOG/CLOUDS DONT BREAK TEMPERATURES COULD STRUGGLE
TO WARM THROUGH THE DAY.

EVEN WARMER AIR ALOFT PUSHES INTO THE REGION TUESDAY ONLY
INCREASING THE STRENGTH OF THE INVERSION...SO EXPECTING SIMILAR
CONDITIONS TUESDAY MORNING WITH EXTENSIVE FOG SOON AFTER
SUNSET...LASTING INTO THE AT LEAST MID/LATE MORNING.


LONG TERM...TUESDAY NIGHT THROUGH SUNDAY...
HIGH AMPLITUDE UPPER LEVEL RIDGE WILL REMAIN OVER THE REGION
THROUGH THE END OF THE WEEK. CAN EXPECT VALLEY INVERSIONS UNDER
THE RIDGE ALONG WITH FOG AND LOW STRATUS. STILL SOME UNCERTAINTY
AS TO HOW WIDESPREAD THE VALLEY STRATUS AND FOG WILL GET UNDER THE
INVERSION SO FORECAST VALLEY TEMPERATURES HAS NOT BEEN EASY. STILL
TRENDING THAT VALLEY TEMPERATURES WILL BE IN THE 30S AND MAYBE THE
LOW 4OS...BUT IF THERE IS NOT AS MUCH MOISTURE TRAPPED UNDER THE
INVERSION AS WE ARE THINKING...VALLEY HIGHS COULD BE MUCH WARMER.
HIGH TEMPERATURES ABOVE THE INVERSION WILL BE IN THE 40S AND LOW
50S.

FRIDAY NIGHT THROUGH SUNDAY...A LOW OFF THE BRITISH COLUMBIA COAST
AND ASSOCIATED FRONTAL SYSTEM WILL MAKE ITS WAY ACROSS OUR AREA
THROUGH THIS PERIOD. INCREASED POPS SATURDAY AM WITH BEST CHANCES
SATURDAY AFTERNOON ESPECIALLY FOR THE WEST CENTRAL MOUNTAINS.
DIFFERENCES IN MODELS AFTER THIS LEADS TO LOW CONFIDENCE FOR TIMING
AND LOCATION OF PRECIPITATION. TEMPERATURES WILL CONTINUE TO BE
ABOVE NORMAL THROUGH THE PERIOD.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...NONE.
OR...NONE.

&&

$$

WEATHER.GOV/BOISE

DISCUSSION...DG
AVIATION.....JT
AIR STAGNATION...DG
PREV SHORT TERM...CR
PREV LONG TERM....JA/JC


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