FXUS65 KMSO 261819
AFDMSO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
1119 AM MST SAT DEC 26 2009

.UPDATE...ISSUED A COUPLE OF MINOR UPDATES THIS MORNING TO
INDICATE FLURRIES FROM THE STRATUS DECK PERSISTING IN THE WESTERN
MONTANA VALLEYS. SEE LITTLE CHANGE THROUGH AT LEAST SUNDAY MORNING
INCLUDING LOCAL EAST WINDS...FLURRIES...LOW CLOUDS AND FOG WHERE
THE STRATUS INTERSECTS TERRAIN. 

&&

.AVIATION...LOW STRATUS WILL PERSIST OVER WEST CENTRAL AND NORTHWEST
MONTANA MUCH OF THE DAY AND TONIGHT UNDER A VERY STABLE RIDGE OF
HIGH PRESSURE. THERE WILL BE HOLES IN THE STRATUS AT TIMES...BUT
KMSO AND KGPI WILL SEE CEILINGS FROM 2-3 THOUSAND FEET AGL THE
MAJORITY OF THE TIME THROUGH AT LEAST THE NEXT 24 HOURS. OTHER
TERMINALS COULD SEE PATCHY FOG AT TIMES...BUT THE THREAT IS
MINIMAL.

&&

.PREV DISCUSSION... /ISSUED 348 AM MST SAT DEC 26 2009/ 

DISCUSSION...A HIGH AMPLITUDE RIDGE WILL CONTINUE OVER THE
WESTERN U.S. THROUGH THE WEEKEND. THIS WILL MAINTAIN STABLE
CONDITIONS AT THE SURFACE. LIGHT EAST WINDS HAVE DEVELOPED DUE TO
COOLER AIR EAST OF THE DIVIDE SETTING UP A WEAK PRESSURE GRADIENT.
THIS HAS KEPT THE DEW POINTS DOWN...AND THUS REDUCED THE POTENTIAL
FOR FOG. HOWEVER...LOW STRATUS TWO TO THREE THOUSAND FEET ABOVE
GROUND LEVEL HAS BEEN AN ISSUE FOR MUCH OF WEST CENTRAL AND
NORTHWEST MONTANA OVERNIGHT. THERE IS NO INDICATION THIS WILL
CHANGE THROUGH SUNDAY MORNING. THE AIR MASS OVER THE REGION IS
COOL FOR THIS TIME OF YEAR...BUT TEMPERATURES WILL BE HIGHLY
DEPENDENT ON THE STRATUS LOCATION. VALLEYS FROM NEAR KALISPELL
SOUTH TO THE BITTERROOT WILL NOT SEE AS MUCH DIURNAL (DAY TO
NIGHT) CHANGE UNDER THE STRATUS. HOWEVER...AREAS WITH NEARLY
CLEAR SKIES SUCH AS SOUTHWEST MONTANA AND NORTH CENTRAL IDAHO WILL
COOL OFF READILY OVERNIGHT.

THE RIDGE IS EXPECTED TO WEAKEN SOME MONDAY ALLOWING MID AND HIGH
LEVEL MOISTURE TO INVADE FROM THE SOUTHWEST. THERE IS ONLY A
SLIGHT CHANCE OF SNOW OUT OF THIS...BUT IT WILL HELP IMPROVE
MIXING AND DIMINISH THE VALLEY INVERSIONS AND LOW STRATUS.

TUESDAY THROUGH SATURDAY...THE UPPER BLOCKING PATTERN WILL
FURTHER WEAKEN AS SHORTWAVES OF ENERGY MOVE INLAND TRANSITIONING
THE NORTHERN ROCKIES INTO A NORTHWESTERLY UPPER FLOW PATTERN. THE
FIRST INCREASE IN MOISTURE AND ENERGY LOOKS TO BE LATE TUESDAY
WHERE VERY LIGHT SNOW IS POSSIBLE. THE ACTIVE PATTERN WILL KEEP
CHANCES FOR SNOW THROUGH THE EXTENDED...ALTHOUGH...BOTH THE GFS
AND ECMWF HAVE A SHORTWAVE RIDGE OVER THE NORTHERN ROCKIES FOR
LATE THURSDAY AND INTO FRIDAY. THIS RIDGE COULD CAUSE A BRIEF DRY
PERIOD BEFORE A POSSIBLE WARM FRONT PASSES FRIDAY AND SATURDAY.

AVIATION...LOW STRATUS WILL PERSIST OVER WEST CENTRAL AND
NORTHWEST MONTANA MUCH OF THE DAY AND TONIGHT UNDER A VERY STABLE
RIDGE OF HIGH PRESSURE. THERE WILL BE HOLES IN THE STRATUS AT
TIMES...BUT KMSO AND KGPI WILL SEE CEILINGS FROM 2-3 THOUSAND FEET
AGL THE MAJORITY OF THE TIME THROUGH AT LEAST THE NEXT 24 HOURS.
OTHER TERMINALS COULD SEE PATCHY FOG AT TIMES...BUT THE THREAT IS
MINIMAL.

AIR STAGNATION...THE HIGH PRESSURE AND VERY STABLE CONDITIONS
WILL CONTINUE THROUGH THE WEEKEND. LOW STRATUS HAS SOMEWHAT
LIMITED THE STRENGTH OF SURFACE INVERSIONS FROM THE BITTERROOT
VALLEY NORTH BY TEMPERING THE RADIATIONAL COOLING. MODEL SOUNDING
INDICATE THE STRATUS WILL LIKELY CONTINUE IN THIS REGION THROUGH
SUNDAY MORNING.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...NONE.
ID...NONE.
&&

$$

UPDATE...GIBSON
LONG TERM....KITSMILLER
AVIATION...WHITMORE