FXUS65 KMSO 071031
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
331 AM MST MON DEC 7 2009
.DISCUSSION...A BROAD TROUGH HAS ESTABLISHED ITSELF OVER THE
NORTHERN ROCKIES... AND WILL BE THE DOMINATE FEATURE FOR THE NEXT 48
HOURS. THIS TROUGH HAS ALLOWED AN ARCTIC AIR MASS TO ENCOMPASS THE
NORTHERN ROCKIES WITH TEMPERATURES TODAY BEING 25 TO 30 DEGREES
BELOW SEASON NORMALS. MANY LOCATIONS WILL ONLY BE IN THE POSITIVE
SINGLE DIGITS FOR HIGHS TODAY. A PRESSURE GRADIENT WILL KEEP
EASTERLY WINDS GOING...ESPECIALLY DURING THE MORNING HOURS. THESE
EASTERLY WINDS IN COMBINATION WITH THE EXTREMELY COLD TEMPERATURES
WILL CAUSE DANGEROUS WIND CHILLS ACROSS WESTERN MONTANA THIS
MORNING. SO THE WIND CHILL WARNINGS AND ADVISORIES REMAIN IN PLACE.
THE PRESSURE GRADIENT WILL RELAX THIS AFTERNOON THROUGH TONIGHT
ALLOWING THE WINDS TO SUBSIDE. ALSO THE SKIES ARE EXPECTED TO BE
FAIRLY CLEAR TONIGHT ALLOWING TEMPERATURES TO DROP WELL BELOW ZERO.
TONIGHT SHOULD BE THE COLDEST NIGHT OF THE WINTER SEASON THUS FAR.
TUESDAY WILL BE A STRUGGLE TO WARM UP SINCE THE DAY WILL BE
STARTING OFF COOLER AND THE LACK OF MIXING...SO TEMPERATURES
SHOULD BE SIMILAR TO TODAY OR POTENTIALLY EVEN COLDER. TUESDAY
NIGHT THE UPPER LEVEL FLOW WILL BECOME NORTHWESTERLY ALLOWING FOR
A MOISTER AIR MASS TO MOVE INTO THE REGION. THIS MOISTURE WILL
PRIMARILY INCREASE CLOUD COVER WHICH IN TURN WILL NOT ALLOW
TEMPERATURES TO DROP AS DRAMATICALLY.
WEDNESDAY THROUGH SUNDAY... VERY WEAK SHORTWAVE ENERGY WILL MOVE
THROUGH THE NORTHERN ROCKIES ON WEDNESDAY. MOISTURE WILL INCREASE
WITH THIS SYSTEM... BUT SNOW WILL BE LIGHT WITH LESS THAN
IDEAL SNOW CRYSTAL GROWTH DUE TO VERY COLD TEMPERATURES. AFTER THE
SHORTWAVE PASSES A NORTHWESTERLY FLOW ALOFT WILL SET UP OVER THE
REGION. THE AIR MASS WILL MODERATE UNDER THIS FLOW PATTERN WITH
SOMEWHAT WARMER TEMPERATURES POSSIBLE. THIS FLOW PATTERN CAN ALSO BE
VERY TEMPERAMENTAL SINCE POTENTIAL SHORTWAVE FEATURES COULD
INTERACT WITH ANY AVAILABLE MOISTURE ALLOWING FOR SNOW... ESPECIALLY
WITH THE SNOW GROWTH REGION LIFTING INTO A MUCH BETTER PORTION OF THE
ATMOSPHERE. ONE WORRY WITH THE NORTHWESTERLY FLOW IS THAT MODEL
GUIDANCE SUGGEST THAT THE JET WILL HAVE ITS ORIGIN IN THE GULF OF
ALASKA...WHICH COULD ALLOW FOR AN INCREASE IN MOISTURE. WILL CONTINUE
SNOW CHANCES FOR THE HIGH TERRAIN TO ACCOUNT FOR THIS POTENTIAL
THROUGH THE END OF THE WEEK.
BOTH THE GFS AND ECMWF PROMOTE A MORE MOIST WESTERLY OR SOUTHWESTERLY
PATTERN FOR THE REGION BY THE END OF THE WEEKEND...HOWEVER... THE
ECMWF HAS A MUCH COLDER AIR MASS OVER WESTERN MONTANA OPPOSED TO
THE WARMER GFS. FOR NOW WILL GO WITH A MORE MODERATE AND MOIST
PATTERN UNTIL GUIDANCE COMES INTO BETTER AGREEMENT ON TEMPERATURES.
&&
.AVIATION...A BROAD TROUGH WILL BE THE PRIMARY FEATURE OVER THE
NORTHERN ROCKIES TODAY CAUSING EXTREMELY COLD TEMPERATURES. ALSO
SOME MOISTURE CONTINUES TO STREAM THROUGH THE TROUGH CAUSING LOW
CLOUDS. THE CLOUDS WILL OBSCURE MOUNTAINS AND PASSES AT TIMES.
A TIGHT PRESSURE GRADIENT IS CAUSING GUSTY EASTERLY WINDS ACROSS
THE NORTHERN ROCKIES...ESPECIALLY TIGHT EAST-WEST ORIENTED
CANYONS. THE CLOUDS AND WINDS ARE EXPECTED TO DECREASE THIS
AFTERNOON AND OVERNIGHT.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WIND CHILL WARNING UNTIL 11 AM MST THIS MORNING FOR
BUTTE/BLACKFOOT REGION-FLATHEAD/MISSION VALLEYS-
POTOMAC/SEELEY LAKE REGION-WEST GLACIER REGION.
WIND CHILL WARNING UNTIL 11 AM MST THIS MORNING FOR
BITTERROOT/SAPPHIRE MOUNTAINS-MISSOULA/BITTERROOT VALLEYS.
WIND CHILL ADVISORY UNTIL 11 AM MST THIS MORNING FOR
KOOTENAI/CABINET REGION-LOWER CLARK FORK REGION.
ID...WIND CHILL ADVISORY UNTIL 11 AM MST THIS MORNING FOR EASTERN
LEMHI COUNTY.
&&
$$
SHORT TERM...SMITH
LONG TERM....KITSMILLER
AVIATION...SMITH
