000
FXUS65 KTFX 201140
AFDTFX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
440 AM MST THU NOV 20 2008
...UPDATE TO AVIATION...
.DISCUSSION...
TODAY THROUGH FRIDAY...AN UPPER LEVEL RIDGE OF HIGH PRESSURE
WILL MOVE INTO EASTERN MONTANA BY 00Z TONIGHT AS AN UPPER LEVEL
TROUGH OVER THE PACIFIC NORTHWEST MOVES TOWARDS WESTERN MONTANA.
A WEAK DISTURBANCE WILL MOVE THROUGH WESTERN AND CENTRAL MONTANA
THIS MORNING FOR MAINLY HIGH CLOUDS. OTHERWISE...CLOUDS WILL
INCREASE FROM THE WEST TODAY WITH A CHANCE OF SNOW OVER THE
CONTINENTAL DIVIDE THIS AFTERNOON. SNOW WILL BECOME LIKELY OVER
SOUTHWEST MONTANA THIS EVENING AND THE ENTIRE AREA BY MIDNIGHT
TONIGHT...AS THE UPPER LEVEL TROUGH SWINGS TROUGH. AS THE
UPPER LEVEL TROUGH PASSES THROUGH...MOST OF THE DYNAMICS WILL
BE OVER SOUTHWEST MONTANA. SNOW AMOUNTS WILL BE GENERALLY
UNDER TWO INCHES OVER THE SOUTHWEST PLAINS WITH HIGHER
AMOUNTS OVER THE SOUTHWEST AND LITTLE BELT MOUNTAINS. THIS
IS A FAIRLY FAST MOVING SYSTEM. THUS...MOST PRECIPITATION SHOULD
BY OUT OF THE REGION BY FRIDAY MORNING...AS AN UPPER LEVEL RIDGE
MOVES INTO THE AREA. GUSTY WEST WINDS WILL DEVELOP ON FRIDAY
AS PRESSURE GRADIENTS BETWEEN SURFACE HIGH PRESSURE OVER
THE GREAT BASIN AREA AND A SURFACE TROUGH OVER SOUTHERN
ALBERTA...EXTENDING INTO CENTRAL MONTANA INCREASES.
THUS...TEMPERATURES WILL WARM TO SLIGHTLY ABOVE SEASONAL AVERAGE.
FRIDAY NIGHT AND SATURDAY...ANOTHER UPPER LEVEL TROUGH MOVES
THROUGH MONTANA FOR ISOLATED TO SCATTERED SNOW SHOWERS...MAINLY
OVER THE SOUTHWEST MOUNTAINS AND ALONG THE EAST SLOPES
OF THE ROCKIES. HOWEVER...GUSTY DOWNSLOPE WINDS WILL KEEP
THE PLAINS MOSTLY DRY. TEMPERATURES WILL REMAIN CLOSE
TO SEASONAL AVERAGE. WILLIAMSON DC
SATURDAY NIGHT THROUGH TUESDAY NIGHT...MEDIUM RANGE MODELS REMAIN IN
DECENT AGREEMENT EARLY IN THE PERIOD WITH SHORTWAVE ENERGY SWINGING
THROUGH SOUTHERN CANADA SATURDAY NIGHT WITH NORTHWEST FLOW ALOFT
OVER MONTANA. THIS PATTERN WILL BE FAVORABLE FOR GUSTY DOWNSLOPE
WINDS OVER THE PLAINS OF NORTH CENTRAL MONTANA WHILE THE MOUNTAINS
NEAR THE CONTINENTAL DIVIDE RECEIVE SCATTERED SHOWERS. AN UPPER
LEVEL RIDGE BUILDS OVER THE PACIFIC NORTHWEST ON SUNDAY THEN MOVES
INTO THE NORTHERN ROCKIES SUNDAY NIGHT BEFORE SHORTWAVE ENERGY
FLATTENS THE RIDGE MONDAY THROUGH TUESDAY. THIS WILL KEEP THE PLAINS
GENERALLY DRY WITH BREEZY WEST WINDS WHILE HIGHER LEVEL CLOUDS
STREAMING ACROSS THE REGION COULD BRING ISOLATED SHOWERS OVER THE
CONTINENTAL DIVIDE AT TIMES. A STRONGER SHORTWAVE COULD APPROACH
WESTERN MONTANA BY LATE TUESDAY NIGHT...BUT MODELS ARE SHOWING
DISAGREEMENTS IN TIMING AND STRENGTH OF THIS FEATURE. FOR NOW WILL
CONTINUE WITH POPS THAT ARE NEAR OR A LITTLE ABOVE CLIMATOLOGY FOR
TUESDAY NIGHT. TEMPERATURES ARE EXPECTED TO REMAIN ABOVE SEASONAL
AVERAGES THROUGH THE ENTIRE MEDIUM RANGE PERIOD. MPJ
WEDNESDAY THROUGH THURSDAY...MODEL DIFFERENCES DURING THIS PERIOD
DETRACT FROM FORECAST CONFIDENCE. A COMPROMISE WAS USED LEANING
TOWARD THE ECMWF SOLUTION...BECAUSE OF BETTER RUN TO RUN
CONSISTENCY. AN UPPER LEVEL RIDGE OVER WESTERN MONTANA IS FLATTENED
AND NUDGED EAST AT TIMES AS WEAK SHORTWAVE DISTURBANCES PASS OVER.
MOSTLY CLOUDY SKIES AND SCATTERED RAIN OR SNOW SHOWERS ARE EXPECTED
OVER THE WESTERN AND SOUTHWESTERN MOUNTAINS...WITH ONLY A FEW
SHOWERS OVER THE SOUTHWEST VALLEYS. DOWNSLOPE WINDS SHOULD KEEP THE
PLAINS AND LARGER VALLEYS MOSTLY DRY. MILD AIR WITH THESE SYSTEMS
SHOULD KEEP TEMPERATURES ABOVE SEASONAL AVERAGES. EK
&&
.AVIATION...
VFR CONDITIONS WILL PREVAIL ACROSS NORTH CENTRAL AND SOUTHWEST
MONTANA TODAY AS A HIGH PRESSURE RIDGE REMAINS OVER THE FORECAST
AREA. SCATTERED TO BROKEN HIGH LEVEL CLOUDS OVER WESTERN PORTIONS
OF THE REGION WILL GRADUALLY EXPAND EAST AS IT THICKENS AND LOWERS
THROUGH THE DAY. THE RIDGE WILL BE PUSHED EAST THIS EVENING AS A
SHORTWAVE TROUGH PUSHES INTO WESTERN MONTANA AROUND 00Z BRINGING
INCREASING LOW AND MID LEVEL CLOUDS ALONG WITH SCATTERED TO
NUMEROUS SNOW SHOWERS BY 06Z. MVFR OR LOWER CONDITIONS ARE LIKELY
AT KHLN AND KBZN AS LIGHT SNOW DEVELOPS AFTER 06Z...AND THERE
WILL BE A CHANCE FOR MVFR OR LOWER CONDITIONS AT KGTF AND KLWT AS
SNOW SHOWERS MOVE THROUGH SOUTHERN PORTIONS OF CENTRAL MONTANA. MPJ
&&
.PRELIMINARY POINT TEMPS/POPS...
GTF 38 29 45 32 / 0 60 20 0
CTB 35 26 44 28 / 0 50 10 10
HLN 45 29 46 30 / 0 60 10 10
BZN 42 29 46 28 / 10 60 10 10
WEY 42 23 32 19 / 10 70 30 10
DLN 45 28 41 29 / 0 70 10 10
HVR 30 21 44 23 / 0 50 20 10
LWT 31 25 42 25 / 0 70 30 10
&&
.TFX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SHORT TERM...WILLIAMSON DC
LONG TERM...MPJ/EK
AVIATION...MPJ
WEATHER.GOV/GREATFALLS
