FXUS65 KTFX 050020
AFDTFX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
515 PM MST FRI DEC 4 2009
...UPDATE TO AVIATION...
.DISCUSSION...
TONIGHT THROUGH SUNDAY...LATEST SATELLITE IMAGERY IS INDICATING
CONVECTIVE SNOW SHOWERS DEVELOPING AHEAD OF THE MAIN COLD FRONT
ALONG ROCKY MOUNTAIN FRONT AND ADJACENT PLAINS. AS OF 3 PM FRIDAY
AFTERNOON...COLD FRONT IS JUST NORTH OF THE US/CANADIAN BORDER AND
WILL CONTINUE TO PROGRESS SOUTHWARD THIS EVENING. STRONG UPPER-
LEVEL TROUGH AND ASSOCIATED COLD FRONT WITH PACIFIC MOISTURE WILL
BRING WIDESPREAD SNOW...BLOWING SNOW AND SHARPLY COLDER
TEMPERATURES TO THE REGION BEGINNING TONIGHT AND CONTINUING
THROUGHOUT THE WEEKEND. EVEN THOUGH QPF VALUES ARE ON THE LIGHTER
SIDE...LIQUID TO SNOW RATIOS OF 20-25 TO 1 WILL ALLOW FOR HIGHER
SNOW AMOUNTS. SUFFICIENT DYNAMICS WITH STRONG LIFT IN THE
DENDRITIC GROWTH ZONE AND AN UPPER-LEVEL JET STREAK WILL ALSO
ALLOW FOR WIDESPREAD SNOW. THUS...HAVE CONTINUED WINTER STORM
WARNING FOR ALL OF NORTH-CENTRAL AND PORTIONS OF CENTRAL MONTANA
WITH SNOWFALL TOTALS OF 6 TO 10 INCHES POSSIBLE IN THE PLAINS AND
12 TO 20 INCHES OF SNOW IN THE MOUNTAINS. THE HEAVIEST SNOW ACROSS
THE NORTHERN PLAINS WILL BE OVERNIGHT AND THEN OVER CENTRAL
MONTANA THE HEAVIEST SNOWFALL IS EXPECTED BETWEEN MIDNIGHT AND
NOON ON SATURDAY. STRONG AND GUSTY NORTHERLY WINDS OF 20 TO 30 MPH
WILL ALSO CAUSE AREAS OF BLOWING SNOW WHICH WILL REDUCE
VISIBILITIES TO LESS THAN ONE QUARTER OF A MILE AT TIMES. IN
ADDITION...COMBINATION OF STRONG WINDS AND VERY COLD TEMPERATURES
WILL YIELD WIND CHILL VALUES AS LOW AS 25 BELOW ZERO AT TIMES.
OVER SOUTHWEST MONTANA...ADVISORY CRITERIA SNOWFALL IS EXPECTED IN
THE VALLEYS WITH HIGHER SNOW AMOUNTS IN THE MTNS AND WILL CLOSELY
MONITOR THIS AREA AS THE FRONT MOVES FURTHER SOUTH AND MAKE ANY
CHANGES NECESSARY.
MODELS SEEM TO BE IN FAIR AGREEMENT FOR SATURDAY NIGHT AND SUNDAY
AS UPPER-LEVEL TROUGH CONTINUES TO MOVE SOUTH. UPSLOPE FLOW OVER
NORTH-CENTRAL AND CENTRAL MONTANA WILL ALLOW FOR SNOW TO CONTINUE
DURING THIS TIME FRAME...HOWEVER SNOW WILL BE LESS INTENSE.
TEMPERATURES WILL DECREASE THROUGHOUT THE DAY ON SATURDAY AND
REMAIN VERY COLD WITH HIGH TEMPS ONLY IN THE SINGLE DIGITS ABOVE
ZERO ON SUNDAY. MLS
SUNDAY NIGHT THROUGH WEDNESDAY NIGHT...MEDIUM RANGE MODELS ARE IN
FAIR AGREEMENT THROUGH MUCH OF NEXT WEEK. PERIOD BEGINS WITH A DEEP
UPPER LEVEL TROUGH AND COLD SURFACE HIGH PRESSURE ENTRENCHED OVER
THE REGION WITH GRADUAL MODIFICATION OF THIS COLD PATTERN THROUGH
THE WEEK UNDER A PERSISTENT NORTH TO NORTHWEST FLOW ALOFT. THE
COLDEST TEMPS OF THE SEASON THUS FAR ARE EXPECTED TO OCCUR SUNDAY
NIGHT THROUGH TUESDAY WITH COLD ARCTIC HIGH PRESSURE IN PLACE OVER
THE REGION. CLOUD COVER SUNDAY NIGHT WILL LIKELY BE THE ONLY
LIMITING FACTOR FOR TEMPS WITH WIDESPREAD OVERNIGHT LOWS IN THE MID
TEENS TO NEAR 20 BELOW ZERO. MONDAY LOOKS TO HAVE THE COLDEST
DAYTIME TEMPS WITH MOST AREAS REMAINING BELOW ZERO THROUGH THE DAY.
MONDAY NIGHT WILL LIKELY BE THE COLDEST WITH THE POTENTIAL FOR AT
LEAST SOME BREAKS IN THE CLOUD-COVER BRINGING ADDITIONAL RADIATIVE
COOLING TO THE ALREADY COLD AIRMASS. BEST POTENTIAL FOR PARTIAL
CLEARING AND THEREFORE COLDEST TEMPS IS ACROSS SW MT WHERE
WIDESPREAD MIN TEMPS AT OR BELOW 20 BELOW ZERO ARE EXPECTED MONDAY
NIGHT. TEMPS REMAIN COLD ON TUESDAY WITH ONLY SLIGHT MODERATION OF
THE AIRMASS ALLOWING FOR HIGH TEMPS TO REACH ABOVE ZERO. THE COLD
SURFACE HIGH BEGINS TO SHIFT EAST TUESDAY NIGHT WITH WESTERLY WINDS
FIRST DEVELOPING ALONG THE EAST SLOPES OF THE ROCKIES AND GRADUALLY
SPREADING EAST ACROSS THE PLAINS WEDNESDAY AND WEDNESDAY NIGHT
BRINGING GRADUAL WARMING. FLURRIES OR LIGHT SNOW SHOWERS WILL LIKELY
PERSIST ACROSS MUCH OF THE REGION SUNDAY NIGHT AND EVEN INTO MONDAY
AS WEAK DISTURBANCES ROTATE THROUGH THE UPPER LEVEL TROUGH AND MOIST
LOW LEVEL UPSLOPE FLOW CONTINUES. ANOTHER DISTURBANCE MOVING THROUGH
NW FLOW ALOFT COULD BRING SOME ADDITIONAL SNOW SHOWER ACTIVITY ON
WEDNESDAY. HOENISCH
THURSDAY AND FRIDAY...GFS CONTINUES TO BRING A COLD AND RATHER
MOIST NORTHWEST FLOW ALOFT. THIS IS IN LINE WITH THE ENSEMBLES.
THUS...WILL LEAN TOWARD THIS MODEL. THE ECMWF SHOW A COLD POLAR
VORTEX LOW OVER WESTERN CANADA WITH A MOIST WESTERLY FLOW ALOFT.
OVERALL...WILL GO WITH COOLER THAN SEASONAL AVERAGE TEMPERATURES.
THE PLAINS WILL REMAIN MOSTLY DRY WITH ISOLATED TO SCATTERED SNOW
SHOWERS OVER THE MOUNTAINS THROUGH THE PERIOD. WILLIAMSON DC
&&
.AVIATION...
UPDATED 0015Z
THE CANADIAN COLD FRONT WILL CONTINUE TO MOVE SOUTH THROUGH NORTH
CENTRAL (KCTB KHVR)...CENTRAL (KGTF KLWT)...AND SOUTHWEST (KHLN
KBZN) MONTANA THROUGH 12Z. SNOW...HEAVY AT TIMES...WITH STRONG AND
GUSTY NORTHERLY WINDS WILL SPREAD SOUTH OVER THE AREA BEHIND THE
FRONT. WIDESPREAD MVFR CONDITIONS WILL DETERIORATE TO IFR CONDITIONS
FROM THE NORTH BY 08Z WITH AREAS OF BLOWING SNOW. IN FACT...SNOW HAS
ALREADY BEGUN AT THE KCTB TERMINAL AS OF 00Z. THE HEAVIEST SNOWFALL
IS EXPECTED ACROSS NORTH CENTRAL AND CENTRAL MONTANA BETWEEN 06Z AND
18Z. COULSTON
&&
.PRELIMINARY POINT TEMPS/POPS...
GTF 13 17 -2 4 / 90 100 100 60
CTB 12 15 -1 2 / 100 100 90 40
HLN 15 18 3 9 / 90 100 90 50
BZN 6 19 -4 3 / 40 90 80 60
WEY -7 11 -14 6 / 40 70 60 40
DLN 6 15 -3 4 / 40 60 60 30
HVR 13 21 3 12 / 90 100 90 40
LWT 11 15 -2 4 / 70 100 100 60
&&
.TFX WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM 5 AM SATURDAY TO 11 AM MST SUNDAY
FOR MTZ008-015-052-053-055.
WINTER STORM WARNING UNTIL 11 AM MST SUNDAY
FOR MTZ009>014-044>051-054.
&&
$$
WEATHER.GOV/GREATFALLS
