FXUS65 KTFX 041753
AFDTFX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1052 AM MST FRI DEC 4 2009
...UPDATE TO AVIATION...
.UPDATE...
INCREASED WINDS OVER THE ROCKY MOUNTAIN FRONT AND ADJACENT PLAINS
THIS MORNING AS PRESSURE GRADIENT TIGHTENS OVER THE AREA.
REMAINDER OF THE FORECAST SEEMS TO BE ON TRACK AS A SIGNIFICANT
STORM SYSTEM APPROACHES. MLS
&&
.AVIATION...
UPDATED 1752Z
COLD FRONT WILL MOVE SOUTH OUT OF CANADA ACROSS OUR AREA BRINGING
SNOW FALL AND STRONG WINDS ACROSS MUCH OF THE HIGH PLAINS LATE TODAY
AND TONIGHT. WHILE VFR CONDITIONS ARE CURRENTLY PERSISTING...MVFR
AND IFR CONDITIONS DUE TO CEILINGS AND OR VISIBILITY WILL BE
POSSIBLE BY THE END OF THE TAF PERIOD AS THE PREDOMINANT WEATHER
CATEGORY FOR ALL SITES. MOST OF THE STRONG SNOWFALL SHOULD OCCUR
BETWEEN 06Z AND 18Z ACROSS THE AREA. SUK
&&
.PREV DISCUSSION...
/ISSUED 534 AM MST FRI DEC 4 2009/
TODAY THROUGH SUNDAY EVENING...CURRENT SATELLITE IMAGERY
SHOWS NARROW BAND OF UPPER LEVEL MOISTURE OVER WESTERN MT MAKING
STEADY EASTWARD PROGRESS. THIS BAND WILL MOVE ACROSS THE STATE
TODAY BUT HAVE LITTLE IMPACT OTHER THAN PRODUCING SOME HIGH CLOUD
COVER. CLOSE BEHIND THE BAND IS A SHARP AND VERY COLD UPPER LEVEL
TROF THAT IS DIGGING ACROSS SW CANADA. THIS SYSTEM WILL BRING AN
ABRUPT CHANGE TOWARD MID-WINTER CONDITIONS FOR THIS WEEKEND ACROSS
NORTH CENTRAL AND CENTRAL MT. A SURFACE COLD FRONT FROM THE SYSTEM
IS EXPECTED TO DROP SOUTH FROM ALBERTA...MOVING EAST OF THE DIVIDE
THIS AFTN. THE FRONT WILL USHER IN A SHARPLY COLDER CANADIAN
AIRMASS...PRIMING THE AREA FOR SIGNIFICANT SNOWFALL WHEN A BROAD
SWATH OF PACIFIC MOISTURE ARRIVES EARLY SATURDAY MORNING AND SPREADS
SOUTH DURING THE DAY. THOUGH THE MOISTURE WILL COVER A BROAD
AREA...QPF VALUES ARE FAIRLY LIGHT. BUT THE VERY COLD LOW-LEVEL
TEMPERATURES WILL PRODUCE SNOW RATIOS OF AROUND 20-25 INCHES PER
INCH OF LIQUID WATER...GENERATING SNOWFALLS OF 6 TO 10 INCHES OVER
THE PLAINS AND 12 TO 20 INCHES IN THE MOUNTAINS BETWEEN THIS EVENING
AND MIDDAY SUNDAY. THUS HAVE UPGRADED THE EARLIER WINTER STORM
WATCH TO A WARNING FOR ALL CENTRAL AND NORTH CENTRAL COUNTIES.
SNOWFALL ACROSS SOUTHWEST MT WILL BE LIGHTER AND NOT EXPECTED TO
REACH WARNING CRITERIA AT THIS TIME. STRONG NORTHERLY WINDS WILL
ACCOMPANY THE SNOW...PRODUCING AREAS OF BLOWING SNOW...LOW
VISIBILITY...AND WIND CHILLS OF ZERO TO MINUS 20 DEG SAT NIGHT AND
SUNDAY.
FORECAST MODELS ARE IN GENERALLY GOOD AGREEMENT ON SLOWLY SWINGING
THE UPPER TROF SOUTH AND WESTWARD INTO THE GREAT BASIN ON SAT/SUN.
THE ACCOMPANYING MOISTURE BEGINS TO THIN OUT SUN AFTN BUT OCCASIONAL
LIGHT SNOW WILL OCCUR INTO SUN EVE. TEMPERATURES WILL STEADILY
DECREASE THRU THE WEEKEND WITH HIGHS ON SAT IN THE TEENS AND ONLY
REACHING THE SINGLE DIGITS ON SUNDAY. WARANAUSKAS
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
&&
.PRELIMINARY POINT TEMPS/POPS...
GTF 31 13 17 -1 / 0 90 100 90
CTB 33 12 15 0 / 30 100 100 90
HLN 29 15 18 7 / 0 90 100 90
BZN 20 6 19 -4 / 0 40 90 80
WEY 12 -7 11 -14 / 0 40 80 70
DLN 26 6 15 -3 / 0 40 60 60
HVR 32 13 21 9 / 0 90 90 90
LWT 31 11 15 -1 / 0 70 100 90
&&
.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 FROM 5 PM THIS AFTERNOON TO 11 AM MST
SUNDAY FOR MTZ009>014-044>051-054.
&&
$$
WEATHER.GOV/GREATFALLS
