FXUS65 KTFX 051135
AFDTFX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
435 AM MST SAT DEC 5 2009
UPDATED AVIATION DISCUSSION
.DISCUSSION...
TODAY THROUGH MONDAY EVENING...LATEST ANALYSIS SHOWS THE
DEEP UPPER LEVEL TROF BRINGING THE SNOW THIS WEEKEND IS ORIENTED
EAST TO WEST FROM THE DAKOTAS...ACROSS SOUTHERN MONTANA...TO CENTRAL
OREGON. MOST OF THE TROF'S MOVEMENT OVERNIGHT HAS BEEN ALONG ITS
FAR WESTERN BOUNDARY...WHICH HAS ACTED TO SLOW THE SOUTHERN PUSH OF
THE TROF THRU MONTANA AND KEPT MOST OF THE SYSTEM'S MAIN MOISTURE
PLUME ALONG AND NORTH OF THE US/CAN BORDER. BUT CURRENT WATER VAPOR
IMAGERY SHOWS THE MOISTURE BEGINNING TO MAKE A MORE DEFINITIVE PUSH
SOUTHWARD...WITH RADAR CONFIRMING THAT LIGHT AND MODERATE SNOW IS
BECOMING MORE WIDESPREAD ACROSS CENTRAL AND NORTH CENTRAL MONTANA.
MODELS ARE IN GOOD AGREEMENT THAT THIS TREND WILL CONTINUE
TODAY...WITH STEADY ACCUMULATIONS EXPECTED THROUGH THE DAYTIME
HOURS. FORECAST LIQUID WATER VALUES REMAIN LIGHT...GENERALLY IN THE
0.3 TO 0.6 INCH RANGE OVER THE NORTHERN TWO-THIRDS OF THE CWA. BUT
THE ACCOMPANYING ARCTIC AIRMASS WILL PRODUCE HIGH SNOW RATIOS...
COMBINED WITH STRONG GUSTY WINDS THAT WILL CREATE BROAD AREAS OF LOW
VISIBILITIES DUE TO BLOWING SNOW...SO THE WINTER STORM WARNING
REMAINS IN EFFECT. MEANWHILE...THE LEADING EDGE OF THE SNOW HAS
BEGUN TO REACH SOUTHWEST MONTANA...WITH SNOW BECOMING MORE
WIDESPREAD THERE TODAY AS WELL. HOWEVER QPF VALUES THERE ARE
GENERALLY A QUARTER-INCH OR LESS OVER THE NEXT 24 HRS...SO ONLY
ADVISORY LEVEL SNOWFALLS ARE STILL ANTICIPATED.
SNOW WILL CONTINUE TONIGHT IN ALL AREAS AS ADDITIONAL MOISTURE
STREAMS IN FROM CANADA BUT WILL GRADUALLY TAPER OFF TO PATCHY
SNOWSHOWERS BY MIDDAY SUNDAY. THE FOCUS THEN SHIFTS TO THE SHARPLY
PLUNGING TEMPERATURES AS THE ARCTIC AIRMASS BEHIND THE UPPER TROF
OVERSPREADS THE ENTIRE CWA. HIGHS ON SUNDAY WILL ONLY REACH THE
SINGLE DIGITS WITH OVERNIGHT LOWS OF 10 TO 20 BELOW ZERO. MOST
LOCATIONS WILL HAVE HIGH TEMPS ON MONDAY THAT WILL NOT MAKE IT ABOVE
ZERO. ACCOMPANYING WIND CHILLS ON MONDAY WILL BE IN THE 25 TO 40
BELOW ZERO RANGE. WARANAUSKAS
MONDAY NIGHT THROUGH SATURDAY...MEDIUM RANGE MODELS REMAIN IN FAIR
AGREEMENT THROUGH MOST OF THIS WEEK BEFORE DIVERGING SOMEWHAT BY
NEXT WEEKEND. PERIOD BEGINS WITH A COLD ARCTIC AIRMASS IN PLACE OVER
THE REGION MONDAY NIGHT INTO TUESDAY. EXPECT WIDESPREAD TEMPS IN THE
TEENS TO TWENTIES BELOW ZERO MONDAY NIGHT AS SOME CLEARING WORKS IN
FROM THE WEST. SURFACE HIGH REMAINS CENTERED OVER NORTH CENTRAL MT
ON TUESDAY WITH MAX TEMPS REMAINING BELOW ZERO ACROSS MOST OF THE
AREA. SURFACE HIGH BEGINS TO SHIFT EAST TUESDAY NIGHT AND WEDNESDAY
WITH PRESSURE GRADIENTS BEGINNING TO SUPPORT SOME WESTERLY DOWNSLOPE
WINDS ALONG THE EAST SLOPES OF THE ROCKIES INITIALLY LATE TUESDAY
NIGHT...SPREADING EAST ACROSS THE NORTH CENTRAL PLAINS THROUGH THE
DAY ON WEDNESDAY. THIS WILL BEGIN A GRADUAL MODERATION TO THE VERY
COLD TEMPS THROUGH THE REMAINDER OF THE WEEK WITH TEMPERATURES
FINALLY RISING TO NEAR AVERAGE LEVELS BY NEXT SATURDAY. THE SW MT
VALLEYS WILL BE VERY SLOW TO WARM DUE TO STRONG INVERSIONS LIKELY
TRAPPING THE COLD AIR. COLD AIR MAY ALSO LINGER A BIT LONGER ACROSS
NE ZONES...ESPECIALLY IN THE MILK RIVER VALLEY. ASIDE FROM A FEW
FLURRIES NO SIGNIFICANT PRECIPITATION IS EXPECTED THROUGH THURSDAY.
DIFFERENCES ARISE IN THE THURSDAY NIGHT THROUGH SATURDAY PERIOD WITH
SOME MEDIUM RANGE MODELS BEGINNING TO INDICATE THE POTENTIAL FOR
ANOTHER ARCTIC AIRMASS TO SLIDE BACK IN FROM THE NORTHEAST AS THE
POLAR VORTEX OVER CENTRAL CANADA ELONGATES. HOENISCH
&&
.AVIATION...
UPDATED 1200Z
A DEEP UPPER LEVEL TROUGH WILL REMAIN OVER THE REGION THROUGH THE
NEXT 24 HRS MAINTAINING A MOIST NORTHERLY FLOW ALOFT. LOW PRESSURE
OVER EASTERN MT AND THE DAKOTAS WILL TRACK OFF TO THE SE WHILE COLD
HIGH PRESSURE BUILDS SOUTH INTO THE REGION FROM THE NW. WIDESPREAD
SNOW ALREADY OVER MUCH OF NORTH CENTRAL MT WILL SPREAD SOUTH THROUGH
SW MT THIS MORNING WITH SNOW HEAVY AT TIMES OVER NORTH CENTRAL MT
TERMINALS. GUSTY N TO NW WINDS WILL ALSO CREATE ADDITIONAL
VISIBILITY RESTRICTIONS IN BLOWING AND DRIFTING SNOW AT TIMES
THROUGH MUCH OF THIS MORNING. WIDESPREAD IFR CONDITIONS WITH
OCCASIONAL LIFR WILL PERSIST THROUGH MUCH OF THIS MORNING WITH
SLIGHT IMPROVEMENT THIS AFTERNOON AS SNOW AND WINDS GRADUALLY
DIMINISH IN INTENSITY. PRECIPITATION WILL FURTHER DIMINISH IN
INTENSITY TO VERY LIGHT SNOW OR FLURRIES LATER TONIGHT THROUGH
SUNDAY...HOWEVER CEILINGS WILL LIKELY ONLY IMPROVE TO MVFR.
HOENISCH
&&
.PRELIMINARY POINT TEMPS/POPS...
GTF 17 -2 4 -20 / 100 100 70 30
CTB 15 -1 2 -23 / 100 100 40 30
HLN 18 3 9 -15 / 100 90 50 20
BZN 19 -4 3 -16 / 90 80 60 20
WEY 11 -14 6 -20 / 70 60 40 30
DLN 15 -3 4 -17 / 60 60 30 20
HVR 21 3 12 -18 / 100 100 50 30
LWT 15 -2 4 -19 / 100 100 60 30
&&
.TFX WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 11 AM MST SUNDAY FOR MTZ008-015-
052-053-055.
WINTER STORM WARNING UNTIL 11 AM MST SUNDAY FOR MTZ009>014-
044>051-054.
&&
$$
SHORT TERM...WARANAUSKAS
LONG TERM...HOENISCH
AVIATION...HOENISCH
WEATHER.GOV/GREATFALLS
