000 FXUS65 KTFX 230410 AFDTFX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GREAT FALLS MT 910 PM MST TUE DEC 22 2009 .EVENING UPDATE... BIG SKY COUNTRY SITS BETWEEN TWO UPPER LEVEL LOWS...ONE WELL TO THE SOUTH NEAR LAS VEGAS AND ANOTHER DROPPING SOUTHEAST OUT OF CENTRAL ALBERTA. FORECAST LOOKS TO BE ON TRACK AT THIS TIME...ONLY CONCERN IS THE TIMING OR ONSET OF SNOW ACROSS THE NORTHWESTERN PORTIONS OF THE FORECAST AREA NEAR THE ROCKY MOUNTAIN FRONT...BUT SOME LIGHT SNOW IS STARTING TO DEVELOP NEAR AND AROUND CUT BANK. COSMETIC TWEAKS TO TEMPS...WINDS..AND CLOUD COVER TO ADD CURRENT INFO...BUT OVERALL FORECAST IN GOOD SHAPE. UPPER LEVEL JET MAX PUSHING SOUTH INTO WESTERN MONTANA WITH AREA IN PREFERRED LEFT EXIT REGION WITH DECENT UPPER LEVEL DIVERGENCE SEEN IN THE H5 TO H3 LAYER...ALLOWING FOR ENOUGH FORCING TO PRODUCE LIGHT SNOWFALL THROUGH THE NIGHT AS THE UPPER LEVEL TROUGH DRIVES ACROSS THE TREASURE STATE. ADVISORY REMAINS IN EFFECT THROUGH WEDNESDAY MORNING. --SCHOTT && .PREV DISCUSSION... /ISSUED 447 PM MST TUE DEC 22 2009/ TONIGHT THROUGH WEDNESDAY NIGHT...A PROGRESSIVE WEATHER SYSTEM WILL MOVE INTO THE NORTHWEST ZONES LATE TONIGHT THEN MOVE EAST THROUGH THE REMAINDER OF THE AREA WEDNESDAY MORNING AND EARLY AFTERNOON. MODELS ARE FASTER WITH THIS SYSTEM THAN THEY WERE ON MONDAY. IN LINE WITH THIS HAVE LOWERED SNOW ACCUMULATIONS OVER THE PLAINS FROM 2 TO 5 INCHES TO 2 TO 3 INCHES. THE ARCTIC SURGE ASSOCIATED WITH THE WEATHER SYSTEM WILL MOVE INTO THE NORTHERN ZONES THIS EVENING THEN SPREAD SOUTH OVER ALL BUT FAR SOUTHWEST MONTANA BY WEDNESDAY MORNING. WINDS ASSOCIATED WITH THIS SURGE IN COMBINATION WITH VERY COLD TEMPERATURES WILL CAUSE WIND CHILLS OF 10 BELOW ZERO TO 20 BELOW ZERO ON WEDNESDAY. EVEN THOUGH EXPECTED SNOW ACCUMULATIONS ARE LESS THAN WAS EARLIER THOUGHT...THE COMBINATION OF THE SNOW AND VERY COLD WIND CHILL TEMPERATURES WILL STILL NECESSITATE THE CONTINUATION OF THE WINTER WEATHER ADVISORY. SINCE THE SYSTEM IS EXPECTED TO BE FASTER WILL DROP THE WINTER WEATHER ADVISORY EXCEPT IN THE FAR EAST FOR WEDNESDAY AFTERNOON. BLANK CHRISTMAS EVE THROUGH TUESDAY...AN UPPER-LEVEL RIDGE OF HIGH PRESSURE BEGINS TO BUILD OVER THE PACIFIC NORTHWEST ON CHRISTMAS DAY WITH DRY NORTHERLY FLOW AND MODERATING TEMPERATURES. OVERALL...A QUIETER PATTERN IS ANTICIPATED FOR THE FORECAST PERIOD WITH THE STRONG UPPER RIDGE PERSISTING OVER THE WESTERN U.S. THROUGH THE HOLIDAY WEEKEND. THOUGH TEMPERATURES WILL TREND TOWARDS AVERAGE...A FRESH SNOWPACK MAY STILL KEEP READINGS AROUND TO SLIGHTLY BELOW SEASONAL AVERAGES. RIDGE AXIS WILL MOVE EAST BY SUNDAY WITH WESTERLY FLOW DEVELOPING. DRY CONDITIONS ARE EXPECTED TO CONTINUE THROUGH MONDAY...WITH INCREASING PACIFIC MOISTURE BY MONDAY NIGHT INTO TUESDAY AS OUR NEXT SHORTWAVE APPROACHES. MLS && .AVIATION... UPDATED 2347Z. LOW CLOUDS AND PATCHY AREAS OF DENSE FREEZING FOG WILL CONTINUE TO IMPACT TAF SITES ACROSS NORTH CENTRAL AND CENTRAL MONTANA. SNOW SHOWERS SHOULD BEGIN TO DEVELOP THIS EVENING ACROSS THE AREA. LIGHT SNOW SHOWERS CONTINUING INTO THE OVERNIGHT PERIOD SHOULD KEEP VIS AND CIGS DOWN IN THE IFR CATEGORY FOR MUCH OF THE TIME. THERE IS THE POTENTIAL FOR SOME PLOWING AND DRIFTING SNOW...AND VISIBILITY COULD DROP TO LESS THAN ONE MILE AT TIMES...HOWEVER THIS IS NOT ANTICIPATED TO BE A PREVAILING WEATHER CONDITION AT ANY SITE FOR AN EXTENDED PERIOD OF TIME. SUK && .PRELIMINARY POINT TEMPS/POPS... GTF 6 8 -6 17 / 80 80 10 0 CTB 0 4 -9 16 / 90 30 0 0 HLN 15 16 -5 16 / 60 50 10 0 BZN 17 18 -7 15 / 50 50 10 10 WEY 10 19 -13 12 / 60 80 20 10 DLN 13 23 -5 14 / 40 40 10 0 HVR 4 5 -16 3 / 90 80 10 0 LWT 8 9 -7 9 / 80 80 10 10 && .TFX WATCHES/WARNINGS/ADVISORIES... WINTER WEATHER ADVISORY UNTIL 11 AM MST WEDNESDAY FOR CASCADE... CHOUTEAU...EASTERN TETON...HILL...JUDITH BASIN...LIBERTY... MEAGHER...SOUTHERN ROCKY MOUNTAIN FRONT. WINTER WEATHER ADVISORY UNTIL 5 PM MST WEDNESDAY FOR BLAINE... FERGUS. WINTER WEATHER ADVISORY UNTIL 11 AM MST WEDNESDAY FOR EASTERN GLACIER...EASTERN PONDERA...NORTHERN ROCKY MOUNTAIN FRONT... TOOLE. && $$ SHORT TERM...BLANK LONG TERM...GUSTAFSON AVIATION...SUK UPDATE...SCHOTT WEATHER.GOV/GREATFALLS