378 FXUS65 KTFX 270314 AFDTFX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GREAT FALLS MT 914 PM MDT THU APR 26 2012 .UPDATE... EXTREME SHORT RANGE MODELS INCLUDING THE HRRR AND RUC HAVE MOVED INTO TIGHT AGREEMENT WITH LATEST NAM AND GFS RUNS SHOWING A SIGNIFICANTLY HIGHER AMOUNT OF MOISTURE OVER THE MOUNTAINS OF SOUTHWEST MONTANA TONIGHT. DUE TO THE SIGNIFICANTLY SHORT TIME FRAME HAVE MADE THE NONTRADITIONAL DECISION TO UPGRADE THE WINTER STORM WATCH TO A WARNING THIS EVENING WITH THIS NEW DATA. DUE TO THE INCREASED CONFIDENCE FOR THE POTENTIAL OF SIGNIFICANT IMPACTS BEFORE THE NEXT STANDARD FORECAST SUITE UPDATE. THE WARNING IS IN EFFECT FOR ELEVATIONS ABOVE 6500 FEET...WITH NO ADVISORY BELOW 6500 FEET DUE TO THE WARMER TEMPERATURES EXPECTED IN THE VALLEY LOCATIONS OF THE SOUTHWEST TONIGHT. THESE ARE THE EXPECTED CONDITIONS AT THIS TIME...HOWEVER THERE IS A SLIGHT POSSIBILITY THAT SNOW ACCUMULATIONS COULD MAKE IT DOWN INTO THE LOWER ELEVATIONS AND ADDITIONAL HIGHLIGHTS MAY BECOME NECESSARY. WITH THIS UPDATE...THE IMPACTS FOR LOCATIONS OUTSIDE OF SOUTHWEST MONTANA LOOK LESS IMPRESSIVE AND DECIDED TO DELAY THE WINTER WEATHER WATCH UNTIL 1800Z FRIDAY. SUK && .AVIATION... UPDATED 2355Z. A TROUGH OF LOW PRESSURE AND COLD FRONT WILL BRING ACTIVE WEATHER TO THE AREA THROUGH THE TAF PERIOD. AN UNSTABLE AIRMASS OVER THE REGION WILL RESULT IN SCATTERED SHOWERS AND THUNDERSTORMS...SOME WITH GUSTY WINDS...THROUGH THE EARLY EVENING HOURS. A COLD FRONT WILL BRING A LINE OF SHOWERS AND THUNDERSTORMS AND WIND SHIFT THROUGH THE AREA FROM WEST TO EAST...AFFECTING KHLN...KCTB...AND KGTF AFTER 01Z AND KBZN... KLWT...AND KHVR CLOSER TO 06Z. WESTERLY WINDS WILL INCREASE ACROSS THE AREA OVERNIGHT AND INTO FRIDAY ALONG WITH AREAS OF PRECIPITATION AND LOWER CEILINGS...RESULTING IN MVFR CONDITIONS AND MOUNTAIN OBSCURATION. LANGLIEB && .PREV DISCUSSION... /ISSUED 555 PM MDT THU APR 26 2012/ TODAY THROUGH SATURDAY EVENING...FORECAST STILL ON TRACK FOR PAIR OF VERY MOIST PACIFIC WEATHER SYSTEMS TO BRING WIDESPREAD AND LONG DURATION PRECIPITATION ALONG WITH MUCH COOLER TEMPERATURES OVER OUR REGION THROUGH THE ENTIRE SHORT-TERM FORECAST AREA. CURRENT SATELLITE IMAGERY SHOWS THE SLIGHTLY WEAKER SYSTEM NOW IN ARIZONA AND BEGINNING TO LIFT NORTHEASTWARD. MEANWHILE...THE MUCH DEEPER UPPER LEVEL TROF IS CLOSING IN ON THE WA/OR COAST AND IS EXPECTED TO MOVE INLAND TONIGHT. ASSOCIATED MOISTURE PLUME WITH THESE SYSTEM COVERS A BROAD AREA OF THE GREAT BASIN AND NRN ROCKIES WITH ADDITIONAL MOISTURE EXPECTED TO ACCOMPANY THE STRONGER TROF. AS OF 230 PM LEADING EDGE OF THE MOISTURE IS LOCATED IN OVER OUR FORECAST AREA WHERE IT HAS BEGUN TO MIX WITH LIFT AND INSTABILITY TO GENERATE ISOLATED SHOWERS AND A FEW THUNDERSTORMS OVER SOUTHWEST AND CENTRAL MT. EXPECT THIS ACTIVITY TO BROADEN IN COVERAGE THIS AFTN/EVE BUT INSTABILITY WILL WEAKEN AFTER SUNSET SO SHOWERS WILL CONTINUE BUT THUNDERSTORMS SHOULD DISSIPATE. BEST NEWS IN THE OUTLOOK IS THAT PRIMARY SHORT-TERM FORECAST MODELS ARE IN MUCH BETTER AGREEMENT ON CONDITIONS FOR TOMORROW AND SATURDAY...CERTAINLY BETTER THAN THE GLARING DIFFERENCES SEEN THE PAST FEW DAYS. THE TWO WEATHER SYSTEMS WILL CONTINUE TO TRACK TOWARD EACH OTHER AND EVENTUALLY MERGE OVER NORTHERN WYOMING LATE FRI...THEN GRADUALLY LIFT INTO FAR SOUTHEAST MT ON SAT. AS IT DOES SO...ADDITIONAL MOISTURE WILL BE PULLED FROM THE DAKOTAS OVER THE NRN FLANK OF THE CLOSED LOW PRESSURE AREA...KEEPING A NEAR CONTINUOUS PERIOD OF RAIN...RAIN/SNOW MIX...AND SNOW GOING THROUGH SATURDAY. MODELS STILL SHOW SOME SLIGHT VARIATIONS IN TOTAL PRECIPITATION AND AMOUNTS BUT GENERALLY EXPECT ONE-HALF TO THREE-QUARTERS OF AN INCH OVER MUCH OF THE REGION. SNOW WILL DEVELOP OVER THE SOUTHWEST MTNS TONIGHT AND GRADUALLY SPREAD INTO THE HIGHER ELEVATIONS OF CENTRAL MT FRI PM-SAT AM. SNOW LEVELS WILL GRADUALLY DROP THROUGH SATURDAY SO A FEW INCHES OF WET SLUSH ARE POSSIBLE AT ELEVATIONS AROUND 3000 FT WITH SIGNIFICANTLY HIGHER SNOW AMOUNTS FOR THE CENTRAL MTN RANGES. THUS CURRENT WINTER WEATHER WATCH FOR ALL BUT OUR NORTHEAST ZONES REMAINS IN EFFECT AND BEGINS AT MIDNIGHT TONIGHT. GIVEN THE EXPECTED PRECIP AMOUNTS...RIVERS AND STREAMS WILL CONTINUE TO RISE. A FEW CREEKS IN GLACIER COUNTY ARE ALREADY NEAR BANKFULL...SO HAVE ISSUED A SMALL STREAM FLOOD ADVISORY THERE. FURTHER DETAILS ARE IN THE HYDROLOGY SECTION BELOW. WARANAUSKAS SATURDAY NIGHT THROUGH THURSDAY...MODERATE AGREEMENT IN MODEL GUIDANCE REGARDING THE TIMING AND LOCATION OF THE UPPER LEVEL LOW AS IT MOVES OUT OF THE REGION. CURRENT FORECAST FAVORS THE GFS SOLUTION AND KEEPS RESIDUAL MOISTURE AROUND AS THE GFS IS SLOWER TO PUSH THE SYSTEM NORTHEASTWARD. PRECIPITATION SHOULD BE LIGHT AND REMAIN AS RAIN...POSSIBLY A RAIN SNOW MIX...IN THE LOWER ELEVATIONS WITH SNOW IN THE MOUNTAINS. A MORE ZONAL FLOW DEVELOPS EARLY NEXT WEEK AND WILL PERSIST THROUGH WEDNESDAY. A SHORTWAVE WILL MOVE OVER THE AREA MONDAY EVENING BRINGING A CHANCE OF PACIFIC MOISTURE...MAINLY TO THE ROCKY MOUNTAIN FRONT AND SW MONTANA...AND THEN AGAIN ON TUESDAY EVENING AFFECTING THE SAME AREAS. BY WEDNESDAY AFTERNOON EXPECT CLEARING AND THE FLOW TO BECOME MORE SOUTHWESTERLY. TEMPERATURES WILL START OFF A LITTLE COOL ON SUNDAY AND APPROACH SEASONAL AVERAGES THE REMAINDER OF THE PERIOD. MERCER && .PRELIMINARY POINT TEMPS/POPS... GTF 39 50 33 47 / 80 60 70 70 CTB 34 48 31 49 / 90 60 70 70 HLN 36 48 32 49 / 90 50 60 60 BZN 31 46 23 46 / 90 80 60 60 WEY 27 35 18 37 / 100 80 70 60 DLN 30 43 24 46 / 100 70 50 50 HVR 38 51 33 45 / 80 70 80 60 LWT 35 47 30 41 / 90 70 70 70 && .TFX WATCHES/WARNINGS/ADVISORIES... WINTER STORM WATCH FROM FRIDAY AFTERNOON THROUGH SATURDAY AFTERNOON BROADWATER...CASCADE...CENTRAL AND SOUTHERN LEWIS AND CLARK...CHOUTEAU...EASTERN GLACIER...EASTERN PONDERA...EASTERN TETON...FERGUS...JEFFERSON...JUDITH BASIN...MEAGHER...NORTHERN ROCKY MOUNTAIN FRONT...SOUTHERN ROCKY MOUNTAIN FRONT. WINTER STORM WARNING UNTIL NOON MDT FRIDAY FOR ELEVATIONS ABOVE 6500 FEET FOR BEAVERHEAD...GALLATIN...MADISON. SMALL STREAM FLOOD ADVISORY FOR GLACIER COUNTY UNTIL NOON SATURDAY. && $$ SHORT TERM...SUK LONG TERM...MERCER AVIATION...LANGLIEB WEATHER.GOV/GREATFALLS