000 FXUS65 KTFX 250455 AFDTFX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GREAT FALLS MT 1055 PM MDT FRI APR 24 2009 .UPDATE...FAIRLY QUIET EVENING ACROSS NORTH CENTRAL AND SOUTHWEST MONTANA. NORTHWEST FLOW ALOFT CONTINUES ACROSS THE REGION AS ONE SHORTWAVE EXITS THE REGION AND ANOTHER ONE GAINS STRENGTH OVER WESTERN CANADA. MODELS ARE SHOWING THAT WEAK SHORTWAVE RIDGING WILL BUILD OVER MY FORECAST AREA TONIGHT WITH THE AIRMASS REMAINING GENERALLY DRY. ISOLATED TO SCATTERED SNOW SHOWERS THAT DEVELOPED THIS AFTERNOON ARE RAPIDLY DISSIPATING AS THE AIRMASS COOLS AND STABILIZES. HAVE UPDATED THE FORECAST TO LOWER THE CHANCE OF SHOWERS FOR THE REMAINDER OF THE NIGHT AND TO REDUCE CLOUD COVER. TEMPERATURE AND WIND FORECAST FOR THE REMAINDER OF THE NIGHT LOOKS PRETTY GOOD SO WILL NOT MAKE ANY CHANGES TO THESE PARAMETERS. MPJ && .AVIATION... UPDATED 0455Z. CLOUDS WILL DECREASE IN COVERAGE OVERNIGHT AS INSTABILITY WANES AND WEAK UPPER-LEVEL RIDGING OCCURS. CONSIDERED ADDING VICINITY FOG FOR KBZN..KLWT..AND KGTF BUT AS SOUTHWEST WINDS DEVELOP AM ANTICIPATING LITTLE TO NO FOG DEVELOPMENT. AN UPPER-LEVEL TROUGH AND EMBEDDED SHORTWAVE WILL MOVE THROUGH THE REGION ON SATURDAY WITH A COOL..MOIST/UNSTABLE AIRMASS PERSISTING. AS THE SHORTWAVE APPROACHES...INCREASING MID-LEVEL CLOUDS ARE EXPECTED FROM NORTH TO SOUTH BEGINNING SATURDAY MORNING. SCATTERED SNOW SHOWERS ARE POSSIBLE WITH THE SHORTWAVE MAINLY AFTER 18Z AT KCTB AND TOWARDS THE END OF THE FORECAST PERIOD FOR REMAINING TAF SITES. OVERALL...DRY/VFR CONDITIONS ARE EXPECTED THROUGHOUT THE PERIOD...HOWEVER SCATTERED SNOW SHOWERS WILL LEAD TO LOCAL MVFR CONDITIONS AND MOUNTAIN OBSCURATION ON SATURDAY AFTERNOON/EVENING. MLS && .PREV DISCUSSION... /ISSUED 605 PM MDT FRI APR 24 2009/ UPPER TROF CURRENTLY MAKING VERY SLOW PROGRESS ACROSS CENTRAL AND SOUTHWEST MT. POCKET OF DRY AIR ALONG AND SLIGHTLY BEHIND THE AXIS PROVIDED SOME CLEARING EARLIER...BUT AFTERNOON INSTABILITY AND SECONDARY SHOT OF MOISTURE WORKING SOUTHWARD FROM BC/ALB IS NOW PRODUCING SOME LIGHT RAIN/SNOW SHOWERS FROM CUT BANK TO HELENA TO LEWISTOWN. SHOWERS WILL CONTINUE INTO THE EVENING...CHANGING OVER TO ALL SNOW...AS REMAINDER OF MOISTURE TRACKS SOUTHWARD. ONLY LIGHT ACCUMULATIONS EXPECTED AS MOISTURE FIELD IS CONSIDERABLY LESS THAN WHAT PASSED THROUGH YESTERDAY. FORECAST MODELS HAVE MAINTAINED CONSISTENCY ON PUSHING THE UPPER TROF INTO ERN MT AND WY OVERNIGHT...BUT GIVEN CURRENT EVOLUTION...TROF MAY LINGER ACROSS SOUTHWEST ZONES INTO SAT MORNING. REST OF CWA SHOULD SEE GRADUALLY CLEARING SKIES OVERNIGHT WITH TRANSIENT UPPER LEVEL RIDGE OVERHEAD TOMORROW. THIS BREAK WILL BE SHORT-LIVED AS NEXT SHORTWAVE TROF ARRIVES FROM SW CANADA ON SAT NIGHT AND WILL BE OVER CENTRAL AND WESTERN MT ALL DAY ON SUNDAY. ALL ZONES SHOULD SEE PERIODS OF PRECIPITATION WITH MIX OF RAIN/SNOW IN THE PLAINS AND SNOW IN THE HIGHER ELEVATIONS...CHANGING TO ALL SNOW BY LATE AFTN AS COLDER AIRMASS COMES IN WITH THIS SYSTEM. MODEL SNOWFALL TOTALS ARE MOSTLY LIGHT AT THIS TIME...BUT ENHANCED INSTABILITY WITH THIS SYSTEM MAY RESULT IN SOME AREAS...ESPECIALLY IN SOUTHWEST MT...REACHING ADVISORY LEVEL CRITERIA. OVERNIGHT TEMPS WILL BE WARMER THAN LAST NIGHT AND HIGHS TOMORROW WILL BE SLIGHTLY BELOW TO NEAR NORMAL UNDER PARTLY CLOUDY SKIES... BUT TEMPS TUMBLE BACK WELL BELOW NORMAL ON SUNDAY. WARANAUSKAS SUNDAY NIGHT THROUGH WEDNESDAY NIGHT...THE MEDIUM RANGE MODELS ARE IN GOOD AGREEMENT...KEEPING THE AREA UNDER A MOIST AND UNSETTLED PATTERN THROUGH THE PERIOD WITH COOLER THAN NORMAL TEMPERATURES. INITIALLY...A BROAD UPPER LEVEL TROUGH OVER THE WESTERN UNITED STATES WILL KEEP SNOW SHOWERS LIKELY IN THE MOUNTAINS SUNDAY NIGHT WITH A GOOD CHANCE FOR RAIN AND SNOW SHOWERS OVER THE PLAINS. FOR THE REMAINDER OF THE PERIOD...THE UPPER LEVEL TROUGH WILL DEEPEN INTO A CLOSED UPPER LEVEL LOW PRESSURE SYSTEM OVER THE INTERIOR WESTERN U.S. BY TUESDAY WHICH MEANDERS THROUGH THE GREAT BASIN THROUGH THE SECOND HALF OF NEXT WEEK. THE SLOW MOVEMENT AND ABUNDANT MOISTURE ASSOCIATED WITH THIS SYSTEM CURRENTLY INDICATED BY THE LATEST MEDIUM RANGE FORECAST MODELS INDICATES THAT THIS STORM MAY HAVE A SIGNIFICANT IMPACT ON THE REGION THROUGH THE MIDDLE AND LATER HALF OF NEXT WEEK. TEMPERATURES WILL REMAIN COOL WITH SNOW LEVELS AS LOW AS 3000 FT THROUGH THE PERIOD...MEANING POTENTIALLY HEAVY SNOW...ESPECIALLY IN THE MOUNTAINS. THE EXACT TIMING...LOCATION AND STRENGTH OF THIS SYSTEM IS STILL UNCERTAIN...HOWEVER LIQUID EQUIVALENT PRECIPITATION AMOUNTS OF 1 TO 2 INCHES ARE POSSIBLE FOR PORTIONS OF CENTRAL AND SW MT FROM TUESDAY THROUGH THURSDAY. HOENISCH/COULSTON THURSDAY AND FRIDAY...A LOW PRESSURE TROUGH MOVING THROUGH THE REGION WILL BE EXITING TO THE SOUTHEAST ALLOWING HIGH PRESSURE TO BEGIN BUILDING IN FROM THE WEST. THE SOUTHWEST AND CENTRAL MOUNTAINS WILL HAVE A CHANCE FOR SCATTERED SHOWERS AS THE LOW PULLS EAST. TEMPERATURES ARE EXPECTED TO BE BELOW SEASONAL AVERAGES. SAUCIER && .PRELIMINARY POINT TEMPS/POPS... GTF 25 52 31 41 / 10 20 40 50 CTB 23 48 28 38 / 10 30 60 50 HLN 26 53 32 47 / 10 20 40 50 BZN 18 51 31 49 / 10 20 40 60 WEY 16 45 22 41 / 20 30 50 70 DLN 22 49 27 45 / 10 20 40 50 HVR 24 56 32 47 / 10 10 40 50 LWT 22 48 29 43 / 20 20 30 50 && .TFX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...MPJ SHORT TERM...WARANAUSKAS LONG TERM...HOENISCH/COULSTON/SAUCIER AVIATION...SYNER WEATHER.GOV/GREATFALLS