000 FXUS65 KTFX 201732 AFDTFX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GREAT FALLS MT 1135 AM MDT MON JUL 20 2009 .UPDATE... TODAY...UPDATE TO ZONES PLANNED BY 930...SURFACE HIGH PRESSURE HAS MOVED SOUTH ACROSS THE BORDER INTO NORTH CENTRAL MONTANA. A FEW SHOWERS REMAIN ALONG THE EXTREME EAST OF THE ZONES BUT CHANCE OF MEASURABLE PRECIP WILL BE VERY LOW. WILL UPDATE TO LOWER POPS ACROSS NORTH CENTRAL MONTANA AND REMOVE MENTION OF PRECIP FROM ZONES. WILL ALSO FRESHEN FIRST PERIOD SKY COVER. TEMPS CONSEQUENTLY WILL BE COOLER AND BACK TO NEAR NORMALS FOR A DAY. ZELZER && .AVIATION... UPDATED 1735Z. THE WEAK COLD FRONT THAT PASSED OVERNIGHT IS KEEPING SOME MID-LEVEL CEILINGS ACROSS EASTERN CENTRAL MT TODAY. EXPECT THESE CLOUDS TO MOVE EAST AND BRING MOSTLY CLEAR CONDITIONS TO ALL TERMINALS BY TONIGHT. WINDS SHOULD GENERALLY BE LIGHT ACROSS THE REGION TODAY. && .PREV DISCUSSION... /ISSUED 530 AM MDT MON JUL 20 2009/ TODAY THROUGH WEDNESDAY...ISOLATED SHOWERS AND THUNDERSTORMS WILL CONTINUE TO DEVELOP ACROSS NORTH CENTRAL AND CENTRAL MONTANA THROUGHOUT THE DAY AS A SHORTWAVE TROUGH MOVES THROUGH THE MOIST AND UNSTABLE NORTHWEST FLOW ALOFT AND CAUSES ELEVATED CONVECTION. THESE STORMS SEEM TO BE LOW-TOPPED AND MAINLY POSE A THREAT FOR BRIEF RAIN SHOWERS AND GUSTY WINDS. AS THE DAY WARMS INTO THE AFTERNOON...ISOLATED THUNDERSTORMS WILL ALSO BEGIN TO DEVELOP OVER THE HIGHER TERRAIN OF SOUTHWEST MONTANA. AGAIN...THE AFTERNOON STORMS SHOULD BE FAIRLY WEAK. TEMPERATURES WILL ONLY WARM TO NEAR NORMAL AS WELL. THE UPPER LEVEL RIDGE OF HIGH PRESSURE OVER THE GREAT BASIN WILL THEN MOVE EAST INTO THE ROCKY MOUNTAINS TONIGHT THROUGH WEDNESDAY FOR A GRADUAL WARM UP...AS TEMPERATURES WARM TO AROUND 10 DEGREES ABOVE NORMAL BY WEDNESDAY. ALTHOUGH THE WARMING TEMPERATURES WILL ALSO BRING INCREASING INSTABILITY TO THE AREA...MOISTURE WILL BE LIMITED TO FAR SOUTHWEST MONTANA ON TUESDAY FOR ONLY A SLIGHT CHANCE OF AFTERNOON AND EVENING THUNDERSTORMS THERE. THE WEAK FLOW ALOFT WITH THE UPPER RIDGE WILL THEN TAP INTO SOME MONSOONAL MOISTURE ON WEDNESDAY...SPREADING THE SLIGHT CHANCE OF AFTERNOON AND EVENING SHOWERS AND THUNDERSTORMS NORTH INTO CENTRAL MONTANA. BECAUSE OF THE WEAK FLOW...THERE WILL BE VERY LITTLE WIND SHEAR...WHICH WILL LIKELY KEEP THE STORMS WEAK AS WELL. COULSTON WEDNESDAY NIGHT THROUGH SATURDAY NIGHT...FORECAST CONFIDENCE FOR THIS PERIOD IS LOW AS EXTENDED RANGE MODELS SHOWING MARKED DIFFERENCES IN SOLUTIONS. ONLY TIME SPAN OF GOOD AGREEMENT IS FROM WED NIGHT UNTIL ABOUT MIDDAY THURS...AS MODELS DEPICT UPPER RIDGE STILL IN PLACE OVER THE GREAT BASIN AND NRN ROCKIES WHILE A SHORTWAVE TROF APPROACHES THE PACNW COAST. OTHER THAN A SLIGHT CHANCE OF THUNDER IN SOUTHWEST MT WED NIGHT...MOST OF THE AREA SHOULD SEE QUIET WEATHER CONDITIONS INTO EARLY THURS. FROM THAT POINT FORWARD ITS HARD TO TELL IF ANY MODEL HAS GOOD READ ON EVOLUTION OF THE PACNW TROF...AS GFS PUSHES IT QUICKLY ACROSS THE REGION BY FRI EVE WHILE EUROPEAN STILL HAS THE TROF BACK IN WA/OR. DIFFERENCES BECOME EVEN MORE PRONOUNCED BY SAT...WITH GFS SHOWING MOST OF MT IN DRY COOL NORTHERLY FLOW ON WESTERN BOUNDARY OF LONGWAVE TROF OVER ERN US AND EUROPEAN MODEL REBUILDING THE UPPER RIDGE AND PUSHING LARGE AMOUNT OF MONSOONAL MOISTURE FOR THUNDERSTORMS NORTHWARD INTO WESTERN/CENTRAL MT. CURRENT FORECAST GRIDS ARE A WEAK COMPROMISE BETWEEN THE MODELS...ACCEPTING THE TREND OF COOLER TEMPS AND KEEPING IN AT LEAST SLIGHT CHANCE OF PRECIP OVER ALMOST THE ENTIRE AREA FROM THUR EVE THRU SATURDAY. FURTHER ADJUSTMENTS ARE LIKELY WITH LATER...AND HOPEFULLY MORE CONSISTENT...MODEL RUNS. WARANAUSKAS SUNDAY THROUGH MONDAY...MODEL DIFFERENCES DURING THIS PERIOD DETRACT FROM FORECAST CONFIDENCE. BOTH THE GFS AND ECMWF HAVE MADE BIG CHANGES FROM PREVIOUS RUNS. THE GFS HAS A HIGH AMPLITUDE RIDGE ALONG THE WEST COAST...WHILE THE ECMWF HAS A LOW AMPLITUDE RIDGE OVER THE NORTHERN ROCKIES SUNDAY THAT IS FLATTENED AND NUDGED WESTWARD AS A SHORTWAVE DROPS SOUTH OUT OF CANADA. DUE TO THESE DIFFERENCES THIS FORECAST KEPT POPS NEAR OR A LITTLE BELOW CLIMATOLOGICAL VALUES. BOTH MODELS INDICATED HEIGHT RISES...SO TEMPERATURES WERE WARMED TO NEAR SEASONAL AVERAGES. EK && .PRELIMINARY POINT TEMPS/POPS... GTF 78 51 85 53 / 10 10 0 0 CTB 75 49 81 51 / 10 10 0 0 HLN 84 55 89 57 / 10 0 0 0 BZN 85 49 88 52 / 10 10 10 10 WEY 76 40 79 42 / 10 10 20 20 DLN 79 49 84 51 / 10 0 10 10 HVR 81 50 85 52 / 10 10 10 10 LWT 74 48 80 51 / 10 20 10 10 && .TFX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ WEATHER.GOV/GREATFALLS