000 FXUS65 KRIW 291902 AFDRIW AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RIVERTON WY 1202 PM MST THU NOV 29 2007 .SHORT TERM...TODAY THROUGH SATURDAY NIGHT UPDATED TO REMOVE SNOW ADVISORY FOR THE TETON AND GROS VENTRE RANGES. LIGHT SNOW WILL CONTINUE AT TIMES THIS AFTERNOON BUT ADDITIONAL SNOWFALL SHOULD GENERALLY BE AN INCH OR LESS. SNOWFALL REPORTS FROM THE AREA INDICATE 4 TO 6 INCHES OF SNOW HAS FALLEN SINCE EARLY THIS MORNING. LIGHT SNOW HAS BEGUN TO FALL IN JACKSON AND BY THE LOOKS OF THE WEB CAMS AT TETON PASS AND AT ALPINE JUNCTION LIGHT TO MODERATE SNOWFALL IS FALLING IN THOSE AREAS NOW. THE SNOW THAT IS OCCURRING THERE NOW APPEARS TO BE ASSOCIATED WITH THE FAST MOVING AREA OF ISENTROPIC LIFT THAT IS OUTRUNNING AN AREA OF QG FORCING TO THE NW. THE AREA OF ISENTROPIC LIFT WILL BE EXITING THIS AREAS SHORTLY. THE AREA OF QG FORCING IS HANGING BACK TO THE NORTHWEST IS NOW JUST APPROACHING YELLOWSTONE PARK. THIS FORCING WILL PRODUCE MOST OF THE SNOW THIS MORNING ACROSS WESTERN WYOMING THIS MORNING. STILL EXPECTING THE BRUNT OF THE SNOWFALL TO OCCUR ACROSS THE TETONS AND THE SOUTHWEST PORTION OF YELLOWSTONE PARK WHERE 5 TO 9 INCHES OF SNOWFALL IS EXPECTED ALONG WITH 4 TO 7 INCHES IN THE GROS VENTRE RANGE BY 19Z TODAY. THE SNOW ADVISORY IS STILL IN EFFECT FOR THESE MOUNTAINS THROUGH 19Z TODAY. 2 TO 4 INCHES OF SNOW IS EXPECTED ACROSS THE JACKSON VALLEY. LESSER AMOUNTS OF SNOW ARE EXPECTED ACROSS THE WYOMING AND SALT RIVER RANGES TO THE SOUTH WHERE THE RIGHT REAR QUAD DYNAMICS OF THE 150 KNOT JET WILL SLIDE SOUTH OF THE MAIN AREA OF FORCING AND DEEPER MOISTURE TO THE NORTH. EXPECTING 2 TO 5 INCHES OF SNOW IN THESE MTNS ALONG WITH 1 TO 3 INCHES OF SNOWFALL IN THE STAR VALLEY. THE WIND WILL NOT BLOW STRONG ENOUGH TO JUSTIFY A BORDERLINE SNOW ADVISORY ANYWHERE ELSE. THIS AFTERNOON WE WILL BE LEFT WITH CONVECTIVE OROGRAPHIC SNOW SHOWERS IN WESTERN WY WHERE ADDITIONAL LOCAL AMOUNTS OF 2 INCHES OR SO OF SNOWFALL CAN BE EXPECTED. THE ASSOCIATED COLD FRONT WILL PUSH SOUTH ACROSS WYOMING EARLY THIS EVENING WHICH WILL HELP SPREAD AREAS OF LIGHT SNOW ACROSS THE DIVIDE TO CENTRAL AND EASTERN WY THIS EVENING. INCREASED THE CHANCES OF SNOW SHOWERS SLIGHTLY ON FRIDAY FOR WESTERN WY AS SOME ISENTROPIC LIFT WILL INTERACT WITH A FAIRLY DEEP LAYER OF MOISTURE THERE. HIGH TEMPERATURES WILL BE ABOUT 10 DEGREES COLDER ON FRIDAY BEHIND THIS EVENINGS COLD FRONT. THE NEXT UPSTREAM SHORTWAVE NOW OFF THE BRITISH COLUMBIA COAST WILL DIVE SOUTHEAST AND INTERACT WITH THE NORTHERN REACHES OF A SUBTROPICAL PLUME OF MOISTURE FRIDAY NIGHT AND SATURDAY. THE ASSOCIATED ISENTROPIC LIFT FROM THIS FEATURE WILL LIKELY SPREAD STRATIFORM SNOWFALL ACROSS SWEETWATER AND NATRONA COUNTIES LATE FRIDAY NIGHT INTO SATURDAY. IN ADDITION...THE GFS/ECMWF HAS BEEN CONSISTENTLY ADVERTISING A CLOSED SURFACE LOW OVER CO SATURDAY IN SPITE OF THE FACT THAT THE ASSOCIATED UPPER LEVEL FEATURE WILL COME THROUGH AS AN OPEN WAVE. THE NAM IS NOW ADVERTISING THIS AS WELL. WITHOUT A CLOSED 700MB LOW LOCATED IN THE SAME PLACE AS THE SURFACE LOW...SNOW AMOUNTS SHOULD BE RELATIVELY LIGHT IN SPITE OF THE TREND TOWARD HIGHER POPS. THE GFS PLACES THE CLOSED 700MB LOW TOO FAR NORTH OVER THE NEBRASKA/SD/WY TRIPLE POINT. THIS WEATHER SYSTEM WILL BE EXITING THE AREA TO THE EAST SATURDAY NIGHT. AS A SIDE NOTE...30 TO 40 MPH SOUTHWEST WINDS WILL BLOW ACROSS THE GREEN MTNS THIS MORNING. BEHIND THE UPPER LEVEL TROUGHPA AND AHEAD OF THE CANADIAN FROPA...WEST WINDS WILL INCREASE TO 25 TO 35 MPH ACROSS SWEETWATER COUNTY WITH NORTHWEST WINDS AROUND THE SAME SPEEDS IN THE WIND RIVER BASIN THIS AFTERNOON. 6 TO 7MB 3 HOURLY SURFACE PRESSURE RISES WILL SPEED UP THE POST FRONTAL NORTHERLY WINDS ACROSS THE WIND RIVER AND BIG HORN BASINS AS WELL AS JOHNSON COUNTY THIS EVENING. .LONG TERM...SUNDAY THROUGH THURSDAY IN THE WAKE OF A TROUGH PASSAGE SATURDAY NIGHT...FLAT RIDGING IS EXPECTED TO DEVELOP ON SUNDAY...AND BECOME A BIT MORE AMPLIFIED ON MONDAY...AND BECOMING MORE ZONAL ON TUESDAY...AHEAD OF A SHORTWAVE TROUGH APPROACHING THE REGION. THIS RIDGING WILL RESULT IN A WARMING TREND THROUGH TUESDAY...AS 700MB TEMPS WARM BETWEEN 0 AND +5 DEG CELSIUS...EXCEPT PERHAPS ACROSS THE FAR NORTHWEST. HAVE TRENDED WARMER...BUT STILL BELOW GUIDANCE...DUE TO UNCERTAINTY OF SNOWFALL THAT OCCURS IN THE SHORT TERM...AND THE STEEP SURFACE INVERSIONS THAT MAY RESULT. THIS WOULD IMPACT THE BASINS AND VALLEYS THE MOST. PRECIPITATION CHANCES WOULD MAINLY BE LIMITED TO THE MOUNTAINS AND WESTERN VALLEYS SUNDAY THROUGH TUESDAY...WITH THE LEAST AMOUNT OF ACTIVITY ON MONDAY WHEN THE RIDGE WILL BE A BIT MORE AMPLIFIED. BY TUESDAY NIGHT AND WEDNESDAY...THE 00Z GFS HAS A SHORTWAVE TROUGH IMPACTING THE FORECAST AREA...WITH PRECIPITATION MAINLY WEST OF THE DIVIDE TUESDAY NIGHT AND THEN SPREADING EAST OF THE DIVIDE ON WEDNESDAY. THE 00Z ECMWF IS FARTHER NORTH WITH THIS FEATURE...WITH LESS IMPACT ON THE AREA. THEN ON THURSDAY THE ECMWF IS SHOWING ANOTHER TROUGH APPROACHING FROM THE WEST...WITH THE GFS MUCH SLOWER. WILL ONLY HAVE SMALL POPS FOR THE WEDNESDAY AND THURSDAY SYSTEMS...UNTIL DETAILS BECOME CLEARER. SEASONAL TO ABOVE AVERAGE TEMPERATURES ARE EXPECTED WEDNESDAY AND THURSDAY. && .AVIATION... AN ADVANCING STORM SYSTEM WILL MOVE THROUGH WESTERN WYOMING TODAY WITH WIDESPREAD LOW CEILINGS AND SNOW SPREADING ACROSS THE MOUNTAINS AND VALLEYS WEST OF THE CONTINENTAL DIVIDE WITH MVFR TO IFR CEILINGS AND VISIBILITIES AND WIDESPREAD MOUNTAIN OBSCURATIONS. LOW LEVEL WINDS WILL BE STRONG OVER THE WEST AND SOUTH THROUGH THE DAY. A COLD FRONT WILL MOVE SOUTH FROM MONTANA IN THE AFTERNOON WITH LOW CLOUDS AND AREAS OF LIGHT SNOW MOVING SOUTH EAST OF THE CONTINENTAL DIVIDE IN THE NORTH AND SPREADING SOUTH ACROSS THE EAST IN THE EVENING. LOCAL MVFR CEILINGS ARE EXPECTED IN THE NORTHERN BIG HORN BASIN AND EAST OF THE BIGHORN MOUNTAINS ACROSS EASTERN JOHNSON COUNTY SPREADING INTO NATRONA COUNTY. AS CONDITIONS WORSEN IN THE NORTH AND EAST... SNOW WILL TAPER OFF IN THE WESTERN MOUNTAINS BUT LOW CLOUDS WILL LINGER THROUGH MUCH OF THURSDAY NIGHT WITH LOCAL SNOW SHOWERS IN THE HIGHER MOUNTAINS. && .FIRE WEATHER... A PACIFIC STORM SYSTEM WILL PRODUCE 4 TO 9 INCHES OF SNOWFALL ACROSS MUCH OF THE WESTERN WYOMING MOUNTAINS THIS MORNING WITH 1 TO 4 INCHES OF SNOW EXPECTED IN THE WESTERN VALLEYS. SNOW SHOWERS WILL CONTINUE THROUGH THE REST OF THE AFTERNOON IN WESTERN WYOMING WHERE ADDITIONAL LOCAL AMOUNTS OF A COUPLE OF INCHES OF SNOWFALL COULD OCCUR. AN ASSOCIATED COLD FRONT WILL SPREAD SOME LIGHT SNOW TO AREAS EAST OF THE DIVIDE TONIGHT. ANOTHER PACIFIC STORM SYSTEM WILL INTERACT WITH THE NORTHERN REACHES OF A SUBTROPICAL MOISTURE PLUME STREAMING IN FROM THE SOUTH AND WILL PRODUCE SOME SNOW ACROSS SWEETWATER AND NATRONA COUNTIES MAINLY ON SATURDAY. WEST SOUTHWEST WINDS WILL INCREASE TODAY AHEAD OF THE PACIFIC WEATHER SYSTEM. NORTH NORTH WINDS WILL INCREASE THIS EVENING EAST OF THE DIVIDE BEHIND THIS EVENINGS COLD FRONT. TEMPERATURES WILL BECOME ABOUT 10 DEGREES COLDER BEHIND THE COLD FRONT BEGINNING FRIDAY. && .RIW WATCHES/WARNINGS/ADVISORIES...NONE. && $$ SHORT TERM...LIPSON/SKRBAC LONG TERM...WM AVIATION...C.BAKER FIRE WEATHER...LIPSON