Issued by NWS Riverton, WY
000 FXUS65 KRIW 222156 AFDRIW AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RIVERTON WY 256 PM MST SUN NOV 22 2009 .SHORT TERM...TONIGHT THROUGH WEDNESDAY QUICK HITTING STORM SYSTEM BRINGING MODERATE SNOW TO PORTIONS OF WESTERN WYOMING THIS AFTERNOON WITH A QUICK 1 TO 3 INCHES SINCE 11 AM. APPROACHING FRONTAL SYSTEM...QG FORCING...INITIAL DIFFLUENT FLOW AND NOSE OF CURVED JET STREAK WILL ADD UP TO CONTINUED SNOWFALL THROUGH THE EVENING...ESPECIALLY WHEN COMBINED WITH STEEPENING LAPSE RATES TO AT LEAST 7.5C/KM AND LOW LI`S. DEBATED A WINTER WEATHER ADVISORY FOR THE TETONS AND STAR VALLEY BUT DECIDED TO JUST GIVE A FEW ADVISORY AMOUNTS TO THE FAVORED AREAS AND FRESHEN UP THE SPS. UPPER LOW IS STILL HOLDING TOGETHER FAIRLY GOOD AND IT MAY BE A LITTLE STRONGER EAST OF THE DIVIDE THAN EARLIER EXPECTED. HAVE TRENDED TOWARDS THE 18Z NAM WITH A BETTER CHANCE OF SNOW IN THE CENTRAL BASINS AND SURROUNDING MOUNTAINS AFTER MIDNIGHT. ALSO KEPT IDEA OF GUSTY WINDS BEHIND THE FRONT IN THE WIND RIVER BASIN. SHOULD ALSO SEE A GOOD QUICK SHOT OF SNOW AND STRONG WIND ALONG THE I80 CORRIDOR THIS EVENING AND HIT THIS AREA A LITTLE HARDER. SOME LIGHT SNOW OR FLURRIES MAY LINGER EARLY MONDAY IN THE EAST AND IN THE MOUNTAINS THROUGH THE DAY. STEEP TEMPERATURE INVERSIONS SHOULD DEVELOP IN THE BASINS AND VALLEYS MONDAY NIGHT AND STICK AROUND FOR A FEW DAYS. THE MOUNTAINS WILL SEE SOME IMPROVEMENT TUESDAY INTO WEDNESDAY. THE NEXT CLIPPER TYPE SYSTEM MAY VERY WELL IMPACT OUR NERN ZONES TUESDAY NIGHT WITH A CHANCE OF SNOW AND GUSTY NORTH WIND. TIMING LOOKS A LITTLE FASTER THAN EARLIER RUNS AND ADJUSTED THIS FASTER BY 12 HOURS. .LONG TERM...WEDNESDAY NIGHT THROUGH SUNDAY WEAK BACKDOOR COLD FRONT MAY BRING SOME LOW CLOUDS AND LIGHT/SNOW FLURRIES DOWN THE INTERSTATE 25 CORRIDOR WEDNESDAY. OTHERWISE DRY AND MILD CONDITIONS WILL PREVAIL ACROSS MOST OF THE AREA WEDNESDAY THROUGH FRIDAY. CHANCES OF SNOW INCREASE ACROSS THE FAR WEST VALLEYS AND WESTERN MOUNTAINS LATE FRIDAY WITH COOLER CONDITIONS AND CHANCES FOR LIGHT SNOW SPREADING EAST OF THE DIVIDE SATURDAY. MAINLY DRY AND COOL WEATHER EXPECTED ON SUNDAY. MEDIUM-RANGE MODELS REVERTING BACK TO STRONGER CLIPPER SYSTEM DIVING INTO THE NORTHERN PLAINS TUESDAY NIGHT DRIVING SHALLOW BACK DOOR COLD FRONT ALONG WITH WEAK ISENTROPIC UPGLIDE ACROSS EASTERN WYOMING. LOW POPS REINTRODUCED DOWN THE INTERSTATE 25 CORRIDOR WEDNESDAY EXPECTING POSSIBLY SOME LOW CLOUDS/FLURRIES BEHIND FRONT. OTHERWISE UPPER RIDGE BUILDS ACROSS REGION WEDNESDAY AND THURSDAY BRINGING MOSTLY CLEAR SKIES WITH ABOVE NORMAL HIGH TEMPERATURES THOUGH VALLEY INVERSIONS WILL SUPPRESS HIGHS THANKSGIVING DAY AND FRIDAY...AND LARGE DIURNAL TEMPERATURE RANGES WITH GOOD RADIATIONAL COOLING AT NIGHT. MODELS SHOW INCREASING SPREAD WITH NEXT INCOMING PACIFIC TROUGH MAKING LANDFALL ON FRIDAY. RUN-TO-RUN AGREEMENT BETWEEN ECMWF AND GFS EXISTS ON A TREND TOWARD SLOWER PROGRESSION. FORECAST FRIDAY NIGHT AND ONWARD A BLEND OF 00Z ECMWF AND GEFS MEAN WITH SPLIT TROUGH PASSAGE EXPECTED FRIDAY NIGHT AND SATURDAY...INCREASING CHANCES OF SNOW ACROSS THE WEST LATE FRIDAY WITH POST-FRONTAL CHANCES OF SNOW EAST OF THE DIVIDE SATURDAY. HOWEVER...AS SEEN WITH THE LAST FEW SPLIT SYSTEMS...PRECIPITATION EAST OF THE DIVIDE HAS BEEN HARD TO COME BY. THUS...IMPACTS TO TRAVEL OVER THE HOLIDAY WEEKEND EXPECTED OVER THE FAR WEST AND WESTERN MOUNTAINS FRIDAY NIGHT INTO SATURDAY MORNING THOUGH IMPACTS EXPECTED TO BE MINIMAL WITH SNOW AMOUNTS BELOW ADVISORY CRITERIA. MAINLY DRY CONDITIONS EXPECTED ON SUNDAY WITH NEAR NORMAL TEMPERATURES. && .AVIATION... A TROUGH WILL MOVE ACROSS WYOMING OVERNIGHT...BRINGING LIGHT SNOW TO THE WEST AND SOUTH AND POSSIBLY TO THE BIGHORNS AND JOHNSON COUNTY. EXPECT MVFR CIGS AND VSBYS WITH SNOW...WITH LOCAL IFR CONDITIONS. THE SNOW WILL TAPER TO SNOW SHOWERS BEFORE 06Z AND MAINLY IMPACTING HIGHER TERRAIN...THOUGH MVFR CIGS WILL LIKELY PERSIST IN THE FAR WEST VALLEYS THROUGH LATE MONDAY MORNING. MTN OBSCURATIONS ARE EXPECTED THROUGH THE OVERNIGHT...IMPROVING SOME AS MONDAY DAYTIME PROGRESSES. NORTHWEST TO NORTH WINDS ARE EXPECTED OVER THE CENTRAL AND SOUTHWEST OVERNIGHT AND MONDAY. && .FIRE WEATHER... A QUICK HITTING STORM SYSTEM WILL MOVE ACROSS THE AREA TONIGHT SPREADING A ROUND OF SNOW AND GUSTY WIND. THE HEAVIEST SNOW WILL CONTINUE OUT WEST THIS EVENING BEFORE DIMINISHING. THE ODDS OF LIGHT SNOW WILL INCREASE EAST OF THE DIVIDE LATER THIS EVENING AND INTO THE EARLY MORNING HOURS. BRISK NORTH TO NORTHWEST WINDS AND COLD CONDITIONS WILL PREVAIL IN MANY AREAS MONDAY WITH SOME FLURRIES IN THE HIGH COUNTRY. STEEP BASIN AND VALLEY INVERSIONS WILL DEVELOP MONDAY NIGHT WITH GRADUALLY WARMING TEMPERATURES IN THE MOUNTAINS THROUGH MIDWEEK. && .RIW WATCHES/WARNINGS/ADVISORIES...NONE. && $$ SHORT TERM...SKRBAC LONG TERM...AEM AVIATION...TWB FIRE WEATHER...SKRBAC
