Issued by NWS Riverton, WY
000 FXUS65 KRIW 220226 AFDRIW AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RIVERTON WY 926 PM MDT TUE OCT 21 2008 .UPDATE... SNOWBANDS TAPERING OFF QUICKLY THIS EVENING ACROSS CENTRAL WYOMING. THEREFORE HAVE CANCELLED THE WINTER WEATHER ADVISORY FOR NATRONA COUNTY...INCLUDING CASPER MOUNTAIN. EXPECT THE BANDS TO PERSIST PAST MIDNIGHT BUT ACCUMULATIONS WILL BE LIGHT AND LOCALIZED. /PREVIOUS DISCUSSION ISSUED 324 PM TUE OCT 21 2008/ .SHORT TERM...TONIGHT THROUGH FRIDAY A VIGOROUS SHORT-WAVE TROUGH OVER WEST-CENTRAL WYOMING THIS AFTERNOON IS BRINGING A STRONG SURFACE COLD FRONTAL PASSAGE AND SCATTERED TO NUMEROUS RAIN AND SNOW SHOWERS TO MANY LOCATIONS OF THE CWA. THE UPPER LEVEL SHORT-WAVE TROUGH IS EXPECTED TO CONTINUE TO DIG TO THE SOUTHEAST AND BE LOCATED OVER NORTHWEST COLORADO AROUND SUNSET. LATEST GLOBAL MODELS CONTINUE THE IDEA OF DIGGING THE TROUGH JUST ABOUT DUE SOUTH LATE TONIGHT. A STRONG AND UNSTABLE NORTHERLY FLOW WILL CONTINUE IN NATRONA COUNTY THROUGH THE NIGHT. DECIDED TO ISSUE A WINTER WEATHER ADVISORY FOR 4 TO 6 INCHES OF ACCUMULATING SNOW FROM THE WALTMAN/HIGHLAND AREA TO CASPER THIS AFTERNOON THROUGH TONIGHT. EXPECTING 8 TO 12 INCHES IN THE CASPER/MUDDY MOUNTAIN AREA DURING THE SAME TIMEFRAME. LOCAL ADVISORY AMOUNTS ARE POSSIBLE IN THE GREEN MOUNTAINS AND RATTLESNAKE HILLS UNTIL ABOUT MIDNIGHT BUT KEPT THEM OUT OF THE WINTER WEATHER ADVISORY WITH A SHORTER SNOWFALL DURATION AND MORE OF A LOCALIZED EVENT EXPECTED. RIVERTON AND LANDER AREAS LIKELY TO EXPERIENCE AN INCH OR TWO OF ACCUMULATION BY MIDNIGHT AS THE COLD POOL ALOFT/INSTABILITY COMBINES WITH MODERATE UPSLOPE NORTHEASTERLY FLOW. SCATTERED TO NUMEROUS SNOW SHOWERS WILL CONTINUE THROUGH TONIGHT IN THE BIG HORN MOUNTAINS AND BRING NEAR ADVISORY AMOUNTS TO PORTIONS OF THE BIG HORNS BUT WITH A MORE SCATTERED NATURE TO THE SNOWFALL...ALSO DECIDED TO LEAVE THE BIG HORN RANGE OUT OF ANY WINTER HIGHLIGHTS. THE UPPER TROUGH CONTINUES TO DIG SOUTHWARD DURING THE DAY WEDNESDAY WITH SNOW GRADUALLY TAPERING OFF DURING THE MORNING IN NATRONA COUNTY AS A HIGH AMPLITUDE RIDGE BEGINS TO TRANSLATE OVER THE STATE. A COLD NIGHT IN STORE FOR MANY AREAS WEDNESDAY NIGHT...AND WITH STRONG WAA TAKING PLACE AT H7 OVER WESTERN AND CENTRAL WYOMING (AS THE HIGH AMPLITUDE RIDGE AT H5 FOLDS OVER)...ANTICIPATING AREAS OF VALLEY FOG EARLY THURSDAY MORNING AND ONCE AGAIN FRIDAY MORNING. .LONG TERM...FRIDAY NIGHT THROUGH TUESDAY OVERALL THIS PERIOD WILL BE CHARACTERIZED BY A RIDGE OF HIGH PRESSURE OVER THE WESTERN U.S. WITH DRY WEATHER AND ABOVE NORMAL TEMPERATURES. BOTH GFS AND ECMWF DETERMINISTIC RUNS AS WELL AS GEFS MEAN TRENDING FURTHER NORTH WITH CLIPPER THAT WILL KICK OUT CUTOFF LOW OVER THE CENTRAL PLAINS FRIDAY AND SATURDAY. THIS CLIPPER WILL HAVE LITTLE IMPACT ON WYOMING...IF ANYTHING A WEAK DRY BACKDOOR FRONTAL PASSAGE ACROSS THE NORTH AND EAST SATURDAY WITH TEMPERATURES A FEW DEGREES COOLER THAN FRIDAY. AFTERWARDS BROAD FLAT UPPER RIDGE EXPANDS ACROSS THE WESTERN AND CENTRAL U.S. THROUGH TUESDAY WITH POLAR JET STAYING WELL NORTH OF THE AREA OVER WEST AND CENTRAL CANADA. H7 TEMPERATURES THROUGH THIS PERIOD WILL AVERAGE AROUND 5C RESULTING IN BASIN AND VALLEY HIGH TEMPERATURES IN THE 55F TO 65F RANGE. && .AVIATION... THE PACIFIC FRONT WILL CONTINUE TO MOVE SOUTH THROUGH SOUTHERN WYOMING THIS AFTERNOON AND EVENING...AND SHOULD BE THROUGH THE STATE TONIGHT. SHOWER ACTIVITY AND GUSTY WINDS ASSOCIATED WITH THIS FRONT WILL CONTINUE THROUGH THIS EVENING...ESPECIALLY EAST OF THE DIVIDE. RAIN SHOWERS WILL CONVERT TO SNOW SHOWERS AS THE AFTERNOON CONTINUES. SHOWER ACTIVITY WILL BRING PERIODS OF IFR TO LIFR CONDITIONS ACROSS IMPACTED AIRPORTS TODAY. SHOWERS WILL THEN BEGIN TO TAPER OFF OVERNIGHT AND INTO WEDNESDAY MORNING. HIGH PRESSURE WILL MOVE INTO THE AREA...WITH DRY CONDITIONS VFR CONDITIONS EXPECTED BY WEDNESDAY AFTERNOON. && .FIRE WEATHER... A STRONG PACIFIC STORM SYSTEM WILL CONTINUE TO BRING ACCUMULATING SNOWS TO CENTRAL WYOMING THIS EVENING THROUGH TONIGHT. THE MOST SIGNIFICANT ACCUMULATING SNOWFALL IS EXPECTED IN JOHNSON AND ESPECIALLY NATRONA COUNTY TONIGHT. SCATTERED SNOW SHOWERS ARE EXPECTED THROUGH THIS EVENING ALONG AND WEST OF THE DIVIDE. HIGH PRESSURE GRADUALLY BUILDS OVER THE DISTRICT WEDNESDAY AND THURSDAY. && .RIW WATCHES/WARNINGS/ADVISORIES...NONE. && $$ UPDATE...TWB SHORT TERM...CHURCH LONG TERM...AEM AVIATION...BRANHAM FIRE WEATHER...CHURCH
