Issued by NWS Riverton, WY
000 FXUS65 KRIW 070739 AFDRIW AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RIVERTON WY 139 AM MDT THU MAY 7 2009 .SHORT TERM...TODAY THROUGH SATURDAY NIGHT 140KT WESTERLY JET STREAM LOCATED OVER CENTRAL WYOMING EARLY THIS MORNING. ALL MODELS PROGGING THE JET CORE TO REMAIN OVER WYOMING THROUGH SATURDAY. SCATTERED OROGRAPHIC SHOWERS CONTINUE TO FORM JUST UPSTREAM FROM THE WESTERN WIND RIVER MOUNTAINS. AT H5...A SHORT-WAVE TROUGH OVER EASTERN WASHINGTON WILL BEGIN DIGGING SOUTHEASTWARD THIS EVENING...MOVING OVER FAR NORTHERN WYOMING BY FRIDAY MORNING. THE ASSOCIATED SURFACE COLD FRONT AND H7 BAROCLINIC ZONE WILL SAG SOUTHWARD THROUGH THE NORTHERN BIG HORN BASIN AND JOHNSON COUNTY LATE THIS AFTERNOON. BEST ATMOSPHERIC LIFT ASSOCIATED WITH MID-LEVEL FRONTOGENETIC FORCING DEVELOPS AROUND SUNSET FROM NORTHERN YELLOWSTONE PARK EAST INTO NORTHEAST JOHNSON COUNTY. INCREASED PRECIPITATION CHANCES FURTHER OVER THE NORTH LATE THIS AFTERNOON THROUGH THE EVENING HOURS WITH SUFFICIENT PACIFIC MOISTURE AND LARGE SCALE LIFT COMBINING TO PRODUCE AREAS OF LIGHT LOW ELEVATION RAIN AND MOUNTAIN SNOW...ESPECIALLY THIS EVENING. CENTER OF H5 SHORT-WAVE TROUGH/VORT LOBE EXPECTED TO REMAIN OVER CENTRAL WYOMING FRIDAY THROUGH SATURDAY. A SECONDARY SURFACE AND MID-LEVEL FRONT MOVES SOUTHWARD THROUGH THE STATE FRIDAY AFTERNOON...WITH THE MAIN CAA EAST OF THE DIVIDE. NORTHEAST UPSLOPE FLOW WILL KEEP BELOW AVERAGE TEMPERATURES...LIGHT WIND AND THE CHANCE OF LOW ELEVATION RAIN AND MOUNTAINS SNOW SHOWERS ALONG AND EAST OF THE DIVIDE THROUGH AT LEAST THE FIRST HALF OF THE WEEKEND. SCATTERED THUNDERSTORMS ARE EXPECTED SATURDAY AFTERNOON AS H5 COLD POOL REMAINS OVER THE AREA AND FAIRLY MOIST LOW LEVELS REMAIN ALONG EAST OF THE DIVE. WITH LOW THETA-E AIR AND NO DIRECTIONAL SHEAR BELOW H7 TO SPEAK OF...EXPECT STORMS TO BE THE GARDEN VARIETY TYPE. JET AND ASSOCIATED H5 TROUGH FINALLY BEGIN TO TRANSITION SOUTHEASTWARD INTO NORTHERN COLORADO LATE SATURDAY NIGHT. .LONG TERM...SUNDAY THROUGH THURSDAY SHORTWAVE EXITING THE AREA SUNDAY WITH SOME LINGERING SHOWERS MAINLY IN THE EAST...AND POSSIBLY THE NORTHWEST. OTHERWISE WILL HAVE FLAT RIDGING BUILDING IN SUNDAY NIGHT...WITH THE FLOW BECOMING ZONAL ON MONDAY. SOME WEAK SHORTWAVE ENERGY MOVES THROUGH MONDAY AFTERNOON/EVENING...WITH A SLIGHT CHANCE OF SHOWERS IN THE NORTHWEST MTNS AND BIGHORN MTNS. A STRONGER WAVE APPROACHES TUESDAY WITH AN INCREASED CHANCE OF SHOWERS AND THUNDERSTORMS ACROSS THE NORTH. THIS WAVE MOVES THROUGH WEDNESDAY WITH THE HIGHER CHANCES OF PRECIP EXTENDING FURTHER SOUTH AND EAST. RIDGING RETURNS THURSDAY AND FRIDAY MORNING...WITH JUST A SLIGHT CHANCE OF SHOWERS IN THE NORTHWEST MTNS. SEASONAL TEMPERATURES EXPECTED INITIALLY IN THE EXTENDED. THEN UNSEASONABLY COLD AIR MOVES IN WEDNESDAY WITH THE STRONG WAVE MOVING THROUGH...THOUGH TEMPERATURES REBOUND QUICKLY POSSIBLY ABOVE SEASONAL NORMS FOR THURSDAY. IT LOOKS TO BE QUITE BREEZY TO WINDY MANY AREAS THROUGH THE EXTENDED UNDER THE FAST PROGRESSIVE FLOW. && .AVIATION... RAIN AND SNOW SHOWERS WILL OCCUR IN THE NW WY MOUNTAINS AS WELL AS IN THE NORTHERN BIGHORN MOUNTAINS TODAY INTO THE EVENING HOURS WITH MVFR CONDITIONS PREVAILING. A FEW AFTERNOON AND EVENING SHOWERS AND THUNDERSTORMS WILL OCCUR OVER THE LOWER ELEVATIONS OF NORTHERN WY. THE CENTRAL AND SOUTH WILL BE MOSTLY DRY WITH WINDY CONDITIONS BY 18Z. WIND SPEEDS OF 20 TO 30 KNOTS WILL OCCUR WITH GUSTS AROUND 35 KNOTS. THE WIND WILL DECREASE BY 03Z FRI IN MOST LOCATIONS. A COLD FRONT WILL MOVE INTO NORTHERN WY AFTER 21Z AND PUSH SOUTH DURING THE EVENING HOURS. AREAS OF MVFR CEILINGS WILL BE POSSIBLE ACROSS NORTHERN WY AFTER 00Z FRI. && .FIRE WEATHER... ANOTHER WINDY DAY IN STORE FOR MANY AREAS OF THE DISTRICT TODAY...ESPECIALLY CENTRAL AND SOUTH. LOW ELEVATION RAIN AND MOUNTAIN SNOW WILL INCREASE OVER THE FAR NORTH TODAY AS A CANADIAN COLD FRONT APPROACHES THIS AREA FROM SOUTHERN MONTANA. WIDESPREAD RAIN AND MOUNTAIN SNOW ARE EXPECTED IN THE NORTHERN ZONES THIS EVENING BEFORE GRADUALLY DISSIPATING AFTER ABOUT MIDNIGHT. A RE- REINFORCING SHOT OF COOL AIR EAST OF THE DIVIDE IS EXPECTED ON FRIDAY...ALONG WITH SCATTERED SHOWERS AND ISOLATED AFTERNOON THUNDERSTORMS. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...CHURCH LONG TERM...TWB AVIATION...AR FIRE WEATHER...CHURCH
