Issued by NWS Cheyenne, WY
000 FXUS65 KCYS 111837 AFDCYS AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHEYENNE WY 1237 PM MDT SAT OCT 11 2008 .UPDATE...BOTH NAM AND GFS HAVE COME INTO CONGRUENCY AND CONSISTENCY ON THIS WINTER WEATHER EVENT. BOTH NAM AND GFS QPFS AS WELL AS CURRENT TEMPERATURES SUGGEST WINTER STORM WARNING CRITERIA SNOW WHEN COMBINED WITH THE WINDS ACROSS THE LOWER ELEVATIONS OF SOUTHERN CARBON COUNTY. HAVE ALSO ADDED A WINTER WEATHER ADVISORY FOR TONIGHT FOR THE WYOMING EASTERN PLAINS AND FOR NORTHWEST NEBRASKA. ALTHOUGH SNOWFALL AMOUNTS WILL BE LIMITED EAST OF THE LARAMIE RANGE...THIS IS THE FIRST SIGNIFICANT SNOW OF THE SEASON AND WANT TO MAKE SURE OUR CUSTOMERS ARE AWARE OF IMPLICATIONS OF SNOWFALL EAST OF THE LARAMIE RANGE TONIGHT AND THUS THE WINTER WEATHER ADVISORY ISSUANCE. RUBIN && .PREV DISCUSSION... /ISSUED 303 AM MDT SAT OCT 11 2008/ SHORT TERM...TODAY AND TONIGHT... UPPER LOW ORGANIZING OVER SRN NEVADA EARLY THIS MORNING WITH ONE PIECE OF VORTICITY HAVING EJECTED NORTHEAST OVER ERN IDAHO. PCPN HAS DIMINISHED OVER THE CWA THIS MORNING AS SOME MID LEVEL DRYING HAS OCCURRED BEHIND LEAD SHORTWAVE. THIS LULL HAS BEEN EXPECTED AND DO NOT SEE MUCH MORE IN THE WAY OF PCPN FOR THE CWA THIS MORNING INTO THE AFTERNOON THUS HAVE SHAVED POPS DOWN SOME. PCPN WILL BEGIN TO INCREASE FROM WEST TO EAST BY LATE THIS AFTERNOON AS UPPER LOW BEGINS TO LIFT NORTHEAST AND MAIN SHOT OF SHORTWAVE ENERGY MAKES ITS APPROACH TO THE AREA. THIS SHOULD BRING AN AREA OF RAIN AND SNOW ACROSS MOST OF THE CWA TONIGHT WITH ACTIVITY TAPERING OFF OVER THE WESTERN CWA LATE TONIGHT. MAIN PROBLEM IS PRECIPITATION TYPE AND TIMING OF CHANGEOVERS. EXPECTED TEMPERATURE PROFILE OF THE ATMOSPHERE AND PROGGED THICKNESS VALUES FAVOR PRECIPITATION BEING MAINLY LIQUID OVER MUCH OF THE CWA OUTSIDE OF THE MTNS TODAY...THEN A GRADUAL CHANGEOVER TO SNOW FROM WEST TO EAST TONIGHT AS MID LEVEL TEMPERATURES COOL WITH APPROACH OF UPPER LOW. STILL APPEARS PCPN WILL REMAIN PRIMARILY RAIN OVER MOST OF THE PANHANDLE THROUGH TONIGHT. CONTINUE TO LEAN TOWARD THE COOLER SIDE OF GUIDANCE TEMPS ESPECIALLY OVER THE ERN WY PLAINS WHERE CLOUDS AND MODEST UPSLOPE FLOW WILL HINDER MUCH IN THE WAY OF WARMING TODAY. BASED ON ABOVE REASONING AND EXPECTATIONS HAVE MADE SOME ALTERATIONS TO TIMING OF HILITES AS WORST WEATHER OUTSIDE OF THE MTNS LOOKS TO HOLD OFF UNTIL THE EVENING. LONG TERM...SUNDAY THROUGH FRIDAY... FORECAST MODELS CONTINUE TO SHOW A QUICK TRANSITION TO ISENTROPIC DOWN-GLIDE ACROSS THE AREA FROM WEST TO EAST AS THE UPPER LEVEL ENERGY EJECTS NORTHEAST FROM THE AREA. HAVE INCREASED PRECIPITATION CHANCES EARLY SUNDAY MORNING IN THE NEBRASKA PANHANDLE...BUT THEN DECREASED FROM INHERITED FORECAST AS EVEN SUPPORT FOR OROGRAPHIC SNOW SHOWERS IN THE SIERRA MADRE AND SNOWY RANGES DO NOT APPEAR AS LIKELY AS EARLIER FORECAST. HAVE LINGERED MID AND HIGH CLOUD COVER ACROSS SOUTHEAST WYOMING AND THE NEBRASKA PANHANDLE THROUGH MONDAY...AS LINGERING ENERGY IN THE BASE OF THE UPPER TROUGH WILL BE SLOW TO MOVE EAST OF THE AREA. THE EUROPEAN AND NAM ARE SHOWING THE LINGERING BASE OF THE UPPER TROUGH OVER COLORADO EVEN BY MIDDAY TUESDAY...WHILE THE GFS IS FORECASTING NEAR ZONAL FLOW TO ALMOST A WEAK UPPER SHORT-WAVE RIDGE. THEN THE GFS AND EUROPEAN SHOW SIMILAR UPPER LEVEL FLOW FEATURES...BUT HAVE VARYING STRENGTH ON THE TROUGH MOVING ALONG THE CANADIAN BORDER THURSDAY INTO FRIDAY. THE EUROPEAN IS MUCH MORE AMPLIFIED WITH THE TROUGH AND THUS CREATES A COLD FRONT AND MOVES IT THROUGH THE FORECAST AREA THURSDAY OR THURSDAY NIGHT. TEMPERATURES THROUGH THE LONG TERM WILL START OFF AROUND 10 DEGREES BELOW NORMAL FOR SUNDAY AND MONDAY...BEFORE WARMING TO SLIGHTLY ABOVE NORMAL VALUES THROUGH THE REMAINDER OF THE WEEK. AVIATION... WIDESPREAD MVFR/IFR CONDITIONS EXPECTED ACROSS THE AREA TODAY MAINLY DUE TO LOW CIGS. MORE WIDESPREAD IFR CONDITIONS FOR LOW VSBYS ALONG WITH LOW CIGS TONIGHT WITH A PERIOD OF RAIN AND/OR SNOW OCCURRING OVER MOST LOCATIONS. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...WINTER STORM WARNING UNTIL 6 AM SUNDAY FOR WYZ061-WYZ062-WYZ063- WYZ064. WINTER STORM WARNING FROM 6 PM SATURDAY TO 6 AM SUNDAY FOR WYZ059. WINTER WEATHER ADVISORY FROM 6 PM SATURDAY TO 6 AM SUNDAY FOR WYZ060-WYZ065-WYZ066-WYZ067-WYZ068-WYZ069-WYZ070. NE...WINTER WEATHER ADVISORY FROM 6 PM SATURDAY TO 6 AM SUNDAY FOR NEZ001-NEZ002-NEZ003. && $$ UPDATE...RUBIN SHORT TERM/AVIATION...RE LONG TERM...CARPENTER
