Issued by NWS Cheyenne, WY
000 FXUS65 KCYS 151039 AFDCYS AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHEYENNE WY 339 AM MST THU JAN 15 2009 .SHORT TERM...TODAY THROUGH FRI NIGHT... FAST NORTH NORTHWEST FLOW ALOFT TODAY AND TONIGHT WILL BACK TO MORE NORTHWESTERLY AND DECREASE SOMEWHAT FRI. A WEAK SHORTWAVE DISTURBANCE WILL IMPACT THE FORECAST AREA FRI NIGHT BUT ONLY SOME HIGH CLOUDS ARE LIKELY DUE TO WEAK LIFTING DYNAMICS AND LIMITED MOISTURE. AT THE SURFACE...A QUASI ARCTIC BACKDOOR TYPE OF FRONT BACKED OVER EASTERN PORTIONS OF THE FORECAST AREA WED EVENING...AND IS CURRENTLY LOCATED OVER EXTREME EASTERN WY BETWEEN THE LARAMIE RANGE AND THE NEB LINE. AREAS OF PATCHY FOG HAVE DEVELOPED EARLY THIS MORNING OVER SOUTHERN PORTIONS OF THE FORECAST AREA...AND THESE AREAS OF FOG WILL LIKELY PERSIST UNTIL 15Z OR SO. OTHERWISE...THE FRONT WILL LIKELY STALL OUT ALONG THE MOUNTAINS AS IT IS ORIENTED PARALLEL TO THE MID AND UPPER LEVEL FLOW THUS LITTLE ADDITIONAL SOUTHWESTWARD PUSH IS ANTICIPATED. EAST AND NORTH OF THE STALLING FRONT...LIGHT SNOW WILL BE POSSIBLE THIS MORNING IN THE SHALLOW SURFACE BASED UPSLOPE. THE COLDEST TEMPERATURES TODAY CAN BE EXPECTED EAST OF THE FRONT OVER NORTHERN PORTIONS OF THE WESTERN NEB PANHANDLE WHERE THE DEPTH OF THE COLD AIR MASS IS THE GREATEST. THE STALLED FRONT WILL BEGIN TO LIFE BACK TO THE EAST LATE TODAY AS A LEESIDE TROUGH. MOISTURE WILL DECREASE AND THIS AFTERNOON SHOULD BE MAINLY DRY. THE LEE TROUGH WILL MOVE EAST TONIGHT AS A WARM FRONT AND WILL BE PUSHED BY SOME FAIRLY BRISK WINDS ALONG AND ADJACENT TO THE LARAMIE RANGE. FRI WILL BE DRY AND MILDER WITH BRISK WINDS AND AFTERNOON TEMPS PUSHING 50 DEGREES EAST OF THE MOUNTAINS. FRI NIGHT WILL REMAIN DRY WITH BRISK DOWNSLOPE SURFACE WINDS EAST OF THE MOUNTAINS. .LONG TERM...SATURDAY THROUGH WEDNESDAY... MODELS FAIRLY SIMILAR FOR THIS PART OF THE FORECAST UP THROUGH TUESDAY. THE LARGE AMPLITUDE RIDGE ALOFT CURRENTLY JUST OFF THE WEST COAST MOVES EAST WITH THE AXIS OVER THE CWA ON TUESDAY. OVER THE WEEKEND SEVERAL SHORTWAVE TROUGHS WILL AFFECT THE CWA AS THEY MOVE OVER THE NORTHERN PLAINS IN THE NORTHERLY FLOW ALOFT. THAT WILL RESULT IN CONSIDERABLE MID AND HIGH LEVEL CLOUD COVER OVER THE CWA NAINLY SATURDAY MORNING AND AGAIN SUNDAY AFTERNOON AND SUNDAY NIGHT. SURFACE HIGH PRESSURE WILL REMAIN CENTERED OVER IDAHO AND UTAH OVER THE WEEKEND...WITH A WEAK SURFACE LOW PRESSURE TROUGH OVER THE EAST PART OF THE CWA. SLIGHT COOLING TAKES PLACE ON SATURDAY AS THE 1000 TO 500 MB THICKNESS VALUES LOWER SLIGHTLY. WITH THE HEIGHTS ALOFT CONTINUING TO RISE...TEMPERATURES WILL BE VERY MILD SUNDAY THROUGH TUESDAY WITH HIGHS SOME 20 DEGREES ABOVE NORMAL ESPECIALLY EAST OF THE LARMAIE RANGE. AND TO TOP IT OFF...SURFACE WINDS WILL BE MUCH LIGHTER THAN WE HAVE RECENTLY. MODELS DIFFER SOME FOR NEXT WEDNESDAY AND THURSDAY AS UPPER LEVEL TROUGH APPROACHES. THE FEATURE IS ALONG THE WEST COAST TUESDAY NIGHT WITH THE ECMWF SLOWER AND DEEPER WITH THE TROUGH THAN THE GFS. THE FEATURE MOVES EAST THROUGH NEXT THURSDAY WITH THE GFS NOW SLOWER THAN THE ECMWF BY 00Z FRIDAY WITH THE TROUGH AXIS IN THE SPLIT FLOW ALOFT. THE GFS HAS THE FEATURE FROM EASTERN MONTANA TO NEVADA AT THAT TIME...WHILE THE ECMWF PLACES THE TROUGH AXIS FROM NORTH DAKOTA TO WYOMING. BOTH MODELS MOVE AN ARCTIC FRONT ACROSS THE CWA WEDNESDAY NIGHT AND THURSDAY MORNING WITH A DEEP SURFACE LOW NEAR SOUTHEAST COLORADO 00Z FRIDAY. MODELS INDICATE THE SOUTHWEST WIND FLOW ALOFT WILL RIDE OVER THE BOUNDARY LAYER COLD AIR. STILL A LONG WAYS OUT...BUT AT THIS TIME IT APPEARS THAT NEXT WEDNESDAY NIGHT AND THURSDAY WILL BE MUCH COLDER WITH LIGHT SNOW. FOR NOW WILL JUST TREND FOR COOLER TEMPERATURES AND HIGHER POPS FOR WEDNESDAY NIGHT AND THURSDAY. && .AVIATION...AREAS OF MVFR CONDITIONS EAST OF THE LARAMIE RANGE EARLY THIS MORNING IN PATCHY FOG. OTHERWISE VFR CONDITIONS EXPECTED TODAY AND TONIGHT WITH DECREASING MOISTURE. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...NONE. NE...NONE. && $$ JR/WEILAND
