FXUS65 KCYS 302120
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
220 PM MST MON NOV 30 2009

.SHORT TERM....TONIGHT THROUGH WEDNESDAY.
CURRENT WATER VAPOR SHOWS A SHORT WAVE MOVING INTO SOUTHERN
CANADA/PAC NW. NAM AND GFS ARE CLOSE IN THE SHORT TERM...WITH THE
GFS A BIT FASTER BRINGING THE SHORT WAVE SOUTH ACROSS THE CWA. NW
FLOW WILL BE ALOFT OVER THE FORECAST AREA TONIGHT AS THE SHORT
WAVE CONTINUES TO MOVE INTO NORTHERN MT AND IDAHO. MOSTLY CLEAR
SKIES AND WESTERLY WINDS WILL CONTINUE OVER THE CWA TONIGHT. THE
SHORT WAVE WILL MOVE ACROSS MT TUES MORNING...WITH THE ASSOCIATED
COLD FRONT MOVING INTO NORTHERN WY BY 12Z TUES. THE UPPER TROUGH
WILL MOVE INTO NW WY BY 18Z TUES WITH THE TROUGH AXIS EXTENDING
FROM THE NORTHERN PLAINS TO THE CENTRAL ROCKIES PER NAM. THE COLD
FRONT WILL RACE SOUTH DURING THE DAY TUES WITH NORTH WINDS AND
MUCH COLDER TEMPERATURES MOVING INTO THE FORECAST AREA BEHIND THE
FRONT. LIGHT SNOW WILL DEVELOP IN THE AFTERNOON ACROSS CONVERSE
AND NORTHERN ALBANY AND CARBON COUNTIES. THE SHORT WAVE WILL
AFFECT THE REST OF THE CWA BY TUES EVENING AS IT MOVES SOUTH AND
EAST. A SECONDARY LOBE WILL ALSO MOVE ACROSS SW WY. SNOWFALL
AMOUNTS WILL BE FROM 3 TO 5 INCHES BY LATE TUES NIGHT IN THE
MOUNTAINS...2 TO 4 INCHES AT THE LOWER ELEVATIONS OF
CARBON...ALBANY AND CONVERSE COUNTIES...AND LESS THAN AN INCH
ACROSS THE REST OF THE AREA. THE SYSTEM IS MOVING FAST AND
SNOWFALL AMOUNTS ARE UNDER ADVISORY CRITERIA...SO DO NOT THINK
HIGHLIGHTS ARE NEEDED AT THIS TIME. SKIES WILL BECOME PARTLY
CLOUDY ON WED...BUT VERY COLD TEMPERATURES WILL OCCUR OVER THE
FORECAST AREA. HIGH TEMPERATURES WILL BE A FEW DEGREES BELOW
NORMAL TUES...AND 15 TO 20 DEGREES BELOW NORMAL ON WED.

.LONG TERM...WEDNESDAY NIGHT THROUGH SUNDAY.
COLD AIR WILL CONTINUE TO FILTER ACROSS WYOMING AND NEBRASKA ON
WEDNESDAY NIGHT INTO THURSDAY. BOTH THE GFS AND ECMWF SHOW 700MB
TEMPERATURES OF MINUS 22 DEGREES CELSIUS. EXPECT SOME LOCATIONS TO
EITHER TIE OR BREAK THE RECORD LOWEST HIGH TEMPERATURE ON
THURSDAY. SOME OF THE LOCATIONS INCLUDE THE CHEYENNE AIRPORT OF
21F DEGREES SET IN 1967...RAWLINS AIRPORT OF 17F DEGREES SET IN
1973...CHADRON AIRPORT OF 20F DEGREES SET IN 1995...AND
SCOTTSBLUFF AIRPORT OF 23F DEGREES SET IN 1935. OTHER LOCATIONS
WILL BE NEAR THE RECORD LOWEST HIGH TEMPERATURE ON THURSDAY. BOTH
THE GFS AND ECMWF INDICATE PRECIPITATION AS THE UPPER LEVEL LOW
SHIFTS EAST ON THURSDAY...MAINLY IN THE MOUNTAINS. THUS HAVE
PLACED A SLIGHT CHANCE OF MAINLY MOUNTAIN SNOW. HOWEVER...BOTH
MODELS INDICATED THE UPPER LEVEL HIGH MOVING DIRECTLY OVER EASTERN
WYOMING THROUGH THURSDAY. THE SUBSIDENCE SHOULD DIMINISH ANY
CHANCES OF PRECIPITATION OUTSIDE OF THE MOUNTAINS. EXPECT UP TO 2
INCHES OF SNOW ACCUMULATION IN THE MOUNTAINS WITH THIS SYSTEM.

BOTH MODELS SUGGEST SHORTWAVE RIDGING TO DEVELOP OVER THE WESTERN
UNITED STATES AND MOVE EAST. HOWEVER...THIS IS WHERE THE GFS AND
ECMWF BEGIN TO DIFFER GREATLY IN THE MODEL SOLUTIONS. THE GFS
RAPIDLY PUSHES THE UPPER LEVEL RIDGE ACROSS THE AREA AND MOVES THE
UPPER LEVEL LOW BEHIND IT ACROSS MONTANA BY SATURDAY MORNING. THE
ECMWF HAS A MORE SLOWER PROGRESSION OF THE SHORTWAVE RIDGE AND
DEVELOPS A MUCH STRONGER UPPER LEVEL LOW ACROSS THE PACIFIC
NORTHWEST. THE GFS ENSEMBLE MEAN INDICATES A UPPER LEVEL TROUGH
DEVELOPMENT BUT HAS A SLOWER PROGRESSION COMPARED TO THE LATEST
GFS MODEL RUN AND THE CANADIAN HAS A MODEL SOLUTION THAT APPEARS
TO LOOK A BLEND BETWEEN THE GFS AND ECMWF. BOTTOM LINE...FRIDAY
THROUGH SUNDAY THERE IS UNSETTLED WEATHER TO BE ANTICIPATED BUT
THERE IS STILL A LOT OF UNCERTAINTY OF THE TIMING. THUS HAVE KEPT
IN LOW GRADE CLIMATOLOGICAL POPS ON FRIDAY NIGHT INTO SUNDAY.

&&

.AVIATION...
VFR CONDITIONS WILL OCCUR ACROSS THE AREA UNTIL LATE
TUESDAY MORNING. AREAS OF MVFR CONDITIONS WILL OCCUR ACROSS THE
NORTHERN ZONES AS A COLD FRONT MOVES SOUTH ACROSS THE AREA. THE
MVFR CONDITIONS WILL SPREAD SOUTH ACROSS THE AREA BY EVENING. SOME
AREAS OF IFR CONDITIONS WILL ALSO OCCUR TUESDAY NIGHT...ESPECIALLY
IN AND NEAR THE MOUNTAINS DUE TO LOW CIGS AND VSBYS. CONDITIONS
WILL IMPROVE BY LATE MORNING WED.

&&

.FIRE WEATHER...
NO FIRE WEATHER CONCERNS AT THIS TIME.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...NONE.
NE...NONE.

&&

$$

SHORT TERM/AVIATION...KMD
LONG TERM...MAK