FXUS65 KCYS 211931
AFDCYS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
1231 PM MST MON DEC 21 2009
.UPDATE...
CANCELLED HIGH WIND WARNING FOR PORTIONS OF SOUTHEAST WY SINCE
WINDS HAVE DROPPED BELOW CRITERIA OVER THE LAST FEW HOURS.
&&
.PREV DISCUSSION... /ISSUED 538 AM MST MON DEC 21 2009/
SHORT TERM...TODAY THROUGH TUESDAY NIGHT...
SHORTWAVE FEATURE IS CURRENTLY MOVING THROUGH WESTERN WYOMING AT
12Z. THE SFC LEE TROUGH IS DEEPENING IN RESPONSE...WHICH WILL HELP
TO INCREASE WINDS THROUGH THE MORNING...WITH THE STRONGEST WINDS
PROGGED TO OCCUR DURING THE LATE MORNING HOURS. WILL CONTINUE WITH
THE CURRENT HIGH WIND WARNING IN EXPECTATION OF WINDS REACHING
CRITERIA IN THE LATE MORNING...ALTHOUGH GUIDANCE HAS BACKED OFF
SOMEWHAT WITH INTENSITY OF THE WINDS SO COULD BE A VERY LOW END
EVENT. TEMPERATURES FOR TODAY WILL REMAIN ON THE WARM SIDE WITH
THE DOWNSLOPE WINDS IN FULL FORCE...IN ADDITION TO INCREASING
INSOLATION AS SKIES CLEAR SOMEWHAT IN THE WAKE OF THE DEPARTING
WAVE. WINDS WILL EASE THROUGH THE EVENING AS THE GRADIENT RELAXES
WITH THE DEPARTING WAVE. ON TUESDAY...A STRONG UPPER LOW WILL DIG
ACROSS THE WESTERN US WITH THE LOW CENTERED OVER ARIZONA BY 12Z
WEDNESDAY. AS THIS OCCURS...FLOW ALOFT WILL BACK TO THE SOUTHWEST
AND WILL ADVECT PACIFIC MOISTURE INTO THE AREA. A PIECE OF ENERGY
WILL BREAK OFF AND MOVE ACROSS THE AREA EARLY ON TUESDAY...
ENHANCING OROGRAPHIC SNOW SHOWERS AND AN INVERTED TROUGH FEATURE
OVER THE ROCKY MOUNTAINS. LOW LEVEL WINDS WILL BECOME SOUTHERLY
ACROSS THE CWA AS THIS OCCURS...THEN SHIFT TO THE NORTHEAST BY
EARLY WED MORNING AS THE TROUGH SHIFTS TO THE SOUTH WITH THE
DEEPENING UPPER LOW. UPPER LEVEL FLOW WILL REMAIN OUT OF THE SOUTH
AND MODELS INDICATED DEEP ISENTROPIC LIFT SETTING UP OVER
SOUTHEAST WYOMING AND NEBRASKA. THUS EXPECT PRECIPITATION TO
INCREASE IN COVERAGE THROUGH THE DAY TUESDAY WITH WIDESPREAD
PRECIPITATION EXPECTED BY TUESDAY NIGHT. NAM SOUNDINGS AND SREF
GUIDANCE SHOW A WARM LAYER ALOFT INITIALLY IN THE DEEP ISENTROPIC
FLOW...THEREFORE EXPECT THE PRECIP TYPE EAST OF THE LARAMIE RANGE
TO BEGIN AS RAIN OR A RAIN SNOW MIX TUESDAY AFTERNOON BEFORE
TRANSITIONING TO SNOW TUESDAY EVENING. AS WINDS SHIFT TO THE
NORTHEAST TUESDAY NIGHT...EXPECT SNOWFALL RATES TO PICK UP QUITE A
BIT WITH THE COMBINATION OF UPSLOPE FLOW AND MID LEVEL LIFT. COULD
SEE A COUPLE INCHES BY WED MORNING OVER AND EAST OF THE LARAMIE
RANGE...WITH LOCALLY HIGHER AMOUNTS IN THE SOUTHERN LARAMIE RANGE
AND ALONG THE CHEYENNE RIDGE.
LONG TERM...WEDNESDAY THROUGH SUNDAY...
BY WEDNESDAY MORNING...THE ELONGATED UPPER CLOSED LOW WILL BE
TRACKING ACROSS THE SRN ROCKIES...AND WILL EMERGE ONTO THE SRN
PLAINS DURING THE DAY ON WEDNESDAY. SFC CYCLOGENESIS WILL OCCUR
EARLY WEDNESDAY...WITH AN EVENTUAL 998MB SFC LOW DEVELOPING ACROSS
WEST TX. THIS SFC LOW WILL COMBINE WITH A 1040MB SFC HIGH ACROSS
SRN CANADA TO CREATE A STRONG NORTHEAST AND EVENTUAL PRESSURE
GRADIENT ALONG THE WYOMING PLAINS AND NEBRASKA PANHANDLE DURING THE
DAY ON WEDNESDAY. A FAVORABLE THERMAL PROFILE WILL FOSTER GOOD
DENDRITIC GROWTH ESPECIALLY EARLY ON WEDNESDAY ACROSS SOUTHEAST
WYOMING AND THE SRN NEBRASKA PANHANDLE. ADDITIONALLY...MOIST ASCENT
ENHANCED BY OROGRAPHICS WILL CONTINUE TO RESULT IN GOOD SNOWFALL
RATES...ESPECIALLY EARLY WEDNESDAY. BY LATE WEDNESDAY SNOW SHOULD
BEGIN TO GRADUALLY TAPER OFF AS THE BEST MOISTURE AND LIFT BEGINS TO
PULL AWAY. BEST SNOWFALL IS EXPECTED TO BE GENERALLY SOUTHEAST OF
VEDAUWOO TO LUSK LINE. AN ADDITIONAL 2 TO 4 INCHES WILL BE POSSIBLE
DURING THE DAY ON WEDNESDAY...BRINGING POSSIBLE STORM TOTALS TO 4
TO 8 INCHES. ALTHOUGH SOME UNCERTAINTY REMAINS ON THE PRECISION OF
SNOWFALL AMOUNTS...MODEL TEMPORAL AND SPATIAL CONSISTENCY
CONTINUES TO ADD TO CONFIDENCE. SO...HAVE OPTED TO HOIST A WINTER
STORM WATCH FOR AREAS SOUTHEAST OF THIS VEDAUWOO TO LUSK LINE
TUESDAY NIGHT THROUGH WEDNESDAY NIGHT...WITH 4 TO 8 INCHES OF SNOW
POSSIBLE.
THE UPPER LOW WILL EJECT NORTHEAST INTO THE MID-MISSISSIPPI RIVER
VALLEY FOR THURSDAY...WITH NORTHERLY FLOW ACROSS THE NRN ROCKIES.
MODELS DIFFER SLIGHTLY ON THE STRENGTH OF A SECONDARY SHORTWAVE
DROPPING SOUTHEAST OUT OF CANADA FOR THURSDAY...WITH THE GFS THE
STRONGEST...AND ECMWF THE WEAKEST. IN EITHER CASE...DO NOT EXPECT
ANY MOISTURE WITH THIS SYSTEM...HOWEVER IT WILL EFFECTIVELY
REINFORCE THE COLD AIR ALONG THE FRONT RANGE. UPPER RIDGE WILL
AMPLIFY ACROSS THE PACIFIC COAST...MAINTAINING DRY NORTHERLY FLOW
THROUGH AT LEAST SATURDAY NIGHT. THE GFS SUGGESTS THAT THIS RIDGE
WILL BUILD IN BY THE LATTER HALF OF NEXT WEEKEND.
AFTERNOON HIGHS WILL ONLY REACH THE 20S ON WEDNESDAY THROUGH FRIDAY.
VALLEYS WEST OF THE LARAMIE RANGE WILL NOT MAKE IT OUT OF THE TEENS
ON THURSDAY AND FRIDAY. TEMPERATURES WILL MODERATE BACK TO NEAR
NORMAL FOR THE WEEKEND.
AVIATION...
WEST WINDS WILL INCREASE AT WYOMING TERMINALS TODAY AS AN UPPER LEVEL
DISTURBANCE RACES BY. SUSTAINED WIND SPEEDS OF 20 TO 25 KTS CAN BE
EXPECTED WITH GUSTS OF 30 TO 35 KTS. WINDS WILL BE LIGHTER ACROSS
THE PANHANDLE TODAY. VFR CONDITIONS WILL OCCUR TODAY...WITH JUST
SOME MID/UPPER-LEVEL CLOUDS PASSING OVERHEAD. IFR CIGS/VSBYS ARE
EXPECTED AT KCDR LATE THIS EVENING AND OVERNIGHT AS WINDS BECOME
LIGHT AND MOISTURE INCREASES.
FIRE WEATHER...
NO FIRE WEATHER CONCERNS AT THIS TIME.
&&
.CYS WATCHES/WARNINGS/ADVISORIES...
WY...WINTER STORM WATCH FROM 6 PM TUESDAY TO 11 PM WEDNESDAY FOR
WYZ102-WYZ106-WYZ107-WYZ108-WYZ116-WYZ117-WYZ118-WYZ119.
NE...WINTER STORM WATCH FROM 6 PM TUESDAY TO 11 PM WEDNESDAY FOR
NEZ002-NEZ003-NEZ019-NEZ020-NEZ021-NEZ054-NEZ055-NEZ095-
NEZ096.
&&
$$
SHORT TERM/AVIATION...WFOCYS
LONG TERM...WFOCYS
