Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 272227
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
327 PM MST THU NOV 27 2014

.SHORT TERM...(TONIGHT THROUGH SATURDAY)
ISSUED AT 322 PM MST THU NOV 27 2014

WARM AND WINDY CONDITIONS CONTINUE ACROSS THE CWA THIS AFTN. A HIGH
WIND WARNING WILL REMAIN IN EFFECT FOR ARL AND BRX THROUGH 5 PM THIS
EVE AS RECENT HIGHWAY OBS HAVE SHOWN SUST WINDS AROUND 35-40 MPH AND
GUSTS NEAR 55 MPH. UNTIL RECENTLY HOWEVER...THE TYPICAL WINDY AREAS
HAVE STRUGGLED TO GUST MUCH HIGHER THAN 50 MPH. BELIEVE THIS IS THE
RESULT OF EXTENSIVE MIDLVL CLOUD COVER PREVENTING THE EROSION OF AN
IMPRESSIVE SFC INVERSION...KEEPING THE STRONGEST FLOW JUST OFF THE
SURFACE. WOULD NOT TAKE MUCH TO ACHIEVE WARNING CRITERIA AS THE CYS
VAD WIND PROFILE SHOWED 50-55 KTS OF FLOW ONLY AROUND 2K FEET AGL AT
18Z TODAY. DECIDED TO CANCEL THE WARNING FOR THE SUMMIT AND ADJACENT
SOUTH LARAMIE RANGE FOOTHILLS AS VEDAUWOO HAS BEEN BELOW 45 MPH ALL
DAY. THE SFC PRESSURE GRADIENT WILL REMAIN STRONG OVERNIGHT SO GUSTY
WINDS SHOULD PERSIST. HOWEVER THE H85 CAG-CPR GRADIENT IS PROGGED TO
FALL BELOW 55 METERS SHORTLY AFTER 00Z SO EXPECT ANY POTENTIAL FOR
60 MPH GUSTS TO QUICKLY DIMINISH AFTER SUNSET.

ANOTHER DAY OF STRONG WESTERLY/DOWNSLOPING WINDS FOR FRIDAY AS THE
LEE SFC TROUGH REMAINS TO THE EAST OF THE CWA. LITTLE CHANGE IN THE
OVERALL SYNOPTIC PATTERN WITH BOTH THE GFS AND NAM SHOWING A 130 KT
H25 JET STREAK ENTERING NORTHERN WY AT 12Z FRI. STRONG SUBSIDENCE IS
EXPECTED IN THE FAVORED RIGHT EXIT REGION AND WINDS ALOFT COULD BE
MUCH HIGHER THAN TODAY PER RECENT MODEL PROGS. 60 TO 70 METER CAG-
CPR GRADIENTS DEVELOPING AFTER 12Z FRI ARE CONCERNING...ESPECIALLY
SEEING GFS/NAM H75 FLOW AS HIGH AS 60 KTS ALONG THE LARAMIE RANGE BY
EARLY AFTN. MUCH LIKE TODAY...GFS AND NAM VERTICAL XSECTIONS SHOW A
HEALTHY 50 TO 60 KT BELT OF FLOW JUST OFF THE SFC ON FRI AFTN. GAVE
STRONG CONSIDERATION TO A HIGH WIND WATCH AFTER 18Z BUT DECIDED
AGAINST IT. NAM SOUNDINGS CONTINUE TO SHOW A STRONG SFC INVERSION
WHICH WAS OBVIOUSLY A MAJOR CONTRIBUTING FACTOR IN THE LIMITED WINDS
TODAY. WAVE CLOUDS WILL ALSO BE AROUND...ALTHOUGH AS A GENERAL RULE
CLOUD COVER SHOULD BE LESS. CHANCES OF HITTING HIGH WIND CRITERIA
ARE PROBABLY HIGHER TOMORROW...BUT POOR LAPSE RATES COULD KEEP WINDS
VERY MARGINAL AT BEST. HIGH WIND POTENTIAL HAS BEEN WELL ADVERTISED
THROUGH THE END OF THE WEEK...SO THE PREFERRED COURSE OF ACTION IS
TO LET THE NEXT COUPLE OF SHIFTS MONITOR TRENDS.

ASIDE FROM THE WINDS...FRIDAY SHOULD BE QUITE WARM ALONG AND EAST OF
THE LARAMIE RANGE. GFS/ECMWF H7 TEMPERATURES ARE IN THE 3 TO 5 DEG C
RANGE IN A FAVORABLE SYNOPTIC PATTERN FOR DOWNSLOPE OR COMPRESSIONAL
WARMING. MOS GUIDANCE SUGGESTS LOW TO MID 60S FOR MOST OF SOUTHEAST
WY AND THE WESTERN NE PANHANDLE...SO CONTINUED TO TREND WARMER WITH
THE ANTICIPATION THAT CLOUD COVER WILL NOT BE QUITE AS THICK DURING
PEAK HEATING GIVEN RH PROGS THROUGH THE COLUMN. THERMAL PROFILES ARE
EXPECTED TO COOL SLIGHTLY ON SAT AHEAD OF A STRONG COLD FRONT MOVING
SOUTH ACROSS THE NORTHERN ROCKIES AND HIGH PLAINS. PRESSURE FALLS IN
ADVANCE OF THIS FEATURE WILL ONCE AGAIN STRENGTHEN LLVL GRADIENTS IN
THE GAP AREAS. CONTINUED SUBSIDENCE AND IMPROVEMENT IN LAPSE RATES
WOULD SUGGEST POTENTIAL FOR HIGH WINDS ON SAT AFTN AS WELL.

.LONG TERM...(SATURDAY NIGHT THROUGH THURSDAY)
ISSUED AT 322 PM MST THU NOV 27 2014

A SHORTWAVE TROUGH WILL MOVE EAST ACROSS THE PAC NW AND NORTHERN
ROCKIES SATURDAY NIGHT...AND ACROSS THE NORTHERN PLAINS THROUGH
SUNDAY. THE COLD FRONT ASSOCIATED WITH THIS TROUGH WILL SLIDE SOUTH
ACROSS THE HIGH PLAINS AND INTO THE CWA ON SATURDAY NIGHT...AND
LOOKS TO STALL JUST WEST OF THE LARAMIE RANGE THRU SUNDAY. BEST
MIDLEVEL FORCING WITH THE WAVE WILL REMAIN NORTH OF THE CWA...WHILE
JET DYNAMICS WILL BE BEST IN COLORADO AND THE CENTRAL PLAINS.
THEREFORE...PRECIP CHANCES FOR THE CWA WILL BE TIED TO FRONTAL
FORCING IN ADDITION TO OROGRAPHICS AS THE FRONT SLIDES WEST OF THE
LARAMIE RANGE AND SFC WINDS TURN NNE. EXPECT TO SEE RAIN/SNOW MIX AT
THE ONSET LATE SATURDAY NIGHT IN THE FAR NORTHERN ZONES BUT WILL
QUICKLY CHANGE TO ALL SNOW AS THE FRONT RUSHES SOUTH AND COLD
CANADIAN AIR FILTERS IN. SHOULD SEE LIGHT SNOW SPREAD NORTH TO SOUTH
ACROSS THE CWA THROUGH SUNDAY AFTERNOON...WITH THE FOCUS SHIFTING TO
THE MOUNTAINS AND IMMEDIATE LEESIDE PLAINS FOR THE EVENING IN
UPSLOPE FLOW. AGAIN...WITH THE LACK OF UPPER SUPPORT...DO NOT EXPECT
MUCH MORE THAN AN INCH OR TWO OF ACCUMULATION. WILL ALSO SEE A BRIEF
RESPITE FROM THE WINDS AS THE FRONT LOOKS TO REMAIN STALLED OVER THE
MOUNTAINS THROUGH SUNDAY NIGHT. BROADSCALE RIDGING WILL TAKE HOLD
OVER THE ROCKIES THROUGH THE REMAINDER OF THE PERIOD WITH THE
LEESIDE SFC TROUGH DEVELOPING ONCE AGAIN LATE MONDAY. THE FRONT
SHOULD KICK EASTWARD SLOWLY BUT SURELY AS THE LEE TROUGH DEVELOPS.
WINDS WILL PICK UP ON MONDAY AS THE FRONT SHIFTS EAST AND
STRENGTHENS THE SW-NE PRESSURE GRADIENT ALONG AND EAST OF THE
LARAMIE RANGE THROUGH TUESDAY MORNING.

TUESDAY WILL BE MILD AND BREEZY WITH DRY CONDITIONS. MODELS THEN
DIFFER WITH THE SYNOPTIC PATTERN WEDNESDAY WITH THE GFS BRINGING A
SHORTWAVE TROUGH ACROSS THE ROCKIES WHILE THE ECMWF/CANADIAN MODELS
MAINTAIN RIDGING ALOFT. TRENDED THE FORECAST TOWARD THE LATTER TWO
MODELS...ALTHOUGH DID NOT COMPLETELY DISCOUNT THE GFS AS THE
ENSEMBLE MEAN HINTS AT THE TROUGH MOVING OVERHEAD AS WELL. KEPT
SLIGHT CHANCE POPS MAINLY IN THE WEST...WITH HIGHER POPS IN THE
MOUNTAINS THROUGH WEDNESDAY NIGHT TO ACCOUNT FOR THIS UNCERTAINTY.
TEMPS WILL REMAIN MILD FOR WEDNESDAY WITH BREEZY WINDS OUT WEST.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z FRIDAY MORNING)
ISSUED AT 1102 AM MST THU NOV 27 2014

MAIN CONCERN FOR THIS TAF PERIOD WILL BE GUSTY WINDS AND MOUNTAIN
WAVE TURBULENCE...MAINLY FOR THE WYOMING TERMINALS. WEST TO
SOUTHWEST WINDS GUSTING TO 35 KTS WILL CONTINUE THIS
AFTERNOON...WITH WINDS QUICKLY INCREASING TO 50-60 KTS JUST OFF
THE SURFACE. DIRECTION OF WINDS ALOFT WILL BE AT A WEST-NORTHWEST
COMPONENT. MOUNTAIN WAVE TURBULENCE IS EXPECTED TO BE MOD TO SEV
AT TIMES ESPECIALLY CLOSEST TO THE PEAKS...AND WILL PERSIST
THROUGH FRIDAY WITH THESE STRONG WINDS IN PLACE. SFC WINDS ACROSS
THE PANHANDLE WILL REMAIN LIGHT THROUGH EARLY AFTERNOON BUT WILL
STEADILY INCREASE FROM WEST TO EAST AFTER 20Z WITH GUSTS TO 25 KTS
THROUGH EARLY EVENING. KCDR MAY BE THE EXCEPTION AS THIS AREA WILL
BE SHELTERED BY THE PINE RIDGE. WINDS WILL DIMINISH SLIGHTLY
OVERNIGHT...BUT WILL INCREASE ONCE AGAIN LATE FRIDAY MORNING. DRY
CONDITIONS WILL PREVAIL WITH MAINLY HIGH CIRRUS MOVING OVERHEAD
FROM THE WEST.

&&

.FIRE WEATHER...
ISSUED AT 250 AM MST THU NOV 27 2014

MINIMAL FIRE WEATHER CONCERNS THROUGH THE HOLIDAY WEEKEND. TEMPERATURES
WILL BE QUITE MILD OVER THE NEXT FEW DAYS. MINIMUM AFTERNOON HUMIDITY
VALUES WILL BE LOWEST ON FRIDAY AND SATURDAY WITH A FEW LOCATIONS TO
THE EAST OF THE LARAMIE RANGE IN THE 15-20 PERCENT RANGE. IT WILL BE
WINDY ACROSS SOUTHEAST WYOMING THROUGH SATURDAY WITH GUSTS TO 35-40
MPH COMMON...ESPECIALLY DURING THE AFTERNOONS.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...HIGH WIND WARNING UNTIL 5 PM MST THIS AFTERNOON FOR WYZ106-110.

NE...NONE.
&&

$$

SHORT TERM...CLH
LONG TERM...RJM
AVIATION...RJM
FIRE WEATHER...CLH/ZF


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