Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 251807
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
1107 AM MST THU DEC 25 2014

.SHORT TERM...(TODAY THROUGH SATURDAY)
ISSUED AT 221 AM MST THU DEC 25 2014

PREVIOUS FORECAST LOOKS RIGHT ON TRACK AS THE MODELS LITTLE CHANGED
FROM YESTERDAY. GFS DOES SHOW SOME LIGHT PRECIPITATION NEAR THE WY
AND NE BORDER EARLY THIS MORNING...WHICH DOES LINE UP NICELY WITH
RADAR ECHOS...BUT LITTLE IF ANY IS REACHING THE GROUND AS IT IS
TAKING SOME TIME TO SATURATE THE BOUNDARY LAYER. AS OF 10Z DOUGLAS
SHOWED 4SM AND LIGHT SNOW...SO STARTING TO SEE SATURATION AT HIGH
ELEVATIONS...PLUS UPSLOPE FLOW NOW KICKING ALONG THE NORTHERN
LARAMIE RANGE AS THE COLD FRONT PUSHES SOUTH. WINDS ARE TURNING
NORTH AS CHEYENNE AT THIS TIME. THE MAIN PUSH OF COLD AIR WILL START
THIS AFTERNOON AND WILL SEE STEADY TO FALLING TEMPERATURES.

WINTER STORM WARNINGS AND ADVISORIES ARE IN EFFECT TODAY AND TONIGHT
ACROSS THE REGION. HIGH END ADVISORY TO LOW END WINTER STORM WARNING
TYPE SNOW AMOUNTS WILL OCCUR ACROSS MOST OF THE FORECAST
AREA...ALTHOUGH SOME HIGHER ACCUMULATIONS ARE LIKELY ALONG PORTIONS
OF THE CHEYENNE RIDGE IN LARAMIE COUNTY AND THE FOOTHILLS OF THE
LARAMIE RANGE DUE TO UPSLOPE FLOW. THE OTHER FEATURE DRIVING HIGH
SNOW AMOUNTS...ESPECIALLY OVER LARAMIE COUNTY IS A POCKET OF
INSTABILITY WHICH THE MODELS HAVE BEEN SHOWING THE LAST FEW DAYS. IT
IS POSSIBLE FOR HIGHER SNOW RATES LATE THIS AFTERNOON AND EVENING
DUE TO THE UNSTABLE BOUNDARY LAYER IN THIS AREA. ALSO THE NAM AND TO
A LESSER EXTEND THE GFS ARE SHOWING AN AREA OF FRONTOGENESIS
COLLOCATED WITH THE UNSTABLE AIR...WHICH MEANS THERE IS A HIGH
LIKELIHOOD FOR A 3 TO 6 HOUR PERIOD WITH A VERY HEAVY SNOW BAND. THE
MOSTLY LIKELY TIME FRAME LOOKS TO BE BETWEEN 21Z AND 3Z. THE MOSTLY
LIKELY LOCATION BEING FROM AROUND LARAMIE TO CHEYENNE. IF A BAND OF
HEAVY SNOW DOES DEVELOP...THEN SNOW AMOUNTS COULD EASILY REACH A
FOOT IN THESE AREAS AND TRAVEL THROUGH THIS AREA WOULD BECOME
EXTREMELY DIFFICULT IF NOT IMPOSSIBLE FOR A TIME.

INTO THE NEBRASKA PANHANDLE SNOW AMOUNTS WILL BE LESS THAN ACROSS
MOST OF SOUTHEAST WYOMING BUT STILL LOOKING AT 2 TO 5 INCHES OVER
MOST OF THE PANHANDLE. WOULD NOT BE SURPRISED TO LOCALLY SEE 6 TO 7
INCHES OF SNOW IN THE COUNTIES ALONG THE WY BORDER. WINDS WILL BE A
BIT GUSTY ACROSS THE NEBRASKA PANHANDLE TOWARDS THIS EVENING AND
INTO THE OVERNIGHT WITH GUSTS 20 TO 30 MPH WHICH WHEN COMBINED WITH
FALLING SNOW WILL REDUCE VISIBILITIES TO 1/2 SM OR LESS. SIMILAR
WIND SPEEDS WILL BE FOUND OVER SOUTHEAST WY EAST OF THE LARAMIE
RANGE LATE THIS AFTERNOON THROUGH THE EVENING HOURS BEFORE
DISSIPATING. SO IF CHEYENNE DOES SEE A BAND OF HEAVY SNOW...WINDS
WILL NOT HELP ANY AS SNOW WILL DRIFT...MAKING TRAVEL EVEN MORE
DIFFICULT.

SNOW WILL TAPER OFF FROM NORTHWEST TO SOUTHEAST ACROSS THE AREA
TONIGHT INTO FRIDAY. THAT IS IF THE NAM AND GFS SOLUTIONS ARE
CORRECT. THE CANADIAN AND ECMWF CONTINUE WITH LIGHT SNOW ACROSS FAR
SOUTHEAST WY AND SOUTHERN NE PANHANDLE THROUGH FRIDAY AFTERNOON AND
INTO THE EVENING AROUND SIDNEY NEBRASKA. THE FORECAST IS BASED OFF
THE NAM/GFS SOLUTIONS BUT WILL HAVE TO SEE WHICH WAY THE 12Z MODELS
TREND AS MAY HAVE TO INCREASE POPS AND SNOW AMOUNTS INTO THE
SOUTHEAST FORECAST AREA FRIDAY. SATURDAY WILL BE DRY BUT COULD SEE
SOME LIGHT MOUNTAIN SNOWS LATE IN THE DAY.

.LONG TERM...(SATURDAY NIGHT THROUGH WEDNESDAY)
ISSUED AT 321 AM MST THU DEC 25 2014

MAIN FORECAST CONCERN FOR THE LONG RANGE IS THE ARCTIC COLD FRONT
FORECAST TO PUSH SOUTH ACROSS THE FORECAST AREA SUNDAY NIGHT...IN
ADDITION TO A PERIOD OF SNOW BEGINNING AS EARLY AS SUNDAY
AFTERNOON. THIS ARCTIC AIRMASS HAS THE POTENTIAL TO BE THE COLDEST
SO FAR THIS SEASON WITH BITTER COLD TEMPERATURES POSSIBLE MONDAY
THROUGH EARLY WEDNESDAY.

MODELS IN GOOD AGREEMENT ON THE GENERAL PATTERN SUNDAY AND
MONDAY...BUT ARE STRUGGLING WITH THE PLACEMENT OF A VIGOROUS
SHORTWAVE TROUGH FORECAST TO DIG SOUTHEAST ACROSS THE PACIFIC
NORTHWEST AND INTO THE IDAHO/WYOMING AREA BY SUNDAY AFTERNOON. THE
MOVEMENT AND PLACEMENT OF THIS SHORTWAVE WILL MAINLY DETERMINE
SNOW AMOUNTS BEGINNING LATE SUNDAY AND POSSIBLY CONTINUING INTO
TUESDAY. WILL HAVE TO WATCH HOW THIS SHORT WAVE INTERACTS WITH JET
ENERGY ALOFT AND POSSIBLE CONDITIONAL INSTABILITY ALONG THE ARCTIC
FRONT. MODELS ARE INDICATING SOME STRONG FORCING AND LIFT ACROSS
MOST OF THE I80 CORRIDOR FOR A PROLONGED PERIOD. HAVE A FEELING
MODELS ARE DOWNPLAYING SNOWFALL POTENTIAL...WITH THE 00Z GFS BEING
THE ONLY EXCEPTION. ARCTIC FRONTS TEND TO BE DIFFICULT TO
PREDICT...IN TERMS OF SNOWFALL AND SPECIFICALLY SNOWFALL RATES 4
TO 6 DAYS OUT. SO WILL HAVE TO CLOSELY MONITOR THIS THROUGH THIS
WEEKEND. CONTINUED TO INCREASE POP BETWEEN 50 TO 70 PERCENT INTO
MONDAY...HIGHEST VALUES OVER THE MOUNTAINS. WOULD NOT BE SURPRISED
IF SOME LOCATIONS OVER THE HIGH PLAINS RECEIVE MODERATE SNOWFALL
ACCUMULATIONS WITH PERIODIC HEAVIER SNOW BANDS. AT THIS TIME...IT
IS TOO DIFFICULT TO DETERMINE THE PRECISE LOCATION AND TIMING.

THE ARCTIC AIRMASS WILL SETTLE OVER THE REGION MONDAY AND MONDAY
NIGHT...WITH MODELS INDICATING 850MB TEMPERATURES BETWEEN -20C TO
-25C. THESE VALUES TRANSLATE TO SURFACE TEMPERATURES BELOW ZERO
ACROSS MOST OF THE FORECAST AREA. FORECAST HIGH TEMPERATURES ON
TUESDAY ARE CURRENTLY BETWEEN 0 TO 5 DEGREES ABOVE ZERO.
HOWEVER...IF CURRENT MODEL TRENDS CONTINUE...MANY LOCATIONS MAY
SEE HIGH TEMPERATURES SOMEWHERE AROUND -5 TO -10. IN
ADDITION...MODELS ARE ALSO INDICATE WARM AIR ADVECTION ALOFT. WITH
A SOLID SNOWPACK ACROSS MOST OF THE AREA...LLVL TEMPERATURE
INVERSIONS WILL BE A MAJOR PROBLEM WITH THE LACK OF
WIND...ESPECIALLY THIS TIME OF THE YEAR WITH A LOW SUN ANGLE. LOW
TEMPERATURES MAY BECOME BITTERLY COLD WITH DANGEROUS WIND CHILLS
TUESDAY AND WEDNESDAY MORNING.

THANKFULLY...THE ARCTIC AIRMASS WILL NOT STICK AROUND TOO LONG AS
HIGH TEMPERATURES WILL RETURN TO THE TEENS BY WEDNESDAY
AFTERNOON...AND POSSIBLY IN THE 20S TO NEAR 30 BY THURSDAY AS
WESTERLY FLOW STRENGTHENS OVER THE HIGH PLAINS. POP WILL BE BELOW
10 PERCENT WEDNESDAY AND THURSDAY WITH DRY CONDITIONS EXPECTED.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z FRIDAY AFTERNOON)
ISSUED AT 1104 AM MST THU DEC 25 2014

WIDESPREAD LIFR AND IFR WILL CONTINUE THIS AFTERNOON AND INTO THE
EVENING HOURS WITH MOUNTAINS OBSCURED. AS SNOW DECREASES IN
COVERAGE AND INTENSITY TONIGHT...WE EXPECT FOG AND LOW CLOUDS TO
REMAIN WIDESPREAD PRODUCING IFR. CONDITIONS SLOWLY IMPROVING TO
MVFR AND VFR FRIDAY MORNING.

&&

.FIRE WEATHER...
ISSUED AT 221 AM MST THU DEC 25 2014

A WINTER STORM SYSTEM WILL BRING SNOW TO THE AREA TODAY AND TONIGHT.
A FEW LINGERING SNOW SHOWERS FOR FRIDAY BUT LITTLE TO NO SNOW
ACCUMULATION EXPECTED. DRY FOR THIS WEEKEND BUT THEN ANOTHER STORM
SYSTEM TO IMPACT THE AREA MONDAY ALONG WITH A STRONG ARCTIC COLD
FRONT WITH BITTERLY COLD AIR. HIGH TEMPERATURES MONDAY AND TUESDAY
WILL BE IN THE SINGLE DIGITS WITH SOME PLACES STRUGGLING TO
REACH ZERO DEGREES FOR HIGHS ON TUESDAY. WIND CHILLS WILL BE MINUS
20 TO MINUS 35 EARLY NEXT WEEK.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...WINTER WEATHER ADVISORY UNTIL 5 AM MST FRIDAY FOR WYZ102-103.

     WINTER STORM WARNING UNTIL 5 AM MST FRIDAY FOR WYZ101-104>119.

NE...WINTER WEATHER ADVISORY UNTIL 5 AM MST FRIDAY FOR NEZ002-003-021-
     055-095-096.

     WINTER STORM WARNING UNTIL 5 AM MST FRIDAY FOR NEZ019-020-054.

&&

$$

SHORT TERM...SML
LONG TERM...TJT
AVIATION...RUBIN
FIRE WEATHER...SML


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