Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 241201
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
601 AM MDT TUE MAR 24 2015

.SHORT TERM...(TODAY THROUGH WEDNESDAY)
ISSUED AT 415 AM MDT TUE MAR 24 2015

OVERNIGHT WATER VAPOR LOOP SHOWED TWO SHORTWAVE TROUGHS...ONE LIFTING
NORTHEAST TOWARD THE DAKOTAS AND THE OTHER MOVING EAST ACROSS THE
COLORADO FRONT RANGE. ELEVATED INSTABILITY NORTH OF A EAST-WEST BOUNDARY
ACROSS THE CENTRAL GREAT PLAINS WAS TRIGGERING SCATTERED SHOWERS/TSTORMS
FROM NORTH/NORTHEAST KANSAS INTO SOUTHEAST NEBRASKA. LIGHTER SHOWERS
WERE MOVING ACROSS THE COLORADO FRONT RANGE AND PLAINS. SKIES WERE
MOSTLY CLOUDY WITH TEMPERATURES RANGING FROM THE UPPER 20S TO LOWER
40S. THE PRESSURE GRADIENT WAS STRENGTHENING OVERNIGHT...WITH WESTERLY
WINDS GUSTING TO 45 MPH ALONG THE I-25 AND I-80 CORRIDORS.

FORECAST CHALLENGES DURING THE NEXT 24 TO 36 HOURS WILL DEAL WITH STRONG
WINDS AND PRECIPITATION CHANCES. LATEST NUMERICAL MODEL GUIDANCE CONCURS
WITH THE EASTWARD PROGRESSION OF THE SHORTWAVE TROUGHS FROM THE FRONT
RANGE INTO THE GREAT PLAINS. TRAILING THE TROUGHS WILL BE AN ELONGATED
VORT LOBE FROM THE PACIFIC NORTHWEST INTO MONTANA AND WYOMING...WHICH
WILL EVOLVE INTO AN AMPLIFYING TROUGH OVER THE HIGH PLAINS WEDNESDAY.
INTENSIFYING SURFACE LOW PRESSURE OVER EXTREME NORTHEAST WYOMING AND
SOUTHEAST MONTANA WILL TRACK NORTHEAST INTO SOUTHERN NORTH DAKOTA
TODAY. THE LARGE GRADIENT BETWEEN THIS SYSTEM AND A STOUT RIDGE
TO THE WEST WILL RESULT IN RATHER STRONG AND GUSTY WEST TO NORTHWEST
WINDS. DEEP LAYER MIXING AND LARGE SCALE SUBSIDENCE BEHIND THE EXITING
TROUGH WILL ALLOW 50-60 KT WINDS ALOFT TO REACH THE SURFACE. THE
CRAIG-CASPER 700-850MB GRADIENT PEAKS IN THE 5OS METERS TODAY...THEN
TAPERS OFF THIS EVENING. HIGH WIND CRITERIA WILL BE MET TODAY FOR
WIND PRONE AREAS OF SOUTHEAST WY AS WELL AS THE LARAMIE VALLEY AND
CHEYENNE. WARNINGS ARE IN EFFECT UNTIL 8 PM THIS EVENING.

AFTER A LULL IN THE PRECIPITATION THIS MORNING...SHOWERS WILL REDEVELOP
THIS AFTERNOON ALONG AND WEST OF THE LARAMIE RANGE AS PACIFIC MOISTURE
AND UPSLOPE WESTERLY FLOW RESUME. ELEVATIONS ABOVE 7500 FEET CAN EXPECT
ACCUMULATING SNOW WITH THE SHOWER ACTIVITY. SNOTEL DATA INDICATED
THAT 2 TO 4 INCHES OF SNOW HAD FALLEN SINCE YESTERDAY AFTERNOON. AN
ADDITIONAL 4 TO 6 INCHES IS FORECAST THROUGH WEDNESDAY. THE SHOWERS
WILL SPREAD EAST ONTO THE HIGH PLAINS THIS EVENING. FAIRLY WIDESPREAD
SHOWER COVERAGE WILL THEN CONTINUE TONIGHT AND WEDNESDAY FOR LOWER
ELEVATIONS. HIGH TEMPERATURES TODAY WILL BE ABOUT 5 TO 7 DEGREES
COOLER THAN YESTERDAY. THE PASSAGE OF A COLD FRONT TONIGHT WILL
USHER IN MUCH COOLER TEMPERATURES WEDNESDAY WITH HIGHS IN THE 30S
AND 40S LOWER ELEVATIONS AND 20S MOUNTAINS.

.LONG TERM...(WEDNESDAY NIGHT THROUGH MONDAY)
ISSUED AT 303 AM MDT TUE MAR 24 2015

ENERGETIC NORTHWEST FLOW OVER THE ROCKIES WILL BRING A SLIGHT CHANCE
TO CHANCE FOR SHOWERS OVER THE MOUNTAINS AND NORTHERN COUNTIES
WEDNESDAY NIGHT...EXPANDING TO COVER MUCH OF THE FORECAST AREA THRU
THURSDAY. STILL THINKING PRECIP TYPE WILL BE SNOW OVERNIGHT WITH A
MIX THURSDAY MORNING CHANGING TO MAINLY RAIN OR A MIX OF RAIN/SNOW
IN THE AFTERNOON AS TEMPS WARM INTO THE 40S AND 50S BELOW 9000 FT.
THINKING SOME OF THE MORE VIGOROUS SHOWERS WITH COOLER DOWNDRAFTS IN
WYOMING WILL FAVOR THE MIX OF RAIN/SNOW. GUSTY NORTHWEST WINDS STILL
LOOK LIKE A GOOD BET ON THURSDAY AS WELL WITH RELATIVELY STEEP LOW
LEVEL LAPSE RATES IN PLACE AND WINDS OF 25 TO 35 KTS PROGGED AT
THE TOP OF THE MIXED LAYER. THE UPPER RIDGE WILL BUILD IN FROM THE
WEST ON FRIDAY WITH H7 TEMPS INCREASING TO BETWEEN 0-4C ON FRIDAY
AND 4-6C ON SATURDAY...TRANSLATING TO SFC TEMPS OF 50S/60S ON
FRIDAY AND 60S/70S ON SATURDAY. WINDS ALOFT WILL REMAIN ROUGHLY
20-25 KTS SO BREEZY CONDITIONS WILL PERSIST BOTH DAYS. AFTERNOON
MIN HUMIDITIES SHOULD DROP INTO THE MID TEENS ON SATURDAY...SO
COULD SEE ELEVATED FIRE WEATHER CONDITIONS IN THE AFTERNOON. GFS
AND ECMWF THEN SHOW SIGNIFICANT DIFFERENCES IN HANDLING THE
EVOLUTION OF THE POTENT SHORTWAVE TROUGH DIGGING SOUTHEAST FROM
CANADA SATURDAY NIGHT AND SUNDAY...AND PASSAGE OF ASSOCIATED COLD
FRONT THROUGH THE CWA. MAINTAINED A MOSTLY DRY FORECAST SATURDAY
NIGHT/SUNDAY DUE TO THESE DIFFERENCES...IN ADDITION TO THE FACT
BOTH MODELS ARE LEANING TOWARDS MOISTURE DEPLETING A BIT BEFORE
REACHING THE CWA. SUNDAY WILL BE COOLER BEHIND THE COLD FRONT THAT
WILL PASS THRU SOMETIME SATURDAY NIGHT/SUNDAY...BUT WILL REBOUND
QUICKLY FOR MONDAY AS HIGH PRESSURE BUILDS BACK IN FROM THE WEST.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z WEDNESDAY MORNING)
ISSUED AT 600 AM MDT TUE MAR 24 2015

MAIN CONCERN FOR THIS TAF PERIOD WILL BE STRONG WESTERLY WINDS
OVER MUCH OF THE FORECAST AREA TODAY. LOOKING AT 30-40 KTS FOR THE
NEBRASKA SITES AND 40-50 KTS AT TIMES FOR THE WYOMING
LOCATIONS...WHICH WILL MAKE FOR TURBULENT CONDITIONS. STRONGEST
WINDS ARE EXPECTED GENERALLY DURING THE 18-00Z TIMEFRAME. WINDS
WILL EASE THIS EVENING AS SUNSET APPROACHES...ALTHOUGH WILL NEVER
TRULY CALM DUE TO A COLD FRONT DROPPING SOUTH ACROSS THE PLAINS
AFTER 06Z TONIGHT. A FEW SHOWERS ARE POSSIBLE WITH THE
FRONT...ALONG WITH LOWERING CIGS. NOT COMPLETELY CONVINCED WILL
BE A FULL ON STRATUS EVENT AS GUSTY NORTHWEST WINDS WILL REMAIN IN
PLACE...WHICH MAY HELP TO KEEP CIGS SCT DUE TO SOME DRYING
DOWNSLOPE EFFECTS. HIGHEST LIKELIHOOD FOR THESE LOWER CIGS WILL BE
AT KAIA...KCDR...AND KRWL.

&&

.FIRE WEATHER...
ISSUED AT 315 AM MDT TUE MAR 24 2015

DEEP LOW LEVEL MIXING AND LARGE SCALE SUBSIDENCE WILL RESULT IN
STRONG WEST TO NORTHWEST WINDS. GUSTS OF 50 TO 60 MPH ARE EXPECTED
FOR WIND PRONE AREAS AND SOUTHEAST WYOMING PLAINS TODAY. MINIMUM
RELATIVE HUMIDITIES WILL FALL INTO THE 15 TO 20 PERCENT RANGE THIS
AFTERNOON. ELSEWHERE...PACIFIC MOISTURE AND UPSLOPE FLOW WILL
BRING RAIN AND HIGH ELEVATION SNOW SHOWERS. A PASSING COLD FRONT
AND UPPER LEVEL TROUGH WILL PROMOTE MORE WIDESPREAD SHOWER COVERAGE
TONIGHT AND WEDNESDAY. A TIGHT SURFACE GRADIENT WILL GENERATE GUSTY
WINDS WEDNESDAY THROUGH FRIDAY.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...HIGH WIND WARNING UNTIL 8 PM MDT THIS EVENING FOR WYZ106-110-116-
     117.

     HIGH WIND WARNING UNTIL 8 PM MDT THIS EVENING FOR WYZ115-118.

     RED FLAG WARNING FROM 10 AM THIS MORNING TO 8 PM MDT THIS
     EVENING FOR WYZ310.

NE...RED FLAG WARNING FROM 10 AM THIS MORNING TO 8 PM MDT THIS
     EVENING FOR NEZ313.

&&

$$

SHORT TERM...MAJ
LONG TERM...RJM
AVIATION...RJM
FIRE WEATHER...MAJ



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