Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 251205
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
505 AM MST WED FEB 25 2015

.SHORT TERM...(TODAY AND FRIDAY)
ISSUED AT 333 AM MST WED FEB 25 2015

CHALLENGING FORECAST TODAY WITH REGARD TO SNOWFALL AMOUNTS ACROSS
SOUTHEAST WYOMING. WATER VAPOR IMAGERY SHOWS SHORTWAVE IN
NORTHWESTERLY FLOW ALOFT BEGINNING TO MOVE INTO WYOMING. MODERATE
LLVL GRADIENT OVER SOUTHEAST WYOMING HAS CAUSED A FEW GUSTS TO 45
MPH IN ARLINGTON IN THE PAST FEW HOURS. NOT EXPECTING THESE WINDS
TO GET MUCH STRONGER AS THE GRADIENT WEAKENS THROUGH THE MORNING.
COLD FRONT WILL PUSH ACROSS THE REGION THIS MORNING. STILL LOOKING
LIKE THE SNOW EVENT THIS AFTERNOON THROUGH THE EVENING WILL BE
DRIVEN BY WEAK INSTABILITY AND LLVL UPSLOPE AS OPPOSED TO DEEP
LAYER ASCENT. THE NAM SHOWS MUCAPE VALUES OF 100-200 J/KG ALONG
AND WEST OF THE LARAMIE RANGE FROM 18Z THROUGH 00Z WITH LESS
INSTABILITY IN THE GFS. ALTHOUGH SLIGHTLY NEGATIVE EPV VALUES ARE
SHOWN TO THE WEST OF THE LARAMIE RANGE DURING THE AFTN...THE
QUESTION WILL BE WHAT WILL FOCUS THIS INSTABILITY INTO BANDS OF
SNOW. THERE IS WEAK LOW TO MIDLVL FRONTOGENESIS OVER THE
FOOTHILLS OF NORTHERN COLORADO BUT NOT MUCH OVER WYOMING.
THUS...CURRENT THINKING IS THAT SNOW AMOUNTS WILL BE HIGHLY
VARIABLE DUE TO THE CONVECTIVE NATURE OF THE SNOW. HAVE GENERALLY
2-3 INCHES FOR LOWER ELEVATIONS ALONG AND WEST OF THE LARAMIE
RANGE WITH AROUND 6 INCHES IN THE SNOWY RANGE. AMOUNTS WILL BE
LOCALLY HIGHER IN HEAVIER SNOW SHOWERS OR BANDS THAT DEVELOP.
STILL HAVE LOWER AMOUNTS TO THE EAST OF THE LARAMIE RANGE (LESS
THAN 2 INCHES) WITH NOT MUCH INSTABILITY OUT THERE.

EVEN WITH THE MARGINAL SNOW AMOUNTS...DECIDED TO GO WITH A WINTER
WEATHER ADVISORY FOR THE SNOWY RANGE...LARAMIE VALLEY...AND
SOUTH LARAMIE RANGE. SOUNDINGS OVER THE INTERSTATE 80 SUMMIT SHOW
THE POTENTIAL FOR GUSTS TO 35 MPH FROM LATE MORNING THROUGH THE
AFTERNOON. THIS COMBINED WITH HEAVIER SNOW SHOWERS WOULD PRODUCE
PRETTY HAZARDOUS TRAVEL CONDITIONS ALONG INTERSTATE 80 IN AREAS OF
FALLING AND BLOWING SNOW. WINDS WILL DROP OFF BY LATE THIS
EVENING WITH SNOW LARGELY ENDING AFTER 06Z. LOWERED TEMPERATURES
FOR TONIGHT BY A FEW DEGREES WITH MOST LOCATIONS IN THE SINGLE
DIGITS. WIDESPREAD CLOUD COVER...SNOWPACK...AND 700MB TEMPS AROUND
-20C WILL KEEP HIGHS ON THURS BELOW THE UPPER TEENS. ALL MODELS
SHOW LIGHT SNOW RETURNING TO THE SOUTHERN LARAMIE RANGE EXTENDING
EASTWARD ALONG THE CHEYENNE RIDGE FROM THURS AFTN INTO EARLY FRI
MORNING. THIS OCCURS AS SOUTHEASTERLY LLVL UPSLOPE STRENGTHENS.
CAN NOT RULE OUT ACCUMULATIONS OF 1-2 INCHES FROM THE INTERSTATE
80 SUMMIT TO CHEYENNE OVER THIS PERIOD...HOWEVER SNOWFALL RATES
WILL BE LIGHT. LOW CLOUDS AND PATCHY FOG WILL PERSIST ALONG THE
CHEYENNE RIDGE THROUGH FRI MORNING. ANOTHER COLD DAY IS IN STORE
FOR FRI...HOWEVER PRECIP CHANCES WILL BE ON THE DECREASE.

.LONG TERM...(FRIDAY NIGHT THROUGH TUESDAY NIGHT)
ISSUED AT 333 AM MST WED FEB 25 2015

A DEEP UPPER TROUGH WILL DEVELOP OVER THE GREAT BASIN THROUGH THE
WEEKEND WITH SOUTHWEST WINDS INCREASING ALOFT. MOIST SOUTHEASTERLY
FLOW AT THE SURFACE ON FRIDAY NIGHT WILL BRING A HIGH LIKELIHOOD FOR
STRATUS ACROSS THE PLAINS. THIS PATTERN IS OFTEN FAVORABLE FOR FOG
DEVELOPMENT AS WELL...HOWEVER FORECAST SOUNDINGS FAVOR MORE OF A
STRATUS VS EVENT FOG SO LEFT MENTION OF FOG OUT OF THE FORECAST FOR
NOW. SFC CYCLOGENESIS WILL OCCUR OVER THE GREAT BASIN IN ADVANCE OF
THE DEEPENING UPPER TROUGH WITH SFC WINDS OVER THE FORECAST AREA
SHIFTING TO THE EAST-NORTHEAST IN RESPONSE TO THE STRONG PRESSURE
FALLS PROGGED OUT WEST. AS MENTIONED PREVIOUS...UPPER WINDS WILL
SHIFT TO THE SOUTHWEST WHICH WILL PROMOTE A FAVORABLE ISENTROPIC
LIFT SCENARIO WITH THE EASTERLY WINDS IN THE LLVLS. MODELS CONTINUE
TO POINT TOWARD THE BEST OVERALL LIFT DOWN SOUTH IN COLORADO ON
SATURDAY...BUT AREAS ALONG THE WY/CO BORDER...ESPECIALLY NORTHEAST
FACING SLOPES...WILL SEE A CHANCE FOR LIGHT SNOW WITH COMBINED
OROGRAPHICS AND ISENTROPIC LIFT.

A SECOND SHORTWAVE TROUGH WILL MOVE SOUTH ACROSS THE NORTHERN
PLAINS SATURDAY NIGHT-SUNDAY MORNING. THIS WAVE WILL INCREASE
BROADSCALE LIFT OVER THE FORECAST AREA WITH SNOW CHANCES SPREADING
TO FURTHER NORTH ACROSS THE PLAINS AS A RESULT. STILL THINKING
SNOW WILL REMAIN LIGHT OVERALL. THE NORTHERN WAVE WILL QUICKLY
SHIFT EAST THRU THE DAY WITH SFC HIGH PRESSURE BUILDING OVER THE
CWA. SFC WINDS WILL WEAKEN AND EVENTUALLY SHIFT BACK TO THE
SOUTHEAST OVERNIGHT REDUCING ISENTROPIC FORCING. THE DEEP UPPER
TROUGH WILL BEGIN TO KICK EASTWARD ACROSS THE DESERT SOUTHWEST AND
INTO THE SOUTHERN PLAINS SUNDAY NIGHT AND MONDAY WITH SUBSIDENCE
MOVING OVERHEAD. THIS WILL KEEP CONDITIONS DRY AND SLIGHTLY WARMER
FOR MONDAY. THE NEXT SHORTWAVE TROUGH LOOKS TO APPROACH FROM THE
NORTHWEST ON TUESDAY WITH INCREASING CHANCES FOR SNOW THROUGH
TUESDAY NIGHT. TEMPERATURES WILL REMAIN BELOW NORMAL THROUGH THE
PERIOD.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z THURSDAY MORNING)
ISSUED AT 505 AM MST WED FEB 25 2015

LIGHT SNOW SHOWERS HAVE DEVELOPED EARLY THIS MORNING OVER THE
FORECAST AREA AND WILL CONTINUE AT TIMES THROUGH THIS EVENING. MVFR
TO IFR CONDITIONS WILL PREVAIL IN THE SNOW SHOWERS...WITH THE
HEAVIEST SHOWERS EXPECTED ALONG AND WEST OF THE LARAMIE RANGE LATE
THIS MORNING THROUGH THE AFTERNOON. THE MOUNTAINS WILL BE
OBSCURED. SNOW WILL DIMINISH THIS EVENING...HOWEVER A MOIST
BOUNDARY LAYER WILL PREVAIL SO STRATUS AND PATCHY FOG WILL LIKELY
PERSIST OVERNIGHT.

&&

.FIRE WEATHER...
ISSUED AT 333 AM MST WED FEB 25 2015

NO FIRE WEATHER CONCERNS.  A FEW INCHES OF WIDESPREAD SNOW IS
EXPECTED TO FALL ACROSS SOUTHEAST WYOMING THIS AFTERNOON THROUGH
EARLY TONIGHT.  A COLD AIRMASS WILL REMAIN IN PLACE THROUGH EARLY
NEXT WEEK WITH HIGH HUMIDITY VALUES.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...WINTER WEATHER ADVISORY FROM 11 AM THIS MORNING TO 2 AM MST
     THURSDAY FOR WYZ114>116.

NE...NONE.
&&

$$

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


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