Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 252023
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
123 PM MST WED FEB 25 2015

.UPDATE...
ISSUED AT 123 PM MST WED FEB 25 2015

ANOTHER QUICK UPDATE EARLY THIS AFTERNOON TO EXPAND THE WINTER
WEATHER ADVISORY TO INCLUDE ARLINGTON AND THE EASTERN FOOTHILLS OF
THE SOUTHERN LARAMIE RANGE. ALTHOUGH KCYS 88D IS OVERSHOOTING MUCH
OF THE ACTIVITY IN THE ARLINGTON AREA...WEBCAMS IN THAT AREA
CONFIRM POOR TRAVEL CONDITIONS AND AT LEAST A FEW INCHES OF
ACCUMULATION ON AND NR THE ROADWAY. ADDITIONALLY...RADAR TRENDS
CONTINUE TO SHOW A FAIRLY PERSISTENT SNOW BAND EAST OF THE I80
SUMMIT OUT TO AROUND THE ROUNDTOP EXIT. UPDATED PRODUCTS HAVE BEEN
SENT.

UPDATE ISSUED AT 1125 AM MST WED FEB 25 2015

SNOW BANDS HAVE BEEN ACTIVELY DEVELOPING ACROSS SOUTHEAST WYOMING
OVER THE PAST 6 HOURS OR SO...MAINLY IMPACTING LARAMIE...ALBANY AND
NOW CARBON COUNTIES. SNOWFALL AMOUNTS HAVE BEEN PRETTY LIGHT BENEATH
THESE INDIVIDUAL BANDS THUS FAR GIVEN THEIR FAIRLY PROGRESSIVE
NATURE. LATEST MODELS CONTINUE TO PROG THAT THESE SNOW BANDS WILL
GRADUALLY SHIFT WEST THRU THE REST OF THE AFTERNOON MAINLY AFFECTING
AREAS ALONG AND WEST OF INTERSTATE 25. EXPECT MOST PLACES TO IN AND
AROUND THE SNOWY AND LARAMIE RANGES TO PICK UP A QUICK 2 TO 4 INCHES
THIS AFTERNOON...WITH ELEVATIONS ABOVE 7000 FEET SEEING UPWARDS OF 4
TO 8 INCHES. VISIBILITIES AND QUICK SNOW ACCUMULATIONS ARE THE MAJOR
CONCERN AS RATES MAY BE PRETTY INTENSE ESPECIALLY WHEN INSTABILITY
IS REALIZED THIS AFTERNOON. FOR THE MORNING UPDATE...THINK THAT THE
CURRENT ADVISORY HAS THINGS HANDLED WELL. BUT WILL BE WATCHING THE
ARLINGTON AREA AND PORTIONS OF CENTRAL LARAMIE COUNTY CLOSELY SHOULD
A BAND BECOME STATIONARY THIS AFTERNOON.

&&

.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 18Z TAFS THROUGH 18Z THURSDAY AFTERNOON)
ISSUED AT 1125 AM MST WED FEB 25 2015

ARCTIC COLD FRONT SWEPT THROUGH WITH COLD AIR PLUNGING SOUTH.
VFR AND MVFR PREVAILS THIS AFTERNOON...WITH AREAS OF IFR AND
MOUNTAIN OBSCUREMENT IN SNOW SHOWERS AND FOG AND BLOWING SNOW...
THEN MVFR PREVAILS TONIGHT WITH AREAS OF IFR IN SNOW...FOG AND
LOW CLOUDS...WITH IMPROVING CEILINGS AND VISIBILITIES AFTER
DAYBREAK ON THURSDAY...WITH LOCALIZED IFR IN SNOW SHOWERS AND
FOG.


&&

.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 UNTIL 2 AM MST THURSDAY FOR WYZ110-
     114>117.

NE...NONE.
&&

$$

UPDATE...CAH
SHORT TERM...ZF
LONG TERM...RJM
AVIATION...RUBIN
FIRE WEATHER...ZF



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