Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 281800
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
1100 AM MST SAT FEB 28 2015

.UPDATE...
ISSUED AT 1053 AM MST SAT FEB 28 2015

REMOVED MOST OF THE MENTION FOR CHANCES OF SNOW TODAY. NO MODLES
SUPPORT CHANCES FOR PRECIPITATION THROUGH 00Z THIS EVENING. RADAR
ONLY SHOWING A FEW LIGHT SNOW SHOWERS WELL WEST OF THE FORECAST
AREA. UPPER LEVEL ENERGY IN COLORADO WILL NOT MOVE INTO THE
FORECAST AREA UNTIL THIS EVENING...BRINGING WITH IT THE MAIN
CHANCES FOR LIGHT SNOW. BASED ON LATEST MODEL DATA THE MAIN
CHANCES FOR SNOW TONIGHT WILL BE ALONG AND SOUTH OF THE COLORADO
BORDER.

&&

.SHORT TERM...(TODAY THROUGH MONDAY)
ISSUED AT 240 AM MST SAT FEB 28 2015

LONG WAVE UPPER LEVEL TROUGH EXTENDS FROM COAST TO COAST WITH A
HIGH AMPLITUDE EASTERN PACIFIC RIDGE REACHING AS FAR NORTH AS
SOUTHERN ALASKA AND WESTERN CANADA. A VIGOROUS SHORTWAVE WAS
CENTERED OVER OREGON. A NORTHERN STREAM TROUGH WAS TRACKING EAST
FROM MONTANA TOWARD THE NORTHERN HIGH PLAINS. 120KT SUBTROPICAL
JET WAS RACING EAST FROM SOUTHERN CALIFORNIA ACROSS THE SOUTHERN
ROCKIES AND PLAINS...WITH PACIFIC MOISTURE SPREADING NORTHEAST
ACROSS THE FOUR CORNERS INTO UTAH AND COLORADO. A WEAK SURFACE
TROUGH FROM WESTERN NORTH DAKOTA BISECTED A LOW OVER NORTHEAST
WYOMING INTO SOUTH CENTRAL WYOMING. RECENT SURFACE/SATELLITE OBS
INDICATED THAT THE STRATUS...LEAVING BEHIND SOME MID LEVEL CLOUDS
ACROSS PORTIONS OF CENTRAL AND EASTERN WYOMING. WINDS WERE MAINLY
LIGHT AND TEMPERATURES WERE VERY COLD...RANGING FROM 10 ABOVE TO 5
BELOW ZERO.

LATEST NUMERICAL MODEL GUIDANCE PROGS THE AFOREMENTIONED SHORTWAVE
OVER THE PACIFIC NORTHWEST TO PROGRESS SOUTHWARD AND FORM A CLOSED
UPPER LEVEL LOW JUST OFF THE CENTRAL CALIFORNIA COAST BY SUNDAY.
MEANWHILE...NORTHERN STREAM TROUGH TRACKS EAST ACROSS THE DAKOTAS
AND UPPER MIDWEST...PHASING WITH ENERGY FROM THE SOUTHWEST AND
SOUTH CENTRAL CONUS. ANOTHER COLD SURFACE HIGH PRESSURE RIDGE WILL
EXTEND FROM MONTANA SOUTHEAST THROUGH THE CENTRAL PLAINS PUSHING A
COLD FRONT SOUTHWESTWARD THROUGH THE CWA TODAY. WINDS WILL SHIFT
FROM WESTERLY TO NORTHEASTERLY. SUBTROPICAL MOISTURE WILL OVERRUN
THE BOUNDARY WITH FAVORABLE UPSLOPE FLOW AND UPGLIDE PRODUCING
WIDESPREAD LIGHT SNOWFALL BEGINNING THIS MORNING OVER THE
SNOWY/SIERRA MADRES AND SPREADING NORTH AND EAST ACROSS SOUTHEAST
WYOMING AND WESTERN NEBRASKA THIS AFTERNOON AND TONIGHT. THE SNOW
WILL LINGER ALONG AND SOUTH OF I-80 INTO THE AFTERNOON...CONTINUING
OVER THE SNOWY/SIERRA MADRES SUNDAY NIGHT. SNOW ACCUMULATIONS THIS
AFTERNOON THROUGH SUNDAY AFTERNOON WILL AVERAGE 1-2 INCHES FOR THE
VALLEYS AND PLAINS TO 3-6 INCHES FOR THE SOUTHERN MOUNTAIN RANGES.
HIGH TEMPERATURES TODAY AND SUNDAY WILL RISE INTO THE TEENS AND
20S. VERY COLD TEMPERATURES AGAIN TONIGHT MAINLY IN THE SINGLE
DIGITS ABOVE ZERO. SHORTWAVE ENERGY TRACKING NORTHEAST ACROSS
COLORADO AND SOUTHERN WYOMING WILL INTERACT WITH SUBTROPICAL
MOISTURE PRODUCING RAIN AND SNOW SHOWERS MAINLY ALONG AND WEST OF
THE LARAMIE RANGE MONDAY. HEAVIER SNOW SHOWERS COULD DROP ANOTHER
3-5 INCHES OF SNOW OVER THE SNOWY/SIERRA MADRES AT THAT TIME.
SOUTHWEST WINDS AND SLIGHTLY WARMER 700MB TEMPERATURES WILL PUSH
HIGH TEMPERATURES INTO THE 30S TO AROUND 40 OUTSIDE THE MOUNTAINS.

.LONG TERM...(MONDAY NIGHT THROUGH SATURDAY)
ISSUED AT 240 AM MST SAT FEB 28 2015

A POTENTIAL WINTER STORM EARLY IN THE PERIOD MAKES FOR A CHALLENGING
FORECAST THROUGH THE EXTENDED. THE MODELS ARE IN GOOD AGREEMENT WITH
LARGE SCALE TROUGHING OVER THE WESTERN CONUS. TWO DISTINCT PIECES OF
MID LEVEL ENERGY SHOULD COMPRISE THIS SYSTEM ON MON NIGHT...ONE OVER
SOUTHERN CA WITH ANOTHER WELL TO THE NORTH OVER SOUTHERN ALBERTA AND
SASKATCHEWAN. THIS DISJOINTED SYSTEM WILL MAKE IT TOUGH TO FOCUS
ANY PROLONGED PERIODS OF HEAVY SNOW...BUT THAT IS NOT TO SAY WE
WILL NOT HAVE A THREAT FOR SIGNIFICANT AMOUNTS. GFS/ECM/NAM ARE IN
EXCELLENT AGREEMENT WITH A STRONG COLD FRONT BLASTING SOUTHEAST
ACROSS THE CWA OVERNIGHT THROUGH TUE MORNING. DESPITE THE INTENSE
150 KT UPPER JET PROGGED BY THE GFS...JET POSITION DOES NOT APPEAR
FAVORABLE FOR ANY NOTABLE DYNAMIC FORCING. IN FACT...QG PROGS
ACTUALLY SHOW SUBSIDENCE ALOFT SPREADING IN OVERNIGHT. WE WILL
PROBABLY NEED TO RELY ON LOWER LEVEL FORCING AND FRONTOGENESIS
ALONG THE SURFACE FRONT TO RESULT IN MESOSCALE BANDING...WHICH
MOST MODELS DO FOR A 6-9 HOUR PERIOD THRU 12Z TUE. QPF AMOUNTS
BETWEEN 0.5-0.8 INCH ARE LIKELY OVERDONE GIVEN THIS SCENARIO AND
HOW THE LAST COUPLE SNOWFALL EPISODES ASSOCIATED WITH COLD FROPAS
HAVE WORKED OUT. INSTABILITY IS ALSO LACKING WITH THE GFS SHOWING
POSITIVE EPVS AND MODEST MID LEVEL LAPSE RATES. MAY SEE ADVISORY
AMOUNTS WHERE THE FRONT SETS UP...WITH THE POSSIBILITY OF WARNING
AMOUNTS IN THE SNOWIES/SIERRA MADRES VIA STRONG OROGRAPHICS.

A DIFFICULT TEMPERATURE FORECAST THROUGH THE EXTENDED AS UNCERTAINTY
REMAINS WITH THE MON NIGHT AND TUE SNOWFALL EVENT. CLOUD COVER COULD
STILL BE AN ISSUE ON TUE NIGHT AS MEDIUM RANGE GUIDANCE STILL SHOWS
THE TROUGH AXIS OVERHEAD. IF SKIES CLEAR EARLY ENOUGH...LIGHT WINDS
AND FRESH SNOW WILL ALLOW TEMPERATURES TO REALLY PLUMMET AS H7 TEMPS
COLDER THAN -20 DEG C PER THE ECMWF POINT TO WIDESPREAD SUBZERO
READINGS. TRENDED COLDER FOR WED AFTERNOON AS MOST AREAS SHOULD SEE
ENOUGH ACCUMULATING SNOW TO AFFECT TEMPERATURES. LOW-LEVEL DOWNSLOPE
RETURNS FOR THU AND FRI WITH RIDGING ALOFT. SHOULD BE A MUCH WARMER
AND RELATIVELY DRY PATTERN SETTING UP LATE NEXT WEEK. WED NIGHT/THU
LOOKS TO BE A WINDY PERIOD WITH THE GFS H85 CAG-CPR GRADIENT PROGGED
TO BE OVER 80 METERS ALONGSIDE H75 WINDS AROUND 45 KTS.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH SUNDAY MORNING)

MVFR...TO NEAR IFR CIGS COULD RETURN TO AREAS ALONG AND EAST OF THE
LARAMIE RANGE BY EARLY AFTERNOON AS ANOTHER COLD FRONT MOVES SOUTH
ACROSS THE AREA. LOW-LEVEL UPSLOPE FLOW WILL PROMOTE SNOW SHOWERS
WITH LOCALLY REDUCED VIS BY THIS EVENING...MAINLY AT KCYS...BUT ALSO
KLAR AND KRWL AS WELL. IFR CONDITIONS ARE EXPECTED TONIGHT WITH LOW
CIGS AND LOW VIS EXPECTED FOR MUCH OF THE PERIOD THROUGH WITH LITTLE
IMPROVEMENT THROUGH SUN MORNING.


&&

.FIRE WEATHER...
ISSUED AT 240 AM MST SAT FEB 28 2015

C0LDER THAN NORMAL TEMPERATURES...AMPLE SURFACE MOISTURE FROM
SNOWFALL...AND HIGH RELATIVE HUMIDITIES WILL PRECLUDE FIRE WEATHER
CONCERNS THROUGH MUCH OF NEXT WEEK.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...NONE.
NE...NONE.
&&

$$

UPDATE...SML
SHORT TERM...MAJ
LONG TERM...CLH
AVIATION...TJT
FIRE WEATHER...MAJ


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