Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 191750 AAA
AFDCYS

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE CHEYENNE WY
1150 AM MDT TUE MAY 19 2015

.UPDATE...
ISSUED AT 1150 AM MDT TUE MAY 19 2015

BACK EDGE OF THE COLDEST CLOUD TOPS AND HEAVIEST PRECIP IS
CURRENTLY MOVING NORTHWARD TOWARD THE COLORADO BORDER. EXPECTING
PRECIP RATES TO DROP OFF ALONG INTERSTATE 80 AFTER 18Z.  TEMPS TO
THE EAST OF THE LARAMIE RANGE HAVE MOSTLY STAYED ABOVE
35F...KEEPING THE PRECIP AS PREDOMINANTLY RAIN. SNOW ACCUMULATIONS
OVER THE SOUTHERN LARAMIE RANGE HAVE BEEN LESS THAN A FEW INCHES
AS MUCH OF THE HIGHER REFLECTIVITY HAS REMAINED OVER LARAMIE
COUNTY AND POINTS EASTWARD. WILL KEEP THE WINTER WEATHER ADVISORY
GOING FOR THE SOUTHERN LARAMIE RANGE THROUGH THE AFTERNOON...EVEN
THROUGH RATES WILL BE LIGHT AND ACCUMULATIONS MAINLY ON THE GRASSY
SURFACES.

&&

.SHORT TERM...(TODAY AND THURSDAY)
ISSUED AT 402 AM MDT TUE MAY 19 2015

VERY TRICKY FORECAST TODAY ACROSS THE AREA...AND ESPECIALLY THE
HIGH PLAINS. THE MAIN FORECAST CONCERN WILL BE WHEN RAIN CHANGES
TO SNOW ACROSS THE AREA AND HOW MUCH...IF ANY...ACCUMULATES.
CURRENT KCYS AND KFTG RADAR LOOPS SHOW A VAST INCREASE IN
PRECIPITATION COVERAGE OVER COLORADO AS A LARGE SHIELD OF
PRECIPITATION DEVELOPS ACROSS THE ROCKY MOUNTAINS AND COLORADO
PLAINS AT THIS HOUR. THIS PRECIPITATION IS QUICKLY EXPANDING
NORTH...BUT ALSO EXTENDS SOUTHWARD INTO SOUTHERN COLORADO DUE
STRONG UPPER LEVEL DIFLUENCE AHEAD OF A PACIFIC JET. MOST OF
SOUTHEAST WYOMING AND WESTERN NEBRASKA WILL BE IN THIS AREA OF
UPPER LEVEL DIFLUENCE THROUGH THE DAY IN ADDITION TO JET DYNAMICS
FROM THE APPROACHING PACIFIC JET. IN ADDITION...A STRONG SURFACE
HIGH OVER THE NORTHERN PLAINS WILL REMAIN IN PLACE AND ACTUALLY
INTENSIFY AND SHIFT WESTWARD TODAY AND TONIGHT...WITH ADDITIONAL
FRONTOGENESIS ALONG SOUTHERN WYOMING AND CENTRAL COLORADO AS
COOLER AIR MOVES INTO THE REGION. WIDESPREAD PRECIP AMOUNTS
BETWEEN 0.50 TO 1.00 LIQUID CAN BE EXPECTED FROM THE LARAMIE RANGE
EASTWARD...WITH LESSER AMOUNTS WEST OF THE LARAMIE RANGE. WOULD
NOT BE SURPRISED TO SEE SOME LOCATIONS RECEIVE 1.50 LIQUID BY
TONIGHT ACROSS THE HIGH PLAINS. AS THE SURFACE HIGH STRENGTHENS
AND THE UPPER LEVEL DISTURBANCE MOVES NORTHEAST...MODELS SHOW
700MB TEMPERATURES LOWERING BETWEEN -3C TO -5C EAST OF THE FRONTAL
BOUNDARY...WHICH SHOULD BE JUST WEST OF THE LARAMIE RANGE. MODEL
SOUNDINGS SHOW NEARLY ISOTHERMAL TEMPERATURES FROM 700MB DOWN TO
THE SURFACE. THIS SUGGESTS THE POTENTIAL FOR HEAVY SNOW ACROSS
MOST OF THE HIGH PLAINS EVEN THOUGH CURRENT TEMPERATURES ARE IN
THE 40S TO LOW 50S. EVEN THE I80 SUMMIT IS HOVERING AROUND 35
DEGREES. MODELS SHOW THESE TEMPERATURES FALLING STEADILY TODAY
WITH RAIN CHANGING TO SNOW. DO NOT EXPECT MUCH SNOW WEST OF THE
LARAMIE RANGE DUE TO THE SHALLOW DEPTH OF THE COLD AIR.

IN TERMS OF IMPACT...SHOULD BE MINOR TO MODERATE ACROSS THE AREA
DUE TO THE TIME OF YEAR AND THE SNOWFALL MAINLY OCCURRING DURING
THE DAY. HOWEVER...THESE IS SOME UNCERTAINTY WITH SOME ENSEMBLE
MEMBERS SUGGESTING HIGH SNOWFALL RATES AND ACCUMULATING SNOW DOWN
TO 4500 FEET. WILL HAVE TO KEEP AN EYE ON PERSISTENT
MODERATE/HEAVY SNOWFALL RATES AS THE RAIN CHANGES TO SNOW ACROSS
THE AREA. IF THESE HEAVY SNOWFALL RATES PERSIST AND BECOME THE
NORM...MAY BE LOOKING AT LOCALLY SIGNIFICANT SNOW ACCUMULATIONS
WITH HEAVY SNOW BANDS MOVING ACROSS THE AREA FROM SOUTH TO NORTH.
NOT CONFIDENT ENOUGH IN THIS SCENARIO TO EXPAND THE WINTER
WEATHER ADVISORY THOUGH...SO ONLY KEPT THE ADVISORY GOING FOR THE
I80 SUMMIT WHICH AS THE BEST POTENTIAL IN SEEING HEAVY SNOWFALL
AND ACCUMULATING SNOWFALL ON THE ROADWAYS. FURTHER EAST...1 TO 4
INCHES MAY ACCUMULATE ON GRASSY SURFACES BEFORE MELTING IN BETWEEN
BANDS...WITH VERY LITTLE ACCUMULATION ON THE ROADWAYS. WILL WATCH
THESE TRENDS THROUGH THE MORNING AND ADJUST ACCORDINGLY. WILL HAVE
TO ALSO MONITOR RIVER LEVELS AND POTENTIAL FLASH FLOODING IF SNOW
LEVELS DO NOT LOWER...AS THE LARAMIE RIVER AND AREA CREEKS AND
STREAMS REMAIN HIGH.

BY TONIGHT...THE UPPER LEVEL DISTURBANCE LIFTS NORTH AND EAST INTO
THE DAKOTAS AND WEAKENS. HEAVY PRECIP RATES SHOULD COME TO AN END
BY MIDNIGHT WITH SNOWFALL BECOMING LIGHTER AND MORE SHOWERY IN
NATURE. DECENT AMOUNT OF COLD AIR ACROSS THE HIGH PLAINS...SO CAN
NOT RULE OUT ADDITIONAL LIGHT SNOW ACCUMULATIONS. DRIZZLE AND FOG
MAY BECOME A PROBLEM ALONG THE SOUTH FACING RIDGES SUCH AS THE
CHEYENNE RIDGE FROM CHEYENNE TO SIDNEY TONIGHT AND INTO EARLY
WEDNESDAY. HIGH TEMPERATURES TODAY AND WEDNESDAY WILL BE 20 TO 30
DEGREES BELOW NORMAL. ALTHOUGH PRECIPITATION WILL GENERALLY COME
TO AN END BY WEDNESDAY...UPSLOPE FLOW AND THE REMNANT STATIONARY
FRONT ALONG THE LARAMIE RANGE WILL RESULT IN SCATTERED SHOWERS AND
ISOLATED THUNDER OVER WYOMING.

WARMER TEMPERATURES ARE EXPECTED ON THURSDAY AS A SHORT WAVE
RIDGES MOVES ACROSS THE AREA. TEMPERATURES WILL STILL BE BELOW
NORMAL THOUGH AND GENERALLY IN THE MID 50S TO MID 60S. BREAK IN
THE PRECIPITATION WILL BE SHORT LIVED...AS ALL MODELS INDICATE
ANOTHER PACIFIC STORM SYSTEM MOVING TOWARDS THE AREA.
INITIALLY...EXPECT A ROUND OF SCATTERED SHOWERS AND TSTORMS LATE
ON THURSDAY WITH THE POTENTIAL FOR STRONG TSTORMS AS FORECAST CAPE
VALUES CLIMB NEAR 1000 J/KG ALONG WITH PLENTY OF UPPER LEVEL
DIFLUENCE AHEAD OF THE NEXT PACIFIC STORM. INCREASED POP BETWEEN
40 TO 60 PERCENT ACROSS SOUTHEAST WYOMING THURSDAY AFTERNOON.

.LONG TERM...(THURSDAY NIGHT THROUGH MONDAY)
ISSUED AT 302 AM MDT TUE MAY 19 2015

IF YOU ARE LOOKING FOR A BREAK IN THE COOL AND WET WEATHER
PATTERN...YOU MAY BE WAITING FOR A WHILE. EXTENDED MODELS SHOW NO
SIGN OF CURRENT WEATHER PATTERN CHANGING ANYTIME SOON. THE HEMISPHERIC
LONGWAVE PATTERN WILL REMAIN LOCKED IN PLACE WITH MANY BLOCKING
FEATURES ACROSS THE NORTHERN HEMISPHERE. NOT THE LEAST BEING THE
HIGH OVER LOW BLOCK ACROSS WESTERN CANADA AND WESTERN CONUS
RESPECTFULLY. A CONTINUOUS STREAM OF PACIFIC SYSTEMS WILL SLIDE
UNDER THE UPPER LEVEL HIGH OVER WESTERN CANADA DURING THE FORECAST
PERIOD AND BEYOND. THIS MEANS CHANCES FOR SHOWERS NEARLY
EVERYDAY...ALONG WITH THUNDERSTORMS AND EVEN THE POTENTIAL FOR
SNOW ON OCCASION.

THE NEXT CHANCE FOR SNOW OUTSIDE OF THE MOUNTAIN MAY COME AGAIN AS
EARLY AS FRIDAY. AT THIS TIME KEPT THE MENTION OF SNOW OUT OF THE
GRIDS BUT WILL HAVE TO KEEP A CLOSE EYE ON THE TRACK AND STRENGTH
OF THE SHORTWAVE TROUGH WHICH IS PROGGED TO MOVE OVER THE AREA
FRIDAY. THE NUMERICAL MODELS ARE WARM ENOUGH TO KEEP PRECIPITATION
AS RAIN BUT THE MODELS HAVE HAD A WARM BIAS WITH THE LAST FEW
SYSTEMS. THIS VERY WELL MAY AGAIN BE THE CASE FRIDAY.
FORTUNATELY...BASED ON THE CURRENT MODEL TIMING THE HEAVIEST
PRECIPITATION WOULD BE DURING THE DAYTIME HOURS...MAKING
ACCUMULATIONS OF SNOW UNLIKELY...ESPECIALLY OVER ROADWAYS.

ECMWF AND GFS SHOWING DISTINCT DIFFERENCES IN THE PLACEMENT OF YET
ANOTHER DISTURBANCE SATURDAY. THE GFS IS SLIGHTLY SLOWER AND MUCH
FARTHER WEST WITH SHORTWAVE ENERGY. NOT CONFIDENT IN ANY ONE
SOLUTION AT THIS TIME...BUT THE DIFFERENCE HAVE SIGNIFICANT
FORECAST IMPLICATIONS. THE ECMWF SOLUTION SUGGESTS COOLER
TEMPERATURES WITH SCATTERED SHOWERS AND ISOLATED
THUNDERSTORMS...AND VERY LITTLE CHANCE FOR SEVERE THUNDERSTORMS.
THE WARMER GFS WOULD SUGGEST A CHANCE FOR SOME SEVERE STORMS INTO
THE PLAINS LATE SATURDAY. BY SUNDAY THE BOTH MODELS PUSH THE MAIN
DYNAMICS INTO THE PLAINS...OF COURSE ANOTHER SYSTEM WILL BE HOT
ON ITS HEALS DROPPING SOUTH ACROSS THE INTERMOUNTAIN WEST.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z WEDNESDAY AFTERNOON)
ISSUED AT 1150 AM MDT TUE MAY 19 2015

FOR THE NOON AVIATION PACKAGE WE CONTINUE WITH IFR/LIFR CEILINGS AT
ALL AIRPORT SITES ACROSS SOUTHEAST WYOMING AND THE WESTERN
NEBRASKA PANHANDLE THROUGH THE OVERNIGHT PERIOD DUE TO PROLONGED
PERIODS OF LIGHT PRECIPITATION WITH FOG. EXCEPTION WILL BE RAWLINS
WHERE THEY WILL PREVAIL AT MVFR FLIGHT CATEGORY THROUGH LATE
EVENING IN LIGHT PRECIPITATION AND LOWERED CEILINGS...THEN RISE
TO VFR TOWARDS MIDNIGHT AS DRIER AIRMASS ON THE BACKSIDE OF THE
DEPARTING LOW PRESSURE SYSTEM MOVES IN.

&&

.FIRE WEATHER...
ISSUED AT 302 AM MDT TUE MAY 19 2015

NO FIRE WEATHER CONCERNS THIS WEEK DUE TO UNSEASONABLY COOL
TEMPERATURES AND HIGH CHANCES OF PRECIPITATION. EVEN SOME SNOW IS
EXPECTED OVER THE MOUNTAINS AND PORTIONS OF THE HIGH PLAINS EARLY
THIS WEEK. DAYTIME RELATIVE HUMIDITIES WILL LIKELY NOT LOWER BELOW
50 PERCENT UNTIL LATE THIS WEEKEND OR EARLY NEXT WEEK.

&&

.HYDROLOGY...
ISSUED AT 302 AM MDT TUE MAY 19 2015

LARAMIE RIVER WILL CONTINUE TO RUN AT RELATIVELY HIGH LEVELS
THROUGH THIS WEEK BUT AT THIS TIME IS EXPECTED TO REMAIN WITHIN
ITS BANKS. HOWEVER...THE FIRST OF A SERIES OF PACIFIC STORMS WILL
IMPACT THE REGION TODAY AND INTO TONIGHT. WILL HAVE TO CLOSELY
MONITOR HOW MUCH PRECIPITATION WILL FALL AS A STRONG AREA OF LOW
PRESSURE MOVES ACROSS THE AREA...WITH ANOTHER POSSIBLE BY THE END
OF THE WEEK. HIGHEST PRECIPITATION AMOUNTS ARE LIKELY ACROSS THE
PLAINS WHICH COULD BE ENOUGH TO PUSH THE LARAMIE RIVER AT FORT
LARAMIE INTO MINOR FLOODING.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...WINTER WEATHER ADVISORY UNTIL 6 PM MDT THIS EVENING FOR WYZ116.

NE...NONE.
&&

$$

UPDATE...ZF
SHORT TERM...TJT
LONG TERM...SML
AVIATION...JG
FIRE WEATHER...TJT
HYDROLOGY...TJT


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