Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 201755
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
1155 AM MDT WED MAY 20 2015

.UPDATE...
ISSUED AT 1059 AM MDT WED MAY 20 2015

LOWERED HIGHS FOR TODAY MAINLY FROM LARAMIE TO CHEYENNE AS LOW
CLOUDS WILL BE QUITE PERSISTENT INTO THE AFTERNOON. THE HRRR DOES
SHOW VIS IMPROVING OVER THE SOUTHERN LARAMIE RANGE AND THE
ARLINGTON AREA AFTER 18Z...AND WYDOT VIS IS SLOWLY STARTING TO
COME UP. DENSE FOG ADVISORY WILL PROBABLY BE EXPIRED AT NOON...BUT
WILL MONITOR OVER THE NEXT HOUR. WITH A STABLE AIRMASS OVER THE
PLAINS...SHOWERS AND AN ISOLATED TSTM WILL MAINLY BE CONFINED TO
THE SNOWY RANGE WESTWARD.

&&

.SHORT TERM...(TODAY THROUGH FRIDAY)
ISSUED AT 346 AM MDT WED MAY 20 2015

THE PACIFIC STORM SYSTEM WHICH BROUGHT RAIN AND EVEN SOME SNOW TO
THE REGION YESTERDAY IS CURRENTLY WEAKENING AND MOVING EASTWARD
OVER THE NORTHERN PLAINS THIS MORNING. THERE IS STILL SOME LIGHT
SNOW BEING REPORTED ACROSS PORTIONS OF FAR EASTERN WYOMING AND
WESTERN NEBRASKA...BUT ACCUMULATIONS HAVE BEEN MINIMAL. OF MORE
SIGNIFICANCE IS THE FREEZING DRIZZLE NEAR THE RIDGE TOPS AND ALONG
THE LARAMIE RANGE...WITH CHEYENNE REPORTING FREEZING DRIZZLE
THROUGH MOST OF THE NIGHT. THERE HAVE BEEN A FEW REPORTS OF BLACK
ICE ON THE ROADWAYS...BUT SO FAR IT HAS BEEN RELATIVELY ISOLATED
WITH MAINLY JUST WET ROADWAYS. WILL CONTINUE TO KEEP AN EYE ON
THIS AS TEMPERATURES CONTINUE TO LOWER THROUGH THE UPPER 20S EARLY
THIS MORNING. HIGH TEMPERATURES TODAY WILL STRUGGLE TO REACH THE
MID TO UPPER 40S DUE TO A PERSISTENT LOW CLOUD DECK AND EVEN SOME
PATCHY FOG/DRIZZLE ALONG THE SOUTH FACING RIDGES.
EVENTUALLY...WINDS WILL SHIFT INTO THE EAST TO NORTHEAST TODAY
WITH CLOUD CEILINGS INCREASING A BIT BY THE AFTERNOON. NOT MUCH
SUN TODAY...WITH VALLEY LOCATIONS WEST OF THE LARAMIE RANGE HAVING
THE BEST CHANCE TO SEE SOME SUNSHINE. WITH SO MUCH MOISTURE AND
NEARLY SATURATED LOW TO MIDLEVELS...IT WILL NOT TAKE MUCH
INSTABILITY TO GENERATE SOME LIGHT SHOWERS. KEPT POP BETWEEN 20 TO
40 PERCENT FOR NOW...WITH SHOWERS LIKELY IN THE MOUNTAINS.

TONIGHT WILL BE MORE OF THE SAME...BUT WITH LESS CLOUD COVER OVER
THE NORTHERN AND WESTERN PORTION OF THE FORECAST AREA. MODELS SHOW
WINDS SHIFTING BACK INTO THE SOUTH OR SOUTHEAST...RESULTING IN
ANOTHER ROUND OF FOG AND/OR DRIZZLE FOR THE CHEYENNE AND PINE
RIDGE...ALONG WITH THE EASTERN FOOTHILLS OF THE LARAMIE RANGE. DO
NOT EXPECT THIS STRATUS DECK TO MAKE IT WEST OF THE LARAMIE
RANGE...SO LARAMIE AND RAWLINS MAY JUST BE PARTLY CLOUDY TONIGHT
BUT RELATIVELY COLD. MAY NEED A FROST OR FREEZE ADVISORY FOR
PORTIONS OF THE NORTHERN NEBRASKA PANHANDLE AS LOW TEMPERATURES
ARE PROGGED TO BE NEAR OR JUST BELOW FREEZING AT CHADRON AND
ALLIANCE...WHICH IS OVER ONE WEEK PAST THE AVERAGE LAST FREEZE.
IT IS UNCLEAR HOW MUCH CLOUDINESS WILL BE PRESENT IN THESE
AREAS...SO WILL HOLD OFF ON ANY HEADLINES FOR NOW.

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 LOW 60S. THERE
SHOULD BE A BREAK IN THE PRECIPITATION ACROSS THE AREA DUE TO THE
MIDLEVEL SUBSIDENCE ASSOCIATED WITH THE RIDGE ALOFT. THE BREAK IN
THE PRECIPITATION WILL BE SHORT LIVED HOWEVER...AS ALL MODELS
INDICATE ANOTHER PACIFIC STORM SYSTEM MOVING TOWARDS THE AREA.
INITIALLY...EXPECT A ROUND OF SCATTERED SHOWERS AND ISOLATED TSTORMS
LATE ON THURSDAY AND INTO THURSDAY EVENING. WILL HAVE TO WATCH THE
NEXT SIGNIFICANT SHORTWAVE TROUGH LIFTING NORTHEAST OUT OF THE
SOUTHWEST LATE THURSDAY AND INTO FRIDAY. MODELS SHOW THIS
SHORTWAVE MOVING ACROSS COLORADO AND WYOMING EARLY ON FRIDAY
BRINGING ANOTHER ROUND OF RAINFALL AND MOUNTAIN SNOWS DUE TO A
LARGE AREA OF UPPER LEVEL DIFFLUENCE. SNOWLEVELS SHOULD BE
BETWEEN 7500 TO 8500 FEET THIS TIME AROUND...SO DO NOT EXPECT ANY
SNOW FLAKES OVER THE HIGH PLAINS AT THIS TIME. THIS SYSTEM HAS A
BETTER CHANCE TO CONTAIN HEAVIER RAIN SHOWERS COMPARED TO
YESTERDAYS SYSTEM...MAINLY BECAUSE OF THE CONDITIONAL INSTABILITY
ALOFT SHOWN BY MODEL SOUNDINGS. AFTER THE MAIN AREA OF
PRECIPITATION...EXPECT PARTIAL CLEARING SKIES FRIDAY AFTERNOON.
WITH THE COLD POOL ALOFT ASSOCIATED WITH THE UPPER LEVEL TROUGH
OVERHEAD...IT WILL NOT TAKE MUCH TO TRIGGER SCATTERED SHOWERS AND
TSTORMS LATE IN THE AFTERNOON EVEN AFTER THE TROUGH MOVES
NORTHWARD...WHICH IS TYPICAL THIS TIME OF THE YEAR. CONTINUED
INCREASING POP BETWEEN 60 TO 80 PERCENT...ESPECIALLY ALONG AND
EAST OF THE LARAMIE RANGE.

.LONG TERM...(FRIDAY NIGHT THROUGH TUESDAY)
ISSUED AT 340 AM MDT WED MAY 20 2015

ACTIVITY WEATHER PATTERN TO REMAIN IN PLACE THROUGH THE LONG TERM
FORECAST PERIOD. CHANCES FOR SHOWERS AND THUNDERSTORMS WILL EXIST
EVERY DAY AS COOL UNSTABLE AIR ALOFT REMAINS IN PLACE ALONG WITH
SEVERAL DISTURBANCES WHICH WILL MOVE ACROSS THE FORECAST AREA
INTO EARLY NEXT WEEK. DIFFICULT TO PINPOINT THE BEST CHANCES FOR
PRECIPITATION GIVEN THE FACT IT IS TOUGH FOR THE MODELS TO RESOLVE
A NEARLY CONTINUOUS STREAM OF UPPER DISTURBANCES...BUT SAFE TO
SAY AFTERNOON SHOWERS AND THUNDERSTORMS WILL BE COMMON DUE TO
DIURNAL HEATING PRODUCING INSTABILITY MANY DAYS. NOT OVERLY
CONCERNED ABOUT SEVERE THUNDERSTORMS THROUGH THE PERIOD BUT IF
THERE IS A DAY TO WATCH IT IS SATURDAY. AT THIS TIME IT LOOKS AT
THROUGH THE BOUNDARY LAYER MAY STAY CAPPED WHILE A DISTURBANCE
MOVES ACROSS THE AREA EARLY IN THE DAY...BUT ELEVATED CONVECTION
MAY BE POSSIBLE TOO. SOMETHING TO KEEP AN EYE ON THE NEXT FEW
DAYS.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z THURSDAY AFTERNOON)
ISSUED AT 1059 AM MDT WED MAY 20 2015

THE MAIN CONCERN WITH THIS TAF ISSUANCE IS THE TIMING OF WHEN THE
IFR/MVFR CEILINGS WILL BEGIN TO CLEAR OUT.

LATEST SURFACE ANALYSIS WAS SHOWING COLD AIR DAMMING PERSISTING ALONG
THE LEE SIDE OF THE LARAMIE RANGE AROUND LUNCHTIME. THE MODELS ARE
CURRENTLY TOO FAST IN MOVING THIS CANADIAN SURFACE HIGH EAST THIS
AFTERNOON. AS A RESULT...WE ARE EXPECTING THE LOW CEILINGS TO
HANG ON TO MUCH OF THE AREA THIS AFTERNOON. THE LOWEST CEILINGS
ARE AROUND CYS (AROUND 800FT) AND POINTS WEST TO THE I-80 SUMMIT
WHERE THEY ARE STILL EXPERIENCING DENSE FOG. THIS SURFACE HIGH
SHOULD GRADUALLY SHIFT EAST LATE THIS AFTERNOON INTO THIS EVENING.
HOWEVER...WE ARE NOT CONFIDENT THAT CYS CEILINGS WILL CLIMB VERY
MUCH DUE TO THE EASTERLY FLOW KICKING IN. WE ARE LITTLE MORE
CONFIDENT THAT THE LOWER CEILINGS SHOULD BEGIN TO SCOUR OUT FROM
NORTH TO SOUTH ACROSS THE PANHANDLE SITES LATE THIS AFTERNOON
BASED ON THE LATEST SATELLITE TRENDS. HOWEVER...THERE IS A
POSSIBILITY THAT SOME PATCHY FOG COULD DEVELOP IN ITS WAKE.
OTHERWISE...WEAK ISENTROPIC ASCENT WILL BRING SOME VFR CEILINGS
INTO THE AREA AFTER MIDNIGHT.


&&

.FIRE WEATHER...
ISSUED AT 246 AM MDT WED MAY 20 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 EARLY TODAY AND ON FRIDAY. DAYTIME
RELATIVE HUMIDITIES ACROSS THE HIGH VALLEYS AND HIGH PLAINS WILL
LIKELY NOT LOWER BELOW 50 PERCENT UNTIL EARLY NEXT WEEK.

&&

.HYDROLOGY...
ISSUED AT 246 AM MDT WED MAY 20 2015

LARAMIE RIVER AT FORT LARAMIE WILL CONTINUE TO RUN AT RELATIVELY
HIGH LEVELS THROUGH THIS WEEK...BUT AT THIS TIME IS EXPECTED TO
REMAIN WITHIN ITS BANKS. THE PACIFIC STORM SYSTEM WHICH PUSHED
ACROSS THE REGION YESTERDAY DID NOT IMPACT RIVER LEVELS TOO MUCH
ACROSS THE AREA. HOWEVER...ANOTHER PACIFIC SYSTEM ON FRIDAY MAY
BRING MODERATE TO HEAVY RAINFALL AMOUNTS TO THE HIGH PLAINS AND
POSSIBLY THE LARAMIE VALLEY...SO WILL NEED TO CONTINUE TO MONITOR
THE LARAMIE RIVER THROUGH LATE THIS WEEK. THE LARAMIE RIVER AT
LARAMIE IS FORECAST TO FALL BELOW ACTION STAGE THIS WEEKEND OR
EARLY NEXT WEEK.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...DENSE FOG ADVISORY UNTIL NOON MDT TODAY FOR WYZ110-116.

NE...NONE.
&&

$$

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



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