Area Forecast Discussion
Issued by NWS Cheyenne, WY

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432
FXUS65 KCYS 182058
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
258 PM MDT MON MAY 18 2015

.UPDATE...
ISSUED AT 252 PM MDT MON MAY 18 2015

LATEST WEBCAMS WAS SHOWING THE DENSE FOG PERSISTING ACROSS THE
I-80 SUMMIT. AS A RESULT...WE ARE EXTENDING THE DENSE FOG ADVISORY
UNTIL 09Z. THIS MAY BE THE TIME WHEN MORE WIDESPREAD SHOWERS/SNOW
SHOWERS BEGINS TO DEVELOP OVER THE AREA.

UPDATE ISSUED AT 1130 AM MDT MON MAY 18 2015

WE WENT AHEAD AND ISSUED A QUICK UPDATE TO ISSUE A DENSE FOG
ADVISORY FOR AREAS BETWEEN BUFORD (MILE MARKER M336) AND VEDAUWOO
(MILE MARKER M330). SOUTHEAST SURFACE WINDS WILL PERSIST THROUGH
THE EARLY AFTERNOON AS THE UPPER LEVEL DIFFLUENCE CONTINUES TO
INCREASE IN ADVANCE OF THE UPPER LEVEL TROF. WE MAY NEED TO EXTEND
THIS ADVISORY AND EXPAND IT FURTHER EAST...BUT WE WILL CONTINUE
TO MONITOR THROUGH THE AFTERNOON.

UPDATE ISSUED AT 633AM MDT MON MAY 18 2015

WEBCAMS SHOWING SOME FOG AROUND ARLINGTON IN THE NORTHERN SNOWY
RANGE AND ALSO INTO THE SOUTHERN LARAMIE RANGE AND FOOTHILLS
BETWEEN CHEYENNE AND LARAMIE. SOME OF THIS FOG WILL BE DENSE THIS
MORNING BUT CONDITIONS SHOULD IMPROVE BY LATE THIS MORNING. ALSO
THE RADAR IS IS SHOWING SOME DRIZZLE AROUND CHEYENNE AND WEST INTO
THE SOUTHERN LARAMIE RANGE. ADDED THE FOG AND DRIZZLE TO THE
FORECAST THIS MORNING.

&&

.SHORT TERM...(TODAY AND THROUGH WEDNESDAY)
ISSUED AT 400 AM MDT MON MAY 18 2015

MAIN FORECAST CHALLENGE WILL BE ON TUESDAY AS A RELATIVELY STRONG
UPPER LEVEL LOW MOVES ACROSS THE AREA PRODUCING WIDESPREAD RAIN
AND SNOW. CONDITIONS THROUGH THIS AFTERNOON WILL MAINLY BE DRY
WITH ONLY A FEW SHOWERS/THUNDERSTORMS ALONG AND WEST OF THE
LARAMIE RANGE THROUGH THIS AFTERNOON...BUT SHOWERS WILL BEGIN TO
SPREAD INTO THE PLAINS THIS EVENING AS THE UPPER LOW APPROACHES
THE AREA AND DIVERGENT FLOW ALOFT DEVELOPS.

PRECIPITATION WILL BECOME WIDESPREAD TUESDAY BUT QUESTIONS REMAIN
AS TO PRECIPITATION TYPE AND QPF AMOUNTS. RAIN WILL CHANGE TO
SNOW LATE TONIGHT INTO TUESDAY MORNING ACROSS THE MOUNTAINS AS
COOLER AIR ALOFT ASSOCIATED WITH THE UPPER LEVEL LOW MOVES INTO
THE FORECAST AREA. THE BIG QUESTION AND CONCERN IS WHETHER OR NOT
SNOW LEVELS WILL DROP FAR ENOUGH FOR SNOW INTO THE PLAINS. 00Z
MODELS RUNS TRENDED SLIGHTLY WARMER WHICH MAKES SENSE GIVEN THE
LOW WILL BE LIFTING OUT OF THE SOUTHWEST CONUS AND STRONG
SOUTHEAST WYOMING WILL PUMP RELATIVELY WARM AND MOIST LOW AIR INTO
THE PLAINS. THIS CONTINUOUSLY REINFORCING WARM LOW LEVEL AIR
SHOULD KEEP PRECIPITATION IN THE FORM OF RAIN...EXCEPT OVER HIGHER
ELEVATIONS SUCH AS ALONG THE CHEYENNE RIDGE AND PINE RIDGE. BUT
EVEN ALONG THESE RIDGE AREAS PRECIPITATION RATES WILL NEED TO BE
HIGH ENOUGH TO COOL THE ATMOSPHERIC COLUMN ENOUGH FOR SNOW TO
REACH THE SURFACE...AT LEAST BASED ON CURRENT MODEL TEMPERATURES.
OF NOTE...THE NAM IS MUCH COOLER THAN ALL OTHER MODELS WITH 700
MB TEMPERATURES -3 TO -6 C TUESDAY. OTHER MODELS ARE 0 TO -2 AT
700 MB. DID NOT BUY OFF ON THE NAM...BUT 700 MB TEMPERATURES WILL
BE SOMETHING TO KEEP AN EYE ON IN FUTURE MODEL SOLUTIONS JUST IN
CASE THE NAM IS ON TO SOMETHING.

BASED ON CURRENT MODEL CONSENSUS...WHICH MAY CHANGE SOMEWHAT
BETWEEN NOW AND TUESDAY...THE HEAVIEST PRECIPITATION SHOULD MOVE
INTO THE I80 CORRIDOR MID TUESDAY MORNING...LIFTING NORTHWARD
THROUGH THE DAY. PLACES LIKE CHEYENNE SHOULD SEE RAIN CHANGE TO
SNOW WITH TEMPERATURES FALLING INTO THE MID 30S WITH BIG HEAVY WET
SNOW FLAKES. LATE AFTERNOON TO EARLY EVENING PRECIPITATION RATES
SHOULD DECREASE WITH SNOW LIKELY CHANGING BACK TO LIGHT RAIN ALONG
INTERSTATE 80. FARTHER NORTH...ALONG THE PINE RIDGE IT WILL BE
SLIGHTLY COOLER ALOFT WHICH MAY BE JUST ENOUGH TO KEEP SNOW AS THE
MAIN PRECIPITATION TYPE EVENING INTO TUESDAY NIGHT. OF COURSE A
SUBTLE ERROR OF ONLY 1 OR 2 DEGREES IN THE CURRENT MODELS CAN MAKE
A HUGE DIFFERENCE IN PRECIPITATION TYPE AND SNOW ACCUMULATION...SO
MUCH UNCERTAINTY STILL EXISTS WITH TOTAL SNOW AMOUNTS.

CURRENTLY THINKING IMPACTS FROM SNOW WILL BE MINIMAL WITH
THIS SYSTEM...MAINLY DUE TO THE TIME OF DAY THAT THE HEAVIEST
SNOW IS EXPECTED TO OCCUR. SNOW RATES WILL LIKELY BE HIGH ENOUGH
FOR SNOW ACCUMULATIONS ON ELEVATED SURFACES AND GRASSY
AREAS...BUT IT WILL BE DIFFICULT FOR SNOW TO ACCUMULATE ON
ROADWAYS. MAIN TRAVEL IMPACT WILL BE LOW VISIBILITIES DURING
TIMES OF HEAVIER SNOW AND GUSTY WINDS.

PRECIPITATION WILL DECREASE IN INTENSITY AND COVERAGE TUESDAY
NIGHT WITH THE MAIN CHANCES FOR ACCUMULATING SNOW SHIFTING NORTH
INTO EAST CENTRAL WYOMING AND NORTHERN PORTIONS OF THE NEBRASKA
PANHANDLE. CHANCES FOR PRECIPITATION WILL CONTINUE INTO WEDNESDAY
BUT ONLY LOOKING FOR RAIN SHOWERS AND POSSIBLY A FEW ISOLATED
THUNDERSTORMS IN THE WEST.

.LONG TERM...(WEDNESDAY NIGHT THROUGH SUNDAY)
ISSUED AT 334 AM MDT MON MAY 18 2015

LONG RANGE MODELS REMAIN IN GOOD AGREEMENT...SHOWING A NEARLY
STATIONARY LONG WAVE TROUGH REMAIN ACROSS THE WESTERN UNITED
STATES. THIS PATTERN WILL NOT CHANGE MUCH IN THE NEAR FUTURE DUE
TO CONSIDERABLE BLOCKING OVER NORTH AMERICA...AND A LARGE REX
BLOCK EXTENDING FROM NORTHERN CANADA SOUTHWARD TOWARDS THE
SOUTHWESTERN UNITED STATES. THEREFORE...EXPECT UNSETTLED AND COOL
CONDITIONS THROUGH LATE NEXT WEEK AND INTO NEXT WEEKEND AS
SEVERAL PACIFIC SYSTEMS MOVE ACROSS THE WESTERN UNITED STATES AND
IMPACT SOUTHEAST WYOMING AND WESTERN NEBRASKA.

YET ANOTHER PACIFIC STORM SYSTEM WILL PUSH ONSHORE ACROSS
SOUTHERN CALIFORNIA AND MOVES NORTHEAST TOWARDS THE ROCKY MOUNTAIN
REGION INTO NEXT WEEKEND...RESULTING IN A CONTINUATION OF RAIN
SHOWERS AND TSTORMS EACH DAY. CURRENT 00Z RUNS ARE HINTING THAT
THE INITIAL SHORTWAVE AHEAD OF THE MAIN LOW WILL BE EERILY SIMILAR
WITH TUESDAYS EVENT...AS A NEGATIVELY TILTED TROUGH QUICKLY MOVES
ACROSS THE AREA FROM SOUTHWEST TO NORTHEAST. 00Z ECMWF IS EVEN
SHOWING 700MB TEMPERATURES AROUND -4C TO -5C ACROSS THE HIGH
PLAINS AS THE MAIN AREA OF PRECIP MOVES NORTHWARD. KEPT POP
BETWEEN 30 TO 50 PERCENT THROUGH FRIDAY. HOWEVER...THURSDAY AND
SATURDAY APPEAR TO BE THE MOST LIKELY DAYS FOR SEVERE OR STRONG
TSTORMS...WITH MODELS NOW SHOWING THE TROUGH AXIS OVERHEAD ON
FRIDAY. IN FACT...TEMPERATURES ON FRIDAY MAY STRUGGLE TO REACH 50
DEGREES DUE TO ONGOING RAINFALL AND NORTH TO NORTHEAST WINDS.

FOR NEXT WEEKEND...MODELS SHOW THE MAIN PACIFIC STORM SYSTEM
PUSHING ACROSS THE AREA ON SATURDAY...WITH WIDESPREAD RAIN SHOWERS
AND SCATTERED TSTORMS. WILL HAVE TO MONITOR THE TRACK OF THIS
STORM...SINCE HEAVY RAINFALL IS LIKELY IF IT WERE TO TRACK SLOWLY
ACROSS THE FORECAST AREA. OTHERWISE...UNSETTLED AND RELATIVELY
COOL CONDITIONS WILL CONTINUE NEXT WEEKEND WITH HIGHS IN THE MID
50S TO LOW 60S.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z TUESDAY AFTERNOON)
ISSUED AT 1151 AM MDT MON MAY 18 2015

MVFR CIGS WILL CONTINUE ACROSS THE AREA THROUGH THE EARLY
AFTERNOON. COULD SEE CIGS LIFT SOME ACROSS THE NEBRASKA PANHANDLE
BY LATER THIS AFTERNOON. SOUTHEAST WINDS WILL GUST TO 25-30 KTS AT
LAR THROUGH MUCH OF THE PERIOD. LIGHT TO MODERATE RAIN WILL MOVE
INTO THE REGION BY EARLY TUES MORNING WITH LOWERING VIS AND CIGS.


&&

.FIRE WEATHER...
ISSUED AT 234 AM MDT MON MAY 18 2015

COOL AND WET CONDITIONS WILL OCCUR THROUGH THE WEEK WITH CHANCES
FOR SHOWERS NEARLY EVERY DAY. THE MAIN CHANCES FOR PRECIPITATION
WILL BE TUESDAY AND FRIDAY AS UPPER LEVEL SHORTWAVES MOVES ACROSS
SOUTHEAST WYOMING AND NEBRASKA PANHANDLE.

&&

.HYDROLOGY...
ISSUED AT 334 AM MDT MON MAY 18 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. THE MAIN QUESTION IS HOW MUCH PRECIPITATION WILL FALL
AS A QUICK MOVING BUT STRONG AREA OF LOW PRESSURE MOVES ACROSS THE
AREA TUESDAY. HIGHEST PRECIPITATION AMOUNTS ARE LIKELY ACROSS THE
PLAINS WHICH COULD BE ENOUGH TO PUSH THE LARAMIE RIVER AT FORT
LARAMIE INTO MINOR FLOODING. ANOTHER SHOT OF WIDESPREAD MOISTURE
IS POSSIBLE LATE THIS WEEK.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...DENSE FOG ADVISORY UNTIL 3 PM MDT THIS AFTERNOON FOR WYZ116.

NE...NONE.
&&

$$

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



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