Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 031617
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
1017 AM MDT THU SEP 3 2015

.UPDATE...
ISSUED AT 1006 AM MDT THU SEP 3 2015

SHOWERS ARE SPREADING NORTHWARD OUT OF NORTHEAST COLORADO AT THIS
TIME. SHOWER ACTIVITY WILL SLOWLY CONTINUE NORTHWARD THROUGH THIS
AFTERNOON AND EVENING IN ASSOCIATION WITH A WEAK SHORTWAVE MOVING
THROUGH SOUTHWEST FLOW ALOFT. A FEW EMBEDDED THUNDERSTORMS WILL
ALSO DEVELOP THIS AFTERNOON BUT THERE IS NO THREAT FOR SEVERE
THUNDERSTORMS.

&&

.SHORT TERM...(TODAY THROUGH SATURDAY)
ISSUED AT 300 AM MDT THU SEP 3 2015

PRETTY QUIET WEATHER EARLY THIS MORNING ACROSS THE FORECAST AREA.
CURRENT WATER VAPOR LOOP SHOWS A RIDGE AXIS BUILDING OVER THE
GREAT PLAINS AS A BROAD UPPER LEVEL TROUGH SETTLES OVER THE
PACIFIC NORTHWEST...BRINGING MUCH NEEDED RAINFALL TO WASHINGTON
AND OREGON. THIS SYSTEM WILL EVENTUALLY IMPACT WYOMING THIS
WEEKEND. BEFORE THIS...CURRENT WATER VAPOR ALSO SHOWS A WELL
DEFINED SHORTWAVE DISTURBANCE ALOFT NEAR THE FOUR CORNERS REGION
WITH MID TO HIGH CLOUDS MOVING NORTHWARD WELL AHEAD OF IT. THIS
FEATURE WILL BE THE MAIN FORECAST CONCERN TODAY. A FEW MODELS
SHOW THIS FEATURE MOVING OVER SOUTHEAST WYOMING THIS AFTERNOON
PRODUCING SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS.
THUNDERSTORMS MAY PRODUCE SOME HEAVY RAINFALL AS SHORT RANGE
MODELS SUCH AS THE HRRR INDICATE REDEVELOPMENT AND TRAINING OVER
THE SAME AREA INTO LATE THIS EVENING. THIS IS DUE TO A POTENT
UPPER LEVEL JET AND GOOD 300MB DIFLUENCE ALOFT. HOWEVER...NOT ALL
MODELS AGREE ON THE TIMING OF THIS FEATURE...WITH THE ECMWF AND
GEM 3 TO 6 HOURS BEHIND THE GFS. DUE TO PEAK HEATING AND AVAILABLE
INSTABILITY...THIS TIMING DIFFERENCE IS SIGNIFICANT. THE NEW 06Z
NAM IS IN GOOD AGREEMENT WITH THE GFS AND HRRR...SO CONFIDENCE IS
INCREASING THAT SEVERAL BANDS OF SHOWERS AND TSTORMS WILL DEVELOP
OVER THE ROCKY MOUNTAINS AND QUICKLY LIFT NORTHEAST ACROSS THE I25
AND I80 CORRIDORS BY THIS AFTERNOON. INCREASED POP ACROSS MOST OF
THE AREA WITH THE EXCEPTION OF CARBON COUNTY. INCREASED POP
BETWEEN 30 TO 60 PERCENT FURTHER SOUTH AND WEST...SINCE THERE IS
STILL LOW CONFIDENCE IN SHOWER/TSTORM COVERAGE NORTH OF THE PLATTE
RIVER VALLEY DUE TO TIMING DIFFERENCES.

FRIDAY WILL BE PRETTY SIMILAR TO THURSDAY WITH HIGH TEMPERATURES
IN THE 80S TO MID 90S AND ANOTHER GOOD CHANCE OF AFTERNOON SHOWERS
AND TSTORMS. IN FACT...CONDITIONS APPEAR MORE FAVORABLE IN THE
EVENING AS THE NAM...GFS...AND ECMWF SHOW QUITE A BIT OF UPPER
LEVEL DIFLUENCE DUE TO JET DYNAMICS ALOFT. THIS DYNAMIC LIFT PEAKS
BETWEEN 00Z TO 06Z FRIDAY EVENING BEFORE LIFTING NORTHEAST.
INCREASED POP UP TO 45 PERCENT ACROSS THE HIGH PLAINS FRIDAY
EVENING.

THE UPPER LEVEL TROUGH OVER THE PACIFIC NORTHWEST WILL MOVE
TOWARDS THE FORECAST AREA ON SATURDAY. IT WILL BE SLIGHTLY COOLER
WITH HIGHS IN THE UPPER 70S TO UPPER 80S...BUT THE PRIMARY COLD
FRONT WILL NOT ENTER SOUTHEAST WYOMING UNTIL LATE SATURDAY NIGHT.
THIS MAIN STORM SYSTEM IS EXPECTED TO MOVE NORTHEAST AND REMAIN
NORTH OF THE FORECAST AREA. KEPT POP BETWEEN 10 TO 20 PERCENT WITH
THE HIGHEST CHANCE ACROSS THE MOUNTAINS...DUE TO SOME DRIER AIR
ALOFT AND A STABLE LAYER AROUND 500MB WHICH WILL INHIBIT WIDESPREAD
CONVECTION.

.LONG TERM...(SATURDAY NIGHT THROUGH WEDNESDAY)
ISSUED AT 230 AM MDT THU SEP 3 2015

THE UPPER LEVEL TROUGH WILL SWING NORTHEASTWARD ACROSS NORTHERN WY
ON SAT NIGHT AND SUN MORNING WITH AN ASSOCIATED FROPA.  WINDS WILL
REALLY INCREASE BEHIND THE FRONT ON SUN MORNING AS THE MODELS SHOW
700MB WINDS OF 35-40 KTS OVER SOUTHEAST WY.  THE GRADIENT DOES BEGIN
TO RELAX BY THE AFTN SO WINDS WILL COME DOWN SOME.  WITH A VERY DRY
AIR MASS IN PLACE ON SUN AND MIN RH VALUES FALLING TO AROUND 15
PERCENT...THERE WILL LIKELY BE SOME FIRE WEATHER CONCERNS DESPITE
THE COOLER TEMPS.  DRY WESTERLY MID LEVEL FLOW WILL REMAIN IN PLACE
THROUGH EARLY NEXT WEEK WITH PW VALUES OF ONLY 0.25-0.5 INCHES AND
NO HINT OF ANY MONSOONAL MOISTURE INTRUSION.  TEMPS WILL BE AROUND
NORMAL FOR EARLY SEP WITH COOLER NIGHTS THAN WE HAVE SEEN RECENTLY.
THE MODEL DIFFERENCES BETWEEN THE GFS AND ECMWF REALLY BEGIN TO SHOW
UP BY TUESDAY AS THE GFS SHOWS A STRONGER SHORTWAVE MOVING ACROSS WY.
REGARDLESS OF THE SOLUTION...A DRY FCST WILL PREVAIL WITH LIMITED
MOISTURE AND DYNAMICS.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z FRIDAY MORNING)
ISSUED AT 532 AM MDT THU SEP 3 2015

VFR CONDITIONS THROUGH THE TAF PERIOD. WINDS WILL MOSTLY REMAIN
BELOW 15 KTS...EXCEPT AT RWL WHERE GUSTS TO 20 KTS WILL BE
POSSIBLE DURING THE AFTERNOON. ISOLATED TO SCATTERED SHOWERS AND
THUNDERSTORMS WILL DEVELOP AFTER 20Z ACROSS SOUTHEAST WY AND THEN
MOVE EASTWARD INTO THE NEBRASKA PANHANDLE THROUGH THE EVENING.
GUSTY AND ERRATIC WINDS ALONG WITH BRIEF HEAVY RAIN WILL BE
POSSIBLE WITH THE STORMS.


&&

.FIRE WEATHER...
ISSUED AT 230 AM MDT THU SEP 3 2015

IT WILL BE PRETTY DRY INITIALLY THIS AFTERNOON OVER EAST CENTRAL
WYOMING BEFORE SOME SUBTROPICAL MOISTURE MOVES INTO THE
AREA...AHEAD OF AN UPPER LEVEL DISTURBANCE. ALTHOUGH FIRE WEATHER
CONCERNS ARE LOW TODAY...WILL HAVE TO MONITOR WIND SPEEDS ACROSS
ZONE 301 AND 302. FOR NOW...WINDS ARE EXPECTED TO BE BELOW 20 MPH
AT THIS TIME. MINIMAL FIRE WEATHER CONCERNS ARE ANTICIPATED FRIDAY
AND SATURDAY AS THUNDERSTORM COVERAGE WILL INCREASE AND RELATIVE
HUMIDITIES WILL TREND HIGHER...MINIMUM DAYTIME RELATIVE HUMIDITIES
WILL BE BETWEEN 20 TO 30 PERCENT BY SATURDAY.

&&

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

&&

$$

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


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