Special Weather Statement, NWS Cheyenne, WY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
546 PM MDT SUN AUG 28 2011

.AVIATION...00Z TAFS

LATEST RADAR IMAGERY SHOWING SHOWING A LINE OF THUNDERSTORMS
DEVELOPING ALONG THE LARAMIE RANGE LATE THIS AFTERNOON. EXPECT
THIS LINE TO MOVE OFF THE RANGE AND POSSIBLY INTENSIFY AS IT MOVES
INTO THE PANHANDLE. AS IT MOVED THROUGH CHEYENNE EARLIER WE SAW
WIND GUSTS TO 30KTS AS THE OUTFLOW BOUNDARY MOVED THROUGH THE
CITY. WOULD EXPECT SIMILAR WIND SPEEDS WHEN IT MOVES THROUGH
KBFF...KIBM AND KSNY IN THE NEXT 2-3 HOURS. WILL NEED TO WATCH
CAREFULLY FOR ANY SEVERE THUNDERSTORM DEVELOPMENT. AREAS RECEIVING
RAINFALL THIS EVENING ARE SUSCEPTIBLE TO STRATUS DEVELOPING IN
UPSLOPING WINDS. HAVE FORECAST KCYS...KSNY AND KBFF DOWN AFTER
MIDNIGHT TO MVFR CONDITIONS. CONFIDENCE GROWING THAT MODERATE
RAINFALL WILL FALL IN THE PANHANDLE MONDAY LATE MORNING THROUGH
THE AFTERNOON AND HAVE GONE MVFR CONDITIONS AFTER 18Z. CONDITIONS
COULD BE LOWER.

CLAYCOMB

&&

.PREV DISCUSSION... /ISSUED 453 PM MDT SUN AUG 28 2011/

UPDATE...WSR-88D REFLECTIVITY LOOPS FROM CHEYENNE AND DENVER RADARS
SHOW WIDESPREAD SHOWERS AND ISOLATED THUNDERSTORMS ACROSS SOUTHERN
ALBANY...FAR SOUTHERN PLATTE AND MOST OF LARAMIE COUNTIES...THUS
HAVE UPDATED THE GRIDDED FORECASTS AND RELATED PRODUCTS TO BOOST
AREAL COVERAGE OF SHOWERS SIGNIFICANTLY THROUGH 800 PM. WELL DEFINED
OUTFLOW BOUNDARY NOTED ON CHEYENNE REFLECTIVITY LOOP WITH THE
BOUNDARY MOVING NORTHEAST ACROSS SOUTHERN PLATTE AND CENTRAL
LARAMIE COUNTIES EAST OF CHEYENNE. EXPECT CONTINUED DEVELOPMENT
OF SHOWERS AND THUNDERSTORMS ALONG AND AHEAD OF THE NORTHEASTWARD
MOVING OUTFLOW BOUNDARY AS MOIST EAST WINDS EAST OF THE BOUNDARY
ADVECT 50 TO 60 DEGREE DEWPOINTS WESTWARD INTO THE BOUNDARY...AS
NOTED BY MID TO UPPER 50 DEGREES DEWPOINTS AT 4 PM AT
SCOTTSBLUFF...ALLIANCE AND SIDNEY. PUBLIC REPORTED 40 TO 50 MPH
WINDS IN THE WAKE OF THE OUTFLOW BOUNDARY ACROSS WESTERN LARAMIE
COUNTY EARLIER...SOME 10 MILES NORTH OF CHEYENNE.  RUBIN

PREV DISCUSSION... /ISSUED 308 PM MDT SUN AUG 28 2011/

SHORT TERM...TONIGHT THROUGH TUESDAY NIGHT
EARLY AFTERNOON WATER VAPOR IMAGERY LOOKS REMARKABLY SIMILAR TO
24 HOURS AGO...WITH THE PERSISTENT UPPER RIDGE STILL PARKED OVER
THE SOUTHWESTERN STATES. SUBTROPICAL MID-LEVEL MOISTURE CONTINUES
TO STREAM NORTHWARD ACROSS THE ROCKIES AND HIGH PLAINS. A CLOSED
UPPER LOW AND ASSOCIATED TROUGH WAS MOVING INTO WASHINGTON AND
OREGON. WEAK SHORTWAVES TOPPING THE RIDGE FROM UTAH AND IDAHO
ACROSS COLORADO AND WYOMING...COMBINED WITH THE MOISTURE AND
LOW/MID LEVEL INSTABILITY WERE TRIGGERING SCATTERED CONVECTION.
THUS FAR...MOST OF THE SHOWERS AND THUNDERSTORMS WERE CONFINED
TO CARBON AND SOUTHWEST ALBANY COUNTIES...AND WERE SLOWLY MOVING
TO THE EAST. THE REGIONAL SURFACE ANALYSIS PLACED A WEAK BACKDOOR
FRONT FROM EASTERN WYOMING INTO THE WESTERN NEBRASKA PANHANDLE.
SURFACE WINDS HAVE SHIFTED TO NORTHEASTERLY ALONG WITH SLOWLY RISING
DEW POINTS BEHIND THE FRONT. THIS FRONT WILL CONTINUE TO DRIFT TO
THE SOUTHWEST INTO SOUTHEAST WYOMING THROUGH LATE THIS AFTERNOON...
WITH ADDITIONAL CONVECTION EXPECTED TO DEVELOP EAST OF THE LARAMIE
RANGE. INSTABILITY WILL BE MARGINAL /SOUNDING CAPES OF 400-800
J/KG/ ALONG WITH INVERTED-V SOUNDINGS TO SUPPORT SOME GUSTY
THUNDERSTORMS THIS AFTERNOON AND EVENING.

THE 12Z SHORT RANGE MODEL SUITE CONTINUES TO ADVERTISE THE FLATTENING
OF THE UPPER RIDGE AS THE PACIFIC NORTHWEST SHORTWAVE TROUGH TRACKS
EAST ACROSS THE NORTHERN ROCKIES AND PLAINS THROUGH TUESDAY. THIS
PATTERN CHANGE WILL RESULT IN A MORE ZONAL FLOW ALOFT. MEANWHILE...
THE BACKDOOR SURFACE BOUNDARY WILL STALL ALONG THE LARAMIE RANGE
TONIGHT AND MONDAY. FAVORABLE UPSLOPE...COMBINED WITH ADEQUATE
MOISTURE AND MARGINAL INSTABILITY WILL KEEP CHANCES FOR SHOWERS AND
THUNDERSTORMS IN THE FORECAST FOR THE NEXT 24 TO 36 HOURS. THE
INSTABILITY/THETA-E AXIS WILL SET UP EAST OF THE LARAMIE RANGE
INTO WESTERN NEBRASKA MONDAY AFTERNOON AND EVENING WITH SBCAPES
OF 500-1000 J/KG AND DEW POINTS IN THE 50S TO LOWER 60S. BULK SHEAR
VALUES LOOK MARGINAL...SO SEVERE THREAT WILL BE MINIMAL. CANNOT
RULE OUT ISOLATED STRONG THUNDERSTORMS WITH WIND GUSTS TO 50 MPH...
SMALL HAIL AND LOCALLY HEAVY RAIN. HIGH TEMPERATURES MONDAY WILL
BE AT LEAST 10 DEGREES COOLER /IN THE 80S/ THAN SUNDAY DUE TO
THICKER CLOUD COVER AND GREATER PRECIPITATION COVERAGE FOR AREAS
EAST OF THE LARAMIE RANGE. TEMPERATURES WILL REMAIN FAIRLY WARM
OUT WEST AS THE BOUNDARY WILL BE STALLED ALONG THE RANGE. ISOLATED
SHOWERS AND THUNDERSTORMS WILL LINGER OVER NORTHEAST PORTIONS OF
THE CWA MONDAY NIGHT AS THE SURFACE BOUNDARY AND SHORTWAVE MOVE
EAST. DRIER AND MORE STABLE AIR WILL FILTER INTO THE CWA FROM THE
WEST...ALONG WITH INCREASING WESTERLY SURFACE WINDS. THIS WILL
HEIGHTEN FIRE WEATHER CONCERNS OVER PORTIONS OF EAST CENTRAL AND
SOUTH CENTRAL WYOMING DURING THE AFTERNOON. ALSO...EXPECT WARMER
TEMPERATURES TUESDAY AFTERNOON /UPPER 80S TO MID 90S/...ESPECIALLY
EAST OF THE LARAMIE RANGE DUE TO DOWNSLOPE WESTERLY WIND COMPONENT.

LONG TERM...WEDNESDAY THROUGH SUNDAY...LONG RANGE MODELS CONTINUE
TO STRUGGLE WITH THE TIMING AND INTENSITY OF NORTHERN PACIFIC
TROUGHS AND ASSOCIATED COLD FRONTS PUSHING SOUTH OUT OF WESTERN
CANADA LATER THIS WEEK AND INTO THE WEEKEND. FAIRLY CONFIDENT THAT
WEDNESDAY AND WEDNESDAY NIGHT WILL BE DRY AND VERY WARM AHEAD OF
THE FIRST COLD FRONT...WITH HIGHS IN THE 90S EAST OF THE LARAMIE
RANGE AND MID TO UPPER 80S ACROSS CNTRL WYOMING. THERE WILL LIKELY
BE ENOUGH MIDLEVEL MOISTURE AND LLVL INSTABILITY TO PRODUCE
ISOLATED THUNDERSTORMS OR RAIN SHOWERS WEDNESDAY AFTERNOON...BUT
EXPECT COVERAGE TO BE LESS TODAY AND MONDAY WITH VERY LITTLE
DYNAMIC FORCING AND MODELS SHOWING ANTICYCLONIC FLOW OVER THE
CNTRL PLAINS. KEPT LOW POP ACROSS THE AREA...UNDER 20 PERCENT.

FOR LATE IN THE WEEK AND INTO NEXT WEEKEND...THE 12Z MODELS ARE
SHOWING POOR CONSISTENCY WITH DIFFERENT SOLUTIONS COMPARED TO
YESTERDAY. THE 12Z ECMWF IS NOW SHOWING A STRONG COLD FRONT AND
MODERATE PRECIPITATION PUSHING SOUTHWARD INTO SOUTHEAST WYOMING
AND WESTERN NEBRASKA STARTING MIDDAY ON THURSDAY...WITH 700MB
TEMPS LOWERING TO 0C JUST NORTH OF THE FORECAST AREA. THIS WOULD
SUGGEST HIGHS ON FRIDAY IN THE 60S TO LOW 70S. HOWEVER...THE GFS
AND THE GEM SHOW THIS FRONT CONSIDERABLY WEAKER COMPARED TO THE
ECMWF AND FURTHER NORTH WITH ONLY SLIGHTLY COOLER TEMPS IN THE UPPER
70S TO LOW 80S AND HARDLY ANY PRECIP. SEVERAL ENSEMBLE MEMBERS ARE
SIMILAR TO THE ECMWF...SO CAN NOT IGNORE THAT SOLUTION. LOWERED
TEMPS TOWARDS CLIMO AND INCREASED POP BETWEEN 15 TO 30
PERCENT ESPECIALLY FURTHER NORTH...WHICH IS IN DECENT AGREEMENT
WITH MODEL CONSENSUS AT THIS TIME. IF ANYTHING...MODELS SHOW
BETTER AGREEMENT FOR DAYS 6 AND 7 AS ANOTHER COLD FRONT PUSHES
SOUTH OUT OF CANADA WITH ALL MODEL SOLUTIONS SHOWING NORTHWEST
FLOW ALOFT AND COOLER TEMPS CLOSER TO NORMAL.

AVIATION...18Z TAF ISSUANCE. WEAK COLD FRONT ACROSS WESTERN
NEBRASKA WILL CONTINUE TO DRIFT SOUTHWEST INTO EASTERN WYOMING
TODAY. THIS WILL RESULT IN WINDS SHIFTING INTO THE EAST TO
GRADUALLY SOUTHEAST TODAY AND TONIGHT. UPSLOPE WINDS WILL PROVIDE
ADDITIONAL MOISTURE TO THE AREA...AND WITH AN UPPER LEVEL
DISTURBANCE EXPECTED TO MOVE ACROSS WYOMING THIS EVENING...RAIN
SHOWERS AND SCATTERED TSTORMS ARE EXPECTED TO DEVELOP BY LATE THIS
AFTERNOON AND PUSH EAST INTO WESTERN NEBRASKA. VFR CONDITIONS WILL
PREVAIL TODAY...BUT PERIODS OF MVFR CONDITIONS CAN NOT BE RULED
OUT...ESPECIALLY AFTER 00Z THIS EVENING.

FIRE WEATHER...SCATTERED SHOWERS AND THUNDERSTORMS ARE EXPECTED
FOR MUCH OF THE DISTRICT THROUGH MONDAY EVENING AS SUBTROPICAL
MOISTURE COMBINES WITH WEAK UPPER LEVEL DISTURBANCES AND A
MEANDERING SURFACE TROUGH OVER SOUTHEAST WYOMING AND THE WESTERN
NEBRASKA PANHANDLE. MINIMUM RELATIVE HUMIDITIES THIS AFTERNOON
WILL BE SLIGHTLY HIGHER THAN RECENT DAYS WITH VALUES IN THE 20S
AND 30S. HUMIDITY RECOVERIES WILL BE GOOD TO EXCELLENT AT NIGHT.
THE AIR MASS WILL BEGIN TO DRY OUT TUESDAY AS THE SURFACE TROUGH
SHIFTS EAST INTO WESTERN PLAINS AND WESTERLY WINDS INCREASE OVER
EAST CENTRAL AND SOUTH CENTRAL WYOMING. MINIMUM RELATIVE
HUMIDITIES WILL PLUMMET TO 10 TO 15 PERCENT OVER PORTIONS OF
CARBON AND CONVERSE COUNTIES TUESDAY AFTERNOON. THESE DRY
CONDITIONS COMBINED WITH GUSTY WEST WINDS UP TO 30 MPH MAY LEAD TO
CRITICAL FIRE WEATHER CONDITIONS AT THAT TIME.

&&

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

&&

SHORT TERM...JAMSKI
LONG TERM/AVIATION...TJT

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