Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 291805
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
1205 PM MDT MON SEP 29 2014

.UPDATE...
ISSUED AT 1130 AM MDT MON SEP 29 2014

FIRST LINE OF SHOWERS AND THUNDERSTORMS ARE NOW EAST OF THE CWFA
FOR THE MOST PART WITH AMPLE SUNSHINE ACROSS MOST OF ALBANY...
LARAMIE AND PLATTE COUNTIES THIS MORNING. ATMOSPHERIC
DESTABILIZATION ALREADY UNDERWAY...WITH HIGH TEMPERATURES ALREADY
BEING REACHED IN THIS AREAS. IN A RECENT UPDATE HAVE BOOSTED HIGHS
3-4F ACROSS AREAS THAT HAVE ALREADY BROKEN OUT LATE THIS MORNING.
THIS WILL SET THE STAGE FOR CONVECTIVE RE-DEVELOPMENT EXPECTED BY
BETWEEN 1-2 PM PER LATEST HRRR OUTPUT. EVERYTHING STILL CONTINUES
TO POINT TO WIDESPREAD CONVECTION BY LATE THIS AFTERNOON AND THIS
EVENING ACROSS THE ERN PLAINS...SOME OF WHICH WILL HAVE SEVERE
POTENTIAL GENERALLY SOUTH AND EAST OF A CHEYENNE TO ALLIANCE LINE.
THE OTHER CONCERN IS WITH FLOODING POTENTIAL OVER THE WESTERN
NEBRASKA PANHANDLE. THESE AREAS HAVE ALREADY RECEIVED 1-2 INCHES
OF RAINFALL THIS MORNING...AND WITH ANOTHER 1-2...POTENTIALLY
HIGHER ON THE WAY LATER TODAY AND TONIGHT...THOUGH IT WAS PRUDENT
TO HOIST A FLOOD WATCH. WILL CONTINUE TO MONITOR DEVELOPMENTS
CLOSELY OVER THE NEXT FEW HOURS.

&&

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 450 AM MDT MON SEP 29 2014

AN ACTIVE DAY OF WEATHER TODAY AS UPPER LEVEL LOW PRESSURE MOVES
INTO THE CENTRAL ROCKIES. SHOWERS AND THUNDERSTORMS CURRENTLY
NOTICEABLE ON RADAR IN ASSOCIATION WITH A VORTICITY LOBE WHICH IS
LIFTING NORTHWARD AHEAD OF THE MAIN UPPER LEVEL LOW. THIS ACTIVITY
SHOULD SUBSIDE SOMEWHAT THROUGH THIS MORNING BUT SHOULD CONTINUE
TO SEE ISOLATED TO SCATTERED SHOWERS AND THUNDERSTORMS. THE MAIN
SHOT OF SHOWERS AND THUNDERSTORMS WILL HOLD OFF UNTIL THIS
MID AFTERNOON AS THE MAIN UPPER LEVEL LOW LIFTS INTO SOUTHEAST
WYOMING AND THE NEBRASKA PANHANDLE. THIS IS A VERY DYNAMIC SYSTEM
AND WILL BRING WITH IT THE POTENTIAL FOR SEVERE THUNDERSTORMS AND
SNOW TO THE MOUNTAINS.

CLOUD COVER THIS AFTERNOON MAY SERVE TO SLIGHTLY LIMIT THE
OVERALL THREAT FOR SEVERE STORMS...BUT REGARDLESS OF SURFACE
HEATING...UPPER LEVEL FORCING SHOULD MORE THAN COMPENSATE. STRONG
DIFFLUENT/DIVERGENT FLOW AND COOLING ALOFT WILL PRODUCE AMPLE
LIFT AND INSTABILITY RESPECTIVELY. THE MAIN SEVERE THREATS WILL BE
HAIL AND HEAVY RAIN...BUT STRONG WIND GUSTS WILL BE POSSIBLE...AS
WELL AS...TORNADOES. LCL HEIGHTS WILL BE VERY CLOSE TO THE GROUND
SUCH THAT ANY STRONG UPDRAFTS WILL NOT HAVE TO DO MUCH WORK TO
STRETCH AND TILT HORIZONTAL VORTICITY. MODELS SOLUTIONS ARE
SHOWING SOUTHEAST SURFACE WINDS OVER FAR SOUTHEAST WYOMING AND
MUCH OF THE NEBRASKA PANHANDLE THIS AFTERNOON AND EVENING. WINDS
ALOFT QUICKLY VEER WITH HEIGHT WHICH IS ALSO FAVORABLE FOR TORNADO
GENESIS. THE MAIN TIME FRAME FOR SEVERE THUNDERSTORMS AND TORNADO
POTENTIAL WILL BE MID AFTERNOON THROUGH MID EVENING.

40 TO 50 KTS OF EFFECTIVE BULK SHEAR ALONG WITH ELEVATED CAPE
AROUND 1000 J/KG WILL BE ENOUGH TO PRODUCE ROBUST UPDRAFTS CAPABLE
OF PRODUCING LARGE HAIL. ALTHOUGH FREEZING LEVELS ARE RATHER HIGH
FOR THIS TIME OF YEAR...THEY WILL ONLY BE 7000 TO 9000 FEET AGL BY
00Z THIS EVENING AND FALLING. THIS MEANS HAILS STONES WILL HAVE
NOT HAVE MUCH TIME TO MELT BEFORE REACHING THE GROUND. WOULD NOT
BE COMPLETELY SURPRISED TO SEE HAIL TO THE SIZE OF GOLF BALLS WITH A
FEW STORMS LATE THIS AFTERNOON AND EARLY EVENING...PARTICULARLY
INTO THE NEBRASKA PANHANDLE. ALSO WORRIED ABOUT HEAVY RAIN. DUAL
POLE RADAR ALREADY SHOWING 1 TO 2 INCHES OF RAIN WITH LAST EVENING
AND OVERNIGHT CONVECTION FOR PORTIONS OF THE AREA. HEAVY RAINS OVER
THESE AREAS LATE TODAY AND TONIGHT MAY BE ENOUGH TO PRODUCE SOME
FLASH FLOODING. FORTUNATELY IT HAS BEEN DRY AS OF LATE SO GENERALLY
THE GROUND SHOULD BE ABLE TO HANDLE HEAVY RAIN...AS LONG AS THERE
IS NOT TOO MUCH TRAINING OF STORMS...WHICH IS POSSIBLE.

THE OTHER MAIN CONCERN IS FOR LATER TONIGHT AS WESTERLY WINDS
INCREASE AND STRONG SUBSIDENCE DEVELOPS BEHIND THE UPPER LOW.
SURFACE PRESSURE GRADIENTS WILL INCREASE AS SURFACE LOW PRESSURE
DEVELOPS IN THE NORTHERN PLAINS AND COLD AIR ADVECTS EASTWARD.
THESE FACTORS WILL LEAD TO THE DEVELOPMENT OF STRONG WEST
WINDS...MAINLY IN WIND PRONE AREAS OF SOUTHEAST WYOMING. HAVE
ISSUED A HIGH WIND WARNING FOR THE WIND PRONE AREAS SUCH AS THE
SOUTHERN LARAMIE RANGE AND FOOTHILLS...ARLINGTON...AND BORDEAUX.
HIGH WIND WARNING IN EFFECT FROM MIDNIGHT TONIGHT TO NOON TUESDAY.
WILL ALSO HAVE TO KEEP AN EYE ON THE NORTHERN NEBRASKA PANHANDLE
FOR WIND ADVISORY POTENTIAL.

.LONG TERM...(WEDNESDAY NIGHT THROUGH SUNDAY)
ISSUED AT 420 AM MDT MON SEP 29 2014

A GOOD CHANCE OF SCATTERED TO NUMEROUS RAIN SHOWERS WILL CONTINUE
THROUGH EARLY WEDNESDAY NIGHT AS THE LAST UPPER LEVEL TROUGH WILL
PUSH EAST OF THE AREA BY EARLY THURSDAY MORNING. THE BEST CHANCE
FOR SHOWERS WILL BE ALONG AND SOUTH OF I80 AS THE TROUGH WILL BE
RATHER PROGRESSIVE. THE HIGHER MOUNTAINS ABOVE 8500 FEET WILL SEE
SNOWFALL INTO THURSDAY MORNING WITH A FEW INCHES OF ACCUMULATION
POSSIBLE. SNOW LEVEL SHOULD REMAIN ABOVE VALLEY FLOORS...ALTHOUGH
IT MAY GET CLOSE TO THE LARAMIE VALLEY FOR A TIME LATE WEDNESDAY
NIGHT. ALL MODELS IN AGREEMENT LATE NEXT WEEK SHOWING STRONG
NORTHWEST FLOW ALOFT RESULTING IN COOL TEMPERATURES FOR THURSDAY
AND FRIDAY...BUT DRIER CONDITIONS ARE EXPECTED AS THE POLAR JET
STREAM SHIFTS NORTHWARD INTO CANADA. TEMPERATURES ON THURSDAY WILL
REMAIN BELOW AVERAGE WITH HIGHS IN THE 50S TO MID 60S.

ALL MODELS IN GOOD AGREEMENT THROUGH THE EXTENDED...SHOWING NORTHWEST
FLOW ALOFT CONTINUING INTO NEXT WEEK. HOWEVER...TEMPERATURES WILL
GRADUALLY INCREASE WITH HIGHS LIKELY AROUND 70 BY SATURDAY AND
SUNDAY AFTERNOON AS A RIDGE AXIS BUILDS ALONG THE WEST COAST AND
THEN DRIFTS EASTWARD. KEPT POP BELOW 10 PERCENT EACH DAY.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z TUESDAY MORNING)
ISSUED AT 1153 AM MDT MON SEP 29 2014

GOING TO GET A BREAK FROM THE RAIN AND LOWER CIGS THROUGH THE
EARLY AFTERNOON. WIDESPREAD MODERATE RAIN WITH EMBEDDED TSTMS IS
EXPECTED TO DEVELOP AT LAR AND CYS AROUND 21Z AND THEN SPREAD
NORTH AND EAST INTO THE NEBRASKA PANHANDLE DURING THE EARLY
EVENING. SINCE RAIN WILL LIKELY BE MODERATE TO HEAVY...WENT WITH
PREVAILING IFR CONDITIONS DURING THE EARLY EVENING OVER THE PLAINS
SITES. RAIN WILL LIKELY LINGER OVER THE NORTHERN PANHANDLE THROUGH
TUES MORNING. SOUTHEAST WINDS WILL BE GUSTY OVER THE PANHANDLE
THROUGH THE AFTN. A COLD FRONT WILL SWING ACROSS THE REGION AFTER
06Z WITH WINDS SWITCHING TO WEST AND SOUTHWEST. IT WILL BE WINDY
AFTER THE FRONTAL PASSAGE WITH GUSTS TO 30-35 KTS THROUGH THE
REMAINDER OF THE TAF PERIOD...ESPECIALLY OVER SOUTHEAST WY.

&&

.FIRE WEATHER...
ISSUED AT 350 AM MDT MON SEP 29 2014

UPPER LEVEL LOW PRESSURE SYSTEM WILL MOVE ACROSS THE FORECAST AREA
TODAY AND TONIGHT. WIDESPREAD SHOWERS AND THUNDERSTORMS WILL BE FOUND
OVER MOST OF SOUTHEAST WYOMING AND NEBRASKA PANHANDLE TODAY. LESS
ACTIVITY IS EXPECTED INTO CARBON COUNTY WITH ISOLATED SHOWERS AND
THUNDERSTORMS OUTSIDE OF THE MOUNTAINS. SNOW ACCUMULATIONS LIKELY
INTO THE SNOWY AND SIERRA MADRE MOUNTAINS. STRONG WEST WINDS TO
DEVELOP TONIGHT OVER PORTIONS OF SOUTHEAST WYOMING. BREEZY TO WINDY
ACROSS THE AREA TUESDAY. COOL WEATHER WITH CHANCES FOR SHOWERS
TUESDAY AND WEDNESDAY. DRYING OUT AND WARMING UP LATE THIS WEEK AND
INTO THE WEEKEND.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...HIGH WIND WARNING FROM MIDNIGHT TONIGHT TO NOON MDT TUESDAY FOR
     WYZ106-110-116-117.

NE...FLOOD WATCH FROM 3 PM MDT THIS AFTERNOON THROUGH TUESDAY MORNING
     FOR NEZ002-003-019>021-054-055-095-096.

&&

$$

UPDATE...HAHN
SHORT TERM...SML
LONG TERM...TJT
AVIATION...FINCH
FIRE WEATHER...SML



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