Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 211051
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
451 AM MDT THU MAY 21 2015

.SHORT TERM...(TODAY THROUGH SATURDAY)
ISSUED AT 445 AM MDT THU MAY 21 2015

PREDAWN WATER VAPOR LOOP DEPICTED A LONG WAVE TROUGH AXIS ALONG
THE PACIFIC COAST WITH EMBEDDED CLOSED CIRCULATIONS. DIFFLUENT
FLOW ALOFT WAS INCREASING BETWEEN THE TROUGH AND DOWNSTREAM UPPER
RIDGE FROM THE SOUTHERN HIGH PLAINS TO WYOMING. THE SURFACE
ANALYSIS SHOWED A RIDGE CENTERED OVER THE GREAT PLAINS...WITH A
WAVY STATIONARY FRONT FROM THE SOUTHERN ATLANTIC COAST THROUGH THE
DEEP SOUTH AND NORTHWEST INTO THE PACIFIC NORTHWEST. MID AND HIGH
LEVEL MOISTURE WAS SPREADING NORTHEAST FROM THE FOUR CORNERS.
PERSISTENT MOIST LOW LEVEL EASTERLY FLOW PRODUCED AREAS OF LOW
STRATUS...DRIZZLE AND FOG ALONG THE I-80 CORRIDOR. THE DENSEST FOG
EXTENDED FROM THE SUMMIT EAST TO BUFORD PER THE WYDOT WEB CAMS.
THE FOG WILL LIKELY PERSIST THROUGH LATE THIS MORNING.

LATEST DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT PROGGING
THE UPPER RIDGE OVER THE CWA TO TRANSLATE EAST INTO THE CENTRAL
AND SOUTHERN GREAT PLAINS AS THE MAIN UPPER LOW OFF THE WEST COAST
OF CA TRACKS SOUTHEAST TOWARD THE DESERT SOUTHWEST DURING THE NEXT
24 TO 48 HOURS. THIS WILL PLACE THE FRONT RANGE AND HIGH PLAINS
IN THE DIFFLUENT FLOW ALOFT WITH SEVERAL PIECES OF SHORTWAVE ENERGY
EJECTING NORTHEAST ACROSS THE REGION. TIMING OF THESE SMALLER
FEATURES AND THEIR INFLUENCE ON CONVECTION ARE USUALLY DIFFICULT
TO RESOLVE. AS THE CENTER OF THE SURFACE RIDGE SLIDES SOUTHEAST
AND WEAK CYCLOGENESIS DEVELOPS OVER EASTERN CO...SOUTHEASTERLY
FLOW WILL CONTINUE TO DRAW LOW LEVEL GULF MOISTURE NORTHWESTWARD
WITH MID/UPPER LEVEL PACIFIC MOISTURE FROM THE SOUTHWEST.

LIGHT SHOWERS WILL DEVELOP LATE THIS MORNING ALONG AND WEST OF
THE LARAMIE RANGE...GRADUALLY SPREADING EAST INTO SOUTHEAST WY
THIS AFTERNOON. INSTABILITY WILL GENERALLY BE WEAK (LESS THAN 500
J/KG SBCAPE) WITH ISOLATED THUNDER POSSIBLE. HIGH TEMPERATURES
WILL MODERATE INTO THE 50S. SHOWERS/ISOLATED THUNDER BECOME
MORE WIDESPREAD THIS EVENING AND OVERNIGHT TONIGHT AS A SHORTWAVE
LIFTS NORTHEAST ACROSS THE CWA. SHOWERS/ISOLATED AFTERNOON THUNDER
ARE LIKELY FRIDAY WITH THE POTENTIAL FOR MODERATE TO LOCALLY HEAVY
RAIN OVER SOUTHEAST WY AND SOUTHERN NE PANHANDLE. IT WILL BE
SLIGHTLY COOLER FRIDAY WITH HIGHS IN THE MID 40S TO MID 50S.

THE MODELS CONTINUE TO TREND TOWARD AN ACTIVE CONVECTIVE DAY
ON SATURDAY AS THE MAIN UPPER LOW MOVES NORTHEAST OVER THE FOUR
CORNERS. THERE ARE DIFFERENCES WITH RESPECT TO TO THE TRACK AND
STRENGTH OF THE SURFACE LOW. AT NOON SATURDAY...THE GFS PLACES THE
LOW JUST SOUTH OF CHEYENNE WHILE THE ECMWF IS NEAR PUEBLO. IF THE
GFS PANS OUT...THERE WILL BE AN MARGINAL RISK FOR STRONG TO
POSSIBLY SEVERE TSTORMS AS FAR NORTH AS CHEYENNE AND SIDNEY WITH
SBCAPES 500-1000 J/KG...DEW POINTS 40S-LOW 50S AND MODEST 0-6KM
SHEAR. CONVERSELY... THE ECMWF SOLUTION WOULD RESULT IN A MORE
STABLE EASTERLY LOW LEVEL FLOW. BETTER CHANCES FOR STRONG TO
SEVERE TSTORMS APPEAR TO BE OVER NORTHEAST CO CLOSER TO THE
INTERSECTION OF THE SURFACE LOW AND TROUGH. WILL SEE A DECREASE IN
THE SHOWERS SATURDAY MORNING...THEN BECOME WIDESPREAD IN THE
AFTERNOON. HIGH TEMPERATURES WILL BE ABOUT 10 DEGREES WARMER
SATURDAY WITH LOW TO MID 60S FOR THE PLAINS.

.LONG TERM...(SATURDAY NIGHT THROUGH WEDNESDAY)
ISSUED AT 415 AM MDT THU MAY 21 2015

SATURDAY NIGHT...UPPER TROUGH PIVOTS ACROSS SOUTHEAST WYOMING WITH
SOME INDICATIONS OF DRY SLOTTING...THOUGH LOOKS LIKE THERE WILL BE
ENOUGH LOW LEVEL CONVERGENCE AND LOW AND MID LEVEL MOISTURE PRESENT
TO SPARK SCATTERED THUNDERSTORMS...SOME STRONG TO SEVERE IN THE
EVENING...MAINLY EAST OF I-25.

SUNDAY...SCATTERED TO NUMEROUS SHOWER COVERAGE WITH ISOLATED
THUNDERSTORMS WITH ADEQUATE MOISTURE PRESENT AND SYSTEM DYNAMICS
MOVING OVERHEAD.

MONDAY...FLOW ALOFT TURNS MORE WEST...WITH SOME SLOW DRYING.
NEVERTHELESS...STILL ENOUGH MOISTURE AND DYNAMICS TO PRODUCE
ISOLATED TO SCATTERED SHOWERS AND THUNDERSTORMS FROM RAWLINS TO
LARAMIE.

TUESDAY...WEAK RIDGE ALOFT...THOUGH WITH ADEQUATE LOW AND MID LEVEL
MOISTURE...AND A LOW LEVEL CONVERGENCE AXIS EVIDENT AT THE SURFACE...
WILL MAINTAIN SCATTERED AFTERNOON AND EVENING COVERAGE.

WEDNESDAY...NEXT SHORTWAVE TROUGH APPROACHES FROM THE GREAT BASIN
STATES. A COLD FRONT WILL ENTER OUR NORTHERN COUNTIES. AGAIN...LOOKS
LIKE A DAY WITH SCATTERED SHOWERS AND THUNDERSTORMS IN THE AFTERNOON
AND EVENING.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z THURSDAY NIGHT)
ISSUED AT 1246 AM MDT THU MAY 21 2015

WYOMING TAFS...MOUNTAINS MOSTLY OBSCURED. IFR PREVAILS IN FOG AND
LOW CLOUDS THROUGH MID MORNING...THEN VFR WITH CEILINGS AROUND 4000
FEET AGL. IN THE LATE AFTERNOON AND EARLY EVENING...ISOLATED TO
SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS PRODUCING VISIBILITIES
4 TO 6 MILES WITH OCCASIONAL MVFR CEILINGS.

NEBRASKA TAFS...VFR AND OCCASIONAL MVFR PREVAIL THROUGH THE PREDAWN
HOURS...EXCEPT AT SIDNEY WHERE IFR IN FOG AND LOW CLOUDS PREVAILS
FROM 10Z TO 15Z. AFTER MID MORNING...VFR PREVAILS WITH CEILINGS
FROM 3000 TO 5000 FEET AGL...THEN MVFR DEVELOPS AGAIN AT SIDNEY
THURSDAY EVENING.

&&

.FIRE WEATHER...
ISSUED AT 345 AM MDT THU MAY 21 2015

THE PERSISTENT COOL AND MOIST UPSLOPE PATTERN WILL PRECLUDE FIRE
WEATHER CONCERNS THROUGH THE WEEKEND. BEST CHANCES FOR WIDESPREAD
WETTING RAIN WILL BE TONIGHT AND FRIDAY.

&&

.HYDROLOGY...
ISSUED AT 400 AM MDT THU MAY 21 2015

WATER LEVELS WILL REMAIN HIGH ALONG THE LARAMIE RIVER AT FORT LARAMIE
AND LARAMIE FOR THE NEXT SEVERAL DAYS. FORT LARAMIE IS FORECAST TO
COME WITHIN A TENTH OF A FOOT OF FLOOD STAGE (7 FT) THROUGH THE
WEEKEND. MODERATE TO LOCALLY HEAVY RAIN WILL BE POSSIBLE FOR THE HIGH
PLAINS AND POSSIBLY LARAMIE VALLEY TONIGHT AND FRIDAY...WHICH MAY
RESULT IN FURTHER RISES.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...DENSE FOG ADVISORY UNTIL 10 AM MDT THIS MORNING FOR WYZ116.

NE...NONE.
&&

$$

SHORT TERM...MAJ
LONG TERM...RUBIN
AVIATION...RUBIN
FIRE WEATHER...MAJ
HYDROLOGY...MAJ


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