Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 211753
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
1153 AM MDT THU MAY 21 2015

.UPDATE...
ISSUED AT 605 AM MDT THU MAY 21 2015

WENT AHEAD AND EXPANDED THE DENSE FOG ADVISORY EAST TO INCLUDE
CENTRAL LARAMIE COUNTY AND THE CHEYENNE AREA AS VISIBILITIES
TANKED TO LESS THAN A QUARTER MILE.

UPDATE ISSUED AT 543 AM MDT THU MAY 21 2015

LATEST CHECK OF WYDOT WEB CAMS INDICATED THAT THE DENSE FOG HAD
SPREAD EAST TOWARD CHEYENNE WITH VISIBILITIES AT A QUARTER MILE
OR LESS. IN A RECENT UPDATE...ADDED THE SOUTH LARAMIE RANGE
FOOTHILLS TO THE DENSE FOG ADVISORY. VISIBILITY AT CHEYENNE
RECENTLY DROPPED TO A QUARTER MILE.

&&

.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 NORTHERN ROCKIES. 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. COULD SEE BETWEEN
ONE HALF AND ONE INCH QPF IN THESE AREAS. 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 18Z TAFS THROUGH 18Z FRIDAY AFTERNOON)
ISSUED AT 1144 AM MDT THU MAY 21 2015

WITH MOIST SOUTH TO SOUTHEASTERLY WINDS IN PLACE...LOW CLOUDS WILL
BE PERSISTENT THROUGH THE AFTERNOON ACROSS THE PLAINS. WILL KEEP
IFR CIGS FOR CYS THROUGH 20Z. BRIEF MVFR CIGS WILL BE POSSIBLE
OVER THE NEBRASKA PANHANDLE...HOWEVER CURRENT THINKING IS IT WILL
BE PREVAILING VFR CIGS. A LARGE AREA OF MOSTLY LIGHT RAIN WILL
MOVE ACROSS THE COLORADO BORDER AND INTO THE REGION BY LATE
TONIGHT AND FRI MORNING. CIGS WILL LOWER WITH THE RAIN AND VIS
WILL BE REDUCED TO MVFR WITHIN POCKETS OF MODERATE RAIN.

&&

.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...NONE.
NE...NONE.
&&

$$

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



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