Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 050948
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
348 AM MDT TUE MAY 5 2015

.SHORT TERM...(TODAY THROUGH THURSDAY)
ISSUED AT 345 AM MDT TUE MAY 5 2015

WEBCAMS ALONG INTERSTATE 80 HAVE CONSISTENTLY SHOWN DENSE FOG FROM
THE SUMMIT TO CHEYENNE OVER THE PAST FEW HOURS. DECIDED TO ADD
CENTRAL LARAMIE COUNTY (CHEYENNE) TO THE DENSE FOG ADVISORY
THRU 16Z WITH THE CYS OBS DROPPING TO ONE QUARTER MILE. THE LOW
CLOUDS AND FOG WILL REMAIN QUITE PERSISTENT THROUGH THE MORNING
WITH THE HRRR MODEL INDICATING LIFR/IFR CONDITIONS THROUGH 18Z
FROM THE INTERSTATE 80 SUMMIT EASTWARD TO CHEYENNE COUNTY. THIS
MAKES SENSE AS THE OVERALL SFC PATTERN OF VERY MOIST SOUTH TO
SOUTHEAST WINDS WILL CONTINUE THROUGH THE MORNING. WATER VAPOR
IMAGERY SHOWS THE SUBTROPICAL UPPER LOW SPINNING OVER NORTHEAST
ARIZONA. THE MODELS MOVE THIS FEATURE TO THE NORTHEAST OVER THE
COURSE OF THE DAY TO NEAR NORTHERN COLORADO BY THIS EVENING.

ONLY AN ISOLATED TO SCATTERED COVERAGE OF SHOWERS IS
EXPECTED THROUGH THE EARLY AFTN ALONG THE COLORADO BORDER.
HOWEVER...MIDLVL HEIGHT FALLS WILL COMBINE WITH INCREASING
DEEP LAYER MOISTURE TO PROMOTE A MORE WIDESPREAD COVERAGE BY THE
LATE AFTN AND EARLY EVENING. MOISTURE IS PLENTIFUL WITH THIS
SYSTEM...IN FACT PW VALUES OF 0.75-1.0 INCHES OVER THE PLAINS ARE 2
STANDARD DEVIATIONS ABOVE NORMAL FOR EARLY MAY. THUS...WITH
RELATIVELY SLOW STORM MOTIONS...LOCALLY HEAVY RAIN WILL BE A
CONCERN ESPECIALLY OVER THE NEBRASKA PANHANDLE. WHILE ISOLATED
TSTMS WILL BE POSSIBLE THIS AFTN...THE VERY WEAK INSTABILITY AND
SHEAR WILL MAKE STRONG STORMS UNLIKELY. SPC HAS DROPPED THE
MARGINAL THREAT OVER THE SOUTHERN PANHANDLE. WITH THE MIDLVL
DISTURBANCE OVERHEAD TONIGHT...WILL KEEP A CHANCE OF SHOWERS
GOING. SOUTHEAST SFC WINDS ARE IN PLACE THRU 12Z WED...SO FOG
WILL AGAIN BE A THREAT OVER PORTIONS OF THE AREA.

THE SHORTWAVE WILL PUSH INTO WESTERN SOUTH DAKOTA/NEBRASKA BY WED
AFTN. THE MODELS SHOW THE HIGHEST QPF OVER THE PLAINS OF SOUTHEAST
WY AS A SFC COLD FRONT INTERACTS WITH GOOD MOISTURE. WILL HAVE
ISOLATED TSTMS DURING THE AFTN WITH SOME LIMITED CAPE.  PRECIP
CHANCES DROP OFF WED EVENING BEHIND THE FRONT AS A DRIER AIRMASS
MOVES IN. THERE COULD ACTUALLY BE A 18-24 HR PERIOD FROM WED
EVENING THRU THURS AFTN WHEN MUCH OF THE CWA WILL BE PRECIP FREE.
SCATTERED STORMS WILL ONCE AGAIN DEVELOP ALONG AND WEST OF THE
LARAMIE RANGE BY THURS AFTN...WITH LIMITED COVERAGE OVER THE
PLAINS IN A MORE STABLE AIRMASS.

.LONG TERM...(THURSDAY NIGHT THROUGH MONDAY)
ISSUED AT 345 AM MDT TUE MAY 5 2015

UNSETTLED WEATHER CONTINUES IN THE LONG TERM. MEDIUM RANGE
GUIDANCE CONTINUES TO ADVERTISE THE POSSIBILITY OF WINTER WEATHER
RETURNING TO THE AREA LATE IN THE WEEKEND.

STARTING OFF THURSDAY NIGHT...WE STILL HAVE THE STALLED SURFACE
BOUNDARY LAYING ALONG THE LARAMIE RANGE...OR SLIGHTLY WEST. BOTH
THE GFS AND ECMWF VERY SIMILAR BRINGING ANOTHER PIECE OF ENERGY
NORTHEAST FROM THE DESERT SOUTHWEST TO INTERACT WITH THIS
BOUNDARY...SO WILL KEEP CHANCE POPS GOING FOR THE THURSDAY NIGHT
TIME FRAME. PROBABLY PRETTY LIGHT SHOWERS THOUGH AS HAS BEEN THE
CASE THE PAST FEW NIGHTS. CONVECTION PICKS UP THROUGH FRIDAY AFTERNOON
WITH BOTH MODELS CONCENTRATING ON THE SOUTHERN PANHANDLE AND
NORTHEASTERN COLORADO. SOUTHEAST UPSLOPING WINDS TO CONTINUE.

GFS AND ECMWF CONTINUE TO AGREE ON A CLOSED LOW ROUNDING THE BASE
OF THE LONGWAVE TROUGH SATURDAY MORNING AND BEGINNING TO LIFT
NORTHEAST INTO SOUTHWESTERN COLORADO SATURDAY AFTERNOON. OUR
FORECAST AREA IS SQUARE IN THE WRAP AROUND MOISTURE TO THE NORTH
OF THIS LOW. ECMWF A LITTLE FASTER ON MOVEMENT OVER THE
GFS...MOVING THE LOW INTO CENTRAL NEBRASKA ROUGHLY 12 HOURS FASTER
THAN THE GFS. BOTH ARE COLD THOUGH ON 700MB TEMPERATURES AND COLD
ENOUGH FOR WIDESPREAD SNOW. BOTH ARE SHOWING 850MB WINDS OF 40 TO
45KTS ACROSS THE PANHANDLE AND EASTERN WYOMING. WINTER IS NOT DONE
WITH SOUTHEAST WYOMING AND WE NEED TO BE WATCHING THE EVOLUTION OF
THIS EVENT AS HEAVY SNOW AND WINTER HEADLINES ARE DEFINITELY A
POSSIBILITY FOR PORTIONS OF THE FORECAST AREA...POSSIBLY EVEN THE
PANHANDLE.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z TUESDAY NIGHT)
ISSUED AT 1055 PM MDT MON MAY 4 2015

WIDESPREAD IFR/LIFR FOR THE OVERNIGHT HOURS FOR MOST SOUTHEAST
WYOMING AND NEBRASKA PANHANDLE AIRPORTS WITH SOUTHEAST TO EAST
UPSLOPING WINDS. EXPECT THIS TO LAST THROUGH THE MID TO LATE
MORNING HOURS BEFORE CONDITIONS IMPROVE SOME...BUT THEN WE WILL BE
DEALING WITH SHOWERS AND THUNDERSTORMS. LOW STRATUS RETURNING
TUESDAY EVENING...BECOMING WIDESPREAD ONCE AGAIN.

&&

.FIRE WEATHER...
ISSUED AT 345 AM MDT TUE MAY 5 2015

A COOL AND WET WEATHER PATTERN WILL KEEP FIRE WEATHER CONCERNS VERY
LOW THROUGH THE WEEKEND. AFTERNOON AND EVENING SHOWERS WILL BE
POSSIBLE TODAY THROUGH THURSDAY...WITH GOOD CHANCES FOR MEASURABLE
RAIN. MORE SUBSTANTIAL PRECIPITATION AMOUNTS ARE LOOKING POSSIBLE
BY LATE FRIDAY INTO SUNDAY ASSOCIATED WITH A STRONG STORM SYSTEM.
MINIMUM RELATIVE HUMIDITY VALUES SHOULD BE WELL ABOVE CRITICAL
THRESHOLDS THROUGH THE PERIOD.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...DENSE FOG ADVISORY UNTIL 10 AM MDT THIS MORNING FOR WYZ116>118.

NE...NONE.
&&

$$

SHORT TERM...ZF
LONG TERM...GCC
AVIATION...GCC
FIRE WEATHER...ZF


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