Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 221758
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
1158 AM MDT FRI MAY 22 2015

.SHORT TERM...(TODAY THROUGH SUNDAY)
ISSUED AT 425 AM MDT FRI MAY 22 2015

A WELL PRONOUNCED LARGE SCALE PATTERN DOMINATED NORTH AMERICA WITH
TROUGHS OVER SOUTHEAST CANADA/NEW ENGLAND AND ACROSS THE WESTERN
CONUS. IN BETWEEN WAS A UPPER RIDGE FROM THE NORTHWEST GULF OF
MEXICO INTO THE NORTHERN GREAT PLAINS. SEVERAL SHORT WAVES WERE
ROTATING AROUND THE LATTER TROUGH...WITH ONE LIFTING NORTH INTO
CO AND ANOTHER ACCOMPANYING THE UPPER LOW OVER SOUTHERN CA. THE
SURFACE ANALYSIS SHOWED TWO FRONTS...ONE BORDERING NEW ENGLAND
ACROSS THE SOUTHERN GREAT LAKES AND INTO THE DAKOTAS...AND THE
OTHER FROM THE NORTHERN GULF COAST EXTENDING NORTHWEST INTO ID.
REGIONAL RADARS INDICATED A LARGE COVERAGE OF SHOWERS AND A FEW
TSTORMS FROM EASTERN UT ACROSS CO AND WESTERN KS SPREADING
NORTHWARD AHEAD OF THE INITIAL SHORT WAVE. ISOLATED LIGHT SHOWERS
HAD ALSO DEVELOPED ALONG AND WEST OF THE LARAMIE RANGE. MOIST
SOUTHEAST FLOW RESULTED IN FAIRLY WIDESPREAD STRATUS AND AREAS
OF FOG OVER SOUTHEAST WY. LOWEST VISIBILITY WAS NEAR THE I-80
SUMMIT AND IN THE VICINITY OF CHEYENNE. THE COVERAGE OF DENSE FOG
WILL NOT BE AS WIDESPREAD AS THURSDAY MORNING WITH THE PRESENCE OF
SHOWER ACTIVITY AND A MORE MIXED BOUNDARY LAYER.

THE NUMERICAL MODELS HAVE BEEN CONSISTENT WITH THE LARGE SCALE
PATTERN...PROGGING THE WESTERN CONUS UPPER TROUGH TO SLOWLY
TRANSLATE EAST TOWARD THE ROCKIES AND HIGH PLAINS THIS WEEKEND.
THE SHORT WAVE TRACKING NORTH ACROSS CO WILL BRING NUMEROUS
SHOWERS. ISOLATED THUNDER WILL DEVELOP ALONG/WEST OF THE LARAMIE
RANGE EARLY AFTERNOON AND EXTEND EAST ONTO MOST OF THE PLAINS
BY LATE AFTERNOON. SHOWER COVERAGE WILL DIMINISH TONIGHT AS THE
SHORT WAVE EXITS NORTH OF THE CWA. IT WILL BE ANOTHER COOL DAY
AS HIGH TEMPERATURES REMAIN IN THE 40S AND 50S.

SATURDAY IS STILL LOOKING LIKE AN ACTIVE DAY FOR THUNDERSTORMS
AS THE SECOND MORE POTENT SHORT WAVE APPROACHES THE CWA. THE
MODELS SHOW A SURFACE LOW FORMING OVER NORTHEAST CO BY SATURDAY
AFTERNOON WITH A TROUGH EXTENDING NORTH-SOUTH ACROSS THE EASTERN
PLAINS. A STRENGTHENING SOUTH-SOUTHEAST BOUNDARY LAYER FLOW WILL
DRAW RICH GULF MOISTURE NORTHWARD ACROSS THE EASTERN PLAINS
SATURDAY AFTERNOON...WITH THE NOSE OF THE THETA-E RIDGE EXTENDING
INTO SOUTHEAST WY AND WESTERN NE. THE NAM AND SREF ARE MORE
BULLISH WITH THE INSTABILITY COMPARED TO THE ECMWF AND GFS WITH
SBCAPES NEAR 1500 J/KG ACROSS THE AFOREMENTIONED AREA. VEERING
WIND PROFILE EAST OF THE LARAMIE RANGE WITH BULK SHEAR VALUES
OF 30-40 KT SUGGEST THE POTENTIAL FOR ISOLATED SUPERCELL STORMS
WITH LARGE HAIL THE PRIMARY THREAT. AN ISOLATED BRIEF TORNADO
CANNOT BE RULED OUT...ESPECIALLY NEAR THE CO BORDER CLOSER TO
THE SURFACE LOW. THE SPC DAY2 OUTLOOK SHOWS A MARGINAL RISK FOR
SEVERE STORMS ALONG AND EAST OF A CHEYENNE-SCOTTSBLUFF-ALLIANCE
LINE. WILL HIGHLIGHT THE SEVERE THREAT IN THE HWO AND WEATHER
STORY. LATER SHIFTS CAN FINE TUNE THE DETAILS BEFORE INTRODUCING
SEVERE WORDING IN THE ZONE FORECASTS. IT WILL BE MILDER SATURDAY
WITH HIGHS IN THE 50S AND 60S.

THE SEVERE STORM RISK MAY EXTEND INTO EARLY SATURDAY EVENING
FOR WESTERN NE BEFORE ENDING WITH DESTABILIZATION AND EXIT OF THE
SHORT WAVE SATURDAY NIGHT. WRAP-AROUND MOISTURE AND COOL AIR
ALOFT SATURDAY NIGHT AND SUNDAY WILL PROMOTE GOOD CHANCES FOR
MORE SHOWERS AND ISOLATED AFTERNOON THUNDERSTORMS. SOMEWHAT COOLER
TEMPERATURES ARE ALSO EXPECTED IN THE WAKE OF THE UPPER LOW.

.LONG TERM...(SUNDAY NIGHT THROUGH FRIDAY)
ISSUED AT 325 AM MDT FRI MAY 22 2015

SUNDAY NIGHT/MONDAY...THE UPPER TROUGH WILL SLOWLY MOVE NORTH OF OUR
COUNTIES DURING THE DAYLIGHT HOURS...WITH MOST OF THE UPWARD
VERTICAL MOTION UP IN COLORADO...THOUGH THERE WILL BE A FAIR SHARE
ACROSS SOUTHEAST WYOMING WITH ISOLATED TO SCATTERED SHOWERS AND
THUNDERSTORMS.

TUESDAY...DESPITE THE RIDGING ALOFT...PRESENCE OF STRONG THERMAL
HEATING AND ADEQUATE LOW AND MID LEVEL MOISTURE...WILL SEE ISOLATED
TO SCATTERED THUNDERSTORMS SOUTH OF A DOUGLAS TO KIMBALL LINE.

WEDNESDAY/THURSDAY...SOUTHWEST FLOW ALOFT IN FULL SWING WITH SEVERAL
WEAK EMBEDDED SHORTWAVE TROUGH ALOFT HELPING TO PRODUCE ISOLATED TO
WIDELY SCATTERED AFTERNOON AND IMPENDING THUNDERSTORMS.

FRIDAY...LESS THUNDERSTORM COVERAGE DUE TO WEST SOUTHWEST FLOW ALOFT
LIKELY STABILIZING THE ATMOSPHERE AND LESS AND LESS LOW AND MID
LEVEL MOISTURE TO ACT AS FUEL TO GENERATE THUNDERSTORMS.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SATURDAY AFTERNOON)
ISSUED AT 1144 AM MDT FRI MAY 22 2015

THE MAIN CONCERN WITH THIS TAF ISSUANCE IS THE TIMING OF WHEN THE
SHOWERS WILL MOVE OUT OF THE AREA THIS AFTERNOON AND LOW CEILINGS
AGAIN TONIGHT.

LATEST RADAR LOOP WAS SHOWING DRIER AIR STARTING TO FILTER INTO
THE NEBRASKA PANHANDLE. HOWEVER...THIS DRY SLOT WAS BEGINNING TO
FILL BACK IN WITH MORE LIGHT RAIN DURING THE LAST COUPLE OF VOLUME
SCANS. WE BASED OUR TAF ON POTENTIAL DRYING THIS AFTERNOON...BUT
WE WILL HAVE TO WATCH RADAR TRENDS TO SEE IF THIS DRY SLOT CAN
REMAIN IN TACT. OTHERWISE...MVFR CEILINGS WILL BE THE TREND THIS
AFTERNOON WITH GUSTY SOUTHEAST FLOW. THE NEXT CONCERN TONIGHT
WILL BE THE POTENTIAL FOR LOWER CEILINGS FILTERING BACK INTO THE
AREA ONCE AGAIN. MEANWHILE...THE UPPER LEVEL TROF BEGINS TO MOVE
INTO WESTERN COLORADO FROM THE GREAT BASIN. THIS FEATURE WILL
INDUCE SURFACE LOW PRESSURE EAST OF THE COLORADO FRONT RANGE
TOMORROW MORNING. AS A RESULT...THE SURFACE FLOW SHOULD BECOME
EASTERLY AND BRING IFR CEILINGS TO MUCH OF THE AREA TONIGHT WITH
THE LIFR CEILINGS POSSIBLE AROUND CYS. THESE LOW CEILINGS SHOULD
GRADUALLY CLIMB AFTER DAYBREAK...BUT MAY HANG ON THROUGH THE MUCH
OF THE MORNING.

&&

.FIRE WEATHER...
ISSUED AT 318 AM MDT FRI MAY 22 2015

A UPPER LEVEL LOW PRESSURE SYSTEM AND ITS ACCOMPANYING DISTURBANCES
OVER THE SOUTHWEST U.S. WILL GRADUALLY MOVE NORTHEAST TOWARD THE
CENTRAL ROCKIES DURING THE NEXT 24 TO 48 HOURS. SHOWERS AND ISOLATED
THUNDERSTORMS WILL BECOME MORE WIDESPREAD TODAY. THE SHOWERS WILL
DECREASE TONIGHT AS ONE DISTURBANCE EXITS THE DISTRICTS THEN INCREASE
ONCE AGAIN SATURDAY WITH THE APPROACH OF A STRONGER DISTURBANCE. A
SURFACE LOW PRESSURE SYSTEM WILL DEVELOP OVER NORTHEAST COLORADO
SATURDAY WITH A BETTER CHANCE FOR THUNDERSTORMS. A FEW STRONG TO
SEVERE THUNDERSTORMS WILL BE POSSIBLE SATURDAY AFTERNOON AND EARLY
EVENING FOR THE SOUTHEAST WYOMING PLAINS INTO MUCH OF WESTERN
NEBRASKA. THE WET PATTERN WILL LINGER INTO SUNDAY BEFORE SOMEWHAT
DRIER AND WARMER CONDITIONS ARRIVE NEXT WEEK.

&&

.HYDROLOGY...
ISSUED AT 325 AM MDT FRI MAY 22 2015

ONGOING WATER RELEASES FROM THE GRAYROCKS RESERVOIR WITH RECENT
RAINFALL PUSHED THE LARAMIE RIVER NEAR FORT LARAMIE ABOVE FLOOD
STAGE. THE RIVER HAS STEADIED AT 7.13 FT OVERNIGHT AND IS FORECAST
TO REMAIN AT 7.1 FT THROUGH THE WEEKEND BEFORE FALLING BELOW FLOOD
STAGE LATE MONDAY. AT LARAMIE...THE RIVER WAS AT 7.35 FT AND IS
FORECAST TO SLOWLY RISE TO 7.8 FT MONDAY THEN FALL TO 7.5 FT
TUESDAY. BASIN AVERAGE RAINFALL OVER THE LARAMIE RIVER IS PROJECTED
TO RANGE FROM A HALF INCH TO AN INCH TODAY THROUGH SUNDAY. THE
LOWER NORTH PLATTE RIVER BASIN COULD RECEIVE ONE TO ONE AND HALF
INCHES OF RAIN FOR THE SAME PERIOD...WHICH WILL RESULT IN SLOW
RISES DOWNSTREAM FROM FORT LARAMIE.

&&

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

$$

SHORT TERM...JAMSKI
LONG TERM...RUBIN
AVIATION...REC
FIRE WEATHER...JAMSKI
HYDROLOGY...JAMSKI


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