Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 310158
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
758 PM MDT SAT MAY 30 2015

.UPDATE...
ISSUED AT 751 PM MDT SAT MAY 30 2015

PRIMARY CHALLENGE FOR OUR OVERNIGHT PERIOD IS THE POSSIBLE
COVERAGE OF LATE NIGHT SHOWERS AND THUNDERSTORMS WITH THE
PROGRESSIVE SHORTWAVE TROUGH ALOFT ADVANCING EASTWARD ACROSS
EASTERN MONTANA AND THE WESTERN DAKOTAS TOWARDS DAYBREAK. LOOKING
AT THE 18Z RUNS OF THE NAM AND GFS...ALONG WITH THE ORIENTATIONS
OF THE 850 MB AND 700 MB THETA-E RIDGES AND ADVECTION...BELIEVE
THERE WILL BE ISOLATED LATE NIGHT SHOWERS AND THUNDERSTORMS ALONG
AND NORTH OF A LUSK TO ALLIANCE LINE FROM 3 AM TO 6 AM...AND
COVERAGE MAY ACTUALLY BE EVEN MORESO...LIKE THE 18Z GFS DEPICTS.

FOR NOW...HAVE ADDED MENTION OF ISOLATED POPS ALONG AND NORTH OF A
LUSK TO ALLIANCE LINE...AND WILL LOOK CLOSELY AT THE 00Z RUNS OF
THE NAM AND GFS TO SEE ABOUT POSSIBLE EXPANSION OF THE AREAL
COVERAGE OF LATE NIGHT SHOWERS AND THUNDERSTORMS LATE TONIGHT.

OTHER METEOROLOGICAL VARIABLES...TEMPERATURES...DEWPOINTS...
WINDS AND CLOUD COVER HAVE BEEN ADJUSTED BASED ON TRENDS FOR OUR
GRIDDED FORECASTS THE REST OF THE NIGHT.

&&

.SHORT TERM...(THIS EVENING THROUGH MONDAY NIGHT)
ISSUED AT 320 PM MDT SAT MAY 30 2015

AFTERNOON WATER VAPOR LOOP DEPICTED A BROAD LOW AMPLITUDE RIDGE
OVER THE WESTERN CONUS. MOISTURE WAS STREAMING INTO THE GREAT BASIN
AND NORTHERN ROCKIES...WELL IN ADVANCE OF A EASTERN PACIFIC TROUGH.
AT THE SURFACE...A COLD FRONT WAS MOVING EAST ACROSS THE MID-LOWER
MS VALLEY ACCOMPANIED BY ACTIVE CONVECTION. A RIDGE EXTENDED
NORTH-SOUTH THROUGH THE GREAT PLAINS...WITH A TROUGH FROM SOUTHEAST
WY INTO WESTERN MT. DIURNAL HEATING RESULTED IN A RATHER EXTENSIVE
CU FIELD OVER MUCH OF WY. RADAR DETECTED A FEW SMALL SHOWERS
DEVELOPING OVER THE SOUTHERN SNOWY AND LARAMIE RANGES...WHICH WILL
LIKELY DISSIPATE BY THIS EVENING. TEMPERATURES WARMED TO NEAR
SEASONAL NORMALS WITH 60S TO LOW 70S.

THE SHORT RANGE MODEL GUIDANCE IS IN GOD AGREEMENT WITH THE LARGE
SCALE FLOW ACROSS THE LOWER 48. THE UPPER RIDGE TO THE WEST SHIFTS
EAST ACROSS THE ROCKIES AND HIGH PLAINS DURING THE FORECAST PERIOD...
WHICH SHOULD PROMOTE A GENERALLY DRY AND WARM WEATHER PATTERN. THE
PROVERBIAL FLY IN THE OINTMENT FOR SUNDAY APPEARS AS A SHORT WAVE
DROPPING SOUTHEAST ON THE ANTICYCLONIC SIDE OF THE UPPER RIDGE
ACROSS EASTERN WY...THE WESTERN DAKOTAS AND WESTERN NE. THERE WILL
BE SOME SHEAR WITH MODEST NORTHWEST FLOW ALOFT OVERLYING A LOW
LEVEL SOUTHEASTERLY FLOW...HOWEVER SBCAPES WILL BE LESS THAN 1000
J/KG ACROSS THE PLAINS. TSTORMS WILL BE WIDELY SCATTERED SUNDAY
AFTERNOON...FAVORING THE HIGHER TERRAIN. CANNOT RULE OUT SMALL
HAIL AND GUSTY WINDS WITH THE STRONGER CONVECTION. TEMPERATURES
SUNDAY WILL BE AT LEAST 10 DEGREES WARMER THAN TODAY WITH HIGHS IN
THE 70S TO LOWER 80S. THE CONVECTION WILL LINGER INTO SUNDAY
EVENING AS A WEAK FRONT NEAR THE FRONT RANGE.

MONDAY LOOKS LIKE A CARBON COPY OF SUNDAY WITH WARMER TEMPERATURES
UNDER INCREASING SOUTHWEST FLOW ALOFT. CONVECTION MAY GET AN
EARLIER START OVER THE MOUNTAINS MONDAY MORNING...THEN INCREASE
IN COVERAGE AND INTENSITY AS IT MOVES EAST OF THE LARAMIE RANGE
DURING THE AFTERNOON. THERE WILL BE ENHANCEMENT ALOFT WITH THE
PASSAGE OF A SHORT WAVE ALONG WITH GREATER INSTABILITY /SBCAPES
1000-2000 J/KG/ AND SHEAR /30-40 KT/. THE SPC DAY3 OUTLOOK
HIGHLIGHTS A MARGINAL TO SLIGHT RISK OF SEVERE TSTORMS FOR THE
PLAINS.

.LONG TERM...(TUESDAY THROUGH SATURDAY)
ISSUED AT 320 PM MDT SAT MAY 30 2015

THE MAIN CONCERN WITH THIS PORTION OF THE FORECAST IS WHEN THE
BEST CHANCE OF CONVECTION WILL BE DURING THE PERIOD.

TUESDAY-TUESDAY NIGHT:
LATEST WATER VAPOR LOOP WAS SHOWING THE UPPER LEVEL TROUGH SPINNING
SEVERAL HUNDRED MILES WEST OF THE CALIFORNIA COASTLINE. THIS
FEATURE WILL SEND PIECES OF ENERGY EAST INTO THE ROCKIES DURING
THE NEXT COUPLE OF DAYS. AS A RESULT...THE UPPER LEVEL FLOW WILL
BECOME SOUTHWEST OVER OUR REGION AND ALLOW FOR MILD AND UNSETTLED
CONDITIONS TO PREVAIL. THIS WILL ALSO BRING BACK THE LOW LEVEL
MOISTURE AND A CHANCE OF AFTERNOON SHOWERS AND THUNDERSTORMS TO
AREAS MAINLY EAST OF THE LARAMIE RANGE ON TUESDAY INTO TUESDAY
NIGHT. THE MODELS ARE ALSO SHOWING THE HIGH INSTABILITY AXIS
SHIFTING SLOWLY WEST INTO THE NEBRASKA PANHANDLE. IF THERE IS
ENOUGH LOW LEVEL CONVERGENCE IT MAY SPARK A STRONG OR SEVERE
THUNDERSTORM LATE TUESDAY AFTERNOON AND EARLY TUESDAY EVENING.

WEDNESDAY-WEDNESDAY NIGHT:
THE MODELS ARE IN PRETTY GOOD AGREEMENT IN SHOWING A STRONGER
PIECE OF ENERGY (POTENTIAL VORTICITY ANOMALY) ROUNDING THE BASE OF
THE UPPER LEVEL TROUGH AND EJECTING INTO SOUTHEAST WYOMING AND
THE NORTHERN NEBRASKA PANHANDLE LATE WEDNESDAY AFTERNOON INTO
WEDNESDAY NIGHT. THIS MAY BE OUR BEST SHOT OF SEEING SHOWERS AND
THUNDERSTORMS. IN FACT...SOME OF THE STORMS MAY BECOME SEVERE DUE
TO SURFACE BASED CAPE VALUES REACHING 2000-3000 J/KG WITH 0-6KM
SHEAR VALUES AROUND 40 TO 50 KT. IN ADDITION...THE MODELS ARE
EVEN HINTING AT A JET STREAK MOVING THROUGH NORTHERN COLORADO
WHICH MAY AID IN THE LIFT. STORM MOTIONS WOULD PRIMARILY BE
SOUTHWEST TO NORTHEAST.

THURSDAY-SATURDAY:
QUITE A BIT MORE UNCERTAINTY IN THIS TIME FRAME ON WHERE THE
INDIVIDUAL POTENTIAL VORTICITY ANOMALIES WILL BE. HOWEVER...THERE
IS GOOD AGREEMENT IN THE CANADIAN/GEM/NAEFS ENSEMBLES THAT THE
UPPER LEVEL TROUGH WILL CONTINUE TO BE STATIONED ALONG THE
INTERMOUNTAIN WEST. THIS WILL BE A FAVORABLE PATTERN TO KEEP
THINGS UNSETTLED THROUGH THE REMAINDER OF THE WEEK AS JET STREAKS
CONTINUE TO PUSH THROUGH THE BASE OF THE TROUGH AND LIFT
NORTHEAST INTO OUR REGION. AS A RESULT...WE ARE KEEPING DIURNAL
CHANCES OF AFTERNOON SHOWERS AND THUNDERSTORMS MAINLY ALONG AND
EAST OF THE LARAMIE RANGE WITH MILD TEMPERATURES PERSISTING.

&&

.AVIATION...(FOR THE 00Z TAFS THROUGH 00Z SUNDAY EVENING)
ISSUED AT 446 PM MDT SAT MAY 30 2015

VFR CONDITIONS EXPECTED ACROSS SOUTHEAST WYOMING AND NEBRASKA
PANHANDLE THROUGH THE NEXT 24 HOURS AS AREA REMAINS UNDER AN UPPER
LEVEL RIDGE OF HIGH PRESSURE. ANY CUMULUS THAT HAS DEVELOPED LATE
THIS AFTERNOON WILL DISSIPATE AFTER SUNSET.


&&

.FIRE WEATHER...
ISSUED AT 220 PM MDT SAT MAY 30 2015

WARMER AND DRIER CONDITIONS WILL CONTINUE THIS AFTERNOON WITH
BREEZY SOUTHERLY WINDS. SUNDAY AND MONDAY WILL BE WARMER WITH
ISOLATED TO SCATTERED AFTERNOON AND EVENING SHOWERS AND
THUNDERSTORMS. MINIMUM RELATIVE HUMIDITIES WILL BE AROUND 25
PERCENT EACH DAY. A TREND TOWARD COOLER AND UNSETTLED WEATHER
WILL RETURN TO THE DISTRICTS BY MID WEEK.

&&

.HYDROLOGY...
ISSUED AT 220 PM MDT SAT MAY 30 2015

THE LARAMIE RIVER NEAR FORT LARAMIE IS CURRENTLY AT 8 FEET AND
IS FORECAST TO REMAIN AT THIS LEVEL THROUGH THE WEEKEND. UPSTREAM
AT LARAMIE...BANKFULL CONDITIONS WILL CONTINUE INTO EARLY NEXT
WEEK WITH A RISE TO FLOOD STAGE EARLY NEXT WEEK.

THE NORTH PLATTE RIVER AT HENRY IS CURRENTLY AT 5.5 FEET AND IS
FORECAST TO FALL BELOW FLOOD STAGE TONIGHT. AT MITCHELL...THE
RIVER IS CURRENTLY AT 9.1 FEET AND IS FORECAST TO SLOWLY FALL TO
MINOR FLOOD STAGE EARLY MONDAY MORNING. AT MINITARE...THE RIVER
WAS STEADY AT 7.4 FEET BUT IS FORECAST TO RISE TO FLOOD STAGE
LATE TONIGHT. AT BRIDGEPORT...THE RIVER WAS AT 7.8 FEET AND MAY
APPROACH BANKFULL STAGE EARLY NEXT WEEK.

SNOWMELT RUNOFF WILL COMMENCE IN THE MOUNTAINS LATE THIS WEEKEND
INTO EARLY NEXT WEEK AS TEMPERATURES RISE INTO THE 60S TO LOW 70S
OVER THE HIGHER ELEVATIONS. THE UPPER NORTH PLATTE RIVER AT SARATOGA
IS FORECAST TO RISE TO BANKFULL STAGE EARLY NEXT WEEK.

&&

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

$$

UPDATE...RUBIN
SHORT TERM...MAJ
LONG TERM...REC
AVIATION...GCC
FIRE WEATHER...MAJ
HYDROLOGY...MAJ



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