Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 281608
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
1008 AM MDT SUN SEP 28 2014

.UPDATE...
ISSUED AT 950 AM MDT SUN SEP 28 2014

AN IMPRESSIVE UPPER-LEVEL LOW CONTINUES TO SPIN ALONG THE CA/NV
BORDER THIS AM. ABUNDANT MOISTURE IS STREAMING NORTHWARD AHEAD OF
THIS FEATURE WITH THE 12Z GJT SKEW-T SHOWING PWATS UPWARDS OF 1.25
INCH. THIS AIR MASS WILL GRADUALLY SHIFT NORTHEAST TODAY AS UPPER
DIFFLUENCE INCREASES OVER THE CWA...WITH PCPN EXPECTED TO INCREASE
IN COVERAGE FROM WEST TO EAST. ADDED HEAVY RAIN TO THE GRIDS THIS
AFTN WITH DEEP MOISTURE IN PLACE. SPC HAS A SEE TEXT FOR PTNL ISO
SEVERE TSTMS LATER TODAY. THIS WOULD MOST LIKELY BE A LATER SHOW
AS WE WAIT FOR BETTER DYNAMIC SUPPORT FARTHER E. THE AREA TO WATCH
WILL BE THE DIFFERENTIAL HEATING BOUNDARY ALONG A LINE FROM CYS TO
AIA. IF MARGINAL INSTABILITY CAN BE REALIZED...LLVL AND DEEP LAYER
SHEAR PROFILES ARE IMPRESSIVE AND WOULD SUPPORT ROTATING UPDRAFTS
ACROSS FAR SOUTHEAST WY. TEMPERATURES ARE THE OTHER CHALLENGE FOR
THIS AFTN. DECREASED HIGHS A COUPLE OF DEGREES WHERE LLVL USPLOPE
IS PROVIDING FUEL FOR DENSE STRATUS.

&&

.SHORT TERM...(TODAY AND TUESDAY)
ISSUED AT 329 AM MDT SUN SEP 28 2014

FORECAST OVER THE NEXT 48 TO 60 HOURS APPEARS ON TRACK AS A
STRONG UPPER LEVEL LOW WILL GRADUALLY MOVE ACROSS THE FORECAST
AREA THROUGH TUESDAY. AS EXPECTED...THE POSITION OF THE LOW
REMAINS OVER THE WESTERN GREAT BASIN REGION EARLY THIS MORNING AS
IT HAS STALLED OVER WESTERN NEVADA. MODELS STILL SHOW THIS LOW
MOVING VERY LITTLE TODAY...AND ACTUALLY RETROGRADING SLIGHTLY INTO
THIS EVENING BEFORE THE DOMINANT LOW PRESSURE CENTER ALOFT FINALLY
EJECTS EASTWARD SOMETIME LATE TONIGHT. RAIN SHOWERS OVER CARBON
COUNTY YESTERDAY HAVE SINCE DISSIPATED WITH ONLY PARTLY CLOUDY
SKIES WEST OF THE LARAMIE RANGE. HOWEVER...SHOWER ACTIVITY IS
STARTING TO REDEVELOP AND PUSH NORTHWARD OUT OF WESTERN COLORADO.
THIS ACTIVITY IS IN ADVANCE OF A 110 KNOT JET MAX IN ASSOCIATION
WITH AN UPPER LEVEL DISTURBANCE. MATTER OF FACT...TSTORMS ARE
STILL ON GOING ACROSS ARIZONA AS THIS DISTURBANCE BEGINS TO ROUND
THE BASE OF THE UPPER LEVEL LOW CENTER. SHOWERS AND ISOLATED
THUNDER WILL GRADUALLY INCREASE IN COVERAGE WEST OF THE LARAMIE
RANGE THIS MORNING...AND THEN EVENTUALLY PUSH INTO THE LARAMIE
RANGE AND ADJACENT HIGH PLAINS LATER TODAY. SHOWERS AND TSTORMS
WILL HOLD OFF UNTIL LATE THIS EVENING FOR WESTERN NEBRASKA...DUE
TO DRY AIR ALOFT AND SOME SUBSIDENCE ON THE WESTERN EDGE OF THE
UPPER LEVEL RIDGE ACROSS THE PLAINS. KEPT POP BETWEEN 60 TO 80
PERCENT ACROSS MOST OF SOUTHEAST WYOMING AND THE NORTHERN NEBRASKA
PANHANDLE THIS EVENING...WITH PROBABILITIES OVER 90 PERCENT ACROSS
THE LARAMIE RANGE. SOME SHOWERS/TSTORMS MAY CONTAIN VERY HEAVY
RAINFALL...GUSTY WINDS...AND SMALL HAIL. HIGHS TODAY WILL
GENERALLY BE 10 DEGREES COOLER COMPARED TO SATURDAY WITH READINGS
RANGING FROM THE MID TO UPPER 60S WEST OF THE LARAMIE RANGE...AND
70S TO NEAR 80 EAST OF THE LARAMIE RANGE. RAINFALL WILL GRADUALLY
DIMINISH FROM SOUTH TO NORTH LATE TONIGHT.

FOR MONDAY...THE MAIN SYSTEM WILL MOVE ACROSS THE CENTRAL ROCKY
MOUNTAINS AND IMPACT THE FORECAST AREA THROUGH MONDAY NIGHT AND
POSSIBLY INTO TUESDAY. WILL HAVE TO CLOSELY MONITOR THIS SYSTEM
FOR THE POTENTIAL OF FLASH FLOODING AND STRONG
THUNDERSTORMS...SINCE MODELS INDICATE THE UPPER LEVEL LOW BECOMING
A NEGATIVELY TILTED OPEN WAVE AS IT MOVES EASTWARD...AND THEN SHOW
THE LOW CLOSING OFF AGAIN SOMEWHERE IN THE VICINITY OF EAST
CENTRAL WYOMING TOWARDS THE BLACK HILLS OF SOUTH DAKOTA. MODELS
SHOW A STRONG JET MAX ALOFT BECOMING TWO INDIVIDUAL JET STREAKS
OVER THE HIGH PLAINS FORMING A FAVORABLE DIFLUENT COUPLET IN
ADDITION TO RAPID LEE-SIDE CYCLOGENESIS. THERE IS THE POTENTIAL
FOR A WIDESPREAD MODERATE TO HEAVY RAINFALL EVENT ALONG WITH RAIN
CHANGING TO SNOW IN THE MOUNTAINS ABOVE 9000 FEET. AFTER A BRIEF
LULL MONDAY MORNING WITH MAINLY DRIZZLE...LOW STRATUS AND PATCHY
FOG...SHOWERS AND TSTORMS WILL QUICKLY DEVELOP AND INCREASE IN
COVERAGE DURING THE AFTERNOON. INCREASED POP BETWEEN 70 TO 90
PERCENT ACROSS MOST OF THE HIGH PLAINS ALONG AND EAST OF THE
LARAMIE RANGE. TEMPERATURES WILL BE CLOSER TO NORMAL ON MONDAY AND
GENERALLY IN THE 60S ACROSS SOUTHEAST WYOMING AND LOW TO MID 70S
ACROSS WESTERN NEBRASKA.

VERY WINDY CONDITIONS MAY BE A CONCERN FOR LATE MONDAY NIGHT AND
TUESDAY AS THE GFS INDICATES WINDS AT 700MB OVER 55 KNOTS...MAINLY
DUE TO WINDS BECOMING WESTERLY AND RAPID CYCLOGENESIS OVER THE
HIGH PLAINS AS THE SURFACE LOW FORMS OVER NE COLORADO AND MOVES
DUE NORTH ACROSS THE BLACK HILLS. A BIT TOO EARLY TO SEE HIGH WIND
CRITERIA...ALONG MODELS HINT TOWARDS DECENT LLVL SUBSIDENCE BY
EARLY TUESDAY MORNING. WILL LIKELY NEED WIND ADVISORIES FOR
PORTIONS OF WESTERN NEBRASKA ON TUESDAY. GUSTS UP TO 55 MPH ARE
POSSIBLE ACROSS THE WIND PRONE AREAS OF SOUTHEAST WYOMING...WITH
GUSTS UP TO 45 MPH ELSEWHERE. TUESDAY WILL BE SOMEWHAT DRIER DUE
TO DOWNSLOPE WINDS...BUT RAIN SHOWERS CAN NOT BE RULED
OUT...ESPECIALLY NORTH OF I80.

.LONG TERM...(TUESDAY NIGHT THROUGH SATURDAY)
ISSUED AT 330 AM MDT SUN SEP 27 2014

EXTENDED MODELS IN RELATIVELY GOOD AGREEMENT THROUGH THURSDAY WITH A
MEAN UPPER LEVEL TROUGH OVER THE WESTERN AND CENTRAL CONUS. THE
DEVIL IS IN THE DETAILS WITH CHANCES FOR PRECIPITATION MAINLY
DEPENDING ON TIMING AND PLACEMENT OF SHORTWAVE ENERGY EMBEDDED IN
THE UPPER LEVEL FLOW. REGARDLESS IT WILL BE COOL WITH TEMPERATURES
BELOW NORMAL WITH CHANCES FOR RAIN SHOWERS OVER MUCH OF THE AREA AND
LIGHT SNOW ACCUMULATION POSSIBLE IN THE MOUNTAINS WITH THE MAIN
CHANCES FOR SNOW ACCUMULATIONS IN THE SNOWY AND SIERRA MADRE RANGES.
HIGH TEMPERATURES WILL GENERALLY BE IN THE 50S WITH SOME 60S INTO
THE LOWER ELEVATIONS INTO THE PLAINS. HIGH TEMPERATURES IN THE
MOUNTAINS WILL BE IN THE 30S AND 40S.

THE 00Z ECMWF AND GFS SHOWING BIG DIFFERENCES IN THE PLACEMENT OF
THE UPPER TROUGH COME FRIDAY AND SATURDAY. THE ECMWF IS ACTUALLY
MORE PROGRESSIVE WITH WARMER AIR MAKING A PUSH ACROSS THE
ROCKIES...WHILE THE GFS DIGS THE UPPER TROUGH SOUTH ACROSS THE GREAT
PLAINS AND IS A GOOD DAY SLOWER BRINGING IN THE WARMER AIR.
TYPICALLY THE EASTWARD PROGRESSION OF AMPLIFIED PATTERNS IS SLOWER
THAN WHAT THE GFS INDICATED...SO KIND OF SURPRISED THE ECMWF IS SO
MUCH FASTER THAN THE GFS. THAT SAID THE ECMWF HAS BEEN PREFORMING
BETTER AT DAYS 5 THROUGH 7 SO HEDGED THE FORECAST TOWARDS THE ECMWF
FOR NOW.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z MONDAY MORNING)
ISSUED AT 330 AM MDT SUN SEP 28 2014

UPPER LEVEL LOW WILL NUDGE ITS WAY INTO SOUTHEAST WYOMING TODAY.
THIS WILL ALLOW FOR THE DEVELOPMENT OF AFTERNOON SHOWERS AND
THUNDERSTORMS...WHICH MAY REACH INTO THE NEBRASKA PANHANDLE LATER
TONIGHT. MAINLY LOOKING AT VFR ACROSS THE FORECAST AREA THROUGH
EARLY THIS AFTERNOON...BUT SOME MVFR CIGS WILL BE POSSIBLE NEAR THE
LARAMIE RANGE. ALREADY LOOKING AT 2000-3000 FT AGL CIGS ALONG THE
NORTHERN LARAMIE RANGE. WIDESPREAD IFR TO MVFR FOR SOUTHEAST WYOMING
TONIGHT WITH SHOWERS BECOMING WIDESPREAD MONDAY AFTERNOON WITH
ISOLATED THUNDERSTORMS.


&&

.FIRE WEATHER...
ISSUED AT 229 AM MDT SUN SEP 28 2014

NO FIRE WEATHER CONCERNS THROUGH THE MIDDLE OF NEXT WEEK. COOLER
TEMPERATURES ALONG WITH INCREASING MOISTURE WILL RESULT IN HUMIDITY
VALUES TRENDING HIGHER OVER THE NEXT FEW DAYS. THE ABUNDANT
MOISTURE WILL COMBINE WITH LIFT FROM AN APPROACHING SYSTEM TO
PRODUCE A GOOD CHANCE OF WIDESPREAD WETTING RAINS ON SUNDAY AND
MONDAY ACROSS MOST OF THE AREA. WINDY CONDITIONS WILL DEVELOP ON
TUESDAY...HOWEVER MINIMUM RELATIVE HUMIDITY VALUES WILL REMAIN
ABOVE 35 PERCENT.

&&

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

$$

UPDATE...HAMMER
SHORT TERM...TJT
LONG TERM...SML
AVIATION...SML
FIRE WEATHER...TJT






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