Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 190341
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
941 PM MDT THU SEP 18 2014

.UPDATE...
ISSUED AT 922 PM MDT THU SEP 18 2014

LATEST WATER VAPOR LOOP WAS SHOWING THE UPPER LEVEL SHORTWAVE
CONTINUING TO TRANSVERSE EAST THROUGH MONTANA. THIS FEATURE HAS
BEEN DRAWING UP A GOOD MOISTURE PLUME THROUGH THE ROCKIES. AS A
RESULT...WE HAVE SEEN THE SHOWERS CONTINUE TO PROGRESS EAST
THROUGH NORTHERN WYOMING. A FEW OF THESE SHOWERS MAY MOVE INTO THE
NORTHERN PARTS OF OUR FORECAST AREA LATE TONIGHT AS THE MOISTURE
TRANSPORT CONTINUES TO SHIFT EAST. THERE IS EVEN SOME UPPER LEVEL
SUPPORT FROM THE RIGHT ENTRANCE REGION OF THE UPPER LEVEL JET
WHICH MAY ALSO SUPPORT THE CONVECTION THROUGH THE NIGHT. THIS
SHOWER ACTIVITY SHOULD BE MAINLY CONFINED ACROSS NORTHERN WYOMING
AND SOUTHWEST SOUTH DAKOTA DURING THE MORNING WHERE THE DEEPER
MOISTURE WILL BE IN PLACE. NOT REAL CONFIDENT ABOUT PRECIPITATION
CHANCES DURING THE AFTERNOON ACROSS THE SOUTHERN HALF OF WYOMING
EXCEPT FOR THE CHEYENNE RIDGE SINCE THE UPSLOPE EFFECTS MAY PLAY
MORE OF ROLE DUE TO THE SURFACE LOW IN NORTHERN COLORADO. WE DID
TRIM BACK POPS ACROSS THE MOUNTAINS DUE TO LIMITED UPSLOPE
DYNAMICS.

&&

.SHORT TERM...(THIS EVENING THROUGH SATURDAY NIGHT)
ISSUED AT 330 PM MDT THU SEP 18 2014

FORECAST CHALLENGES IN THE SHORT TERM DEAL WITH PRECIP CHANCES AS
A WEAK COLD FRONT MOVES SLOWLY INTO THE AREA FRIDAY LATE.

CURRENTLY...COLD FRONT ANALYZED ON SURFACE ANALYSIS CURRENTLY LAYS
FROM NORTH CENTRAL MONTANA...THROUGH BILLINGS INTO NORTHWESTERN
WYOMING AND SOUTHWEST INTO NORTHEASTERN NEVADA THIS AFTERNOON.
CONVECTIVE ACTIVITY HAS BEEN CONFINED TO NORTHWESTERN WYOMING
ALONG THE FRONT. SOUTHEAST WYOMING HAS BEEN WARM AND DRY AS HIGH
PRESSURE DOMINATES THE AREA . LATEST MESOWEST OBSERVATIONS HAVE
SHOWN WESTERLY TO SOUTHWESTERLY WINDS 10 TO 12 MPH WITH GUSTS TO
THE UPPER TEENS. LOOKING AT LATEST NAM SOUNDINGS...WE HAVE PRETTY
MUCH MIXED OUT THE LOW LEVEL INVERSIONS THAT WERE EVIDENT ACROSS
THE FORECAST AREA THIS MORNING. WITH UPPER LEVEL WINDS FAIRLY
LIGHT...DO NOT EXPECT THESE WINDS TO GET ANY STRONGER THAN WHAT
THEY ARE RIGHT NOW. A PRETTY WARM AFTERNOON ALREADY THIS AFTERNOON
WITH LOW 90S IN THE PANHANDLE...MID TO UPPER 80S EAST OF THE
LARAMIE RANGE RANGE AND UPPER 70S/LOW 80S WEST OF THE LARAMIE RANGE.

LATEST GFS...NAM AND ECMWF RUNS ARE SHOWING A SLOW DOWN OF THE
FRONT FOR FRIDAY. NOW SHOWING THE FRONT POSSIBLY MAKING ITS WAY
INTO OUR WESTERN ZONES BY LATE FRIDAY AFTERNOON. GUIDANCE
SUGGESTING THAT CONDITIONS SHOULD BE DRY FOR MOST OF THE DAY LIGHT
HOURS FRIDAY. DID BACK OFF ON POPS SOME FOR FRIDAY AFTERNOON AS
LATEST SOUNDINGS SHOWING PRETTY DRY CONDITIONS UP THROUGH 600MBS
FOR CHEYENNE BY 18Z FRIDAY. PWATS OUT NEAR RAWLINS INCREASE TO
.75 INCHES AT THE SAME TIME WITH THE FRONT MOVING INTO THE AREA.
SO DID NOT REMOVE POPS ALTOGETHER. INCREASING CLOUDS SHOULD LIMIT
HIGH TEMPERATURES FROM TODAYS HIGHS...BUT STILL WARM WITH NEAR 90S
IN THE PANHANDLE AND MID 80S ALONG THE EAST SLOPES OF THE LARAMIE
RANGE. MODEL SOUNDING SHOWING SOME PRETTY STRONG WINDS FRIDAY
AFTERNOON. NAM SHOWING WINDS IN EXCESS OF 25KTS AT RAWLINS TO
35KTS UP BY DOUGLAS SO WE COULD SEE SOME OF THESE WINDS MIXING
DOWN WITH THE TSTMS. ADDED GUSTY WINDS INTO THE WEATHER GRIDS WITH
THESE THUNDERSTORMS.

GET A GOOD SURGE OF NORTHEASTERLY WINDS FRIDAY NIGHT INTO SATURDAY
AS THE FRONT MOVES INTO THE PANHANDLE. 700MB TEMPERATURES COOL TO
+8 TO +10C SATURDAY INTO SUNDAY MORNING...DOWN FROM THE CURRENT
+14 TO +16C TEMPERATURES WE ARE SEEING. SO HIGHS OVER THE WEEKEND
MAINLY IN THE 70S.

.LONG TERM...(SUNDAY THROUGH THURSDAY)
ISSUED AT 310 PM MDT THU SEP 18 2014

LONG RANGE MODELS IN REASONABLE AGREEMENT THROUGH NEXT WEEK...WITH
A SLOW MOVING CLOSED LOW BEING THE MAIN FORECAST CONCERN THROUGH
EARLY IN THE WEEK AS IT MOVES NORTHEAST ACROSS THE GREAT BASIN
REGION AND EVENTUALLY IMPACTING THE NORTHERN AND CENTRAL ROCKY
MOUNTAINS. NOT MUCH IN THE WAY OF DYNAMICS WITH THIS SYSTEM AS IT
PUSHES EASTWARD OUT OF THE EASTERN PACIFIC THIS WEEKEND...BUT LLVL
INSTABILITY...SOME WEAK JET DYNAMICS...AND COOLER AIR ALOFT WILL
BE ENOUGH TO INITIATE SHOWERS AND THUNDERSTORMS BEGINNING EARLY ON
SUNDAY. THIS ACTIVITY IS EXPECTED TO CONTINUE THROUGH AT LEAST
TUESDAY AFTERNOON...ESPECIALLY ACROSS THE HIGH PLAINS...AS THE
TROUGH AXIS WILL SLOWLY PUSH ACROSS EASTERN WYOMING TUESDAY
AFTERNOON. BEHIND THE TROUGH AXIS...DRIER AIR WILL GRADUALLY WORK
ITS WAY INTO WYOMING AND NEBRASKA ON WEDNESDAY AS MODELS SHOW AN
UPPER LEVEL RIDGE BUILDING NORTHWARD ALONG THE CONTINENTAL DIVIDE.
INCREASED POP BETWEEN 40 TO 70 PERCENT ACROSS THE FORECAST
AREA...HIGHEST ACROSS THE SOUTHEAST WYOMING MOUNTAINS. DIFFICULT
TO PIN POINT WHERE THE HEAVIEST AND/OR MORE WIDESPREAD
PRECIPITATION WILL BE DUE TO SLIGHT DIFFERENCES IN POSITION AND
TIMING OF SHORTWAVE ENERGY...BUT IT APPEARS MONDAY AND MONDAY
EVENING MAY BE THE MOST FAVORABLE TIME PERIOD FOR SOME MODERATE TO
HEAVY RAINFALL ACROSS THE HIGH PLAINS. ALTHOUGH TEMPERATURES WILL
LOWER ON SUNDAY AND MONDAY...THEY WILL STILL BE NEAR NORMAL FOR
THE TIME OF THE YEAR WITH HIGHS IN THE 70S AND PERHAPS UPPER 60S
ACROSS THE HIGH VALLEYS OF SOUTHEAST WYOMING. OVERNIGHT LOWS WILL
BE RELATIVELY MILD THIS TIME OF THE YEAR DUE TO EXPECTED CLOUD
COVER AT NIGHT.

MODELS INDICATE THE UPPER LEVEL RIDGE DRIFTING EASTWARD OVER THE
FORECAST AREA BY MIDWEEK...WHICH WILL ALLOW HIGH TEMPERATURES TO
INCREASE INTO THE MID 70S TO LOW 80S WITH DRY CONDITIONS.

FURTHER OUT...A STRONG PACIFIC STORM SYSTEM WILL MOVE ACROSS THE
GREAT BASIN AND PUSH TOWARDS THE ROCKY MOUNTAINS FOR THE LAST
WEEKEND OF SEPTEMBER.

&&

.AVIATION...(FOR THE 00Z TAFS THROUGH 00Z FRIDAY AFTERNOON)
ISSUED AT 541 PM MDT THU SEP 18 2014

THE MAIN CONCERN WITH THIS TAF ISSUANCE IS THE TIMING OF THE
FRONTAL BOUNDARY TOMORROW MORNING AND SHOWER CHANCES DURING THE
AFTERNOON.

LATEST WATER VAPOR LOOP WAS SHOWING CLUSTERS OF SHOWERS AND
THUNDERSTORMS MOVING THROUGH NORTHWEST WYOMING THIS AFTERNOON DUE
TO A SHORTWAVE MOVING THROUGH MONTANA. THIS SHORTWAVE WILL
CONTINUE TO SHIFT EAST INTO THE NORTHERN PLAINS BY TOMORROW
EVENING. AS IT PUSHES THROUGH...IT WILL CAUSE A FRONTAL BOUNDARY
TO PUSH THROUGH THE AREA. THIS BOUNDARY WILL BE FAIRLY WEAK...BUT
IT WILL CAUSE THE WIND TO SHIFT TO THE NORTH AND BRING A CHANCE
OF SHOWERS AND THUNDERSTORMS TOMORROW AFTERNOON. THE BEST CHANCE OF
THUNDERSTORMS APPEARS TO BE MAINLY AFTER 18Z ACROSS THE NORTHERN
NEBRASKA PANHANDLE AND CLOSER TO 21Z ACROSS THE WYOMING TAF SITES.
WE WILL HOLD ON MENTIONING VCTS IN THE TAFS AT THIS POINT...BUT IT
IS SOMETHING WORTH CONSIDERING IN LATER ISSUANCES.


&&

.FIRE WEATHER...
ISSUED AT 330 PM MDT THU SEP 18 2014

ONE MORE HOT DAY THIS AFTERNOON BEFORE COOLER TEMPERATURES MOVE
INTO THE AREA. AFTERNOON HUMIDITIES ARE QUITE LOW THIS AFTERNOON.
FORTUNATELY THOUGH...GUSTY WINDS HAVE BEEN ON THE LIGHT SIDE.
GUSTY WINDS WE ARE SEEING ON AREA SENSORS ARE NOT EXPECTED TO GET
MUCH HIGHER THAN WHAT WE ARE CURRENTLY EXPERIENCING THROUGH THE
REMAINDER OF THE AFTERNOON. A WEAK COLD FRONT LOOKS TO MOVE INTO
THE AREA FROM THE NORTHWEST FRIDAY WITH INCREASING CHANCES FOR
SHOWERS AND THUNDERSTORMS. BEST COVERAGE OVER THE MOUNTAINS FRIDAY
AFTERNOON AND EVENING. COOLER TEMPERATURES AND HIGHER HUMIDITIES
ARE EXPECTED BEHIND THE FRONT THROUGH THE WEEKEND.

&&

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

$$

UPDATE...REC
SHORT TERM...CLAYCOMB
LONG TERM...TJT
AVIATION...REC
FIRE WEATHER...CLAYCOMB






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