Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 300457
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
1057 PM MDT WED APR 29 2015

.UPDATE...
ISSUED AT 926 PM MDT WED APR 29 2015

INHERITED GRIDDED FORECASTS FOR TONIGHT ARE IN NICE SHAPE FROM
OUR DAY SHIFT FORECAST TEAM AND THE DAY SHIFT SHORT TERM
FORECASTER...SO HAVE ONLY MADE SOME MINOR MODIFICATIONS TO
TEMPERATURES...DEWPOINTS...WINDS AND CLOUD COVER FOR THE REST OF
THE NIGHT BASED ON TRENDS AND THE 00Z MODEL RUNS. QUICK PERUSAL OF
THE 30/00Z NAM SHOWS OUR FORECAST IS ON TRACK FOR THE LAST DAY OF
APRIL SHOWERS...SPRINGING COOLER TEMPERATURES AND MAY FLOWERS FOR
THE FIRST DAY OF MAY ON FRIDAY.

&&

.SHORT TERM...(THIS EVENING THROUGH FRIDAY NIGHT)
ISSUED AT 225 PM MDT WED APR 29 2015

BEAUTIFUL SPRING DAY ACROSS THE REGION WITH CLEAR SKIES...LIGHT
WINDS...AND TEMPS IN THE UPPER 60S AND 70S.  RIDGE AXIS IS CURRENTLY
OVERHEAD WITH 700MB TEMPS OF 7-9C.  THIS RIDGE WILL TRANSLATE
EASTWARD BY TONIGHT AND THURS AS A SHORTWAVE MOVES ACROSS MONTANA.
IT WILL BE ANOTHER WARM DAY ON THURS...HOWEVER A WEAK COLD FRONT
WILL MOVE ACROSS THE REGION BY MIDDAY.  AS SFC WINDS BECOME
NORTHEASTERLY BY AFTN AND CONVERGENCE INCREASES ALONG THE LARAMIE
RANGE...SCATTERED SHOWERS AND TSTMS WILL DEVELOP.  THE NAM AS USUAL
SHOWS MORE INSTABILITY (LI VALUES OF -4C)...BUT LOOKS OVERDONE
COMPARED TO THE GFS WHICH INDICATES LI VALUES OF -2C.  GENERALLY
HAVE 20-40 POPS DURING THE AFTN...WITH 30-50 POPS DURING THE
EVENING.  THE STORM THREAT WILL DIMINISH BY EARLY THURS NIGHT AS THE
ATMOSPHERE BEGINS TO STABILIZE.

IT WILL BE COOLER ON FRI ESPECIALLY OVER THE PLAINS IN THE POST
FRONTAL AIRMASS.  THE MODELS SHOW SFC WINDS BECOMING SOUTHEASTERLY
AFTER 18Z...WHICH WILL LEAD TO ISOLATED TO SCATTERED AFTN CONVECTION
DEVELOPING ONCE AGAIN ALONG THE LARAMIE RANGE.  THE BEST COVERAGE OF
STORMS WILL LIKELY BE SOUTH OF THE BORDER IN COLORADO.  LI VALUES OF
-3C ARE PROGGED IN AREAS ALONG INTERSTATE 80...AND WITH FLOW ALOFT
AND DEEP LAYER SHEAR INCREASING...CAN NOT RULE OUT A STRAY STRONGER
STORM IN SOUTHEAST WY.  HOWEVER BELIEVE THAT THE LIMITED HEATING AND
MARGINAL LLVL MOISTURE WILL REALLY LIMIT THAT THREAT.  CONVECTION
WILL DROP OFF BY LATE FRI EVENING WITH A QUIET FRI NIGHT IN STORE.

.LONG TERM...(SATURDAY THROUGH WEDNESDAY)
ISSUED AT 225 PM MDT WED APR 29 2015

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

SATURDAY-MONDAY:
LATEST WATER VAPOR LOOP WAS SHOWING A FAIRLY PRONOUNCED UPPER
LEVEL TROF DIGGING SOUTH ACROSS THE EASTERN PACIFIC WITH A LOT OF
JET ENERGY AT THE BASE OF THIS TROF. THIS TROF IS PROGGED TO
REACH THE SOUTHERN CALIFORNIA COASTLINE BY MONDAY AFTERNOON.
MEANWHILE...OUR REGION IS EXPECTED TO BE IN A ZONAL FLOW REGIME
WHICH WOULD SUPPORT AFTERNOON OROGRAPHIC CONVECTION. AS A
RESULT...WE ARE FAIRLY CONFIDENT THAT THE MOUNTAINS WILL SEE DAILY
AFTERNOON CONVECTION. HOWEVER...WE ARE NOT AS CONFIDENT ABOUT THE
CONVECTION CHANCES ACROSS THE REMAINDER OF THE AREA. THERE IS ARE
SOME HINTS OF SUBTLE JET STREAKS MOVING THROUGH WYOMING ON
SATURDAY AND SUNDAY WHICH COULD PROVIDE ADDITIONAL LIFT OVER THE
AREA AND GIVE WAY TO UNSETTLED CONDITIONS. IN ADDITION...THERE IS
DECENT INSTABILITY (500-1000J/KG) IN PLACE WITH A WEAK SURFACE
TROF WHICH WILL PROVIDE THE TRIGGER FOR SCATTERED THUNDERSTORMS.

MONDAY-WEDNESDAY:
THIS TIMEFRAME HAS A LOT MORE UNCERTAINTY ESPECIALLY WITH THE
TRACK OF THE UPPER LEVEL TROF IN THE SOUTHWEST. THE GFS IS
TRACKING THIS TROF TO INTENSIFY AND LIFT NORTHEAST TOWARDS THE
NEBRASKA PANHANDLE BY WEDNESDAY NIGHT. MEANWHILE...THE ECMWF IS
MORE PROGRESSIVE AND TENDS TO KEEP IT AN OPEN WAVE. THE ENSEMBLE
MEMBER MEAN IS SHOWING A MORE PROGRESSIVE SOLUTION AS WELL. WE
WILL FAVOR THIS SOLUTION FOR NOW WHICH WOULD CONTINUE TO SUPPORT
UNSETTLED CONDITIONS OVER THE AREA THROUGH WEDNESDAY AS THIS WAVE
PULLS IN GULF MOISTURE. AS A RESULT...WE HAVE KEPT CHANCES OF
AFTERNOON/EVENING SHOWERS AND THUNDERSTORMS ACROSS THE AREA
THROUGH MID WEEK. TEMPERATURES SHOULD ALSO REMAIN MILD WITH HIGHS
GENERALLY IN THE 50S AND 60S.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z THURSDAY NIGHT)
ISSUED AT 1057 PM MDT WED APR 29 2015

MOSTLY CLEAR SKIES AND LIGHT WINDS WILL CONTINUE THROUGH THE
OVERNIGHT PERIOD. A COLD FRONT WILL DROP SOUTH ACROSS THE PLAINS
BEGINNING AFTER SUNRISE THURSDAY WITH WINDS SHIFTING TO THE
NORTH-NORTHWEST AND BECOMING GUSTY AT TIMES WITH THE FRONT. THESE
WINDS WILL THEN SHIFT TO THE NORTHEAST THROUGH THE AFTERNOON AS
THE FRONT SLIDES WESTWARD INTO THE MOUNTAINS...WITH SHOWERS AND
THUNDERSTORMS DEVELOPING OFF THE MOUNTAINS AND MOVING TO THE
NORTHEAST THROUGH THE AFTERNOON AND EVENING. STILL EXPECT THIS
ACTIVITY TO AFFECT WYOMING TERMINALS AFTER 19Z...AND NEBRASKA
LOCATIONS AFTER 00Z FRIDAY.

&&

.FIRE WEATHER...
ISSUED AT 225 PM MDT WED APR 29 2015

NO FIRE WEATHER CONCERNS THROUGH THE WEEKEND.  AFTER A SLIGHTLY
COOLER DAY BEHIND A WEAK COLD FRONT ON FRIDAY...TEMPERATURES WILL
WARM ONCE AGAIN BY SATURDAY.  MINIMUM AFTERNOON HUMIDITY VALUES WILL
DROP TO AROUND 20 PERCENT ON SATURDAY AFTERNOON WITH A FEW GUSTS TO
20 MPH ACROSS SOUTHEAST WYOMING.  HOWEVER...THE RECENT RAINFALL AND
GREENUP WILL KEEP THE FIRE WEATHER THREAT LOW.

&&

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

$$

UPDATE...RUBIN
SHORT TERM...ZF
LONG TERM...REC
AVIATION...RJM
FIRE WEATHER...ZF



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