Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 300305
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
905 PM MDT FRI AUG 29 2014

.UPDATE...
ISSUED AT 904 PM MDT FRI AUG 29 2014

UPDATED THE GRIDS TO REMOVE ALL MENTION OF SHOWERS AND THUNDERSTORMS
OVERNIGHT BASED ON RADAR TRENDS AND LATEST MODEL GUIDANCE. ALSO
ADJUSTED TEMPERATURES...DEWPOINTS...WINDS AND CLOUD COVER BASED
ON TRENDS.

UPDATE ISSUED AT 600 PM MDT FRI AUG 29 2014

GRIDDED FORECASTS UPDATED TO MODIFY THE EVENING POPS WITH ISOLATED
THUNDERSTORMS FROM SARATOGA TO CHEYENNE...THEN DRY AFTER 9 PM.

&&

.SHORT TERM...(THIS EVENING THROUGH SUNDAY NIGHT)
ISSUED AT 150 PM MDT FRI AUG 29 2014

A WELL-DEFINED SHORTWAVE CONTINUES TO SLIDE EASTWARD ACROSS THE CWA
THIS AFTN ALONG THE NORTHERN FRINGE OF BROAD UPPER-LEVEL RIDGING IN
PLACE OVER THE CENTRAL ROCKIES. WATER VAPOR SATELLITE SHOWED THIS
FEATURE OVER NORTHEAST WY AS OF 19Z MOVING QUICKLY TOWARD SOUTHWEST
SD AND THE NORTHERN PANHANDLE. CONVECTIVE CHANCES ARE THE MAIN ISSUE
IN THE NEAR TERM. ASIDE FROM MOUNTAIN CONVECTION...SHOWER AND TSTM
CHANCES WILL BE ASSOCIATED WITH LARGER SCALE FORCING IN ADVANCE OF
THE AFOREMENTIONED WAVE. THE NAM IS STILL THE MOST BULLISH WITH QPF
OVER OUR NORTHEAST ZONES WHILE THE HRRR COMPOSITE REFLECTIVITY SHOWS
FAIRLY MODEST CONVECTIVE COVERAGE. CONFIDENCE IS FAIRLY LOW IN JUST
HOW WIDESPREAD THINGS WILL BE TODAY GIVEN WEAK SUBSIDENCE BENEATH
THE RIDGE...AND RECENT RADAR ECHOS HAVE NOT BEEN ANYTHING TO WRITE
HOME ABOUT. BELIEVE WE WILL SEE SOME MID AFTN TSTM ACTIVITY THOUGH
OVER THE HIGH PLAINS. CURRENT SPEED OF THE TROUGH AXIS SUGGESTS THE
BEST FORCING FOR ASCENT AND MOISTURE WILL SHIFT EAST OF THE CWA BY
ABOUT 22Z...AFTER WHICH TIME TSTMS SHOULD DIMINISH. DESPITE GOOD SFC
HEATING TODAY...LATEST SPC MESOSCALE ANALYSIS INDICATES LESS THAN
1000 J/KG OF SBCAPE. LLVL FLOW OUT OF THE NORTH OR NORTHWEST IS NOT
CONTRIBUTING TO ANY REASONABLE SHEAR PROFILES EITHER SO ANY STRONGER
STORMS SHOULD NOT BE WIDESPREAD THOUGH THEY ARE POSSIBLE. IT SHOULD
BE A QUIET OVERNIGHT WITH AS SUBSIDENCE INCREASES OVER THE CWA. DO
NOT EXPECT FOG DEVELOPMENT TONIGHT WITH UNFAVORABLE LLVL WINDS...BUT
CANNOT RULE IT OUT IN THE PLATTE RIVER VALLEY IF WINDS ARE MORE
SOUTHEAST THAN CURRENTLY PROGGED.

SAT STILL LOOKS TO BE THE WARMEST DAY OF THE PERIOD WITH A VIGOROUS
UPPER-LEVEL TROUGH LURKING OVER THE PACIFIC NW. THERMAL RIDGE AXIS
BUILDS INTO THE CWA WITH NAM H7 TEMPS INCREASING TO 13-14 C FROM THE
LARAMIE RANGE EASTWARD. MOST MOS GUIDANCE SUGGESTS HIGHS IN THE 83
TO 93 DEGREE RANGE OVER THE PLAINS...AND THIS LOOKS REASONABLE GIVEN
THE SYNOPTIC SETUP. THE PROGRESSION OF THE TROUGH AND ASSOCIATED SFC
COLD FRONT APPEARS A BIT SLOWER THAN EARLIER MODELS SUGGESTED...SO
THIS SHOULD DELAY PCPN CHANCES UNTIL MID TO LATE AFTN ON SAT. MIDLVL
QG FIELDS SHOW GOOD ASCENT SPREADING FROM WEST TO EAST AFTER 21Z ON
SAT. THIS OCCURS ALONG WITH LLVL CONVERGENCE ASSOCIATED WITH A
STRONG SFC COLD FRONT MOVING ACROSS THE CWA...SO AM CONFIDENT IN AT
LEAST SCATTERED COVERAGE OF SHOWERS AND TSTMS. INSTABILITY COULD BE
WANING BY THE TIME CONVECTION MOVES FAR ENOUGH EAST...BUT CAN NOT
RULE OUT A STRONG OR LOCALLY SEVERE STORM WITH NAM CAPES AROUND 1500
J/KG BENEATH STEEP MIDLVL LAPSE RATES AND ABOUT 30 KTS OF H5 FLOW.
IT WILL BE BREEZY ON SAT AFTN AND EARLY EVE...ESPECIALLY ALONG AND
WEST OF THE LARAMIE RANGE AS HEIGHT GRADIENTS INCREASE ALOFT AND A
STRONG SFC LOW MOVES ACROSS NORTHERN WY. WSW H7 WINDS AROUND 35 TO
40 KTS SUGGEST STRONGER WINDS OVER THE WIND PRONE AREAS...SO BUMPED
UP GUSTS TO AROUND 45 MPH AT ARL AND BRX.

SHOWERS WILL INCREASE OVERNIGHT THROUGH SUN AM...AND COULD CONTINUE
THROUGH MUCH OF THE AFTN WITH COLDER AIR ALOFT AS THE TROUGH MOVES
OVERHEAD. THE MODELS SHOW H7 TEMPS PLUNGING INTO THE MID SINGLE
DIGITS C ON SUN IN THE POST-FROPA AIR MASS. LOOKING FOR HIGHS TO BE
SOME 10 TO 15 DEGREES COOLER THAN SAT. FLOW ALOFT BRIEFLY BECOMES
MORE ZONAL BY LATE SUN NIGHT...WITH THE ECMWF FINALLY STARTING TO
BACK OFF ON QPF BY ABOUT 06Z MON.

.LONG TERM...(MONDAY THROUGH FRIDAY)
ISSUED AT 150 PM MDT FRI AUG 29 2014

SHORTWAVE TROUGH DRIVEN BY A 100+KT JET STREAK WILL SWEEP ACROSS
THE CWFA ON MONDAY. A QUICK SHOT OF MOISTURE ALONG WITH A WEAK
COLD FRONT WILL PRODUCE PERHAPS SOME AM SHOWER ACTIVITY ACROSS
CARBON AND CONVERSE COUNTIES. BETTER CONVECTIVE CHANCES WILL AWAIT
MONDAY AFTERNOON AS WEAK INSTABILITY IS REALIZED. PROGD SBCAPES
OF AROUND 500 J/KG SUGGEST THAT MOST OF THE CONVECTION WILL BE OF
THE WEAK VARIETY. HOWEVER WITH A STRONG JET OVERHEAD RESULTING IN
AROUND 50 KNOTS OF DEEP LAYER SHEAR...CANT ENTIRELY RULE OUT A FEW
STORMS BECOMING STRONG...ESPECIALLY OVR THE WRN NEBRASKA PANHANDLE
LATE IN THE AFTERNOON. CONVECTIVE CHANCES WILL COME TO AN END
QUICKLY MONDAY EVENING. A DRY ZONAL PATTERN WILL ENSUE ACROSS THE
INTERMOUNTAIN WEST BEHIND MONDAYS DEPARTING SHORTWAVE TROF. THIS
WILL BRING A DRY AND UNSEASONABLY WARM PERIOD INTO THE REGION. H7
TEMPERATURE PROGS INCREASE TO 14-16C...WHICH WILL YIELD DAILY HIGH
TEMPERATURES THAT WILL REACH THE 80S AND LOW 90S EAST OF THE
LARAMIE RANGE...WITH 70S OVR CARBON/ALBANY COUNTIES. THESE ABOVE
AVERAGE TEMPERATURES WILL COMBINE WITH BREEZY CONDITIONS AND AN
ALREADY DRY AIR MASS TO HEIGHTEN FIRE WEATHER CONCERNS WEDNESDAY
AND THURSDAY. LATEST MODEL GUIDANCE CONTINUES TO POINT TO EITHER
DEVELOPING ACROSS ACROSS THE PACNW OR PERHAPS A CUT-OFF JUST OFF
THE CALIFORNIA COAST LATE IN THE WEEK. IN EITHER CASE...SOUTHWEST
FLOW INTO THE ROCKYS WILL PROVIDE A FAVORABLE TRAJECTORY FOR
MOISTURE TO RETURN TO THE CWFA AS EARLY AS THURSDAY NIGHT...BUT
MORE SO FOR FRIDAY. TEMPERATURES WILL ALSO BE A TAD COOLER FRIDAY
AS A THICKER CLOUD CANOPY PREVENTS FULL INSOLATION.

&&

.AVIATION...(FOR THE 00Z TAFS THROUGH 00Z SATURDAY AFTERNOON)
ISSUED AT 435 PM MDT FRI AUG 29 2014

CONVECTION REALLY DECREASING LATE THIS AFTERNOON AS THE UPPER
SHORTWAVE MOVES OFF TO THE SOUTHEAST. ONLY AIRPORT THAT MAY SEE
SOMETHING LATE TODAY WOULD BE KLAR. CONVECTION RETURNS SATURDAY
AFTERNOON WITH MOST AIRPORTS SEEING SOME THUNDER BY MID TO LATE
AFTERNOON.


&&

.FIRE WEATHER...
ISSUED AT 150 PM MDT FRI AUG 29 2014

NO CONCERNS FOR CRITICAL FIRE WEATHER IN THE NEAR FUTURE. A STRONG
WEATHER DISTURBANCE AND ASSOCIATED COLD FRONT WILL SWEEP ACROSS THE
AREA ON SAT AFTN THROUGH SUN MORNING. SHOWERS AND TSTMS WILL DEVELOP
FROM WEST TO EAST ACROSS SOUTHEAST WY AND THE WESTERN NE PANHANDLE
DURING THAT TIME. GUSTY WEST-TO-SOUTHWEST WINDS WILL OCCUR ALONG AND
WEST OF THE LARAMIE RANGE ON SAT WITH GUSTS OF 35 TO 45 MPH...BUT
MINIMUM RH VALUES SHOULD REMAIN ABOVE 20 PERCENT. COOLER ON SUN WITH
CONTINUED BREEZY CONDITIONS AND ISOLATED TO SCATTERED SHOWERS AND
WEAK TSTMS.

&&

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

$$

UPDATE...RUBIN
SHORT TERM...HAMMER
LONG TERM...HAHN
AVIATION...CLAYCOMB
FIRE WEATHER...HAMMER






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