934
FXUS65 KCYS 162320
AFDCYS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
520 PM MDT WED SEP 16 2026
KEY MESSAGES
- A WET PATTERN IS EXPECTED THROUGH THE END OF THE WEEK, WITH
ALMOST DAILY CHANCES FOR SHOWERS AND THUNDERSTORMS.
- THERE IS A SLIGHT CHANCE OF EXCESSIVE RAINFALL AND FLOODING
ISSUES THURSDAY AS A STRONGER THAN AVERAGE LATE-SEASON
MONSOON WITH TROPICAL MOISTURE MOVES INTO THE REGION.
SHORT TERM /THROUGH THURSDAY NIGHT/
ISSUED AT 240 PM MDT WED SEP 16 2026
THE NEXT 36 HOURS WILL BE QUITE ACTIVE WITH MULTIPLE ROUNDS OF
STORMS EXPECTED TO IMPACT THE AREA FROM A STRONG MONSOON PUSH.
CURRENTLY, MUCH OF THE CWA SITS IN THE WARM SECTOR BEHIND A WARM
FRONT THAT MOVED ACROSS THE AREA EARLIER. MOISTURE ADVECTION FROM
SOUTHERLY FLOW AND DAYTIME HEATING FROM LITTLE TO NO CLOUD COVER,
COULD CAUSE STORMS TO DEVELOP LATER THIS AFTERNOON, MAINLY FOR AREAS
EAST OF THE LARAMIE RANGE. HI-RES GUIDANCE KEEPS BACKING OFF ON
STORM POTENTIAL, LESSENING THE INSTABILITY ACROSS THE AREA.
NONETHELESS, CANNOT RULE OUT AN ISOLATED THUNDERSTORM OR TWO AHEAD
OF THE EXPECTED APPROACHING COLD FRONT. FURTHER WEST, SHOWERS AND
STORMS CONTINUE ON RADAR WITH MORE PRECIPITATION ENTERING CARBON
COUNTY FROM UTAH AND NORTHWEST COLORADO. LATER THIS EVENING, A
SURFACE COLD FRONT WILL MOVE THROUGH THE CWA, STALLING AROUND THE
WYOMING/COLORADO BORDER. THIS WILL TEMPORARILY PAUSE PRECIPITATION
ACROSS THE AREA AS FORCING WEAKENS FROM THE STALLED FRONT.
MORE WIDESPREAD PRECIPITATION CHANCES ARE FAVORED FOR THURSDAY. AN
UPPER-LEVEL SHORTWAVE WILL MOVE INTO WYOMING DURING THE DAY, WITH A
SURFACE LOW DEVELOPING IN NORTHEAST COLORADO. THIS SHORTWAVE WILL
NOT ONLY CARRY THE TYPICAL MONSOON MOISTURE PLUME FROM SOUTHWEST
FLOW, BUT WILL ALSO HAVE THE REMNANT MOISTURE OF THE TROPICAL LOW
THAT FORMED IN THE GULF OF CALIFORNIA. THIS WILL RAISE OUR PWATS TO
CLIMATOLOGICAL MAX ACCORDING TO NAEFS PERCENTILES. PWATS WILL RANGE
ANYWHERE FROM AN INCH TO 1.5 INCHES OVER THE FORECAST AREA. EVEN
WITH RECORD HIGH PWATS FORECAST, QPF AMOUNTS REMAIN RATHER
UNIMPRESSIVE. MOST MODELS AND ENSEMBLE MEANS ONLY SHOW BETWEEN 0.3
AND 0.7 INCHES ACROSS SOUTHEAST WYOMING AND WESTERN NEBRASKA.
HOWEVER, IT IS IMPORTANT TO NOTE, THAT ESPECIALLY FOR LOCATIONS EAST
OF THE LARAMIE RANGE, THERE IS A LOT OF SPREAD BETWEEN MEMBERS. THIS
SIGNALS LOWER CONFIDENCE IN POSSIBLE PRECIPITATION AMOUNTS. IT IS
ALSO LIKELY THAT MANY OF THE MODELS ARE UNDERDOING PRECIPITATION
AMOUNTS GIVEN THE HIGH PWATS SO WOULD NUDGE TOWARDS THE HIGHER RANGE
OF MEAN QPF. BASED OFF OF HI-RES GUIDANCE, PRECIPITATION WILL LIKELY
MOVE INTO THE CWA THURSDAY MORNING. THE MORE STABLE ENVIRONMENT
WILL LEAD TO MORE STRATIFORM PRECIPITATION TO START. STRATIFORM
PRECIPITATION WILL ALSO BE MOST LIKELY ALONG AND WEST OF THE
LARAMIE RANGE. AS THE DAY PROGRESSES AND DESTABILIZATION OCCURS
FROM DAYTIME HEATING AND THE STALLED FRONT, CONVECTIVE STORMS
WILL BECOME INCREASINGLY LIKELY LATER IN THE AFTERNOON AND
EVENING, ESPECIALLY EAST OF THE LARAMIE RANGE. HI-RES GUIDANCE
SUPPORTS THUNDERSTORM DEVELOPMENT AS WELL AS MODEL SOUNDINGS.
SOUNDINGS SHOW A TALL, SKINNY CAPE PROFILE, INDICATING A HEAVY
RAIN THREAT. CLOUD LAYER WINDS WILL ACTUALLY BE QUITE FAST SO
ANY FLOODING THAT OCCURS WILL NOT BE CAUSED BY SLOW MOVING
STORMS, BUT RATHER TRAINING STORMS OVER AN AREA. ALTHOUGH THE
MAJORITY OF LOCATIONS WILL NOT SEE LARGE AMOUNTS OF QPF, A FEW
ISOLATED LOCATIONS COULD SEE OVER 2 INCHES OF RAIN AND FLASH
FLOODING FROM THUNDERSTORMS. THE FLOODING THREAT WILL LIKELY BE
GREATEST EAST OF THE LARAMIE RANGE, AND LATER IN THE
AFTERNOON/EVENING WHEN THUNDERSTORMS ARE EXPECTED. THE HEAVY
RAIN THREAT WILL CONTINUE UNTIL ABOUT MIDNIGHT WHEN STORMS
FINALLY MOVE OUT OF THE AREA. FOR NOW, DECIDED TO HOLD OFF ON A
FLOOD WATCH, MAINLY DUE TO LOW CONFIDENCE FROM ENSEMBLE MEMBER
SPREAD. HOWEVER, CONSISTENCY FROM RUN TO RUN IN HI-RES MODELS
COULD INCREASE THIS CONFIDENCE AND LEAD TO THE ISSUANCE OF A
WATCH.
LONG TERM /FRIDAY THROUGH WEDNESDAY/
ISSUED AT 240 PM MDT WED SEP 16 2026
FRIDAY WILL BE ANOTHER ACTIVE AND POTENTIALLY WET DAY, AS WE REMAIN
UNDER SOUTHWESTERLY FLOW THAT WILL PROVIDE US WITH ANOTHER PUSH OF
MONSOONAL MOISTURE FROM BAJA CALIFORNIA. AS A RESULT, PWAT VALUES
WILL BE ABOVE THE 90TH PERCENTILE WITH A VIGOROUS CLOSED 500 MB LOW
LOCATED OVER EASTERN OREGON AND WESTERN IDAHO. THE QUESTION WILL BE,
WILL WE HAVE ENOUGH MID AND UPPER-LEVEL FORCING FOR ASCENT TO
TRANSLATE MOST (OR ANY?) OF THIS MOISTURE TO THE SURFACE AS
PRECIPITATION? ENSEMBLE GUIDANCE SHOWS A SERIES OF LOW-AMPLITUDE
SHORTWAVE TROUGHS EMBEDDED IN THE SW FLOW REGIME COMBINED WITH WEAK
ISENTROPIC ASCENT/WARM ADVECTION PRIMARILY OVER SE WYOMING, WHICH
WOULD FAVOR ANOTHER ROUND OF SHOWERS AND THUNDERSTORMS FRIDAY
AFTERNOON AND EVENING. ANY SHOWERS AND THUNDERSTORMS WILL BE CAPABLE
OF PRODUCING HEAVY RAINFALL AND EVEN SOME LOCALIZED FLOODING, HENCE
WPC PLACING MUCH OF SE WYOMING AND THE EXTREME W NEBRASKA PANHANDLE
IN A MARGINAL RISK FOR EXCESSIVE RAINFALL GIVEN THE ANOMALOUSLY
MOIST AIR MASS. HIGH RESOLUTION GUIDANCE ALSO HINTS AT SOMEWHAT OF A
STATIONARY FRONT DRAPED ACROSS THE I-80 CORRIDOR LATE FRIDAY
AFTERNOON, WHICH MAY SERVE AS AN ADDITIONAL FOCAL POINT FOR SHOWERS
AND THUNDERSTORMS. NOT EXPECTING STORM MOTION TO BE AN ISSUE IN
REGARDS TO FLOODING, AS CLOUD-LAYER MEAN WINDS ARE PROGGED TO BE 25-
30 KNOTS WHICH WILL KEEP STORMS MOVING FAST ENOUGH. HOWEVER, ANY
BACKBUILDING CONVECTION WILL POSE A FLOODING THREAT, AS CLOUD-LAYER
MEAN WIND VECTORS ARE ORIENTED AT ROUGHLY A 25 TO 45 DEGREE ANGLE
RELATIVE TO THE STATIONARY FRONT (SO NOT PARALLEL, BUT ALSO NOT
PERPENDICULAR TO THE BOUNDARY). WHILE NOT AN IDEAL FLASH FLOODING
SETUP, SOME DEGREE OF BACKBUILDING MAY OCCUR ESPECIALLY IF
CONVECTION REMAINS ANCHORED TO THE AFOREMENTIONED STATIONARY
FRONT/BOUNDARY GIVEN THE VERY MOIST AIRMASS (PWAT 0.9"-1.1") FOR
THIS TIME OF YEAR. HIGHS ON FRIDAY WILL DEPEND ON CLOUD COVER AND
PRECIPITATION COVERAGE. CURRENT LREF GUIDANCE HAS A MEDIAN HIGH
TEMPERATURE FOR CHEYENNE IN THE MID 70S, HOWEVER I WOULDN'T BE
SURPRISED TO SEE THIS TREND COOLER OVER TIME, ESPECIALLY DEPENDING
ON CONVECTIVE DEBRIS AND COVERAGE OF OVERNIGHT SHOWERS/STORMS ON
THURSDAY INTO FRIDAY MORNING, IN ADDITIONAL TO THE EXACT PLACEMENT
OF THE STATIONARY FRONT.
HEADING INTO SATURDAY, ENSEMBLE GUIDANCE HAS BACKED OFF ON SHUNTING
OFF OUR SUPPLY OF MONSOONAL MOISTURE AS THE CLOSED LOW TO OUR
NORTHWEST BEGINS TO LIFT TO THE NORTH AND EAST. WHILE NOT AS
IMPRESSIVE AS THE PREVIOUS COUPLE OF DAYS, PWAT VALUES ARE STILL
EXPECTED TO RANGE BETWEEN THE 85TH AND 95TH PERCENTILES RELATIVE TO
CLIMATOLOGY, KEEPING ADDITIONAL SHOWER AND THUNDERSTORM CHANCES IN
THE FORECAST. THE BULK OF THE FORCING FOR ASCENT LOOKS TO BE FROM
LATE SATURDAY EVENING INTO THE OVERNIGHT HOURS, WITH AN ADDITIONAL
CHANCE FOR SOME MUCH NEEDED RAINFALL. AS IT STANDS, ENSEMBLE
GUIDANCE HAS 55-65% CHANCES OF ALL AREAS EAST OF THE I-25 CORRIDOR
SEEING AT LEAST 0.1" OF RAIN FROM MIDNIGHT TO NOON ON SUNDAY, WITH
THE HIGHEST VALUES ACROSS THE NEBRASKA PANHANDLE. HOWEVER THE SPREAD
REMAINS RATHER LARGE, WITH THE 25TH PERCENTILE QPF BEING ESSENTIALLY
0.01"-0.05", WHICH WOULDN'T EXACTLY DO MUCH TO HELP OUR DROUGHT
CONDITIONS. FORECAST SOUNDINGS ACROSS THE PANHANDLE SHOW SKINNY CAPE
PROFILES THROUGHOUT THE TROPOSPHERE, PWAT VALUES AROUND THE 70TH
PERCENTILE, AND WARM CLOUD LAYERS UP TO ~3 KM, SO ANY SHOWERS AND
STORMS ARE EXPECTED TO BE RATHER EFFICIENT RAINFALL PRODUCERS.
UNSURPRISINGLY, HIGH TEMPERATURES ON SUNDAY WILL BE RATHER COOL
(ALTHOUGH NOT TERRIBLY FAR BELOW AVERAGE) IN THE 60S FOR SE WYOMING,
AND LOW 70S IN THE PANHANDLE.
HEADING INTO THE WORK WEEK, A WARMING TREND IS EXPECTED AS FLOW IS
MORE OUT OF THE WNW, RESULTING IN DRIER CONDITIONS AS ANOTHER RIDGE
BUILDS TO OUR WEST OVER THE GREAT BASIN. EXPECT TEMPERATURES TO BE
AROUND OR SLIGHTLY ABOVE SEASONAL VALUES ON MONDAY AND TUESDAY IN THE
UPPER 60S TO MID 70S (WARMEST TEMPS ACROSS THE PANHANDLE), AND
LITTLE TO NO CHANCES FOR PRECIPITATION.
ON WEDNESDAY, ENSEMBLE GUIDANCE HINTS AT THE RIDGE BUILDING AND
SHIFTING EAST, PLACING SE WYOMING AND W NEBRASKA IN MORE WSW FLOW
ALOFT, ALLOWING FOR SOME RETURN OF MONSOONAL MOISTURE. AT THIS TIME,
ANY MONSOONAL MOISTURE SURGE DOESN'T LOOK NEARLY AS IMPRESSIVE AS
THE ONE IN THE SHORT/MEDIUM RANGE, BUT LOW PRECIPITATION CHANCES LOOK
TO RETURN COME MID NEXT WEEK.
AVIATION /00Z TAFS THROUGH 00Z FRIDAY/
ISSUED AT 520 PM MDT WED SEP 16 2026
A TRICKY FORECAST FOR THIS TAF PERIOD IN REGARDS TO SHOWERS AND
THUNDERSTORMS THAT WILL ROLL THROUGH THE REGION OVER THE NEXT 24
HOURS, IMPACTING ALL THE TERMINALS AT VARYING TIMES. THE MAIN
AVIATION HAZARDS WITH THIS PRECIPITATION WILL BE A REDUCTION OF VIS
AND/OR CIGS DOWN TO MVFR, IN ADDITION THESE STORMS HAVE THE
POTENTIAL TO BRING GUSTY WINDS UPWARDS OF 35 KNOTS. CONFIDENCE IS
HIGH ENOUGH AT THIS TIME BASED OFF OF IN-HOUSE GUIDANCE TO INCLUDE
PROB30 AND TEMPO GROUPS FOR -TSRA AND -SHRA, RESPECTIVELY AT ALL THE
WYOMING TERMINALS ALSO KBFF AND KAIA. THEN BETWEEN 18Z AND 21Z,
CONFIDENCE IS HIGH ENOUGH TO INCLUDE A FM GROUP THAT INCLUDES VIS
REDUCTIONS TO 2SM IN +TSRA FOR ALL THE SITES EXCEPT KSNY.
CYS WATCHES/WARNINGS/ADVISORIES
WY...NONE.
NE...NONE.
SHORT TERM...SF
LONG TERM...NB
AVIATION...RZ
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