446
FXUS65 KCYS 151146
AFDCYS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
545 AM MDT MON SEP 15 2025
KEY MESSAGES
- WARMER TEMPERATURES ARE EXPECTED MONDAY WITH A FEW ISOLATED PM
SHOWERS AND THUNDERSTORMS CAPABLE OF PRODUCING GUSTY WINDS.
- ANOTHER COLD FRONT AND POTENT PACIFIC SYSTEM WILL PUSH ACROSS
WYOMING TUESDAY AND WEDNESDAY, RESULTING IN COOLER
TEMPERATURES, THUNDERSTORMS, AND MORE WIDESPREAD RAINFALL
THROUGH THE END OF THE WEEK.
- SLIGHT WARMING TREND POSSIBLE FOR NEXT WEEKEND.
SHORT TERM /THROUGH WEDNESDAY/
ISSUED AT 250 AM MDT MON SEP 15 2025
RELATIVELY QUIET WEATHER THIS MORNING. IR SATELLITE LOOP SHOWS
YESTERDAY'S UPPER LEVEL TROUGH, WHICH BROUGHT LOCALLY HEAVY
RAINFALL AND THUNDERSTORMS TO MUCH OF THE AREA, LIFTING NORTH
ACROSS NORTH DAKOTA AND WEAKENING EARLY THIS MORNING. ALL MODELS
SHOW A SHORTWAVE RIDGE AXIS MOVING INTO THE FORECAST AREA FOR
YOUR MONDAY, RESULTING IN TEMPERATURES RETURNING BACK TO NEAR
NORMAL FOR THIS TIME OF THE YEAR. EXPECT DRIER CONDITIONS TODAY
AS WINDS SHIFT INTO THE WEST OR SOUTHWEST FOR THE DAY WITH WIND
GUSTS UP TO 40 MPH POSSIBLE WEST OF INTERSTATE 25...MAINLY IN
THE WIND PRONE AREAS. MODEL GUIDANCE SHOWS SURFACE WINDS BACKING
INTO THE SOUTH OR SOUTHEAST MID TO LATE AFTERNOON, ALLOWING SOME
MOISTURE ADVECTION INTO THE REGION, BUT THIS WEAK MOISTURE SURGE
IS TRENDING LATER IN THE DAY WITH LITTLE IN THE WAY OF FORCING.
LOWERED POP AND PROB THUNDER ACROSS THE AREA, BUT KEPT ISOLATED
TO WIDELY SCATTERED COVERAGE AT BEST OVER THE HIGH PLAINS. IN
FACT, THE MOST FAVORABLE TIMING FOR THUNDERSTORMS APPEARS TO BE
AROUND OR JUST AFTER SUNSET AS SURFACE WINDS EVENTUALLY SHIFT
INTO THE SOUTHEAST...ALLOWING SOME LOW LEVEL CONVERGENCE ALONG
AND EAST OF THE I-25 CORRIDOR. GUSTY WINDS LOOK LIKE THE MAIN
CONCERN WITH THIS ACTIVITY DUE TO A PRETTY DRY MID TO UPPER
BOUNDARY LAYER. TODAY STILL APPEARS LIKE OUR WARMEST DAY OF THE
WEEK WITH HIGH TEMPERATURES RETURNING TO THE MID 70S TO MID 80S
UNDER A SHORTWAVE RIDGE AXIS. THE NEXT UPPER LEVEL PACIFIC
TROUGH WILL BE MOVING EAST ACROSS IDAHO AND WESTERN WYOMING,
SETTING THE STAGE FOR UNSETTLED WEATHER AND MUCH COOLER
TEMPERATURES FOR MIDWEEK.
FOR TUESDAY AND WEDNESDAY, MODELS ARE COMING INTO MUCH BETTER
AGREEMENT WITH THE NEXT WEATHER EVENT. MODELS SHOW THE UPPER
LEVEL PACIFIC SYSTEM SLOWING DOWN ON TUESDAY AND DIGGING
SOUTHEAST ALONG THE WYOMING/COLORADO BORDER. THE TROUGH AXIS
TAKES ITS TIME DRIFTING EAST...AND IS SHOWING SIGNS OF CLOSING
OFF ACROSS FAR SOUTHERN WYOMING OR NORTHERN COLORADO BY
WEDNESDAY. THIS MAKES SENSE, SINCE THE MIDDLE TO EASTERN PART OF
THE COUNTRY WITH BE UNDER THE INFLUENCE OF AN OMEGA BLOCK WHICH
WILL EVENTUALLY TRANSITION TO A REX BLOCK THROUGH THE WEEK.
DETERMINISTIC MODEL GUIDANCE AND ENSEMBLES HAVE TRENDED WETTER
AND COOLER TUESDAY THROUGH THURSDAY OF THIS WEEK WITH A GOOD
CHANCE OF SOME WIDESPREAD DECENT RAINFALL AMOUNTS (0.50 TO
1.00) THROUGH LATE WEDNESDAY NIGHT. CONTINUED TO POP FOR ALL
FORECAST PERIODS AFTER AM TUESDAY. THE ECMWF IS THE MOST
AGGRESSIVE, SHOWING A 575 CLOSED LOW FORMING ACROSS FAR
NORTHEAST COLORADO ON WEDNESDAY WITH LOTS OF WRAP-AROUND
MOISTURE BACKING INTO MOST OF SOUTHEAST WYOMING. GOOD FORCING
WITH THIS SYSTEM DUE TO A COMBINATION OF LOW TO MIDLEVEL
INSTABILITY (SURFACE BASED AND CONDITIONAL), ACTIVE UPPER LEVEL
JET WITH SEVERAL DISTURBANCE LIFTING NORTH AND EAST ACROSS THE
REGION, AND LOW LEVEL MOISTURE ADVECTION OUT AHEAD OF THE TROUGH
AXIS. WEDNESDAY LOOKS LIKE THE COOLEST DAY WITH HIGHS POSSIBLY
NOT REACHING THE LOW 60S FOR PORTIONS OF SOUTHEAST WYOMING. KEPT
HIGHS CLOSER TO THE NBM 50TH PERCENTILE FOR NOW. ANOTHER ROUND
OF SNOW IS LIKELY ABOVE 9000 FEET AS 700MB TEMPERATURES LOWER TO
1C TO 3C LATE TUESDAY AND THROUGH WEDNESDAY. SNOWLEVELS MAY BE A
LITTLE LOWER THAN THE PREVIOUS SYSTEM, WITH SNOW ACCUMULATIONS
UP TO 5 INCHES DOWN TO AROUND 9500 FEET. CAN'T RULE OUT THE I-80
SUMMIT SEEING THEIR FIRST SNOW FLAKES OF THE SEASON, BUT NO
ACCUMULATION IS EXPECTED AT THIS TIME. HIGH TEMPERATURES ON
WEDNESDAY ARE FORECAST TO BE IN THE UPPER 50S TO UPPER
60S...WITH MID 30S TO MID 40S IN THE MOUNTAINS.
AS FOR THUNDERSTORMS, WILL NEED TO MONITOR TUESDAY FOR STRONG
THUNDERSTORMS WITH MODEL SOUNDINGS SHOWING SBCAPE OF 800 J/KG TO
1400 J/KG...HIGHEST ACROSS WESTERN NEBRASKA AHEAD OF THE MAIN
SYSTEM. PRETTY GOOD SHEAR UP TO 35 KNOTS EXPECTED ACROSS
PORTIONS OF THE AREA WITH SOME RISK FOR HAIL AND STRONG WINDS.
ALTHOUGH SOME THUNDER IS EXPECTED ON WEDNESDAY, IT WILL BE FAR
MORE STABLE AND MUCH COOLER WITH MORE OF A STEADY RAINFALL
EXPECTED.
LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY NIGHT/
ISSUED AT 315 AM MDT MON SEP 15 2025
CHANGES TO THE LONG RANGE FORECAST MAINLY REVOLVE AROUND THE
DIFFERENCES IN THE MIDWEEK SYSTEM WHICH IS FORECAST TO SLOW DOWN
AS IT MOVES ACROSS THE AREA. ALTHOUGH WEDNESDAY WILL BE OUR
COOLEST DAY, THURSDAY WILL NOT BE MUCH BETTER WITH ALL MODELS
AND MOST ENSEMBLE RUNS SHOWING NORTHERLY FLOW ON THE BACKSIDE OF
THE UPPER LEVEL LOW ACROSS WESTERN/CENTRAL NEBRASKA AND SOUTH
DAKOTA. MODELS CONTINUE TO SHOW THE SECONDARY UPPER LEVEL
DISTURBANCE DIGGING SOUTH OUT OF CANADA LATE ON THURSDAY,
BRINGING AN ADDITIONAL SHOT OF COOLER AIR. THEREFORE, KEPT THE
TEMPERATURE FORECAST ON THE COOLER-THAN-AVERAGE SIDE INTO FRIDAY
UNDER NORTH TO NORTHEAST WINDS. ENSEMBLE SPREADS ARE STILL
RELATIVELY HIGH, BUT A LITTLE BETTER THAN THE PREVIOUS FEW RUNS.
LOW TEMPERATURES ARE GOING TO BE TRICKY EACH MORNING SINCE ANY
ADDITIONAL RADIATIONAL COOLING THIS TIME OF THE YEAR WILL BRING
LOWS CLOSE TO FREEZING WITH FROST AND FREEZE CONCERNS. HOWEVER,
MOST MODELS SHOW MOSTLY CLOUDY SKIES UNTIL EARLY FRIDAY MORNING.
FURTHER OUT, MODELS SHOW THE UPPER LEVEL RIDGE AXIS ACROSS THE
FOUR CORNERS REGION TAKES ITS TIME TRANSLATING NORTHEAST INTO
THE FORECAST AREA NEXT WEEK. DECIDED TO KEEP HIGH AND LOW
TEMPERATURES CLOSER TO CLIMO NEXT WEEK AND INTO EARLY NEXT WEEK.
THE GFS IS WARMEST BY A MILE COMPARED TO THE OTHER
MODELS...SHOWING HIGHS IN THE 80S. THE ECMWF, CANADIAN, AND MOST
OF THE ENSEMBLE SPREAD SHOW TEMPERATURES A LITTLE ABOVE AVERAGE
WITH HIGHS IN THE 70S TO LOW 80S, AND LOWS IN THE MID TO UPPER
40S TO NEAR 50. THERE REMAINS LOW CONFIDENCE IN PRECIPITATION
CHANCES SINCE MODELS DISAGREE WITH THE POSITIONING OF THE UPPER
LEVEL JET.
AVIATION 12Z TAFS THROUGH 12Z TUESDAY/
ISSUED AT 545 AM MDT MON SEP 15 2025
DRIER WESTERLY FLOW ALOFT EXPECTED TODAY OVER SOUTHEAST WYOMING AND
WESTERN NEBRASKA WITH A LOWER CHANCE FOR SHOWERS AND THUNDERSTORMS
MONDAY AFTERNOON.
HAZARDS/WEATHER TRENDS: VFR CONDITIONS EXPECTED OVER THE NEXT 24
HOURS. THERE IS A SMALL CHANCE FOR A FEW THUNDERSTORMS AFTER 20Z
MONDAY, BUT COVERAGE IS EXPECTED TO BE LIMITED (~10% TO 20%)...SO
WILL LEAVE OUT ANY MENTION IN THE TAFS FOR NOW. BRIEF GUSTY WINDS
WILL BE THE MAIN CONCERN.
CYS WATCHES/WARNINGS/ADVISORIES
WY...NONE.
NE...NONE.
SHORT TERM...TJT
LONG TERM...TJT/RUBIN
AVIATION...TJT
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