353
FXUS65 KCYS 171205
AFDCYS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
605 AM MDT THU APR 17 2025
KEY MESSAGES
- AN IMPACTFUL WINTER STORM IS EXPECTED TODAY THROUGH FRIDAY.
WIDESPREAD ACCUMULATING SNOW IS LIKELY. WINTER STORM WARNINGS
AND WINTER WEATHER ADVISORIES ARE IN EFFECT FOR PORTIONS OF
SOUTHEAST WYOMING AND WESTERN NEBRASKA.
- REMAINING UNSETTLED DURING THE WEEKEND THROUGH EARLY NEXT WEEK
WITH CHANCES OF SHOWERS EACH DAY.
SHORT TERM /THROUGH FRIDAY NIGHT/
ISSUED AT 350 AM MDT THU APR 17 2025
MOSTLY QUIET TONIGHT ACROSS THE REGION AS A FEW SHOWERS DRIFT FROM
SOUTHWEST TO NORTHEAST ACROSS THE CWA. RAIN SHOWERS ARE ONGOING IN
TORRINGTON AS OF 09Z WITH MOSTLY CLOUDY SKIES ACROSS THE REGION.
TEMPERATURES ARE MILD TONIGHT, IN THE 40S AND 50S, WITH GUERNSEY IN
THE LOW-60S. SHOWERS ARE EXPECTED TO INCREASE IN COVERAGE OVER THE
NEXT FEW HOURS AS THE INCOMING SYSTEM PUSHES INTO THE REGION.
AN UPPER-LEVEL, POSITIVELY TILTED TROUGH IS SLOWLY MOVING TOWARDS
THE REGION TONIGHT. A MODEST JET ALOFT WITH FAVORABLE JET DYNAMICS
IS ENABLING THE ISOLATED TO SCATTERED SHOWERS ACROSS THE CWA THIS
MORNING, ALONG WITH SOME LOBES OF 500MB VORTICITY EJECTING OUT AHEAD
OF THE APPROACHING SYSTEM. LOOKING FURTHER THROUGH THE ATMOSPHERE,
THE APPROACHING SYSTEM IS VERTICALLY STACKED ACROSS THE WESTERN
CONUS, WITH A VERY SLOW-MOVING EVOLUTION EXPECTED WITH THE TROUGH
NOT EXITING THE REGION UNTIL THIS WEEKEND. MULTIPLE LOBES OF 500MB
VORTICITY WILL EJECT OUT AHEAD OF THE MAIN VORTICITY MAXIMUM OVER
NORTHERN UTAH. THESE LOBES WILL FURTHER ENHANCE ONGOING SYNOPTIC
LIFT OVER THE REGION, WITH ADDITIONAL LIFT PROVIDED BY THE
APPROACHING TROUGH. FURTHER DOWN AT 700MB, THE VERTICALLY CONTINUITY
OF THE SYSTEM CONTINUES, WITH A WELL-DEFINED LOW OVER IDAHO THIS
MORNING. THIS LOW WILL ADVECT TO THE SOUTHEAST AND BECOME DISTURBED
BY THE ROCKY MOUNTAINS AND STRUGGLE TO KEEP THE LOW CLOSED AND WELL
DEFINED. WARM AIR ADVECTION AT 700MB WILL COME TO AN END EARLY THIS
MORNING AS THE 700MB FRONT MOVES THROUGH AND 700MB TEMPERATURES
BEGIN TO DROP QUICKLY. EXPECT A FAIRLY EARLY HIGH TEMPERATURE FOR
THE DAY AS MOST MODELS HAVE THE SURFACE COLD FRONT MOVING THROUGH
THE REGION BY 15Z. LOCATIONS NORTH WILL STRUGGLE TO SEE THE 40S
TODAY, DESPITE FURTHER SOUTH LOCATIONS SEEING MID-50S TO LOW-60S.
OVERNIGHT LOWS WILL DROP INTO THE TEENS AND 20S ACROSS THE CWA,
FAVORING FROZEN PRECIPITATION IN THE OVERNIGHT HOURS.
AN IMPORTANT FACTOR TO NOTE WITH THIS SYSTEM IS THAT DESPITE ITS
ORIGIN IN CANADA, IT WILL BE VERY MOIST. WITH THE ORIENTATION OF
THIS TROUGH, THE SOUTHERN PORTION HAS BEEN ADVECTING IN REMNANT
PACIFIC MOISTURE, LEADING TO A MOIST BUT VERY COLD SYSTEM.
PRECIPITATION WATER VALUES ARE IN THE 90TH PERCENTILE ACCORDING TO
THE NAEFS MEAN, LEADING TO INCREASED PRECIPITATION AMOUNTS IN THE
FORECAST. HOWEVER, DESPITE THE COLD NATURE OF THIS SYSTEM THE REGION
HAS STILL BE ABOVE AVERAGE FOR THE PAST SEVERAL DAYS AND THE GROUND
TEMPERATURES ARE VERY WARM. AS A RESULT, THE FIRST INCH OR SO OF
FALLING SNOW WILL LIKELY MELT UPON CONTACT WITH THE GROUND.
THEREFORE, TOTAL SNOW FALL WILL LIKELY BE LARGER THAN THE
ACCUMULATED SNOW FALL FOR THIS EVENT. THE SAVING GRACE, HOWEVER, IS
THAT MOST OF THE PRECIPITATION IS EXPECTED TO FALL IN THE OVERNIGHT
HOURS WHEN TEMPERATURES ARE MUCH LOWER THAN THE DAYTIME AND SUN
ANGLES ARE NOT PLAYING A CRITICAL ROLE. THESE CONFLICTING FACTORS
MAKES THE SNOW FALL FORECAST QUITE DIFFICULT. IN ADDITION, THE
NORTHERLY WINDS ACROSS THE AREA WILL FAVOR THE PINE RIDGE AND
CHEYENNE RIDGE, AS WELL AS THE NORTH LARAMIE RANGE. HOWEVER, BOTH
THE PINE RIDGE AND CHEYENNE RIDGE HAVE BEEN VERY WARM THE PAST
SEVERAL DAYS. THE MAIN QUESTION IS HOW LONG WILL IT TAKE FOR GROUND
TEMPERATURES TO COOL ENOUGH TO ALLOW FOR THE ACCUMULATION OF SNOW.
THE MOUNTAINS LOOK TO DO REALLY WELL WITH THIS EVENT, ESPECIALLY THE
NORTH LARAMIE RANGE WITH PROLONGED, MOIST, UPSLOPE FLOW. NORTHERN
CARBON COUNTY SHOULD SEE SIGNIFICANT SNOW FALL AS WELL, WITH THE
FAVORABLE WIND DIRECTION AND AMPLE MOISTURE. AS A RESULT, UPGRADED
THE MAJORITY OF ZONES WEST OF THE I-25 CORRIDOR TO WINTER STORM
WARNINGS AND UPGRADED TO LARAMIE VALLEY AND SARATOGA VALLEY TO
WINTER WEATHER ADVISORIES. MORE UNCERTAINTY COMES INTO PLAY EAST OF
THE I-25. DID DECIDE TO UPGRADE THESE ZONES TO A WINTER WEATHER
ADVISORY AND ADD THE NORTHERN PANHANDLE WHERE THE PINE RIDGE LIES.
IF SNOW BEGINS TO ACCUMULATE QUICKER THAN CURRENTLY EXPECTED, SOME
HEADLINE CHANGES MAY BE NEEDED.
FRIDAY WILL BE VERY COLD BEHIND THE SYSTEM, WITH MOST LOCATIONS
STRUGGLING TO REACH THE 30S AND 40S. SNOW IS EXPECTED TO SLOWLY
TAPER OFF THROUGHOUT THE DAY FRIDAY, WITH MINIMAL PRECIPITATION
EXPECTED BY SATURDAY MORNING. SATURDAY MORNING LOWS WILL BE VERY
COLD, IN THE TEENS AND LOW-20S FOR MOST OF THE REGION. IT WILL BE A
STARK CHANGE AS A FEW WEEKS OF SPRING. HOWEVER, THE MOISTURE
ASSOCIATED WITH THIS SYSTEM WILL BE BENEFICIAL FOR BOTH MOUNTAIN
SNOW PACK AND MAYBE PUTTING A SMALL DENT INTO THE DROUGHT.
LONG TERM /SATURDAY THROUGH WEDNESDAY/
ISSUED AT 350 AM MDT THU APR 17 2025
DETERMINISTIC MODELS AND ENSEMBLES IN REASONABLE AGREEMENT THIS
WEEKEND AND NEXT WEEK...SHOWING AN UNSETTLED WEATHER PATTERN
THROUGH THE EXTENDED. THANKFULLY, IT IS EXPECTED TO BE MORE
SPRING-LIKE WITH MILDER TEMPERATURES BUT WITH A DAILY CHANCE OF
SHOWERS AND SOME THUNDERSTORMS.
FOR THE WEEKEND, ALL MODELS INDICATE THE BEGINNING OF A LONG WAVE
PATTERN SHIFT AS A BROAD UPPER LEVEL TROUGH REPLACES THE GENERAL
MID TO UPPER LEVEL RIDGING THAT HAS PERSISTED ACROSS THE WESTERN
UNITED STATES OVER THE LAST 4 TO 6 WEEKS. STILL REMAINING ON THE
COOL SIDE THIS WEEKEND, ALTHOUGH MODELS SHOW A SLOW WARMING TREND
HEADING INTO SUNDAY. NORTHERLY FLOW ALOFT WILL REMAIN OVER THE
AREA SATURDAY AND SATURDAY NIGHT WITH THE UPPER LEVEL FLOW BACKING
INTO THE WEST ON SUNDAY. REMAINING CHILLY ON SATURDAY WITH HIGHS
STRUGGLING TO REACH 40 DEGREES FOR MOST OF SOUTHEAST WYOMING,
AND 40S TO NEAR 50 BELOW 4500 FEET. OFF/ON RAIN AND SNOW SHOWERS
ARE POSSIBLE SATURDAY, BUT NOT EXPECTING ANY ADDITIONAL SNOW
ACCUMULATION OUTSIDE OF THE MOUNTAINS. SUNDAY LOOKS SOMEWHAT
DRIER WITH MODERATE DOWNSLOPE WESTERLY WIND EXPECTED. IT WILL
ALSO BE MILDER ON SUNDAY WITH TEMPERATURES NEAR NORMAL FOR MID
APRIL (50S TO LOW 60S).
NEXT WEEK LOOKS MOSTLY UNSETTLED AS A LONG WAVE TROUGH SETTLES
OVER THE EASTERN PACIFIC AND WESTERN THIRD OF THE UNITED STATES.
THIS WILL RESULT IN A PROGRESSIVE STORM TRACK ACROSS WYOMING
AND THE ADJACENT HIGH PLAINS OF COLORADO AND WESTERN NEBRASKA.
ALTHOUGH MODELS ARE IN VERY GOOD AGREEMENT WITH THE OVERALL
PATTERN, THERE ARE SIGNIFICANT TIMING AND PLACEMENT DIFFERENCES
BETWEEN INDIVIDUAL MODELS RELATING TO SHORTWAVE DISTURBANCES
AND FRONTAL PLACEMENT. THESE DIFFERENCES DO SHOW UP IN THE
LATEST ENSEMBLE GUIDANCE WITH RELATIVELY HIGH SPREADS IN SURFACE
MOISTURE/PWAT, CLOUD COVER, AND PRECIPITATION CHANCES. THE MORE
POTENT DISTURBANCES ARE FORECAST TO MOVE OVER THE FORECAST AREA
MONDAY NIGHT, WEDNESDAY, AND AGAIN ON THURSDAY AS A TRAIN OF
PACIFIC STORM SYSTEMS MOVE ONSHORE BETWEEN OREGON AND CENTRAL
CALIFORNIA. THE MOST NOTABLE WEATHER EVENT MAY BE TUESDAY NIGHT
AND WEDNESDAY ACROSS SOUTHEAST WYOMING, WITH THE GFS THE MOST
AGGRESSIVE MODEL...SHOWING A STATIONARY FRONT ALONG THE
MOUNTAINS AND I-80 CORRIDOR AS AN UPPER LEVEL DISTURBANCE MOVES
PARALLEL TO THE SURFACE FRONT. GFS SUGGESTS THAT THIS FRONT AND
A MIDLEVEL UPPER LEVEL DISTURBANCE/COLD POOL WILL BE THE FOCUS
FOR NUMEROUS TO WIDESPREAD SHOWERS AND THUNDERSTORMS THROUGH
LATE WEDNESDAY BEFORE THE FRONT DISSIPATES. 00Z GFS SHOWING SOME
PRETTY HIGH QPF TOTALS WITH MOSTLY RAINFALL BELOW 7000 FEET,
BUT SOME SOLID SNOWFALL TOTALS FOR THE MOUNTAINS. HOWEVER, THE
OTHER MODELS ARE NOT AS AGGRESSIVE AND SHOW THE FRONT
DISSIPATING MUCH SOONER COMPARED TO THE GFS. INCREASED POP OVER
THE NBM, BUT DID NOT GO AS HIGH AS 'LIKELY' QUITE YET DUE TO THE
UNCERTAINTY. OTHERWISE, WE ARE EXPECTING A SLOW WARMING TREND
NEXT WEEK WITH NEAR OR SLIGHTLY ABOVE AVERAGE TEMPERATURES IN
THE MID 50S TO MID 60S. WILL NEED TO MONITOR THE POTENTIAL FOR
STRONG THUNDERSTORMS SINCE MODELS AND ENSEMBLE GUIDANCE ARE
SHOWING A GRADUAL INCREASE IN LOW LEVEL DEWPOINTS UNDER EASTERLY
UPSLOPE FLOW DURING THE WEEK.
AVIATION /12Z TAFS THROUGH 12Z FRIDAY/
ISSUED AT 610 AM MDT THU APR 17 2025
STRONG COLD FRONT FOR THIS TIME OF THE YEAR WILL PLUNGE SOUTHWARD
ACROSS SOUTHEAST WYOMING AND WESTERN NEBRASKA TODAY. SCATTERED RAIN
SHOWERS AND ISOLATED THUNDERSTORMS ARE POSSIBLE THIS MORNING THROUGH
THE MIDDLE OF THIS AFTERNOON AS THE FRONT HEADS TOWARDS THE I-80
CORRIDOR. RAIN IS FORECAST TO QUICKLY CHANGE TO SNOW THIS EVENING
AND OVERNIGHT TONIGHT.
HAZARDS/WEATHER TRENDS: VFR CONDITIONS EXPECTED UNTIL 18Z TO 21Z
TODAY WITH OCCASIONAL SHOWERS. CONDITIONS WILL GRADUALLY DETERIORATE
TO MVFR LATER THIS AFTERNOON...AND THEN TO IFR/LIFR BETWEEN 23Z
TODAY AND 05Z FRIDAY. THERE IS A GOOD CHANCE FOR THUNDERSTORMS FOR
KLAR, KCYS, AND POSSIBLY KAIA AND KBFF LATE THIS MORNING AND THIS
AFTERNOON, SO ADDED PROB30 GROUPS TO THE MORE LIKELY TERMINALS. RAIN
WILL CHANGE TO SNOW BETWEEN 21Z AND 00Z THIS AFTERNOON FOR THE
SOUTHEAST WYOMING TERMINALS, AND BETWEEN 00Z AND 06Z THIS EVENING
FOR WESTERN NEBRASKA.
CYS WATCHES/WARNINGS/ADVISORIES
WY...WINTER STORM WARNING FROM NOON TODAY TO NOON MDT FRIDAY FOR
WYZ101-105.
WINTER STORM WARNING FROM 3 PM THIS AFTERNOON TO NOON MDT
FRIDAY FOR WYZ102-109.
WINTER STORM WARNING UNTIL NOON MDT FRIDAY FOR WYZ103-112-114.
WINTER STORM WARNING UNTIL 6 AM MDT FRIDAY FOR WYZ104.
WINTER STORM WARNING FROM 6 PM THIS EVENING TO 6 PM MDT FRIDAY
FOR WYZ106.
WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO 6 PM MDT
FRIDAY FOR WYZ107-108-113-115.
WINTER STORM WARNING FROM 3 PM THIS AFTERNOON TO 6 PM MDT
FRIDAY FOR WYZ110.
WINTER STORM WARNING FROM 9 AM THIS MORNING TO NOON MDT FRIDAY
FOR WYZ116.
WINTER STORM WARNING FROM 9 PM THIS EVENING TO 6 PM MDT FRIDAY
FOR WYZ117.
WINTER WEATHER ADVISORY FROM 9 PM THIS EVENING TO 6 PM MDT
FRIDAY FOR WYZ118-119.
NE...WINTER WEATHER ADVISORY FROM 3 PM THIS AFTERNOON TO NOON MDT
FRIDAY FOR NEZ002-095.
SHORT TERM...AM
LONG TERM...TJT
AVIATION...TJT
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