010
FXUS65 KCYS 061151
AFDCYS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
551 AM MDT TUE MAY 6 2025
KEY MESSAGES
- TRAVEL IMPACTS ARE EXPECTED IN THE HIGHER ELEVATIONS OF I-80
DUE TO FOG THIS MORNING, FOLLOWED BY A CHANCE FOR SLUSHY SNOW
THIS AFTERNOON AND EVENING.
- A SLOW-MOVING SPRING STORM SYSTEM WILL CONTINUE CHANCES FOR
SHOWERS AND ISOLATED THUNDERSTORMS THROUGH WEDNESDAY.
- A WARMING TREND WILL BEGIN THURSDAY AND LAST THROUGH THE
WEEKEND, BUT CHANCES FOR PM SHOWERS AND STORMS WILL CONTINUE.
SHORT TERM /THROUGH WEDNESDAY NIGHT/
ISSUED AT 415 AM MDT TUE MAY 6 2025
A SPRAWLING UPPER LEVEL LOW MEANDERING ACROSS THE SOUTHWEST WILL
KEEP A CHALLENGING FORECAST IN THE PICTURE FOR THE NEXT FEW DAYS.
CURRENT GOES SATELLITE IMAGERY SHOWS THE UPPER LEVEL CIRCULATION
CENTERED ROUGHLY OVER THE AZ/NM BORDER THIS MORNING, WITH A
ROBUST PLUME OF MOISTURE WRAPPING AROUND ITS EASTERN AND
NORTHERN FLANKS ACROSS THE SOUTHERN PLAINS AND INTO SOUTHEAST
COLORADO. OUR AREA REMAINS ON THE NORTHERN PERIPHERY OF THE
SYSTEM, AND THE FLOW REGIME IS SOMEWHAT DISORGANIZED OVER THE
AREA. RADAR SHOWS A FEW SHOWERS CONTINUING TO MOVE NORTHWARD
ACROSS THE AREA, BUT SO FAR THIS IS JUST DROPPING A FEW
HUNDREDTHS OF AN INCH OF RAIN AT BEST. A SURFACE FRONTAL
BOUNDARY IS EXPECTED TO INCH SOUTHWARD TODAY ACROSS THE AREA,
TURNING THE SURFACE WINDS TO EAST OR NORTHEAST RATHER THAN THE
PERSISTENT SOUTHEAST WE HAVE SEEN THE LAST SEVERAL DAYS. THIS
BOUNDARY IS VISIBLE ON RADAR JUST NORTH OF CHEYENNE AT THIS
HOUR. MOIST LOW-LEVEL UPSLOPE FLOW IS ALREADY LEADING TO A SOLID
DECK OF LOW CLOUDS, WHICH IS INTERSECTING THE TERRAIN ABOVE
ABOUT 7500 FEET IN ELEVATION. FAIRLY DENSE FOG HAS BEEN REPORTED
OVER THE I-80 SUMMIT AND ARLINGTON AREAS OVER THE LAST FEW
HOURS, CONSISTENTLY BELOW ONE QUARTER MILE. AS A RESULT, DECIDED
TO ISSUE A DENSE FOG ADVISORY THROUGH THE MORNING HOURS. WHILE
WE MAY SEE SOME BREAKS THIS AFTERNOON, FOG SHOULD PUSH BACK IN
THIS EVENING AND INTO WEDNESDAY MORNING FOR THE SAME AREAS.
AS THE UPPER LEVEL LOW WRAPS UP TO OUR SOUTH TODAY, WE WILL ENTER
INTO A MORE FAVORABLE PERIOD FOR RAINFALL THIS AFTERNOON AND
EVENING. WINDS WILL SHIFT MORE EASTERLY BEGINNING AT THE SURFACE AND
THEN EXPANDING UPWARDS THROUGH THE DAY UNTIL WE HAVE EASTERLIES
COVERING NEARLY THE ENTIRE TROPOSPHERE BY THE LATE EVENING HOURS.
DEEP MOIST UPSLOPE FLOW AND ISENTROPIC LIFT IN THE LOW TO MID LEVELS
WILL INCREASE THE COVERAGE OF SHOWERS THROUGH THE DAY. THE MAIN
QUESTION REMAINS THE PRESENCE OF CONVECTIVE INSTABILITY, WHICH WILL
LIKELY MAKE THE DIFFERENCE BETWEEN LIGHT RAINFALL TOTALS AND MORE
APPRECIABLE AMOUNTS. FORECAST SOUNDINGS THIS AFTERNOON SHOW SOME
SHALLOW INSTABILITY CAPPED A LITTLE ABOVE 500-MB BY A WARM LAYER
ALOFT. THIS LEADS TO FAIRLY LOW EQUILIBRIUM LEVELS, BUT ISENTROPIC
LIFT SHOULD ALLOW FOR SOME ISOLATED STORMS TO TAP INTO THE UNSTABLE
LAYER AND LOCALLY ENHANCE RAINFALL RATES. OVERALL, RECENT MODEL
GUIDANCE HAS SHIFTED FAIRLY DRAMATICALLY SOUTH WITH THE UPPER LEVEL
LOW TRACK, WHICH PUTS OUR AREA MORE ON THE FRINGE OF THE STRONG
MOISTURE PLUME OUT OF THE SOUTHEAST. HOWEVER, A DISORGANIZED UPPER
LEVEL HIGH TO THE NORTHEAST MAY STILL TIGHTEN THE FLOW ACROSS THE
AREA ENOUGH TO KEEP DECENT MOISTURE ADVECTION IN PLACE DESPITE THE
MORE SOUTHERLY TRACK. WHILE QPF AMOUNTS STILL CAME DOWN, WE'RE NOT
QUITE LOOKING AT A TOTAL BUST JUST YET. THE HIGHEST RAINFALL TOTALS
ARE EXPECTED IN LARAMIE, PLATTE AND GOSHEN COUNTIES, AS WELL AS THE
HIGHER ELEVATIONS AND EASTERN SLOPES IN ALBANY COUNTY. THE
OROGRAPHIC LIFT APPEARS TO BE A NECESSITY TODAY, AND THIS WON'T BE
PARTICULARLY STRONG OVER THE NEBRASKA PANHANDLE. CONVECTIVE
INSTABILITY SHOULD STILL ALLOW FOR SOME AREAS TO GET OVER 0.1" OF
RAINFALL, BUT IT LIKELY WON'T BE THE ENTIRE AREA. THE LAST CONCERN
WITH TODAY'S PRECIPITATION WILL BE THE SNOWFALL POTENTIAL. SNOW
LEVELS WILL BE GENERALLY HOVERING AROUND 7500 TO 8500 FEET, WHICH IS
ABOVE MOST POPULATED AREAS. HOWEVER, THIS PUTS AREAS SUCH AS THE I-
80 SUMMIT RIGHT ON THE EDGE. IF PRECIPITATION RATES END UP ON THE
HEAVIER END OF MODEL GUIDANCE THIS AFTERNOON AND EVENING, IT WOULD
NOT BE SURPRISING TO SEE RAIN IMMEDIATELY CHANGE OVER TO SNOW HERE
AND LEAD TO SEVERAL SLUSHY INCHES OF SNOWFALL. RIGHT NOW, THE MOST
LIKELY SCENARIO IS A SUB-ADVISORY 1-3" NEAR THE I-80 SUMMIT, BUT
THERE IS ABOUT A 20% CHANCE TO REACH CLOSER TO 6". THE OTHER AREA TO
WATCH FOR SNOW WILL BE THE EAST SIDE OF THE SNOWY RANGE. THE DEEP
MOISTURE PLUME SETTING UP SHOULD ALLOW FOR FAVORABLE UPSLOPE FLOW ON
THE EAST SIDE OF THE RANGE, DROPPING DOWN TO ABOUT CENTENNIAL. WHILE
SNOWFALL WON'T BE WIDESPREAD ACROSS THE FORECAST ZONE, THERE IS
POTENTIAL FOR 6 TO 10" OF ACCUMULATION EAST OF THE CREST OF THE
RANGE. TO THE WEST, EXPECT A RAPID DROP OFF IN SNOWFALL. OPEN AREAS
ALONG WY-130 ARE LIKELY TO SEE ACCUMULATING SNOW, BUT THE LOWER
ELEVATIONS OF WY-230 ARE A BIT MORE UNCERTAIN. WITH THE PLENTIFUL
MOIST UPSLOPE FLOW EXPECTED, DECIDED TO ISSUE A WINTER WEATHER
ADVISORY, BUT SPECIFIED THESE IMPACTS ARE ONLY EXPECTED AT HIGH
ELEVATION AND ON THE EASTERN SIDE OF THE RANGE. THE SIERRA MADRE
RANGE IS EXPECTED TO BE SHADOWED BY THE TALLER SNOWY RANGE TO THE
EAST, AND THUS WILL RECEIVE LIGHT, IF ANY, SNOWFALL.
MODEL GUIDANCE IS IN FAIRLY GOOD AGREEMENT SHOWING DRIER AIR BEING
ADVECTED INTO THE AREA OUT OF THE EAST AROUND DAYBREAK WEDNESDAY.
THIS SHOULD BRING AN END TO SHOWER ACTIVITY, BUT DEEP EASTERLY FLOW
WILL CONTINUE JUST WITH A FEW DRY LAYERS IN THE MIDDLE ATMOSPHERE.
THIS SHOULD ALLOW FOR MORE CLEARING WEDNESDAY COMPARED TO TUESDAY
WHICH MAY ALLOW FOR ANOTHER ROUND OF ISOLATED THUNDERSTORMS TO
DEVELOP. COVERAGE SHOULD BE CONSIDERABLY LOWER THAN TUESDAY THOUGH.
EASTERLY FLOW WILL BREAK DOWN WEDNESDAY NIGHT AS THE UPPER LEVEL LOW
WEAKENS AND PUSHES OFF TO THE SOUTHEAST.
LONG TERM /THURSDAY THROUGH MONDAY/
ISSUED AT 415 AM MDT TUE MAY 6 2025
A STRONG RIDGE WILL EXPAND ACROSS THE ROCKIES BEGINNING ON THURSDAY.
EXPECT THIS TO STRENGTHEN AND EXPAND TO COVER THE CENTRAL PLAINS AS
WE HEAD INTO THE WEEKEND. 500-MB HEIGHTS WILL APPROACH THE
CLIMATOLOGICAL 99TH PERCENTILE BY SATURDAY. 700-MB TEMPERATURES WILL
BE ABOUT A DAY BEHIND AS USUAL, BUT SHOULD PUSH THE 90TH PERCENTILE
BY SUNDAY PER LREF MEANS. WHILE TEMPERATURES WILL BE QUITE WARM
DURING THIS PERIOD, WE MAY NOT DRY OUT COMPLETELY AS MOST MODEL
GUIDANCE SHOWS LINGERING MOISTURE PUSHED UP AGAINST THE EASTERN
FLANK OF THE ROCKIES AND SURFACE WINDS FAILING TO TURN WESTERLY.
THIS SHOULD KEEP AT LEAST AFTERNOON SHOWERS AND THUNDERSTORMS IN THE
PICTURE FOR MOST DAYS IN THIS PERIOD.
AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/
ISSUED AT 544 AM MDT TUE MAY 6 2025
A SLOW MOVING SPRING STORM SYSTEM WILL CONTINUE TO REDUCE FLIGHT
CONDITIONS TODAY. WYOMING TERMINALS WILL BE MOST AFFECTED THIS
MORNING, WITH DENSE FOG NEAR KCYS, LOW CIGS AT KLAR, AND A FEW
SNOW SHOWERS PRODUCING IFR VIS AT KRWL. EXPECT SOME IMPROVEMENTS
LATER THIS MORNING INTO THE AFTERNOON HOURS, BUT ON AND OFF
SHOWERS (MIXED WITH SNOW AT KRWL AND KLAR) MAY CAUSE BRIEF
FLUCTUATIONS IN FLIGHT CATEGORY. IN NEBRASKA, MVFR CIGS HAVE
MOVED INTO KCDR AND MAY REACH KAIA FOR A FEW HOURS THIS MORNING.
HEADING INTO THE AFTERNOON, EXPECT AN INCREASE IN SHOWERS WITH
ISOLATED EMBEDDED THUNDERSTORMS MID AFTERNOON THROUGH THE
OVERNIGHT HOURS. BRIEF VIS DROPS ARE EXPECTED IN MODERATE TO
HEAVY RAINFALL. LOW CIGS WILL EXPAND ONCE AGAIN TONIGHT.
CYS WATCHES/WARNINGS/ADVISORIES
WY...DENSE FOG ADVISORY UNTIL NOON MDT TODAY FOR WYZ110-116-117.
WINTER WEATHER ADVISORY UNTIL 6 AM MDT WEDNESDAY FOR WYZ114.
NE...NONE.
SHORT TERM...MN
LONG TERM...MN
AVIATION...MN
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