488
FXUS65 KCYS 172126
AFDCYS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
226 PM MST TUE DEC 17 2024
KEY MESSAGES
- A WINTER STORM WARNING IS IN EFFECT UNTIL 8 PM TONIGHT FOR THE
SNOWY RANGE, WITH A WINTER WEATHER ADVISORY UNTIL 8 PM TONIGHT
FOR THE SIERRA MADRE RANGE.
- ANOTHER HIGH WIND EVENT IS LIKELY FOR LATE TUESDAY NIGHT
THROUGH LATE WEDNESDAY NIGHT FOR ARLINGTON, BORDEAUX, THE I-80
SUMMIT AND FOOTHILLS AND FOR NORTHERN CARBON COUNTY.
- RELATIVELY MILD AND DRY WEATHER WILL CONTINUE FOR THURSDAY
THROUGH THE WEEKEND.
SHORT TERM /THROUGH WEDNESDAY NIGHT/
ISSUED AT 121 PM MST TUE DEC 17 2024
A PROGRESSIVE PATTERN CHARACTERIZED BY FAST WESTERLY FLOW
REMAINS IN PLACE OVER SOUTHEAST WYOMING AND WESTERN NEBRASKA
THIS TUESDAY AFTERNOON. EMBEDDED WITHIN THIS WESTERLY FLOW ARE
SEVERAL WEAK SHORTWAVE TROUGHS. ONE OF THESE FEATURES IS
CURRENTLY PASSING THROUGH EASTERN WYOMING, BRINGING WITH IT
LIGHT TO MODERATE SNOW IN THE HIGHER TERRAIN AND A FEW BANDED
SNOW SHOWERS IN THE LOWER ELEVATIONS. HIGH CLOUDS HAVE THINNED
SOMEWHAT THIS AFTERNOON, ALLOWING FOR SOME WEAK INSTABILITY IN
THE LOWER MID LEVELS. 100-300 JKG OF CAPE MAY BE ENOUGH TO FORCE
SOME CONVECTIVELY DRIVEN SNOW SHOWERS THIS AFTERNOON BEFORE
SURFACE HEATING IS LOST IN A FEW HOURS. THE NET EFFECT OF THIS
WOULD BE A FEW SNOW SHOWERS THAT MAY COAT THE GROUND IN PLACES
NEAR AND NORTH OF CHEYENNE UP TOWARD CHUGWATER AND WHEATLAND, AS
WELL AS IN PORTIONS OF THE LARAMIE BASIN GENERALLY NORTH OF THE
TOWN OF LARAMIE ITSELF. ALL PRECIPITATION SHOULD QUICKLY COME TO
AN END AFTER 8PM AS THIS SYSTEM MOVES EAST INTO THE CENTRAL
PLAINS LEAVING GENERALLY CLEAR SKIES AND A SEASONABLY CRISP
OVERNIGHT.
A VERY FAST TRANSITION FROM THE DEPARTING SHORTWAVE TROUGH TO
WEAK RIDGING EMBEDDED UNDERNEATH THE FAST WESTERLY JET WILL
CREATE A NOTABLY TIGHT PRESSURE GRADIENT OVER THE MOUNTAINS OF
SOUTHEAST WYOMING LATE TONIGHT THROUGH WEDNESDAY AND WEDNESDAY
NIGHT. THIS STRONG GRADIENT BETWEEN TROUGH AND WEAK RIDGE WILL
RESULT IN ANOTHER HIGH WIND EVENT FOR THE TYPICAL WIND-PRONE
LOCATIONS OF SOUTHEAST WYOMING. LOCAL IN-HOUSE MACHINE LEARNING
GUIDANCE IS SHOWING VERY HIGH CONFIDENCE OF EXCEEDING 60 MPH
WIND GUSTS WITH THIS EVENT IN THE SNOWY RANGE FOOTHILLS ALONG
I-80, ALONG I-25 BETWEEN CHUGWATER AND WHEATLAND, AND IN
NORTHERN CARBON COUNTY IN THE GAP FLOW REGIONS BETWEEN THE
SHIRLEY AND FERRIS HILLS. GIVEN THE HIGH-CONFIDENCE NATURE OF
THIS EVENT BOTH WITH THIS GUIDANCE AND SEEN IN DETERMINISTIC
HIGH-RES MODELS, WE HAVE UPGRADED THE HIGH WIND WATCH TO A HIGH
WIND WARNING FOR THIS UPCOMING EVENT. OUTSIDE OF THE HIGH WIND
THREAT, EXPECT MILD TEMPERATURES FOR MID-DECEMBER THANKS TO
RIDGING TO THE WEST AND A SLIGHT DOWNSLOPE EFFECT OFF THE HIGHER
TERRAIN. AFTERNOON HIGHS ON THURSDAY WILL RUN NEAR 15 DEGREES
ABOVE AVERAGE WITH VERY DRY WEATHER ACROSS THE REGION. FORECAST
CONFIDENCE IS HIGH THROUGHOUT THE SHORT TERM WITH TEMPERATURES
AND WIND FORECASTS ADJUSTED UPWARD TOWARD THE 90TH PERCENTILE OF
GUIDANCE BLENDS.
LONG TERM /THURSDAY THROUGH MONDAY/
ISSUED AT 121 PM MST TUE DEC 17 2024
THURSDAY MORNING THERE IS A SMALL MID LEVEL (700MB) SHORTWAVE
THAT MOVES THROUGH THE FORECAST AREA THROUGH THE LATE AFTERNOON.
THE WINDS WILL BE PICK UP IN THE MORNING BUT START TO DIE DOWN
IN THE AFTERNOON AS THE SHORTWAVE DIVES SOUTHEAST. LOOKING AT
THE GFS OMEGA FIELD THERE ISNT ANY LIFT ASSOCIATED WITH THIS
PASSING WAVE SO NO PRECIPITATION IS EXPECTED FROM THIS WAVE. AS
THE PACIFIC RIDGE STARTS TO MOVE EAST INTO OUR REGION OUR WINDS
SHOULD BE RELATIVELY CALMER AND WARMER TEMPERATURES THROUGH THE
DAY. LOOKING AT CLUSTER ANALYSIS THERE IS SOME UNCERTAINTY IN
THE AMPLIFICATION OF THE RIDGE AS IT MOVES OVER US. THIS
UNCERTAINTY IS MINOR BECAUSE TEMPERATURES WILL STILL BE ABOVE
AVERAGE DURING THE DAY AND SLIGHTLY ABOVE AVERAGE AT NIGHT. THE
GEFS MEAN TEMP AND M-CLIMATE AND THE NAEFS STD ANOMALY AGREE
WITH THIS IDEA LEADING TO HIGHER CONFIDENCE INTO THE TEMPERATURE
FORECAST.
THIS WEEKEND AND BEYOND... THE SYNOPTIC RIDGE/TROUGH PATTERN ALOFT
IS RATHER PROGRESSIVE WITH A FEW PASSAGES THROUGH MIDWEEK. THE ONE
RIDGE AXIS BISECTS THE FORECAST AREA LATE SATURDAY AND IS QUICKLY
FOLLOWED BY A TROUGH ON SUNDAY. FLOW BECOMES MORE QUASI-ZONAL TO
START THE WORKWEEK BEFORE THE NEXT RIDGE AMPLIFIES LATER TUESDAY.
TOWARD THE SURFACE, A COUPLE RELATIVELY WEAKER FRONTAL PASSAGES LINE
UP WITH THE UPPER TROUGH. DESPITE THE ACTIVITY, MOISTURE IS HARD TO
COME BY. REALLY THE ONLY PRECIPITATION POTENTIAL EXISTS MONDAY IN
THE FORM OF MOUNTAIN SNOW. OVERALL, CONDITIONS CONTINUE TO TREND
MUCH MILDER THAN NORMAL WITH HIGHS IN THE 40S WEST OF THE DIVIDE AND
50S TO THE EAST. THE WARMEST GUIDANCE AS SHOWN BY THE UPPER QUARTILE
IN THE NBM ENVELOPE EVEN BRUSHES 60F IN THE SOUTHERN NEB PANHANDLE.
ECM EFI CONSISTENTLY STAYS AROUND 0.6-0.7/1 FOR MAX TEMPS THIS
WEEKEND, WHILE NAEFS CLIMO SUGGESTS H5-7 TEMPS AROUND THE 90%ILE.
ALSO, MINIMUM HUMIDITY VALUES APPROACH CRITICAL THRESHOLDS NEAR AND
EAST OF THE CHEYENNE RIDGE WITH THE INFLUENCE OF DOWNSLOPE WESTERLY
WINDS THIS WEEKEND. AS FOR WIND, WILL NEED TO MONITOR THE TYPICAL
SPOTS AROUND ARLINGTON AND BORDEAUX FOR HEADLINES, PRIMARILY SUNDAY.
THE PATTERN STILL SUGGESTS LOW POTENTIAL OF A WHITE CHRISTMAS FOR
THE MAJORITY OF THE AREA, BUT A WAVE OF MOISTURE MAY BRING MOUNTAIN
SNOW SHOWERS ON CHRISTMAS EVE. A CLOSED UPPER LOW CROSSES THE
SOUTHERN ROCKIES RIGHT AFTER THE HOLIDAY AS DEPICTED IN THE LONG
RANGE GUIDANCE. IF A NORTHWARD SHIFT IN THE TRACK IS NOTICED, SNOW
CHANCES MAY INCREASE FOR SOUTHEAST WYO AND NEB PANHANDLE.
AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/
ISSUED AT 1046 AM MST TUE DEC 17 2024
A FRONTAL PASSAGE WILL LIKELY PRODUCE SOME BRIEF AND LIGHT SNOW
SHOWERS FOR KLAR AND KCYS STARTING AROUND 19Z AND ENDING BY 23Z. THE
WESTERLY WINDS ADDS SOME UNCERTAINTY TO THESE SHOWERS AS SOME
DOWNSLOPING MAY PREVENT SNOW FROM REACHING THE GROUND IN OUR LOWER
ELEVATIONS. AROUND 08Z ANOTHER WIND PUSH (GUST UP TO 30KTS) WILL
START AT RAWLINS AND WORK ITS WAY EAST.
FOR THE NEBRASKA TERMINALS, VFR WILL LIKELY CONTINUE THROUGH THE
FORECAST PERIOD, EXCEPT FOR THE NORTHERN PANHANDLE (CDR). LOW
STRATUS HAS SETTLED IN NEAR THE PINE RIDGE, BUT SHOULD LIFT LATER
THIS AFTERNOON. OTHERWISE, THE PRIMARY AVIATION WEATHER CONCERN WILL
BE WIND. NORTHWEST WINDS WILL CONTINUE TO GUST THROUGH EARLY
EVENING, ESPECIALLY IN THE CENTRAL AND SOUTHERN PANHANDLE (AIA-SNY),
WHERE GUSTS EXCEED 25 KTS. SURFACE WINDS SHOULD TAPER OVERNIGHT, BUT
LOW LEVEL WIND SHEAR CONDITIONS ARE POSSIBLE AT BFF. GENERALLY
SOUTHWEST WINDS STRENGTHEN MIDDAY WITH THE HIGHER GUSTS OCCURRING
AFTER 18/18Z.
CYS WATCHES/WARNINGS/ADVISORIES
WY...HIGH WIND WARNING FROM 2 AM WEDNESDAY TO 5 AM MST THURSDAY FOR
WYZ104-106-110-116-117.
WINTER WEATHER ADVISORY UNTIL 8 PM MST THIS EVENING FOR WYZ112.
WINTER STORM WARNING UNTIL 8 PM MST THIS EVENING FOR WYZ114.
NE...NONE.
SHORT TERM...MAC
LONG TERM...DS/MM
AVIATION...DS/MM
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page