textproduct: Cheyenne

KEY MESSAGES

- The next storm system will bring strong winds and another round of mountain snowfall will move in this afternoon into Friday morning.

- Multiple additional rounds of mountain snowfall and strong winds are likely Friday through Monday.

SHORT TERM /THROUGH SATURDAY/

Issued at 324 AM MST Thu Dec 26 2024

Cold front moved through the CWA and is now currently located well east of the Nebraska Panhandle. Satellite imagery showing mostly clear skies over the CYS CWA. Night time fog/low stratus imagery continues to show low clouds/fog over our extreme eastern Nebraska Panhandle zones as well as southern Carbon County. Laramie still reporting some low visibility in very localized fog there in the Laramie Valley. Did add areas of fog in the weather grids to cover these areas.

Further west...next Pacific storm system is moving onshore the Washington State/Oregon coast with dense clouds extending east into the Idaho/Wyoming state line. Craig to Casper 850/700mb height gradients on the increase this morning...up to 45/35mtrs. GFS 700mb winds out across northern Carbon County up near 45kts and we are seeing winds up near Muddy Gap up to 47 mph at 2AM. Halleck Ridge starting to come up near Arlington as well...currently reporting 44mph gusts. So High Wind Warnings are looking good as gradients forecast to continue increasing up to 60/52mtrs by sunrise.

GFS 700-800mb winds really peak out around 18Z today with 50-55kts through that layer from northern Carbon County to western Converse County. One final note on the winds...Bordeaux was upgraded to a warning with Random Forest guidance for Bordeaux up near 45 percent by 18Z. GFS 700mb winds 60kts over Bordeaux with 750mb winds near 60kts as well. Confidence was high enough to upgrade this watch.

Moisture from the approaching Pacific low pressure system begins to impact the Sierra Madre and Snowy Ranges this afternoon. Sierra Madre stands the best chance for the strong upslope snow amounts. Currently have 8-12 inches on west facing slopes of the Sierra Madre Range. GFS and ECMWF also hitting the Snowy Range with fairly good snow amounts of 6 to 9 inches...so decided to add them into a Winter Weather Advisory as well. These amounts match up well with WPC Day 1 snow amounts...so confidence was high for adding the Snowy Range.

Another strong wind/mountain snow event looks to be taking shape Friday evening into Saturday. Craig to Casper 850/700mb gradients up above 60mtrs by 12Z Saturday and climb well above 70mtrs by 18Z Saturday. This will likely be a widespread strong wind event that will impact adjacent zones to the wind prone areas...so anticipate a more widespread strong wind event. Persistent moderate snow accumulations in the Snowy and Sierra Madre Ranges may produce snow amounts well over a foot as well.

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

Issued at 324 AM MST Thu Dec 26 2024

Windy conditions persist into the beginning of the extended forecast as a 250 mb jet sits over the CWA. An exiting upper-level shortwave will allow high pressure to build over western Colorado while a lee- side surface trough remains east of the Laramie Range. A decent pressure gradient will exist along and west of the Laramie Range, leading to the potential for high winds Saturday night and into the day Sunday. 700 mb winds will respond to the gradient, with winds maxing out around 60 kts. Strong subsidence over the wind prones will likely help bring these winds down to the surface. High wind headlines will likely be needed for at least the wind prone areas of southeast Wyoming Saturday night. Wind prone adjacent areas like central Laramie County and east Platte County may also need headlines as 60 kts winds with decent subsidence also exist over these areas. High winds will likely continue into the day Sunday, at least for the wind prones, as an incoming shortwave tracks a surface low across the CWA. As the shortwave moves into the CWA Sunday evening, expect winds to continue overnight.

To go along with the prolonged period of high winds, a prolonged period of mountain snow is also expected. With shortwave after shortwave, a steady stream of moisture will be funneled into Wyoming. Given the nearly zonal flow pattern in the mid-levels, orographic induced snow is expected. With snow expected Saturday night through the day Monday, over a foot of accumulation is possible in both the Sierra Madre and Snowys. Luckily, snowfall rates will be on the lower side, with snow accumulating gradually over time. Will areas outside of the high terrain receive any precipitation? It is possible. The Sunday night into Monday shortwave has the best model agreement for precipitation in the lower elevations. Good mid-level moisture and lift from a cold front could be enough to generate snow across other areas of the CWA. However, QPF amounts are fairly light, so accumulations will likely be minimal at best.

Heading into mid-week, the weather finally begins to quiet down. Some lingering showers will be possible during the day Tuesday as yet another weak disturbance moves into Wyoming. Luckily, winds will be on the lighter side with high winds not expected. Upper-level ridging will begin to build over the west coast on Wednesday, leading to a drier and less breezy day.

AVIATION /12Z TAFS THROUGH 12Z FRIDAY/

Issued at 500 AM MST Thu Dec 26 2024

Lingering fog and low CIGs are present this morning, with BR at KAIA and MVFR CIGs at KRWL. Fog and low CIGs will likely lift by mid- morning. By the time clouds begin to lift, another shortwave will make its way into the area. Some scattered showers will be possible, mainly at southeast Wyoming terminals, during the day today. Low CIGs could once again be possible. Any showers moving over a terminal could have the potential to lower visibility. Gusty winds are also expected with this system. The strongest winds will be at southeast Wyoming terminals with gusts over 30 kts possible. Breezy conditions are also possible at western Nebraska terminals this afternoon with gusts over 20 kts.

CYS WATCHES/WARNINGS/ADVISORIES

WY...High Wind Warning from 9 AM this morning to 8 PM MST this evening for WYZ101. High Wind Warning until 8 PM MST this evening for WYZ104-110. High Wind Warning from 8 AM this morning to 11 PM MST this evening for WYZ106. Winter Weather Advisory from noon today to 5 AM MST Friday for WYZ112-114. NE...None.