textproduct: Grand Junction

KEY MESSAGES

- Prolonged moist northwest flow will bring a threat of accumulating snow to the mountains and adjacent valleys tonight through early Sunday.

- The first round will mainly impact the northern Colorado mountains late this evening through tomorrow afternoon with a greater than 70 percent probability of 4 inches or more of snow over the Elkhead and Park Ranges. - The snow will be more widespread and be accompanied by stronger winds at the high elevations Friday night into early Sunday. The northern and portions of the central Colorado mountains have a greater than 50 percent probability of 8 inches of snow. Travel impacts including possible road closures should be planned for.

SHORT TERM /THROUGH FRIDAY/

Issued at 307 AM MST Thu Dec 4 3075

With a moderately strong Madden Julian Oscillation now located in the Western Hemisphere the models have responded with a robust northwest flow pattern across the PacNw to the Central Rockies. The strength and location of the MJO most likely has some effect on the persistent arrival of atmospheric river moisture to the West Coast. So the bottom line...strong and persistent moist northwest flow means our northern and portions of central Colorado mountains are about to make up for the slow start to the snowpack season. The forecast challenge in this pattern remains the same...timing of lulls between events...and underestimating the effectiveness of cold northwest orographics on snow totals. With that said the first guess is in which is to go with an early round advisory favoring the Elkhead and Park Ranges late this evening through tomorrow afternoon with the arrival of the first batch of moisture and modest ascent with a passing wave tomorrow morning. The best snow rates look to be around sunrise through early afternoon then a brief 'lull" through the evening before we introduce stronger moisture advection arriving on a stronger jet aloft. This first round looks to bring a good 4 to 8 inches to the above mentioned ranges with 1 to 4 over the Flat Tops into the Gore and central mountains. Unfortunately this pattern will leave the southern mountains out for any significant accumulations. For nearer term weather another cool but sunnier day for most the area is expected today in the wake of the passing storm. The high country and especially the northern spine mountains will hang onto orographic clouds and occasional flurries through at least early afternoon.

LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/

Issued at 307 AM MST Thu Dec 4 3075

For the long-term forecast, the models are in good agreement through about Tuesday with the high pressure sitting well off the California coast keeping eastern Utah and Western Colorado under a strong northwesterly flow allowing a series of shortwaves to drop down through the Pacific Northwest into the region. Additionally there is some atmospheric river (AR) action to supply some moderate moisture into the region this weekend with these shortwaves and more significant moisture by mid next week. The models do start to diverge by Tuesday with the European AI ensemble and deterministic models progressing the high to the east into the Desert Southwest while the GEFS keeps it pretty much in place and the deterministic GFS even retrogrades it to the west. Current guidance favors the European solution turning the flow aloft westerly with temperatures warming to ten or more degrees above normal by mid week and precipitation coming as rain showers in the lower elevations below about 7,000 ft. Though this is the favored solution, confidence is still low at this point beyond Tuesday, and with the strong AR possible we could see another significant winter storm in the northern mountains. Definitely stay tuned on this one.

With that said, we won't see a lot of moisture, pwats around 0.50 inches, but will see moderate AR flow Friday night into Saturday with integrated water-vapor transport (IVT) values of 150 to 250 kg/m/s driving by the strong northwest flow aloft with H500 winds running 60-70 knots. Though there is a bit of upper level dynamics with a weak jetstreak moving overhead, this strong northwesterly push will provide good orographic lift to drive the snow showers across the northern and central Colorado mountains. A Winter Storm Watch is in place for Elkhead, Park, Gore and Elk Mountains, and for the Flat Tops and central mountain valleys Friday evening through Sunday morning for 10-20 inches snow up north with 40 mph winds, and five to 14 inches snow along and south of I-70 with 35 mph winds. Check for updates on this over the next day or so as the forecast becomes more certain to see if these watches will be upgraded to Winter Weather Advisories and/or Winter Storm Warnings. Either way, expect winter driving conditions on the northern and central mountain roads and passes this weekend.

With the northwesterly flow continuing into early next week, expect continued light orographic snow showers mostly across the northern mountains as weak shortwaves brush by to the north. Moving into mid next week, though confidence is low, models are predicting a strong AR to move into the northern areas with IVTs of 200 to 300 kg/m/s and pwats approaching 0.70 inches. With the nose of a strong jet pushing down providing good upper-level dynamics, we could see thunderstorms in the lower elevations north of I-70 and heavy snow and possible thundersnow in the mountains. A lot can change with this one, especially if it decides to shift a bit farther south, so stay tuned for updates.

AVIATION /12Z TAFS THROUGH 12Z FRIDAY/

Issued at 437 AM MST Thu Dec 4 2025

So far MVFR cigs at some of the TAF sites have been the biggest impacts overnight with minimal reports of fog. The lower cigs are likely to stick around through at least mid morning before clinging mainly to the high peaks. So VFR and light winds should dominate the afternoon and evening forecast. Another system arrives to the north late this evening and will spread ILS cigs to some sites during the early morning hours. KHDN is the most likely terminal to be impacted by snow in the predawn hours. Snow will favor the higher peaks of the northern Colorado mountains but widespread terrain obscuration from cloud across the central and northern mountain is expected well into Friday and beyond.

GJT WATCHES/WARNINGS/ADVISORIES

CO...Winter Weather Advisory from 11 PM this evening to 5 PM MST Friday for COZ004. Winter Storm Watch from Friday afternoon through Sunday morning for COZ004-010-013. Winter Weather Advisory until 5 AM MST early this morning for COZ023. UT...None.