Marine Weather and Tide Forecast for Brighton, CO

UPDATE
Issued at 839 PM MST Thu Jan 23 2025

Cross-sections show potential for a mtn wave overnight into Fri morning with cross barrier flow from 35-45 kts. Thus will see gusty winds develop over the higher terrain after midnight which will linger thru Fri morning. The winds may mix down to the base of the foothills as well, so have added areas of blowing snow in the normal windy spots, which may affect HWY 93 late tonight thru mid morning on Fri. Gusts from 55 to 65 mph will be possible, at times, if the winds do mix down to the base of the foothills.

SHORT TERM /Through Friday/
Issued at 200 PM MST Thu Jan 23 2025

It's a pretty albeit chilly afternoon as temperatures hover within a few degrees of the freezing mark across our lower elevations. In the eastern plains, there's enough elevated moisture and marginal instability for a few convective high-based snow showers, as evidenced by the photogenic cumulus striations on satellite.
However, with dewpoints holding in single digits to low teens, conditions remain too dry to support more than 15% PoPs for a few hours this afternoon.

As we transition into the evening and overnight hours, winds aloft will begin their shift to a northwesterly component ahead of a developing trough over the northern Rockies. Meanwhile, surface pressure gradients will tighten considerably, helping drive enhanced cross-barrier flow of around 40 kts over the Front Range mountains.
A mountaintop critical layer and favorable shear profile should support modest mountain wave development, strongest between midnight tonight and mid-morning Friday, although the prevailing NW/WNW flow isn't optimal for significant amplification. Peak gusts of 60-70 mph look quite achievable from our central foothills, although isolated higher gusts will be possible, particularly in the most wind-prone locations of Boulder County. The winds will help taper overnight lows in the lower foothills and adjacent areas, but overall it'll be another cold night with lows mostly in the single digits and teens for the plains, and into the negatives for the mountain valleys and sky cover clears out.

Daytime conditions on Friday will be milder, with 700mb temperatures rising some 10 degrees C and compressional warming further boosting temperatures in the lee of the Front Range, so highs should make it into the low to mid 40's for areas near the base of the foothills and roughly south of I-76. In the high country, mid-level moisture will increase through the afternoon, with favorable orographics supporting development of snow showers around mid-afternoon in the northern mountains. A Winter Weather Advisory will go into effect at 5pm for our mountain zones - see the discussion below for more details on the incoming snow. For the lowlands, despite a cold front sweeping south through the afternoon, low-level moisture will be lagging behind, so no precipitation is expected until later in the evening or overnight.

LONG TERM /Friday night through Thursday/
Issued at 230 PM MST Thu Jan 23 2025

Main concern for this forecast period will be the snow and renewed cold starting Friday night and continuing through Saturday, and staying as late as late Saturday night/early Sunday morning in the mountains.

The northern mountains and foothills are getting themselves into a more favorable pattern for moderate snow accumulations. A couple things going for this will be persistent frontogenesis straddling the Wyoming/Colorado border, weak Q-G lift, and favorable residence in the right rear entrance region of the upper level jet. This will also be a prolonged event due to these forcing mechanisms staying linked up for a good 18-24 hours, so while overall moisture amounts are rather limited (700-500 mb specific humidity <1.5 g/kg), accumulations should mount up over an extended period.
Furthermore, we'll have a weak upslope component on this side of the Divide, so northern foothills and at least the western-most communities of the I-25 Corridor near the foothills should do reasonably well. A deep DGZ (dendritic growth zone) of up to 7,000 feet is expected initially, which should bring 20:1 snow to liquid ratios (SLR). However, those will likely drop off later in the event toward late morning/mid day Saturday as we lose the DGZ in the shallow upslope layer, and the DGZ total depth drops closer to 2,000 feet.

With all things considered, initial indications would be for 6-12" of snow in the northern mountains, 4-9" amounts in the foothills of Boulder and Larimer Counties, and 2-5" for the southern Front Range Foothills and western Denver suburbs to the west side of Fort Collins. We've opted to issue Winter Weather Advisories for the locations of heavier snowfall, while the lighter amounts for the I-25 Corridor and southern foothills would be shy of criteria and outside of peak commute times. Amounts will likely taper off pretty fast to the east, with some low level dry air intrusion. One thing that could offset that however, is the proximity of the mid level f-gen. We'll also have to watch the I-70 mountain corridor for potential impacts on the ski weekend, but so far snowfall rates there look pretty limited and there has been just a slight trend northward with the latest QPF forecasts.

Snow should essentially taper off across the lower elevations Saturday afternoon and evening, and then across the mountains Saturday night as we lose the forcing mechanisms.

Mainly dry weather will settle into the area under a col for later Sunday through most of Tuesday. There is a little more concern toward Tuesday night or Wednesday that the upper low could kick out far enough north to bring a return of snow to the forecast area. That's still a relatively low probability (20-30%), but latest ensembles are trending toward potential for additional accumulating snow. One cycle doesn't make a strong trend, but this is something to watch over the next few days. We probably won't know a lot more until the upper low finally settles off/near the California coast Sunday.

AVIATION /06Z TAFS through 06Z Saturday/
Issued at 1041 PM MST Thu Jan 23 2025

Winds have increased at KBJC as a mountain wave as developed.
Some guidance suggest that gusts could surpass the 45kt mark in the next few hours at KBJC (<30% chance), but the more likely outcome will be gusts to 25-30kts until ~12Z at this location.
Drainage winds should remain through the morning at KDEN/KAPA. A cold front will shift winds to the NNE in the afternoon followed by lowering ceilings as moisture increases in the mid and lower levels as our next winter system approaches. MVFR conditions should develop close to midnight and IFR is expected by early morning as snow showers could lower visibilities to near 1SM a few hours prior to sunrise.

BOU WATCHES/WARNINGS/ADVISORIES
Winter Weather Advisory from 5 PM Friday to 5 AM MST Sunday for COZ031-033.

Winter Weather Advisory from 9 PM Friday to 5 AM MST Sunday for COZ035.