Boulder, CO Weather

Key messages

- Areas of fog, some dense, across the plains and I-25 Corridor tonight into Tuesday morning.

- Warmer and mostly dry much of this week.

- Unsettled weather potentially returning late in the weekend and early next week, with a 20-30% chance of season's first snow in lower elevations toward Tuesday.

.DISCUSSION /Through Monday/: Issued at 123 PM MDT Mon Oct 26 2026

The upper level trough that brought the overnight/early morning rain and mountain snow is pushing well to our northeast, with drier and more subsident airmass building in behind it. However, abundant low level moisture remained in place east of the Front Range, with the stratus deck only slowly burning off. While we do expect more clearing through the late afternoon hours, areas of low clouds and fog will redevelop tonight in the moist boundary layer. This will be a hybrid radiational/advection fog event, with contributions from a developing weak Denver Cyclone. The expected location of the cyclone developing near/south of metro Denver and pushing slowly northeast overnight into early Tuesday morning favors fog over the eastern and northern plains, but then also wrapping around the back side of the cyclone down the I-25 Corridor into at least the northern suburbs by/shortly after midnight. There's a 50-60% chance it moves as far south as the southern Denver suburbs before daybreak. Given the hybrid event and relatively narrow T/Td spreads, we think some areas will experience dense fog. It's quite difficult to narrow the exact locations of thickest fog development, but something to watch for the overnight and early morning hours with potential for a Dense Fog Advisory. We'll continue to monitor through the evening and overnight.

On Tuesday, we're not sure that we'll see the magnitude of warming we earlier anticipated. That's because of the morning stratus and fog, and lack of lee troughing. We've generally cut temperatures a few degrees across the lower elevations but still mostly lower to mid 60s for highs (but a 30-40% chance we only hit the mid to upper 50s over the eastern plains). What we are more certain about is that tomorrow will start a 3-4 day stretch of dry weather.

That dry weather will be courtesy of drier and brisk west to southwest flow aloft. There will be warm advection and flat ridging by Thursday, which should support highs pushing to near 70F across the plains and I-25 Corridor.

Ensembles are still showing a subtropical push or partial remnants (or at least water vapor transport) of Tropical Storm Vance in the eastern tropical Pacific getting drawn up as early as Friday. However, the main impacts of this would very likely (>80%) chance stay to the east of our forecast area. We'll only have a slight chance of showers reaching the far eastern plains by late Friday.

A larger trough along with a possible subtropical moisture connection/jet off the central and eastern Pacific reaches the far southwest U.S. by late Saturday and Sunday. While that is fairly well agreed upon by ensemble systems, models diverge significantly by the time we reach Sunday into early next week. Yes, there is general troughiness and storminess expected, but some models move the trough through as an open or even shearing wave, while others develop a deep closed low somewhere between the Midwest and Colorado. It was interesting to note the EPS showed a rather marked increase in precipitation and even snow potential as we head toward Tuesday of next week, while the GEFS showed no discernible trend. The EPS had close to 30% of the members showing snow for Denver and the plains, while GEFS supported much lower odds (10-20%). Some of these runs, of course, were much colder than the vast majority of the ensembles, as evidenced in the large temperature variations (25th-75th percentiles from highs in the mid 40s to mid 60s for Monday and Tuesday, but full range of outcomes from highs in the upper 20s to lower 70s). Stay tuned for the latest trends and what could maybe, just maybe, be our first snowfall in the lower elevations should the deeper/slower solutions verify.

.AVIATION /00Z TAFS through 00Z Wednesday/: Issued at 547 PM MDT Mon Oct 26 2026

Current observations shows the Denver cyclone slightly stronger than previously anticipated, with easterly winds of 9-12 kts over DEN and slightly stronger at APA. For this reason, have increased winds accordingly at both airports, with speeds gradually weakening this evening and tonight as the cyclone moves northeast. For BJC, generally light east/northeast winds remain on track before transitioning more northerly/northwesterly tonight.

Fog is still likely for DEN and BJC this evening, although guidance has been hinting at an earlier onset timing this afternoon. Have bumped up impacts to visibility beginning 03Z with patchy fog, although we could start seeing it as early as 02Z. Visibilities will then drop drastically by 06/07Z (down to as low as 1/4SM at DEN) with only small improvements by 09/10Z. As daytime heating occurs, fog will clear out by 16/17Z Tuesday morning. There is still some uncertainty with fog at APA this evening and tonight, as the airport could be just north of stronger southerly winds with weaker westerly winds. Have opted in to keep west winds in tonight, with fog as early as 07Z through early morning. However, if the stronger southerly winds stick around tonight, APA would likely not see fog.

Generally light and VRB winds expected for all terminals on Tuesday morning with clearing skies by the afternoon. Short-range guidance does favor a cyclone forming again, with increasing easterly winds (8-11 kts) by 20/21Z. Some uncertainty with the winds at APA in the afternoon, and will depend on where exactly the cyclone forms (light southwest winds vs light easterly winds).

Watches, warnings and advisories

None.