textproduct: Denver/Boulder

KEY MESSAGES

- Today...strong downslope winds will continue across our mountains, foothills and parts of the urban corridor, with widespread critical fire weather conditions anticipated under a dry and (record) warm air mass. Wind gusts exceeding 100 mph are possible from the foothills to the adjacent plains.

- Particularly Dangerous Situation Red Flag Warning for the lower foothills of Larimer, Boulder, and Jefferson Counties, and the CO-93 corridor for high winds and low relative humidity. - Light to moderate snow returns to the mountains Saturday, with deteriorating road conditions expected along and north of I-70 through the mountains. A winter weather advisory is in effect from midnight tonight through midnight Sunday.

UPDATE

Issued at 308 AM MST Fri Dec 19 2025

A few updates were made to the forecast for the next 24-48 hours. First are the winds and expected red flag conditions, including areas highlighting a Particularly Dangerous Situation. Winds are already howling in the foothills early this morning, with gusts in the 55-70 mph range where we have the high wind warning. We continue to analyze available high resolution model data to get a sense of how strong the winds could be, and just as important, how far east of the foothills the mountain wave sets up. In terms of magnitude, there is good agreement that the foothills of Larimer, Boulder, and Jefferson Counties (as well as Clear Creek/Gilpin and extreme NE Park County) will see the highest winds today, with gusts in the 80-105 mph range. In fact the HREF has 80-99% probabilities of exceeding 80 mph every hour from now through about 2 PM in those areas above 7,000 ft. The highest probabilities are in western Boulder County south to the northeast corner of Park County, where the mountain wave should be positioned this morning. Wind speeds in general associated with the mountain wave and plain old brute force winds at 700 mb are expected to decrease after 3 PM, but gusts 50-80 mph are likely to continue across the foothills through 8 PM. All highlights for the foothills remain in place unchanged.

There continues to be considerable uncertainty in how far east the mountain wave sets up. The latest high res models have been consistent with keeping the 80+ mph gusts along and west of CO-93. The wind gradient is going to be tremendous as just a few miles east of the maximum wind speeds near the base of the foothills will see considerably less wind this afternoon, and 3-5 miles east of the mountain wave will probably even see east winds 10-15 mph, consistent with the development of a rotor. There is roughly a 25 percent chance the mountain wave does not make it to CO-93 and remains above 6,500 ft elevation, based on your flavor of CAM. But overall, the ingredients are in place such that the mountain wave should reach the CO-93 corridor around midday but not get beyond there until later this evening. North of Fort Collins along the Wyoming border, which does not need a mountain wave to produce high winds, should see gusts 60-75 mph this afternoon, especially north of Wellington. Fort Collins itself is not likely to see much wind this afternoon, but could see 30-40 mph gusts after 6 PM like much of the rest of the urban corridor. A strong jet is moving in from the north this evening and when it does, an influx of subsidence via the right exit region of the jet should cause the mountain wave to shift east and broaden out. Wave amplification is probably pretty weak by this evening given the increase in wind shear via the jet, and the loss of the inversion above ridgetop. However, perhaps enough subsidence is the reason CAMs bring strong winds east across the I-25 corridor and much of metro Denver after 6 PM. From 6 PM through midnight, most areas across the I-25 corridor (especially east of I-25) are likely to see their strong winds of the day, after having light winds this morning throughout the afternoon. Gusts 35-60 mph are expected this evening all the way out to DIA. The forecast was refined with the higher confidence in the magnitude and timing of the evening's winds.

In terms of the fire weather threats, nothing much has changed since yesterday's forecast. RH is still expected to drop below 15% where the winds are highest from CO-93 westward into the foothills up to 8,000 ft. These areas from Larimer to Jefferson to Boulder counties continue be a concern for rapid fire growth from this morning through the evening hours. It continues to be a Particularly Dangerous Situation. We implore all to not do anything that could spark a fire in these areas. Elsewhere, the RFWs remain in place especially when the winds increase late afternoon/evening.

Finally, after winds and low RH improve, there is a good amount of Pacific moisture coming into Colorado tonight through Saturday. Snowfall amounts were updated this morning for up to a foot of snow in isolated areas (but more like 4-10 inches outside of the summit of the Park Range), and given moderate snowfall rates and winds 40-60 mph Saturday morning, feel impacts will be great enough to warrant a winter weather advisory from midnight tonight through midnight Sunday (24 hours). Travel conditions are expected to deteriorate across the mountains, especially across the mountain passes, by Saturday morning. Travel conditions should improve significantly by Sunday morning.

DISCUSSION /Through Thursday/

Issued at 307 PM MST Thu Dec 18 2025

Unfortunately we are not done with the high winds yet. As the strong 500 mb trough moves further east into the Great Lakes Friday morning, flow across Colorado flattens to almost due west. The pressure gradient between the ridge to the south and the long wave trough to the north means zonal flow along will increase tonight into Friday morning. Cross-barrier flow near 70kts (mean wind direction at ridge top is around 280) should be in place early Friday morning, which is also when winds in the higher elevations of the Front Range will increase. We have a high wind warning in place for the mountains and foothills above 6,000 ft starting 9 PM tonight. Sustained winds 40-60 mph with gusts approaching 100 mph are expected from the latter half of the night into early Friday. The windiest spots on the tops of peaks/ridges will probably exceed 100 mph by Friday morning. There is some lingering uncertainty regarding how quickly winds will develop and begin to advance down into the lower foothills. It's possible stronger winds don't develop until closer to daybreak for locations below 7,000ft MSL... as hinted by a few of the daytime CAMs.

Attention then turns to the fire weather conditions driven by strong downslope winds throughout the day Friday. Initially, high resolution model soundings show a fair amount of wind shear at 600 mb and above. Though lapse rates remain weak at ridge top, the forward wind shear should prevent a mountain wave from forming across the lower elevations, at least initially. The inversion aloft is close to 600 mb Friday morning. These factors seems to the reason why all high res models keep the high winds above 6,000 to 6,500 feet until early afternoon. At that time winds between 700-500 mb reach 70-80 kts, there are hints of an inversion, and winds decrease with height. Most but not all high res models bring those high winds down the east slopes to the CO-93 corridor, from Lyons to western Boulder and then Jefferson County to Golden.

Friday afternoon looks like a textbook steady state, narrow mountain wave event with gusts up to and exceeding 100 mph, and more than likely those gusts will make it to the CO-93 corridor by the late morning or early afternoon hours. For now, we expect that the winds should weaken rapidly east of the mountain wave such that eastern Boulder, Larimer, and Jefferson Counties may not see much wind at all. The exception will be the wind prone areas north of Fort Collins along the Wyoming border, where gusts to 75 mph are possible (e.g. north of Wellington). High resolution guidance offers a variety of solutions in this timeframe, and the blend of the HRRR/RRFS/REFS/RGEM/HiRes CAIC WRF is reasonably close to our deterministic forecast.

Another thing to note is that after sunset the high res models push the mountain wave eastward. This is likely in response to the right exit region of the jet approaching our area around that time. While wind speeds should be significantly lower when this occurs, it could result in a few hours of gusts 35-60 mph across areas that didn't see much wind throughout the day, including all of the I-25 urban corridor and metro Denver. There is surprisingly more confidence in this period compared to the "steady state" portion of the event during the day.

The concern is the combination of the dry air, critical fuels, and high winds. Record warmth is expected, plus bone dry downslope winds, and the result are RHs dropping below 15 percent. RH will start off low in the foothills which is why we've hoisted an RFW starting at 12Z, which is highly unusual. Once the winds increase midday, red flag conditions are expected to spread east. The RFW for the I-25 corridor eastward includes all of metro Denver, Weld County, and the Palmer Divide through midnight. That's the other unusual aspect of this RFW, the fact that RH recovery is going to be poor well into Friday evening. It is possible (and perhaps likely) that a majority of the Red Flag conditions in the Denver area hold off until near/after sunset and continue into the early overnight. Finally, uncertainty remains in the extent of the strong winds across E. Adams/Arapahoe/Elbert and Lincoln Counties, thus we are keeping a fire weather watch in effect for those areas through Friday evening.

In the mountains, Pacific moisture works its way into Colorado with the zonal flow. Snow chances increase first across the northern mountains Friday evening. Wind direction is expected to be right around 280 and lapse rates above ridge top will allow for snow to continue into Saturday evening. Sometime early Sunday lapse rates and west winds both weaken, though there is still moisture in the westerly flow through much of Sunday. So while it may still be snowing above 9,000 ft in the mountains on Sunday, snowrates are expected to be much lighter than on Saturday. The moisture plume from Saturday into Sunday stays mainly across the northern mountains, which is also where we expect to see travel impacts (US 40 and CO-14), but Saturday morning through midday a few inches of snow looks likely (>80%) for the I-70 corridor as well so probably some slick/slushy spots from Georgetown to Vail Pass as well. Total snow amounts should range from 6-12 inches across the northern mountains, and 2-6 inches for the I-70 corridor. A winter weather advisory may be needed for the mountains on Saturday, but will punt to the midnight shift given the significant focus on the fire weather conditions tomorrow.

Elsewhere, despite the strong zonal flow, a shallow cold front is expected to move across the plains of Colorado throughout the day. Because the front is shallow and there is west flow over the Rockies, the front will wash out quickly especially across the I-25 corridor. However, behind the front and as it is in the process of washing out, there is a period of shallow north- easterly upslope flow later Saturday morning into the afternoon. Precipitation chances will remain low and any accumulation will be light. Wet Bulb temperatures should be 34 degF or warmer so P-type will probably be rain across the I-25 corridor and eastern plains below 6,500 ft. For now, PoPs are 15-30 percent across the plains Saturday late morning through the afternoon. Temperatures look similar both Saturday and Sunday with highs in the 50s across the plains. The mountain valleys will be in the upper 30s Saturday, warming to the mid 40s on Sunday afternoon with more sunshine.

Next week will be depressing for snow lovers, but about as good as you can imagine for those that don't like winter. A developing trough off the California coast builds an impressive ridge aloft across Colorado and the Central Plains of the U.S. Monday afternoon the ridge builds significantly overhead, then slides east Tuesday and Wednesday. Those three days will be exceptionally warm, with highs across the plains and I-25 corridor in the mid-upper 60s. It may hit the 70s across the I-25 corridor on Monday. It should be dry across the area Monday through late Wednesday. 700 mb temps warm to between +2 and +4 degC (which is about 2 standard deviations above climatology), meaning temperatures across the mountain valleys and east slope foothills will be in the 50s.

After the spring like warmth Wednesday, A strong Pacific storm moves onshore, and has an impressive moisture plume that moves northward across Colorado ahead of it starting late Wednesday or early Thursday. 7 days out predictability is naturally quite low, but an ensemble analysis clearly has a signal of increasing snow chances for the mountains, especially the central mountains but with the flow out of the SSW, at this point not getting too excited for a good mountain snow. Additionally, ensemble mean 500 mb plots stall the main trough along the coast, meaning synoptic forcing will be weak across Colorado, and precipitation production will rely mostly on upslope and lapse rates. Unfortunately for the northern mountains and plains, SSW flow is a challenge for precipitation production so despite an influx of Pacific moisture, it sure looks warm and dry Christmas Eve and Christmas for now.

AVIATION /06Z TAFS through 06Z Saturday/

Issued at 11106 PM MST Thu Dec 18 2025

VFR through Saturday. Winds are going to be a significant issue later today and into the evening hours. First though, winds at all terminals should be out of the south tonight into Friday morning. A strong mountain wave will form tonight, initially keeping the high winds west of the terminals over the foothills. However, downstream of the main mountain wave occasionally ribbons of stronger west winds will extend across the terminals, probably starting around 21Z at DEN and APA. For now will handle these more transient west winds with a TEMPO group. After 02Z however, stronger and more persistent west winds are expected with gusts over 30 kts. At BJC, with its proximity to the main mountain wave, will see west winds earlier, and stronger. After 01Z west wind gusts are expected to surpass 50 kts.

West winds should weaken at all terminals between 09-12Z, and turn more WSW. But through 12Z, west gusts could still reach 30 kts. It's a bit beyond the current TAF window, but a cold front with a wind shift to ENE via the formation of a Longmont Anticyclone is expected between 15-18Z Saturday at DEN. The cold front may not even reach BJC or APA. There is still a lot of uncertainty in the evolution of the winds Saturday morning, we'll keep tabs on it as it gets closer.

FIRE WEATHER

Issued at 247 PM MST Thu Dec 18 2025

A dry airmass, combined with a potentially significant downslope wind event, will lead to a large area of critical fire weather conditions beginning as early as Friday morning. The current airmass is quite dry across the I-25 corridor and most of the high country, with dew points generally in the -10 to +5F range this afternoon, and only modest moistening of the airmass is expected by tomorrow. Overnight humidity recovery of 30-50 percent is expected for most of the Front Range with daytime humidity falling in the 10-20 percent range (lower in the Denver metro).

The greatest uncertainty at this time is how downslope winds evolve in the Foothills into the I-25 corridor during the daytime hours. There is still some notable differences across guidance regarding the amount of overlap between the strongest winds and driest air in the foothills. Similarly, there is fairly low confidence in how far east the mountain wave advances east during the day. A blend of the last several cycles of the HRRR and RRFS (and its ensemble) would suggest that a majority of the wind stays in the foothills below 7500ft into the immediately adjacent plains (Highway 93 corridor, for example). Guidance does eventually push some gusts - generally in the 35-55 mph range - into the I-25 corridor by late afternoon and evening.

Combined with what's been a very dry past 60 days (<50% of normal precipitation) in the same areas, and one of the warmer starts to December on record would suggest that critical fire weather conditions will develop across most of the Foothills, I-25 corridor, and adjacent plains over the course of Friday/Friday evening. After extensive discussion among our forecast staff and partners, we have added the Particularly Dangerous Situation (PDS) wording into the Red Flag Warning, primarily for a section of northern Jefferson and central Boulder counties (between 5500-9000ft MSL) where an extended period of >70 mph winds and low enough humidity overlaps.

BOU WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory from midnight tonight to midnight MST Saturday night for COZ031-033-034.

High Wind Warning until 8 PM MST this evening for COZ033>036.

Red Flag Warning until midnight MST tonight for COZ215-216.

Red Flag Warning from 10 AM this morning to midnight MST tonight for COZ238>243.

High Wind Warning from noon today to 8 PM MST this evening for COZ038-039.