227
FXUS65 KBOU 212118
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
318 PM MDT Tue Apr 21 2026

.KEY MESSAGES...

- Critical fire weather conditions will occur on Wednesday due to
  gusty winds and very low humidity. Elevated to critical
  conditions expected on Thursday.

- Dry and unseasonably warm weather continues through Wednesday.
  Highs will be nearly 20 degrees above normal today and
  Wednesday.

- Increasing chances of strong winds across the Front Range
  mountains overnight Wednesday.

- Pattern change to cooler and unsettled weather Friday through the
  upcoming weekend.

&&

.DISCUSSION /Through Tuesday/...
Issued at 302 PM MDT Tue Apr 21 2026

Hot and dry conditions expected for today as we are located under
upper level ridging. This will lead to highs in the low-to-mid 80s
for today across the plains. For tomorrow, critical fire weather
conditions remain on track. The Red Flag Warning will remain in
place across the plains and Front Range foothills. An upper level
trough moves into the western US through the day tomorrow bringing
increasing SW flow as well as a 700 mb jet into the region by
Wednesday afternoon. A developing lee trough and tightening of
pressure gradients will lead to increasing wind gusts (up to 45 mph)
as Wednesday progresses. Relative humidity values will be extremely
low (5% to 10%) for most of the day on Wednesday.

Wednesday also brings the risk for severe weather for the far NE CO
plains including the potential for dry lightning. Sedgwick,
Phillips, and parts of Logan counties have been placed under a
marginal risk by the SPC. This risk is in association with a dryline
that will be in place across far eastern CO on Wednesday afternoon.
Dry adiabatic lapse rates, MUCAPE around 500 J/kg, and DCAPE values
between 1000 to 1500 J/kg are expected. Therefore, any high-based
shower that forms could produce gusty microburst winds. There is
also a potential for hail if there is more moisture available.
Finally, with mid-level moisture being advected into the mountains
and the arrival of the cold front, overnight mountain snow is
possible Wednesday night. The highest chances are for our northern
mountains, with accumulations up to 1-3 inches.

Some uncertainty remains in the forecast of mountain wave
amplification creating a brief window of strong winds up to 40-50
mph for the foothills and areas along the 93 corridor Wednesday
night into early Thursday morning (between 3am and 9am). Some
favorable ingredients will be in place such as subsidence, minimal
wind shear, cross barrier flow, and the potential for critical
layer development. The aforementioned cold front will also move
through on Wednesday night affecting the RH recoveries, with many
areas only reaching 25 to 35% RH overnight on Wednesday into
Thursday morning. Given the uncertainty in wind speeds and RH
recovery above thresholds for fire weather conditions, we have
left the Red Flag Warning in place only until midnight Wednesday
night. This could be extended in time in future packages if the
confidence in wind speeds/mountain wave enhancement increases
and/or the RH values decrease.

After the brief window of mountain wave amplification, there is
increasing confidence of a bora event taking place Thursday morning
into the afternoon given NW flow, cold air advection, and subsidence
aloft. The 700 mb jet is forecast to be located over our northern
tier of counties and into southern WY. Wind gusts could exceed 45 to
50 mph at times in our northern tier counties, including Weld County
north of Fort Collins. However, wind gusts will be lower across the
rest of the plains and Urban Corridor- around 25 to 30 mph- but are
still expected to meet the criteria for elevated to critical fire
danger. RH will be between 10 and 15% on Thursday afternoon.
Therefore, we have issued a Fire Weather Watch for the Front Range
foothills and plains for Thursday. The Northern Front Range
foothills are expected to have slightly higher RH values, so they
have not been included in the Fire Weather Watch at this time.

The weather pattern Thursday night through early next week will
likely be dominated by a longwave trough, with an upper level
closed low sitting around southern Canada. Multiple embedded
shortwaves in the upper level flow will bring daily precipitation
chances, mainly for the mountains, and possibly the plains as
well. As of right now, the best chance for widespread (and
measurable) precipitation is Sunday, as guidance indicates a more
defined shortwave trough trekking across Colorado. Ensembles are
in good agreement of QPF amounts between 0.10-0.25" for the
plains. However, recent model runs of the ECMWF and GFS ensembles
have trended towards less QPF due to a slightly more northerly
track of the shortwave. Stay tuned for updates as this could
change as models get a better handle on the system!

&&

.AVIATION /18Z TAFS through 18Z Wednesday/...
Issued at 1140 AM MDT Tue Apr 21 2026

VFR conditions are expected through this TAF package. Winds are
light and variable across the terminals early this afternoon.
Winds will turn more easterly over the next hour or two,
increasing in speed from the SE around 23Z today. Wind gusts
around 19kts are expected at KAPA for a few hours this evening,
with a low chance for occasional gusts around 18kts at KDEN as
well. Winds will continue to turn clockwise, becoming more
southerly late this evening and increasing wind speeds closer to
16kts at KDEN for a a few hours. Winds overnight will turn more
SSW at all terminals, with speeds around 8 to 11kts. KAPA and
KDEN will start to see stronger SSW winds move into the area near
16Z, with gusts around 20 to 25kts. KBJC is a bit more uncertain
as to when the stronger winds will get there. It will likely be
around 18Z give or take an hour or two. So we`ve left mention of
gusts out of KBJC at this time, since it`s right at the end of
this TAF package.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 10 AM Wednesday to midnight MDT Wednesday
night for COZ214>216-238>251.

Fire Weather Watch from Thursday morning through Thursday
evening for COZ214-216-238>251.

&&

$$

DISCUSSION...MAI/MV
AVIATION...AP

NWS BOU Office Area Forecast Discussion