739
FXUS65 KBOU 171136
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
536 AM MDT Sat May 17 2025

.KEY MESSAGES...

- Scattered thunderstorms over the mountains this afternoon, with
  just a slight chance of storms on the plains.

- Threat of severe storms on Sunday afternoon, mainly in the
  northeast corner of Colorado. There is uncertainty due to the
  track of the low pressure system and timing.

- Cooler and windy with more showers/storms Monday and possibly
  Tuesday, but a lower severe threat.

&&

.UPDATE...
Issued at 312 AM MDT Sat May 17 2025

A couple of targeted forecast updates to note this morning for the
day 1-3 period...

1) Modest surface lee cyclogenesis over southern Colorado this
afternoon ahead of the approaching upper-level trough should
promote advection of higher dewpoints into the eastern plains in
the afternoon. Most CAMs indicate sufficient MLCAPE of 300-700
J/Kg to allow for convective initiation by late afternoon/early
evening along a weak surface convergence axis. With pockets of
bulk shear above 40 kts, would not be surprising to see one or two
strong to severe storms develop, most likely focused over
Washington County and into the northeast plains through the
evening. Have increased PoPs a good amount in these areas to
better message this potential.

2) Sunday`s severe weather threat remains in play, although the
latest guidance does seem to have nudged the developing dryline
and warm front northward slightly. There should be a fairly steep
north to south gradient in precipitation/thunderstorm potential as
a result, with the best chances of severe thunderstorms focused
across our northern and especially northeastern plains. Some
question as to whether storm intensity will pick up quickly enough
for more than a brief window of severe weather potential before
lifting north out of our area, but ingredients do appear to be
there for both a hail and tornado threat over a relatively narrow
axis.

3) For Monday we`ll still retain a potential for precipitation as
cyclic shortwaves continue to emerge from the parent trough, but
NBM PoPs seemed much too high in general, and rather misplaced.
With a cooler and more stable post-frontal airmass, a lack of jet
support and QG lift, and favored prevalence of north/northwest
surface winds, simply couldn`t justify the 70-80% PoPs advertised
by the NBM. Thus, shifted the greater precipitation potential
southward towards the Palmer Divide (upslope) rather than for our
northern areas.

&&

.DISCUSSION /Through Friday/...
Issued at 330 PM MDT Fri May 16 2025

It`s looking like a pretty quiet late afternoon/evening as we`ve
still got stable air over the mountains, evidenced by the trapped
waves in the mid level cloud deck. Winds have finally started to
ease a bit and that will continue into the evening. We did update
the forecast for more cloud cover through Saturday as we`ll have a
weak shortwave trough passing and then the edge of the moisture
stalling over us ahead of the developing western trough. This will
keep temperatures from rising too much on Saturday despite a
warming air mass. With the moistening, we`ll return to scattered
afternoon thunderstorms over the mountains, but the clouds will
limit the eastward progression. The combination of lift from the
trough and increasing low level moisture could produce a cluster
of stronger storms over the northeast corner Saturday evening.

There`s been a little model convergence for the Sunday through
Tuesday time frame, though we`ll need a better handle on details
before we know what it means. There`s clearer agreement on the two
jet streaks rotating around the trough, bringing a lead wave
kicking out of the trough on Sunday and then the second part of
the trough sliding east Monday night or Tuesday. Unfortunately,
there`s still a lot of disagreement on the relative strength of
the jet streaks and their associated troughs. In general, there`s
a trend towards a bit weaker trough for Sunday, but we don`t have
a lot of confidence in that. This could result in a northward
shift in the position of the QG lift, but it could also mean that
there`s less southwest flow to push the developing dryline
eastward out of Colorado. The main threat for severe weather is
with convergence along the developing dryline which may start in
northeastern Colorado Sunday afternoon, or it may focus a bit
further east or north and be a few hours later. There should be
enough ingredients for all kinds of severe weather somewhere in
this area. The severe threat is probably greatest in bordering
areas of Kansas and Nebraska in the evening, but there may be a
better threat of strong isolated storms in our northeast corner at
the beginning. There may be critical fire weather conditions on
our southern flank, but with green grass for the moment, the
threat is diminished.

Monday is in between the two pieces of the trough, and while
details about any possible lift or the mid level flow have some
uncertainty, we will be in a moist and cooler air mass, probably
with decent north or northeast winds. If there`s not much cooling,
this could be a showery/stormy day, though it may be too cool for
much of a severe threat. That will depend on how much cooling
there is. The current trends towards a more sheared large scale
trough point towards either a faster departure or more westerly
flow on Tuesday, but again there`s not a lot of confidence in
that. NBM gradual decrease in PoPs without warming seems fine for
now. Then pretty solid warming and drying in westerly flow with
weak ridging for Wednesday into the later part of the week. There
may be a shortwave passing north of us in the Thursday/Friday time
frame, but this would probably just generate a bit of wind.

&&

.AVIATION /12Z TAFS through 12Z Sunday/...
Issued at 528 AM MDT Sat May 17 2025

VFR conditions are expected to prevail for all terminals through
the TAF period.

Light drainage winds at the current hour will increasingly battle
against a Denver cyclone setup as the morning progresses. Models
are struggling to agree in its positioning, but for the most part
keep it hovering in the vicinity of KDEN through at least early to
mid-afternoon. As a result, there is a high uncertainty in
prevailing wind directions for most of the day, and an increased
likelihood of VRB directions, especially between 17-20Z (11am-2pm
MT). For now, will favor mostly SE flow with cyclone remaining
just north of KDEN.

After ~20Z (2pm MT), ISO-SCT high-based -SHRA/-TSRA will develop
in the area, and may impact Denver area terminals through
approximately 01Z this evening, creating the potential for brief
outflow gusts. There`s a possibility of more organized convection
developing in the eastern plains in the evening. If this occurs,
easterly outflow winds with gusts 20-30 kts can`t be ruled out,
especially for KDEN/KAPA.

Lastly, an approaching warm front Sun AM will lead to increased
potential for lower CIGS 010-015 starting near 10Z (4am MT) Sun.
Bases should remain high enough to limit vis restrictions at the
terminals.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE...Rodriguez
DISCUSSION...Gimmestad
AVIATION...Rodriguez