994
FXUS65 KBOU 012012
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
212 PM MDT Sun Jun 1 2025

.KEY MESSAGES...

- More widespread and locally heavy rainfall still expected late
  Monday - Monday night. Average precipitation amounts have
  decreased somewhat, however.

- Snowmelt plus rainfall will lead to high flows on mountain streams.

- Cooler Tuesday with a decrease in showers. Then gradual warming
  trend rest of the upcoming week, but a round of scattered
  showers/storms most days.

&&

.DISCUSSION /Through Sunday/...
Issued at 212 PM MDT Sun Jun 1 2025

Scattered showers and storms are getting going over the high
country, and CIN is weakening sufficiently to allow some of these to
drift onto the I-25 Corridor and adjacent plains. Despite greater
CIN on the eastern plains, we do expect enough lift from gusty
outflows to generate at least isolated convection if not a bit more
on the eastern plains through this evening.  Gusty outflow winds to
45+ mph will be the main threat with DCAPE (downdraft) of
1300-1500 J/kg, but MLCAPE (updraft) near 1000 J/kg so couldn`t
totally rule out a brief severe/near severe storm with marginal
hail/wind.

The main concern for this forecast cycle shifts to potential for
locally heavy rainfall late Monday afternoon and Monday night.  As
early discussed, there is not great linkage between the trough
ejecting from the Desert Southwest and the trough arriving from the
northwest. However, there is still a brief period very late
Monday afternoon to late evening where things do line up,
providing our highest threat of locally heavy rainfall then.
Ingredients come together in this sense; deeper moisture
associated with the southwest ejecting trough arrives 20Z-00Z,
coincident with peak heating and instability. That should set off
the first round of storms, spreading from the mountains onto the
plains. A few of those storms could become severe on the northeast
plains as MLCAPE increases to 1200-1800 J/kg as surface dewpoints
hold in lower to mid 50s. The greatest risk of severe would be
roughly east of line from Cheyenne to Fort Morgan to Akron. This
is also when more uncertainty starts to enter the forecast. We`ll
still be in favorable synoptic lift area and a cold front arrives
5 pm - 8 pm. Thus, storms could either regenerate behind the
initial round of storms, OR the stronger storms could congeal into
a more organized MCS and head into Nebraska - and then we just
redevelop a more showery/isolated embedded storm event into the
Monday evening and overnight hours. We think the latter is the
most likely scenario at this time. That`s another reason both
ensemble and deterministic QPF amounts have decreased over the
last 24 hours. Nonetheless, we think most areas along/east of the
Front Range have a high probability (>70% chance) of greater than
0.50" of rain, and a 40% chance of greater than 1". Those
probabilities would be slightly lower/higher the farther
north/south one progresses, respectively. Also worth noting,
stronger storms will certainly be capable of producing locally
heavy rain. There is a small risk of flash flooding from 1) an
isolated strong storm moving over a burn scar Monday afternoon and
2) stronger storms merging or training on the northeast plains in
the late afternoon and evening. More in the hydro discussion
below.

For Tuesday, we expect most precipitation to be shifting off to the
south/east as the northern U.S. trough continues to push
southeast, and rather rapid drying/stabilization occurs behind
it. Forecast soundings show it will be difficult to regenerate any
convection with daytime heating as it will be limited by the
cooler boundary layer, so main chance of showers/isolated storms
for the afternoon would be over the high country.

Wednesday through the end of the week...The airmass will gradually
warm and destabilize. Another trough is expected to eject out of the
Desert Southwest late Wednesday or Wednesday night which should
bring another round of at least scattered or greater coverage of
showers and thunderstorms. This pattern is likely to continue
through the latter part of next week as it appears modest mid
level moisture will combine with additional shortwaves (these in
the northern branch). MLCAPE is generally on the modest side
through the week, but it`s early June so certainly any of these
days could see a threat of a couple severe storms.


&&

.AVIATION /18Z TAFS through 18Z Monday/...
Issued at 1149 AM MDT Sun Jun 1 2025

Main concern will be for gusty and variable outflow winds from
passing showers/isolated storms. Surface analysis shows that DCAPE
has increased to >1300 J/kg and will likely peak out near 1500
J/kg. Cumulus is already growing in/near the mountains, and
expect them to grow into showers/storms and push into the
vicinity of the TAF sites 21Z-01Z. Gusty outflow winds will be the
main threat as they will be relatively high based, and enough
threat to push us into TEMPO for VRB winds G35-40kts, and
potential (20-30% chance) for 45kt gusts given expected DCAPE.
Activity should be east of metro TAF sites by/before 01Z. Then
fairly normal diurnal wind patterns will prevail through tonight.
Storm chances will increase further Monday afternoon, with TEMPO
TSRA expected after 21/22Z. VFR conditions will persist for this
period.

&&

.HYDROLOGY...
Issued at 212 PM MDT Sun Jun 1 2025

Continued snowmelt and modest rainfall will be enough to cause
additional rises on streams in the mountains. At this time, only
the upper Colorado River is under enough threat of rises for
minor flooding, and a Flood Watch remains in effect for a small
section above Grand Lake Monday evening through Tuesday.

The main threat of flooding along and east of the Front Range
would be more localized and "flashier", given storms will still be
capable of producing heavy rain, but the total amounts will likely
be lower than earlier anticipated. The risk of flash flooding
would be from 1) an isolated strong storm moving over a burn scar
Monday afternoon or 2) stronger storms merging or training on the
northeast plains in the late afternoon and evening. Warm cloud
depth briefly increases to 6,000 feet on the plains, so storms
would be efficient rainers. A stronger storm in the foothills
could produce a quick inch of rain (less deep warm cloud depth) in
30 minutes, while a stronger storm on the eastern plains in a
richer environment could produce 2 inches of rain in 45 minutes,
so something to still watch considering potential for training or
merging storms. Meanwhile, the I-25 Corridor seems to be caught
in between for the most part, and we earlier collaborated with WPC
to reduce the flash flood threat to Marginal from Slight Risk.

&&

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

&&

$$

DISCUSSION...Barjenbruch
AVIATION...Barjenbruch
HYDROLOGY...Barjenbruch

NWS BOU Office Area Forecast Discussion