National Weather Service Forecast Discussion

000
FXUS65 KBOU 012102
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
302 PM MDT Sun Aug 1 2021

.SHORT TERM...(This evening through Monday)
Issued at 250 PM MDT Sun Aug 1 2021

Current satellite and radar shows the bulk of convection staying
well to our west over western Colorado, under the deeper moisture
plume. We don`t see this plume shifting much through the evening,
as low levels are still drying slightly and only the upper levels
are attempting to moisten. We`ve trimmed back PoPs a little more
for the high country, but isolated to scattered convection is
still possible along/west of the Continental Divide. Temperatures
will be relatively cool and slightly below normal for tonight.

On Monday, we do see the deep monsoonal moisture plume shifting
ever so slightly and slowly eastward. This should result in an
uptick of convective coverage and intensity across our mountain
areas, and with that, an increased risk of flash flooding to burn
scars mainly west of the Front Range. The deeper moisture is
expected to arrive slowly given the amplitude of the ridge and
slow advection of monsoonal moisture ahead of the upper level
disturbance moving across the Great Basin. However, by late in the
afternoon and evening we should start to see a few storms work
east down the Front Range. Most of those may end up being outflow
dominated and weakening, reducing the threat of heavy rainfall.
Most of the plains east of I-25 should stay dry through the
afternoon. Temperatures will continue to warm, with mid to upper
80s for highs across the plains.

.LONG TERM...(Monday night through Sunday)
Issued at 212 PM MDT Sun Aug 1 2021

We should see at least a few showers continue through the early
overnight hours tomorrow night as our moisture plume slowly drifts
to the east. Instability should decrease fairly quickly after
sunset as we lose daytime heating, but there could be a continued
heavy rain threat through the evening. After midnight, most of our
activity will die off, but our specific humidities are likely to
moisten.

Tuesday is shaping up to be a bit wetter than Monday, with better
moisture across most of our CWA. Model soundings over the high
country show more than adequate instability in addition to PWATs
approaching an inch. Forecast guidance is still somewhat
inconsistent with the overall setup, with the GFS and GEFS are a
bit drier/stable over the high terrain while the Euro ensemble
suggesting high probabilities of surface CAPE > 500 J/kg. Have
opted towards the Euro and wetter solutions for now, but
confidence in the overall setup is still relatively low. However,
if guidance continues to advertise a good parameter space for
heavy rain, and additional Flash Flood Watches could be needed for
the high terrain. Stay tuned.

Tuesday night into Wednesday could see some activity drift off
into the plains as moisture continues to slide to the south and
east. Have increased PoPs a little over the plains compared to NBM
but overall precip potential is fairly low. Model soundings show
at least a weak cap Tuesday evening, but we could transition to a
light rainfall event into Wednesday morning if the Euro solution
is reasonable. Will have to watch trends there as recent runs have
been more optimistic at getting measurable precip into the metro.

Beyond Wednesday, the pattern quiets down a little bit with most
of our moisture shunted out of the region. Warmer and drier
weather looks likely Thursday and Friday, with temperatures
warming back into the 90s. Another monsoon push of moisture is
possible by next weekend, but for now will maintain just chances
of afternoon storms across the high country.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 250 PM MDT Sun Aug 1 2021

Satellite shows a considerable amount of smoke aloft, and there`s
even a slight restriction to visibilities at the surface. Thus,
ILS landing conditions will persist through the rest of the day
with limited slant range visibility. The same can be expected
again on Monday, as there is very little advection noted. Generally
light southeasterly winds will prevail through 03Z, then
gradually transition to normal south/southwest drainage winds by
06Z. We don`t expected any thunderstorms through most of Monday,
as we`ll be too stable. There`s just a slight chance of a storm
toward 00Z Tuesday.

&&

.HYDROLOGY...
Issued at 250 PM MDT Sun Aug 1 2021

It appears most of the convection through this evening will stay
west of the burn areas. There is just a limited threat for flash
flooding west of the Continental Divide (East Troublesome and
Williams Fork burn areas). There is still a relatively high
amount of moisture there so if a storm can develop, a quick one
half inch of rain is possible.

Deeper moisture gradually returns to the forecast area Monday,
although the best arrives late in the afternoon or evening hours.
Right now, it still looks like areas along and west of the Front
Range will have the main threat of heavier rainfall. Precipitable
water and 700-500 mb specific humidity should be sufficient for
decent precipitation efficiency, and rainfall rates up to 1 inch
in 40 minutes. However, we are uncertain regarding destabilization
and the exact timing of arrival of the better moisture as the
upper level disturbance over the Great Basin will be slow to move
into the upper level ridge axis. We`ve issued a Flash Flood Watch
for the burn areas mainly along and west of the Front Range,
although it`s not impossible the threat could shift into the lower
elevations of the Cameron Peak burn toward late afternoon or
evening. The Watch could be expanded later if moisture arrives
faster than currently expected.

Flash flooding concerns will continue on Tuesday. The moisture
plume should be over our forecast area, with PWATs near 150% of
normal with very weak steering flow. It looks like there`s a
little bit more forcing over the region during the afternoon, and
thus there should be more widespread coverage across the high
country during the day. With instability increasing yet again, in
combination with the aforementioned weak steering flow, slow
moving thunderstorms would be likely. We`re still far enough out
that this could change, but it is certainly possible that another
Flash Flood Watch could be needed for the high terrain. Some of
the rainfall could drift into the I-25 corridor, but there is less
confidence in that happening.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from Monday afternoon through Monday evening
for COZ032>034.

&&

$$

SHORT TERM...Barjenbruch
LONG TERM...Hiris
AVIATION...Barjenbruch
HYDROLOGY...Barjenbruch/Hiris

NWS BOU Office Area Forecast Discussion