Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 141533
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
933 AM MDT Mon Aug 14 2017

.UPDATE...
Issued at 910 AM MDT Mon Aug 14 2017

Latest HRRR shows the same type of pattern regarding thunderstorms
this aftn developing over the higher terrain west and south of
Denver then moving east/southeast. This mornings soundings not
much different form yesterday either so will base the aftn
forecast off of persistence. At least bump the coverage from
isolated to scattered for the urban corridor this aftn. No changes
planned elsewhere. Any severe potential will be low and confined
to far eastern portion of the cwa. Overall, stronger storms may
produce brief heavy rainfall, one half inch in 30 minutes, wind
gusts 35-45 mph and small hail.

&&

.SHORT TERM...(Today and tonight)
Issued at 341 AM MDT Mon Aug 14 2017

The flow aloft will gradually become more wsw today and then swly
by tonight.  Overall there still should be enough moisture for
wdly sct to sct tstms over the higher terrain this aftn with outflow
boundaries triggering wdly sct storms across the urban corridor. By
this evening deeper low lvl moisture may advect back into the far
ern plains so tstm activity may become better organized and
stronger.  By mid to late evening tstm activity should gradually end
over the far ern plains.  Elsewhere shower and tstm activity should
end by early evening.  As for highs 850-700 mb temps will rise a few
degrees so aftn readings should end up a few degrees warmer than
yesterday.

.LONG TERM...(Tuesday through Sunday)
Issued at 341 AM MDT Mon Aug 14 2017

Operational models continue to march in step through Thursday with
only subtle differences in the details. All show an unusually strong
jet for this time of year diving down the Canadian west coast and
carving out an upper trough over the Pacific Northwest and later
over the Great Basin by Tuesday. Downstream over Colorado, the flow
aloft will have already become southwesterly by Tuesday with the
500mb trough axis passing over Idaho and Utah. Subtropical moisture
brought up from the Desert Southwest by this flow will steadily
increase mean layer PW values over northeast Colorado through the
day. Weak instability waves embedded in the mid-level flow will
further destabilize the high plains atmosphere, particularly across
the northeast corner of the CWA where the NAM, GFS and ECMWF
indicate CAPEs in excess of 2200 j/kg during the afternoon and early
evening hours. SPC`s Day 2 Convective Outlook has this area in a
marginal to slight risk for severe wx. It may take the passage of
cold front or strong outflow boundary to erode the remaining CIN out
there, setting off isolated to scattered storm development. Models
differ on the arrival time of this boundary. The ECMWF and CMC show
passage in the area before 00z/Wed, while the NAM and GFS during the
evening. Large hail and damaging winds are likely to be the
principle threats, but can`t rule out a brief tornado or two with
the moderate to strong vert shear along the front. Locally heavy
rainfall also a possibility. Closer to the Front Range, the chance
for t-storms on Tuesday does not appear to be as great as the
ambient conditions are not expected to be as moist and unstable. As
the upper trough passes over head Tuesday evening, an 80-90kt jet
max passing to our south-southeast may keep t-storms going on the
plains, particularly acrs the southeast corner of the CWA, well into
the night with sfc dewpts possibly staying in the upper 50s/lower
60s.

By Wednesday...models continue to show the upper trough move east of
the CWA and northwest flow spreading a cooler, less humid airmass
over the fcst area during the day. Storm chances look even lower
for the high country, particularly west of the Continental Divide.
Don`t see t-storm chances much more than 20-30 percent along and
east of the Front Range. However, by evening the NAM, GFS and ECMWF
show anticyclonic upslope flow on the plains, which could increase
chances for storms a bit during the evening, particularly east of
the I-25 corridor where moisture levels will be slightly higher.
Wednesday will likely be to the coolest day of the week with highs
on the plains in the upper 70s and lower 80s. Max temps in the high
country should only be a shape lower than those on Tuesday.

By Thursday, the upper trough will be well east of the region which
allows a shortwave ridge to quickly take its place. Should see a 3-5
deg f rebound in temperatures with warming aloft. The new airmass
looks drier, except acrs the far eastern plains where a fairly
strong sly boundary layer flow will keep dew points and PW values
elevated, keeping the chance for late day storms on the leading to
high side.

For Friday and Saturday, growing differences in the models reduces
my confidence in the forecast out that far. Generally speaking,
models show a strong upper ridge building over the Great Basin
and northwest flow over Colorado by Friday. Models also show a
shortwave at mid-levels passing over the area during the day.
However, the GFS is packing a fair amount of moisture and lift
with this feature, while the other models keep things relatively
dry around here. For really no reason, will side with the drier
solution for now. All models then indicate drier and slightly
warmer conditions for Saturday with passage of the upper ridge.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 910 AM MDT Mon Aug 14 2017

VFR through Tuesday. Isolated thunderstorms are expected after
21z. If a storm moves directly over the terminals then stronger
winds gusts to 40 kts possible. Ceilings may drop to around 080
kft with a passing thunderstorm. Otherwise, would expected outflow
winds from storms in the vicinity to produce variable winds gusts
to 30 kts.


&&

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

$$

UPDATE...Cooper
SHORT TERM...RPK
LONG TERM...Baker
AVIATION...Cooper



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