Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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487
FXUS65 KBOU 140200
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
800 PM MDT Wed Sep 13 2017

.UPDATE...
Issued at 800 PM MDT Wed Sep 13 2017

Isolated showers and thunderstorms remain over the area. Expect
them to decrease and end with in the next two hours. Adjusted the
forecast to remove pops a couple hours earlier. Otherwise,
forecast is on track.

&&

.SHORT TERM...(This evening through Thursday)
Issued at 327 PM MDT Wed Sep 13 2017

Latest GOES-16 mesoscale 1 min data combined with radar imagery
has shown that convection has been relatively easy to initiate
today across the Palmer Divide and Front Range mountains and
foothills, but having a fairly hard time sustaining itself across
the lower elevations. With inverted V soundings and surface
dewpoints in the 30s, convection should continue to be on the weak
side of the spectrum the rest of today. The exception may be our
northern counties Larimer/Weld/Morgan/Logan/Phillips/Sedgwick
where slightly better low-level moisture and higher precipitable
water exists. Have increased PoPs this afternoon across our
northern counties based on latest observational trends and HRRR
guidance. Early convection across Weld/Morgan County has already
generated a outflow boundary that should continue to be a focus
for more showers and thunderstorms the rest of the afternoon. In
general, storms will be capable of gusty winds and cloud to ground
lightning until the weaken and/of move into Kansas by about 10
PM. Expect it to be dry area wide after 10 PM.

Tonight a cirrus shield from upstream convection should keep
overnight temperatures warm, with lows near 60 across the I-25
metro corridor, upper 50s rest of the Plains, and generally 40s in
the mountains. All quite warm for mid-September standards. During
the day Thursday a strong trough moves into the Great Basin and
Colorado is under weak southwest flow, and thickness values
decrease across the state. High temperatures should be 3-7 degF
cooler than today area-wide. By Thursday afternoon weak synoptic
lift spreads over the state, and combined with a modest increase
in total precipitable water values (0.80-1.00" Plains, 0.5-0.6
mountains), shower and thunderstorm should be slightly more
numerous Thursday afternoon relative to Wednesday but still not
enough for a free watering of lawns and gardens in most areas
across the Plains. Mountains and foothills, as typical, should
have better shower and thunderstorm coverage. Vertical wind shear
despite southwest flow is still pretty weak, while instability
will again be limited to around 500 J/kg across the Plains. Thus,
main impacts with the afternoon storm will be gusty winds to 35
mph and cloud-to-ground lightning.

.LONG TERM...(Thursday night through Wednesday)
Issued at 327 PM MDT Wed Sep 13 2017

Longwave trough will remain upstream over the west through the
period. One shortwave trough will kick out of the trough in the
Friday/Saturday time frame, with another shortwave deepening the
trough to our west early next week. On a smaller scale, a lesser
wave will produce some lift ahead of the trough and bring up some
mid level moisture Thursday night. This should be the best chance
of precipitation for most of the area with enhanced convective
activity. CAPES will be low however, so not expecting much
strength to the storms. With a bit more moisture there could be
more rain, but gusty winds are probably still the main threat.

There will be a bit of a break before the bigger piece of the
trough comes through. There is some disagreement on the timing of
this, sometime late Friday or early Saturday. This trough will
bring some cooling aloft and drag a significant cold front across
the plains, but there is a sharp south edge to the mid level
moisture that will be over our mountains or perhaps near the
Wyoming border. Lower confidence on the details, but typically in
these situations the cold front will roll right along without much
regard for the timing of the higher trough. If the front comes
early enough it could hold back highs on Friday with falling
temperatures late in the afternoon. It looks like there will be
enough cooling for some snow in the higher mountains where the
moisture is. Snow level should stay above 9000 feet and probably
above 10 thousand. At any rate, amounts should be light and
temperatures warm, so not expecting any accumulation at or below
pass elevations. I did try to smooth out the tight gradient in
PoPs in the models in case the moisture winds up clipping the I-70
corridor.

Saturday will be cooler and may have some stratus in the morning.
There is also some discrepancy about how dry the new air will be.
Drier solutions would have patchy stratus in the morning, then
clearing. The moister solutions would have enough for some diurnal
convection mainly over the mountains.

For Sunday into the early part of next week, recovering
temperatures as dry southwest flow aloft develops downstream from
the reloading trough. It could get to be quite dry, warm and
breezy Monday or Tuesday which could raise the fire danger. Fuels
have dried out in most areas over the last several weeks, so we
could be getting closer to Red Flag conditions. Most model runs
are keeping the large trough west of us with some kind of
shortwave kicking out of it Tuesday/Wednesday. Most solutions are
fairly dry, and could have a day of stronger winds in this
pattern. There are some outliers that just keep us warm through
Wednesday, or move the trough over us for stronger cooling and a
bit better chance of showers. But most likely, it will just be a
little cooler by the middle of the week after a couple of quite
warm and dry days.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 800 PM MDT Wed Sep 13 2017

Northeast winds will turn clockwise and become southerly by 06Z. The
threat for storms has ended at the Denver airports. Another round of
high based showers and thunderstorms is expected Thursday afternoon.
Main threat will be wind shifts from the convection.

&&

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

$$

UPDATE...Meier
SHORT TERM...Schlatter
LONG TERM...Gimmestad
AVIATION...Meier



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