Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 210136
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
736 PM MDT Sun Aug 20 2017

.UPDATE...
Issued at 736 PM MDT Sun Aug 20 2017

A few weak showers and storms will remain possible through this
evening over southern and eastern parts of the area. Cloud cover
will linger for most of the night, making for mild overnight lows.
Will make minor changes to line up with current trends.

An upper level trough will lift northeast across New Mexico and
be over east central Colorado by 18Z Monday. Expect a good amount
of cloud cover from this trough tonight and early Monday. The
showers and storms should remain over southern Colorado. As this
trough moves into eastern Colorado, subsidence behind it is
expected to produce some clearing. There will be enough moisture
around for scattered mid and high clouds, late Monday
morning/early afternoon.

&&

.SHORT TERM...(This evening through Monday)
Issued at 325 PM MDT Sun Aug 20 2017

For the remainder of today instability and shear combinations
continue to look too weak to support sustained convection east of
the mountains. The upslope flow didn`t bring anywhere near enough
moisture in the low-levels, with dewpoints in the 30s to low 40s
west of US 85 and along the Palmer Divide. Better moisture exists
across the northeast quarter of Colorado but the atmosphere is
capped with 700 mb temps around +14C and an inversion around 750
mb. Still, rich low-level moisture exists far eastern Plains so
any storms that can develop along outflow boundaries/gust fronts
have a chance of producing strong winds and cloud to ground
lightning. Overall though confidence in any storm forming is low
that far east. Closer to I-25 isolated high based showers and
thunderstorms could occur through 9 PM today but most activity
should diminish area-wide after that.

For Monday and the solar eclipse viewing there is an offshore
upper-level trough off the coast of Central California, and
Colorado is under southwest flow in the upper levels. A weak short
wave departs Colorado this evening and in its wake subsidence
spreads across our area overnight into early Monday. Another weak
short wave rotates out of out the low offshore Monday around mid
morning and provides weak lift, primarily over the northeast part
of the state. If the short wave trough moves quickly enough
northeast, skies could clear out before the eclipse. Have trimmed
sky cover grids just a tad to reflect the trend for more
optimistic cloud cover around midday. Moisture is limited again
with PWATs less than an inch east of the Rockies and low near-
surface dewpoints, so it looks like at worst a scattered cirrus
cloud deck and a few mid- level clouds across northeast Colorado
during the eclipse. Overall viewing conditions west of a Sterling
to Akron to Limon line should be quite good, with partly cloudy
skies expected east of that line. Temperatures should be around 80
degrees most areas across the lower elevations during the
eclipse. With dry low levels and weak forcing overall, no
precipitation is expected until later in the day. A surface high
to the north driven by the departing short wave trough should
generate northeast upslope flow in the low levels, and combined
with surface heating, storms should form by 2 PM storms across the
Continental Divide and adjacent mountains, then move east. Any
storms that form are not expected to survive east of I-25, but
isolated, widely scattered weak storms capable of producing gusty
winds are still possible across the Plains. Low-level moisture is
much better across the far eastern counties, and any storms that
form out there could become marginally severe with outflow winds
and small hail. The question regarding storms is what would be a
focus mechanism for storm initiation. Not seeing a front in that
area so there would need to be outflow from convection across the
high Plains further east. At this point it`s too far out to
pinpoint timing and areal coverage, so have included a 20-30%
chance of storms far NE Plains. Elsewhere slightly cooler
conditions in the upslope flow should result in highs in the mid
80s lower elevations, with 60s to mid 70s 70s in the mountains.

.LONG TERM...(Monday night through Sunday)
Issued at 325 PM MDT Sun Aug 20 2017

Twenty-five to 50 knot west and west-southwesterly flow aloft is
expected for the forecast area Monday night through Wednesday
night. The upper ridge axis is just west of the CWA Monday night
into Wednesday morning, then it is over the CWA into Wednesday
evening and just east by 12Z Thursday morning. There is no
synoptic scale energy in place for the CWA Sunday night through
Wednesday night. The low level pressure and winds fields show
upslope winds Monday night behind a cold front. There are easterly
winds on early Tuesday, the south easterly by Tuesday afternoon.
Tuesday night through Wednesday night, normal diurnal patterns are
likely. Precipitable water values are in the 0.50 to 1.00 inch range
west to east Monday night into Tuesday. Tuesday afternoon through
Wednesday night, values are around 1.00 inch on the plains and
0.50 to 1.00 inch over the mountains and foothills. Surface dew
points are in the 50s to lower 60s F over the plains much of the
time with mostly 40s F in the mountains. There is pretty high CAPE
over the northeast corner Monday evening. On Tuesday the best
CAPE over the southwestern quarter of the CWA, but there is more
over the plains then yesterday`s model`s indicated. There is more
widespread CAPE late day Wednesday, but it is not as strong as
Tuesday`s. The QPF fields have limited amounts of measurable
rainfall over the plains Monday evening, with best amounts over
the mountains. Late day Tuesday shows decent amounts for the
mountains and foothills with nothing for the plains. Late day
Wednesday is just fair, with the best amounts over the high county
once again. For pops, 20%-40%s are good for Monday evening. Will
do with 30%-60%s late day Tuesday, mainly in the mountains and
foothills. Same goes for late day Wednesday, but will also have
some 10%-20%s out on the plains. For temperatures, Tuesday`s highs
are 0-3 C cooler than Monday`s highs. Wednesday`s readings are
1-4 C warmer than Tuesday`s. For the later days, the upper ridge
axis pushes east of Colorado on Thursday. There is upper
troughiness around Thursday night into Saturday. The upper ridge
builds back in late Saturday night and Sunday. The best days for
moisture are Thursday and Saturday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 736 PM MDT Sun Aug 20 2017

A wind shift or two will remain possible through 04Z from earlier
convection. Appears the threat for thunderstorms in the Denver
area is over. Winds will settle at a south-southwest direction by
06Z. On Monday, north to northeast winds will prevail behind a
weak cold front. There will be a slight chance for thunderstorms
Monday afternoon and evening. Outflow winds from nearby storms
will likely produce a wind shift or two late in the day.

&&

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

$$

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


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