Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 222130
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
330 PM MDT Tue Aug 22 2017

.SHORT TERM...(This evening through Wednesday)
Issued at 330 PM MDT Tue Aug 22 2017

GOES-16 and current radar trends continue to show a stable
atmosphere across the eastern Plains.  As expected, convection is
primarily confined to the East slopes of the mountains. Storms
have been having a difficult time getting out of the foothills so
far. Thus, despite the 50s dewpoints hanging on along the I-25
corridor, cooler temperatures and poor lapse rates aloft have led
to stable conditions and we do not expect any convection to
survive very far east of the foothills. A Denver Cyclone has set
up near DIA with an attendant convergence zone across the eastern
side of metro Denver. A bubbling Cu field is in the vicinity of
the convergence zone and we will watch the feature the remainder
of this afternoon and evening for development given potential
instability should remain limited. Isolated storms should continue
through this evening across the mountains and foothills along and
east of the Continental Divide, diminishing with the loss of
daytime heating. Skies should become mostly clear overnight area-
wide with seasonal low temperatures.

On Wednesday the West Coast trough axis is located along the
California Coast with an upper-level ridge axis right across
Colorado. Weak westerly flow should exist in the mid and upper
levels. Models are hinting at some mid-level moisture being
advected north Wednesday afternoon and increasing PWATs a little,
up to 0.85" west of a Ft. Morgan to Limon line, and about a half
inch in the mountains. In the low levels, shallow moisture
remains in place tomorrow morning but the high res models want to
mix it out by Thursday afternoon, particularly west of Ft. Morgan
to Limon line. Eastern Colorado will have the deepest low-level
moisture and by far the best PWATs, up to 1.25" or better. Despite
the good moisture, not seeing a well defined forcing mechanism to
initiate convection across the eastern quarter of Colorado during
the daylight hours. Expect thunderstorms to form over the
mountains early afternoon and attempt to move east. If the low-
level moisture mixes out as anticipated, rain chances will be low
again along and east of the I-25 corridor. With the moisture mixed
out, temperatures will rebound across the Plains to the upper 80s
to near 90. Another decent chance of afternoon/evening
thunderstorms for the mountains and foothills, similar to the
previous few days. Mountain temperatures will be similar to
Tuesday`s with the ridge in place and lack of significant moisture
in the mid- levels.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 330 PM MDT Tue Aug 22 2017

Mid and high level moisture will increase through
Thursday. At low levels, there will be some moisture over the
eastern parts of the plains, with less near the mountains. This
means we will be marginal for surface based convection over the
plains, but likely to have storms developing over the mountains.
In addition, there is some weak synoptic scale lift moving over
on Thursday that should cause increased convection in the moist
neutral airmass. Drier air moves in on Friday, and then the
upstream ridge builds bringing the flow aloft more northerly. This
will also promote drying. There is little temperature change either
at the surface or aloft through the weekend as the northerly flow
will generally offset the subsidence.

Our forecast shows modestly higher rain chances Wednesday night
through Thursday night, then lower chances after that, mainly
over the mountains. Only minor temperature variations, probably
a few degrees cooler on Thursday due to cloud cover, then warmer
over the weekend with less cloud, and possibly a little cooler
again on Monday or Tuesday as a weak cold front drops across
the plains.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 330 PM MDT Tue Aug 22 2017

VFR conditions will prevail through Wednesday. Denver cyclone has
set up along and just west of the airfield at KDEN and has sloshed
east. For now this feature is not moving much but it is playing
havoc with the LLWS winds across the airport. Thankfully overall
flow is weak so despite varying wind directions depending on the
runway, overall speeds are around 10 kts. Where this feature moves
will obviously continue to influence the wind patterns at the TAF
sites. As of now, there does not appear to be any forcing
"ingredient" around that would add confidence to where and if the
cyclone will move the remainder of today. Thus, best bet for now
is to keep the cyclone near the airport with generally southerly flow
at DEN and APA and weak northerly flow at BJC. Tonight drainage
winds should prevail at the TAF sites after 05Z and be in the
10-15 kt range at DEN, less than 10 kts at APA and BJC.

On Wednesday flow will be light from midday through the evening
hours, with generally north and northeast flow 5-10 kts. There is
a better chance of thunderstorms tomorrow, but still only 20%
after 21Z.

&&

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

$$

SHORT TERM...Schlatter
LONG TERM...Gimmestad
AVIATION...Schlatter


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