Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 300306
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
906 PM MDT MON AUG 29 2016

.UPDATE...
Issued at 857 PM MDT Mon Aug 29 2016

Plains convection continues to push into the foothills as the low
spins over the south western portion of the state. The low is
helping to create a deformation zone pulling moisture up from the
south helping to feed late night convection. Storms will
transition to light to moderate showers with the loss of heating
and are expected to continue until 06z slowly diminishing over the
foothills west of I-25 and to the south. Water vapor shows drying
upstream followed by moisture over the northern portions of NM.
This will allow some clearing over night over the eastern portions
of the CWA with some high clouds moving in by the early morning.
The clearing with high dewpoints and cool temperatures could lead
to some patchy fog over southern Lincoln county in the early
morning ours. Had to adjust forecast slightly to account for the
prolonged showers and gusty winds.

&&

.SHORT TERM...(This evening through Tuesday)
Issued at 327 PM MDT Mon Aug 29 2016

Temperatures reaching the lower and mid 80s earlier this afternoon
de-stabilized the airmass enough for scattered thunderstorms to
develop. The weak upper circulation is currently over the san luis
valley, causing storms to move from the northeast towards the
foothills. Brief and locally heavy rainfall has been occurring
with the showers so far. Precipitable water values are around 0.80
inches, which is around normal for late August. The synoptic set-
up for tomorrow is nearly the same as today as the weak upper
circulation remains over south-central Colorado. Steering winds
are going to remain out of the northeast with a moist airmass
remaining over the region. Low clouds lingering overnight will
help hold temperatures down through most of tomorrow morning. By
midday, heating of the moist airmass should cause it to become
unstable with showers developing. Once again the main focus of the
showers should be along the Palmer Divide, where slightly higher
terrain and upslope northeasterlies will help lift the airmass.
Brief heavy rain will once again be the main threat.

.LONG TERM...(Tuesday night through Monday)
Issued at 327 PM MDT Mon Aug 29 2016

Tuesday night and Wednesday: The anomalously deep trough off the
West Coast located around 135W continues to reinforce the mid and
upper level ridge across Colorado on Wednesday. Winds at 500mb are
forecast to be less than 10 knots and temperatures are in the -7C
range. There is a decent amount of moisture available in the mid
and lower levels of the atmosphere. Anticyclonic flow around the
700mb ridge across SD/Nebraska brings some moisture north into
Colorado, particularly southern Colorado. At the surface dewpoints
in the low 50s are will be place along the urban corridor with
mid 50s across the eastern forecast zones. With weak subsidence in
the mid levels and lack of a westerly component to the weak
steering flow, generating or sustaining convection over the Plains
will be quite challenging. Still expect storms to form early
afternoon in the mountains and along the Palmer Divide. The
highest PWATs are across SE CO, but values over an inch will be in
place across eastern Colorado with about 0.75-1.00" along the
urban corridor. Storm coverage will be limited but any storms that
form could produce locally heavy rainfall given the available
moisture and expected slow movement of the storms. Temperatures on
the Plains should reach the low 80s, with low 60s in the
mountains.

Thursday: On Thursday the ridge axis slides just enough east that
Colorado will see westerly mid-level flow, though it will still be
weak. Temperatures should be a few degrees warmer across the CWA.
Available moisture decreases slightly from Wednesday to Thursday
but surface dewpoints will still be around 50 in the urban
corridor to low 50s across the far eastern part of the state.
PWATs near an inch remain in place east of the mountains. All the
medium range models hint at weak synoptic lift across the state
during the afternoon hours. This, combined with diurnal heating
will produce thunderstorms over the mountains that should be able
to move east and continue over the lower elevations. The models do
not show much if any precipitation across the Plains but given the
return of westerly flow and moisture/instability that looks to be
in place, have raised PoPs slightly above model guidance across
the Plains but still just an isolated chance. Storms that form
will have a chance of locally heavy rainfall.

Friday through Monday: Friday looks to be a repeat of Thursday
with ample moisture in place. Storms that form with afternoon
heating should be capable of producing locally heavy rain but
storm coverage east of the mountains will be isolated. A long
wave trough comes onshore late Friday into Saturday with several
short wave troughs rotating around the base of the trough. The
bulk of the synoptic-scale lift associated with the upstream
long wave trough and associated short wave troughs will remain
north of the state through Sunday. However, both the ECMWF and GFS
place the the long wave trough axis over Utah by late Sunday but
quickly differ on whether enough moisture and lift will combine
into precipitation across the state and when the best chances may
be. Followed the blended guidance for PoPs in the Saturday-Monday
time frame given the uncertainty in the evolution of the West
Coast trough.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 857 PM MDT Mon Aug 29 2016

VFR conditions are expected for period with tempo for storms
upstream that could move over the airport between 03 and 05z.
Storms are starting to lose heating which could result in light to
moderate rainshowers and gusty winds over the field. winds will
switch back around to drainage by 06z with a high level deck
through the evening from lingering moisture. By tomorrow winds
will move westerly with a slight chance of afternoon thunderstorms
tomorrow by 19z.

&&

.BOU Watches/Warnings/Advisories...
None.
&&

$$

UPDATE...Bowen
SHORT TERM...Dankers
LONG TERM...Schlatter
AVIATION...Bowen



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