Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 182008
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
208 PM MDT THU AUG 18 2016

.SHORT TERM...(This evening through Friday)
Issued at 133 PM MDT Thu Aug 18 2016

Convection is well underway across mountains and foothills and
will spread over the plains through early evening. Initial outflow
already pushing over Northern urban corridor which will likely
spark more showers through late afternoon. Overall convection has
been on the weaker side with gusty winds to 35 mph and some
rainfall. Rainfall rates initially have been up to a third of an
inch per hour in the mountains. PW values will continue to slowly
increase and will likely be around an inch by this evening over
the Eastern plains with the potential for heavier rainfall as the
airmass becomes more saturated. Could see a break behind the
initial band of convection from West to East from mid afternoon
through early evening over the mountains and adjacent plains given
decent clearing now in Northwest Colorado. However, with the upper
low in Southwest Wyoming expect storms and showers to redevelop
and fill in back in the mountains and plains during the evening.

Current cold front moving into Southeast Wyoming from Casper to
Lusk and will move into Northeast Colorado this evening. Given
the approach of upper trof expect showers to go through midnight
then taper off through the night. Behind the front later
tonight...expect and increase in low level moisture and stratus
forming across the plains.

As for Friday afternoon, expect a round of convection and likely
stronger thunderstorms than today given higher CAPE values around
1000j/kg and adequate shear. Could be a few severe storms on the
plains...especially from Denver south across the Palmer Divide.
Appears there will be another frontal passage over the plains by
late afternoon which begin to shift focus for storms along and
south of the frontal boundary. Strong subsidence may end showers
over far Northern sections by late in the day. Temperatures will
be much cooler on Friday with highs mainly in the 70s over the
plains.

.LONG TERM...(Friday night through Thursday)
Issued at 133 PM MDT Thu Aug 18 2016

By Saturday morning the broad trough axis will be located over
the upper Mississippi Valley putting Colorado in weak WNW flow
aloft with the 80kt jetstream on the west side of the trough
staying well northeast of Colorado. The northwest flow aloft will
help drive a cool front at the surface that will bring increased
north-northeast winds and a reinforcing shot of cool temperatures
though still believe Saturday will be a bit warmer than Friday
across the Plains. The air behind the front is fairly dry and the
best chance of showers and thunderstorms Saturday
afternoon/evening will be over the Central Rockies and south of
I-70 along the Palmer divide, though any storms that form will be
isolated. PWATs across the area decrease relative to Friday to
around the 0.6-0.8" range resulting in less precipitation for the
storms to work with. Skies will be mostly cloudy across the
southern 2/3 of the CWA especially along and near the Palmer
Divide given north/northeast low level flow. Further north across
northeast Colorado the models are showing partly cloudy to mostly
clear skies after noon given the dry northwest flow.

On Sunday, models are in agreement and continue to indicate a
warming and drying pattern as the upper trough slides further east
and a ridge builds over Colorado.  Temperatures on Sunday will warm
to the low 80s across the Plains with lower 60s in the higher
terrain. Moisture across eastern Colorado will be limited with
PWATs decreasing to the 0.50" range across the CWA and surface
moisture pushed east and south of the CWA. Surface dewpoints will
remain in the mid 40s plains and 30s in the mountains. Winds at
the surface will be light across the CWA wide with southerly flow
across far eastern colorado and East/southeast flow north of the
Palmer divide along the foothills. It should be dry across the
entire CWA on Sunday other than a slight chance of an isolated
thundershower in the mountains south of I-70.

On Monday the ridge shifts east and the mid- and upper-level flow
pattern out of the south and southwest hints at some monsoonal
moisture. As a result rain chances go up in the mountains driven by
daytime heating. Across the plains, low level moisture will be
limited and rain chances should remain low. With mostly clear
skies through the early afternoon and dry low level air, the
plains should warm into the upper 80s to near 90 before afternoon
cloud cover moves from west to east driven by convection in the
mountains.

Tuesday through Thursday the GFS and ECMWF both indicate a weak
upper level trough passing over Colorado mid week. Models in
agreement that Wednesday and Thursday will be cooler than Monday
and Tuesday, and have decreased temps accordingly in the forecast.
There is a lot of uncertainly about precipitation chances but with weak
westerly flow aloft and lack of a well-defined moisture source,
did not see a need to adjust the Tuesday - Thursday low
precipitation chances.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 133 PM MDT Thu Aug 18 2016

Initial outflow from mountain showers will move into local
terminals between 2030-22z with gusts up to 30kt from the
Northwest. Additional showers look to form along and ahead of the
outflow through late afternoon. Could be a break later
afternoon/early evening before frontal passage and additional
showers/storms through midnight. Expect stratus to develop later
tonight as low level moisture increases over the area.

&&

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

$$

SHORT TERM...Entrekin
LONG TERM...Schlatter
AVIATION...Entrekin



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