Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 171753

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1153 AM MDT Sun Sep 17 2017

Issued at 1153 AM MDT Sun Sep 17 2017

Morning soundings show strong low level stability with cool air on
the plains. Expect that surface temperatures will not warm up
enough to support convection in general across the plains.
However, outflow boundaries from the mountains and convergence on
the Denver cyclone boundary may be enough to poke convection
through the weak remaining cap late this afternoon. With high
temperatures in the low to mid 70s, this generates CAPES of
400-600 J/kg, so the severe threat appears low. There is some lift
for this evening that could keep storms going on the plains until
around midnight. Cooled high temperatures slightly, and nudged
PoPs downward and focused on early evening for this update.


.SHORT TERM...(Today and tonight)
Issued at 428 AM MDT Sun Sep 17 2017

Moderate southwesterly flow aloft will continue over Colorado
today as an upper trough moves over Nevada and Utah. A short wave
disturbance embedded in the southwest flow will move across
central and eastern Colorado through the afternoon hours,
generating a round of showers that move out of the foothills early
in the afternoon and then across the plains through the evening.
NAM model soundings over the Denver area show CAPE values peaking
at nearly 1500 j/kg with precipitable waters of 0.80 inches. LCL
values will be around 11000 feet MSL, with mid level winds around
40 knots. Gusty winds up to 40 mph, brief rainfall and hail up to
dime size will be the primary threats from the passing storms.

Temperatures will be 5 to 10 degrees warmer than yesterday due to
the southwesterly flow bringing in air from the desert southwest.
Mountain temperatures will start the day on the chilly side, but
no precipitation is expected until early afternoon when
temperatures will be warm enough for showers to be in the form of
rain. Showers will end over the foothills and urban corridor by
late afternoon when shower activity shifts out onto the plains
and lasts through the evening and into the early morning hours.

Drying is expected over the mountains overnight as flow aloft
switches to westerly, in the wake of the passing upper level

.LONG TERM...(Monday through Saturday)
Issued at 428 AM MDT Sun Sep 17 2017

Dry WSW flow aloft will be over the area on Mon as a sfc lee trough
intensifies over nern CO.  Thus expect a dry day with highs rising
into the lower to mid 80s across the plains.  For Tue into Tue night
an upper level trough will move across the nrn Rockies with rather
brisk WSW flow aloft. There may be just enough moisture for a
slight chc of showers in the mtns nr the WY border otherwise it
will remain dry. At the sfc, low pres will intensify across nern
CO with a pacific fnt moving across the plains Tue night. In
advance of this fnt, expect another day of abv normal temps as
readings reach the mid to upper 80s across the plains. Furthermore
with increasing flow aloft expect there will be gusty winds over
the higher terrain and across the plains as decent mixing occurs.

On Wed, dry SW flow aloft will be over nrn CO as an upper level
trough moves into the nwrn US. Thus expect no pcpn with slightly
cooler temps as highs range fm the upper 70s to lower 80s across
the plains. By Thu an upper level trough will be over the wrn US
with mainly dry SWLY flow aloft remaining over the area. Thus will
continue with a dry fcst as highs continue to be fm 8 to 12
degrees abv normal over nern CO.

For Thu night into Fri the flow aloft will become more SSW as some
moisture eventually becomes embedded in the flow.  Thus will mention
a chc of showers in the mtns on Fri with a slight chc over the
plains.  Meanwhile the GFS has a cool fnt moving across the plains
by Fri morning while the ECMWF holds this fnt off until Fri evening.
As a result the GFS has highs in the 60s on Fri while the ECMWF has
readings in the lower to mid 80s over nern CO. At this point will go
with the blended solution and keep highs in the mid to upper 70s.

By Fri night into Sat the ECMWF keeps the main upper level trough to
the west of the area while the GFS moves the trough across the area.
As a result the GFS has a cooler and wetter solution while the ECMWF
would be drier and not as cool.  Once again blended solutions are
probably the best way to go due to all of the uncertainty.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1153 AM MDT Sun Sep 17 2017

VFR through tonight. Scattered thunderstorms will develop over
the mountains this afternoon, with isolated storms in the Denver
area from 21z- 01z. Variable winds gusting to 30 knots are likely
for an hour or two in this time window.


Issued at 428 AM MDT Sun Sep 17 2017

Fire Danger will increase from Monday into Tuesday over the higher
terrain and plains due to above normal temperatures, low humidities
and increasing wind.  Fire Danger will likely remain elevated from
Wednesday into Thursday as well across much of the area.




SHORT TERM...Dankers
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