Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 290941
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
341 AM MDT Thu Jun 29 2017

.SHORT TERM...(Today and tonight)
Issued at 340 AM MDT Thu Jun 29 2017

Just a few exiting storms over far eastern Colorado early this
morning aided by sagging upper jet now across southern portions
of the state. Showers will have dissipated before sunrise under
mainly clear skies.

Today will be characterized with a relatively dry moderate
westerly flow aloft then an upper trof will approach Northern
Colorado by later tonight with increasing QG ascent ahead of the
trof. Jet streak associated with the trof will also be draped
across northern Colorado helping to aid showers and thunderstorms
for much of tonight. Building high pressure over the norther high
plains will keep much of northeast Colorado under shallow upslope
flow this afternoon and tonight. Temperatures will continue to
cool by 3-5 degrees this afternoon with behind shallow low level
cooling tonight. Readings will mainly be in the mid 80s across the
plains.

High resolution models continue to focus on Elbert and Lincoln
counties again for thunderstorms and even the chance for a severe
storm with surface based CAPES ranging from 1000 to 2000j/kg.
Further north and east, CAPE values are generally under 800j/kg
and lower dewpoints especially along the Front range with readings
dropping into the 30s/lower 40s. Initially this afternoon, storms
will mainly have gusty winds with only brief rain. With
increasing upslope flow this evening, low level moisture will
increase allowing a better chance for showers. Have increased pops
tonight due to the upslope flow and the lift moving in tonight.May
even some stratus developing later tonight so will bump up cloud
cover with mvfr conditions.

.LONG TERM...(Friday through Wednesday)
Issued at 340 AM MDT Thu Jun 29 2017

Foresee a number of changes in the large scale height/circulation
pattern over Colorado and the swrn U.S. during the extended period.
The first of these changes involves a marked cool down on Friday
with a passing mid-level trough and a moist post-frontal upslope
flow on the plains. Latest model guidance continues to trend
temperatures downward for Friday. For instances, 24 hours ago, NAM
MOS temp guidance gave Denver a high of 77 on Friday. The 29/0000Z
NAM MOS now indicates a max temp of only 71. On the other hand, the
last few runs of the GFS, ECMWF and Canadian Gem have not been as
cool or as wet as the NAM as they do not maintain the moist upslope
flow east of the mtns as long, and are not as cool aloft. For now,
will use a blend of these models and stick with mid 70s for highs on
the plains. However, should the upslope flow be as deep and persist
as long as the NAM now indicates, high temps may end up being 3-5
degs F cooler, particularly should a heavy cloud cover linger for
most of the day up against the foothills. As for precip chances,
will raise PoPs slightly above those now in the forecast, with the
higher PoPs on the plains during the morning hours and over and near
the Front Range during the afternoon with heating and continued
orographic forcing. Overall, 12-hr QPF should be light, generally
under a quarter inch. Meanwhile, on the west slope, dry and warm
conditions will persist with 70s in the valleys and 60s on the high
slopes. With relatively light winds, the fire danger will ease some.
By nightfall, precip east of the mtns should quickly decline from
west to east with subsidence and drying kick in behind the exiting
trough.

The next change in our weather will happen this weekend with an
upper-level ridge building in from the Four Corners region.
Subsidence and warming aloft on the front side of this ridge should
pretty much suppress deep convection for all but the higher
elevations of the Front Range mtns on Saturday. Weakly developed
updrafts at higher elevations may manage to produce a few, widely
spaced t-storms. With temps warming aloft, so will sfc temps. Should
see highs on Saturday 8-12 degs F above those predicted for Friday.
By Sunday...the 500mb ridge axis and warmest air aloft are predicted
to be over western Colorado by morning. GFS shows the ridge moving
east across the state as a weak shortwave trough drops down over
Utah. Other models show the ridge retrograding resulting in
northwest flow aloft over the CWA. This appears to generate a dry
and warmer downslope flow off the nrn fthls and Cheyenne Ridge which
warms sfc temps another 4-7 deg f on the plains. T-storms on Sunday
should be isolated in coverage, and mainly over the Front Range and
Palmer Divide.

Sunday night and Monday, models differ in the details, but all show
a weak shortwave trough passing over the fcst area during the
period. Slightly cooler temperatures, mid-level instability and
slightly elevated PW should be adequate to go with a slight chance
of afternoon t-storms mtns and plains.

Independence Day and Wednesday...should see the large upper ridge
over the Great Basin returning to Colorado with 700 mb temps warming
to around 17C. The airmass accompanying the ridge appears quite dry,
although we may still see a few gusty storms form initially over the
high terrain and drifting over the nearby plains late in the day.
Highs both days expected to be in the lower 90s, with very warm 70s
and lower 80s in the high country.

One final note, medium range models are now hinting at the formation
of the North American summer monsoon which they show organizing over
western Mexico and lower deserts of American southwest. No impact on
us quite yet, but perhaps by late next week. Stay tune.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 340 AM MDT Thu Jun 29 2017

Mainly clear skies through this morning. Approaching upper trof
and shallow upslope flow will result in isolated high based storms
by later this afternoon, then as moisture increases this evening a
better chance for scattered showers and storms continuing through
the night. Will keep winds at an northeast component through
tonight.

&&

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

$$

SHORT TERM...Entrekin
LONG TERM...Baker
AVIATION...Entrekin



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