Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 171721
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1121 AM MDT Sat Jun 17 2017

.UPDATE...
Issued at 1121 AM MDT Sat Jun 17 2017

Today`s cold front passage will be the usual mess. We`ve already
had a couple of shallow surges of slightly cooler air out of the
showers across southern Wyoming and the Nebraska panhandle, the
prefrontal winds across southeast Wyoming are also helping to
force and anticyclonic turning of the wind toward the north and
northeast across northeastern Colorado, and the pressure surge has
gotten out ahead of the temperature gradient. The end result will
be a more gradual trend toward increasing northerly winds this
afternoon. There is a shower band aloft, but it is no longer
working on low level convergence or unstable air, so this is
fading as it slides east. The prospect of storms right along or
behind a sharp frontal boundary is fading, though there`s still a
chance of coupling the cooler but moister air over NE Colorado
with the forcing aloft. Lowered PoPs a bit more and not too
concerned about anything strong anymore as the best instability
will be shifting into Kansas and Nebraska. There is still a sharp
temperature gradient upstream with 20 degree colder temperature
from Casper toward the Black Hills, so temperatures falling off in
the late afternoon still looks good.

&&

.SHORT TERM...(Today and tonight)
Issued at 245 AM MDT Sat Jun 17 2017

For today, an increasing northwesterly flow aloft will develop as
an upper level jet (100+ kts) over the northern Rockies, spreads
south and east across southeastern WY and NE by 00z this evening.
The models also indicate moderate downward qg in the mid levels
spreading southward across the cwa around 00z. Both the NAM12 and
GFS show computed CAPES in the 500-1000 j/kg range at 18z then
show a sharp decrease in those values fm north to south through
late afternoon. At the sfc, gusty west to northwest winds will be
in place early this afternoon ahead of an approaching cold front
which will spread south across the cwa fm roughly 21z-00z this
aftn. Although it will not be as hot as yesterday, high
temperatures will still be in the mid to upper 80s today, 4-8
degrees above normal.  Will maintain 10-20 percent pops for
thunderstorm coverage, but may not have much by this evening with
increasing subsidence and diminishing CAPES after 00z. Best chc by
that time will be south and southwest of Denver. Overnight, winds
will generally be northeasterly over the northeast plains with
spatial cross-sections showing mostly cloudy conditions overnight
in the 750-600 mb layer. Fire danger will be elevated today with
the gusty winds and warm temperatures, but minimum relative
humidities should be high enough, in the upper teens to mid 20s
with the driest conditions south and southwest of Denver.

.LONG TERM...(Sunday through Friday)
Issued at 245 AM MDT Sat Jun 17 2017

By Sunday morning, the forecast area will be under relatively strong
northwest flow aloft on the back side of the departing upper trough.
At the same time, a strong surface high is progged to drop down
from Wyoming and slowly migrate southward across ern Colorado during
the day bringing with it cooler temperatures and light breezes to
the CWA. Out ahead of this bubble of high pressure, models show a
swath of low to mid-level clouds passing over ern portions of the
CWA during the morning, together with weak nely/upslope flow. By
early afternoon, the sfc high will be in sern Colorado, creating
s-sely bndry layer winds up here. This wind shift should clear
skies and help moderate the cooler post-frontal airmass. Temps on
Sunday will still be coolish compared to today`s readings with
highs around the 80 degree mark over northeast Colorado. And, with
a strong capping inversion between 700-600 mbs, there`s virtually
no chance for t-storms even in the high country. By Sunday eve,
models show a secondary wave of mid-level moisture passing over
eastern sections of the CWA. However, showers are not expected
with this moisture with neutral ascent at best. After midnight,
should see this moisture flush out by what appears to be a rather
stiff sly low-level jet /LLJ/ which models show forming just east
of the I-25 corridor/Denver metro area around midnight and then
shifting east to the CO/KS state line by dawn. This LLJ marks the
start of what will likely be a very warm if not hot and mostly dry
week ahead for the fcst area.

For next week, a large dome of hot and dry air building over the
Intermountain West Sunday into Monday is predicted to flatten as
it gradually migrates southeastward over the central and southern
Rocky Mountain region by Tuesday. This causes the northwest flow
over Colorado to turn zonal by late Tuesday and remain that way
through Thursday. Before the big warm-up, a relatively moist
easterly/upslope flow east of the mtns Monday should hold max
temps to the low/mid 80s on the plains and possibly generate a
few gusty, poorly organized t-storms late in the day over the
foothills. In areas from the Continental Divide westward, the
warm-up will be underway with highs in the lower 80s on the
valley floors.

For the period Tuesday-Thursday, downslope flow and warming aloft
will combine to produce daily max temps in the 90s at lower
elevations. Can not rule out a new record high somewhere. High
country temps will also run well above average for mid June with
highs generally in the 70s and 80s. It should be dry through the
period, although a weak upper air disturbance passing late on
Wednesday could produce a few gusty late day t-storms over higher
elevations.

By Friday, the GFS and ECMWF show another weak mid-level shortwave
embedded in west-northwest flow aloft brushing the CWA late in
the day. Prior to its passage, models also show a cold front
backing into nern Colorado during the morning hours. Nely flow in
its wake will presumably usher in slightly cooler temperature and
higher humidity, as well as elevate the chance for showers and
t-storms across the CWA.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1121 AM MDT Sat Jun 17 2017

VFR through tonight. The threat of thunderstorms is diminishing as
the cold front coming through this afternoon is no longer well
defined and instability appears to be inadequate. North winds will
increase with gusts in the 20-30 knot range, then diminish after
00z.

&&

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

$$

UPDATE...Gimmestad
SHORT TERM...Cooper
LONG TERM...Baker
AVIATION...Gimmestad



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