Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 171606
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1006 AM MDT Wed Jul 17 2019

.UPDATE...
Issued at 1006 AM MDT Wed Jul 17 2019

We are keeping a close watch on the outflow boundary making it`s
way into the northeast corner of Colorado. An MCS across southern
South Dakota early this morning has generated a rather large
outflow boundary, clearly visible in GOES-16 and in surface
observations. The stratus deck behind the boundary has made it as
far as the CO border north of Crook, and it continues to ooze
southwestward. The boundary itself based on surface
winds/pressure/temperature trends is well into Colorado, currently
extending from northern Weld County southeast into Washington
County. Depending on how far south and west this boundary gets is
critical to the eventual high temperature for this afternoon, and
may become a focus for convection this afternoon. It`s awfully
cool (from an "awesome" and temperature standpoint) where there
is cloud cover behind the gust front, with temps in the 60s across
the Nebraska panhandle and southeast Wyoming. With mostly clear
skies immediately behind the boundary, it`s being rapidly modified
by the July sun so in theory should be washing out without
getting too much further south and west. However, it should cool
the northeast corner of Colorado several degrees, and the previous
forecast had this well in hand. Elsewhere across the plains the
downslope flow today is on track, and highs 95-100 look easily
attainable.

What about convective chances? PW has continued to decrease area-
wide and current GOES-16 imagery shows very little cumulus
development across the mountains, much less than any previous
days this week at the same time of day. Thus, do not expect
anything more than a few clouds there and across the I-25 urban
corridor and plains out to I-76 or so. The exception is across
eastern Elbert and Lincoln Counties. The previously mentioned gust
front with a nice shot of low-level moisture set up a strong
moisture gradient, and at max heating that gradient should be
across Lincoln or western Elbert County. The HRRR has consistently
picked up on this feature, and generated thunderstorms after 3
PM. PoPs will remain in the 20-35% range in this area for this
possibility, including Washington County where storms that form
may cross the southern/eastern part of the County along the
moisture gradient. Storms would be capable of gusty winds, small
hail, and move quickly east-northeast out of our area during the
evening. Made minor adjustment to PoPs to reflect a bit more
confidence in the convective evolution for this afternoon.

&&

.SHORT TERM...(Today and tonight)
Issued at 335 AM MDT Wed Jul 17 2019

This morning features an upper trough extending from the Northern
Rockies down into the Great Basin while an upper ridge covers the
Great Plains. Moisture the southeastern portion of the forecast
area and a jet to the north has kept isolated showers and
thunderstorms going overnight and into the early morning hours.
This may continue for another couple hours based off trends in
water vapor imagery, cloud top temperatures in IR imagery and
radar echoes. A surface trough over the eastern plains will deepen
today to produce a warming and drying downsloping pattern. This
will bring another 2 to 3 degrees of warming today for the area,
with more warming along the foothills and western urban corridor.
The exception will be over the far eastern plains where slight
cooling and moisture will push in from the north due to outflow
from thunderstorms currently over South Dakota and a weak cool
front. This will likely result in a couple degrees cooler than
yesterday, and also provide enough moisture for isolated showers
and thunderstorms this afternoon, mainly southeast of I76.

Tonight will have slightly cooler temperatures compared to early
this morning with mostly clear skies and weaker winds.

.LONG TERM...(Thursday through Tuesday)
Issued at 335 AM MDT Wed Jul 17 2019

The main weather story for Thursday and Friday will be the heat
and possible record breaking temperatures. There will be a
moderate westerly flow aloft over Colorado which will bring a
drier airmass in from the west with little or no chance of
thunderstorms through Friday. Meanwhile, 700mb temperatures climb
to near +20c which will result in afternoon high temperatures
around 100 degrees across lower elevations. The best chance for
breaking the record at Denver is on Thursday where the previous
high temperature is 99 degrees and we should have a good chance of
at least tying or breaking that record.

Some relief is in sight for the weekend as the upper trof which
slides eastward across the northern high plains drops down a cold
front into northeast Colorado on late Saturday. This will bring a
better chance for thunderstorms as moisture levels increase along
with shallow upslope flow. The best chance for storms will be over
the mountains and adjacent plains. Temperatures will cool down,
especially on Sunday with readings back into the 80s across the
plains.

For next week, the high pressure ridge aloft will rebuild over the
eastern Great Basin with a resultant north to northwest flow aloft
over Colorado. This will keep the region susceptible to surges from
the north and persistent low level upslope flow. Precipitable water
values will surge to over an inch so main threat will be heavy rain
with the storms. This pattern looks to remain over the region
through much of next week. The best chance for storms will again
be over the mountains and adjacent plains as airmass may be a bit
too stable over the far plains.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1006 AM MDT Wed Jul 17 2019

VFR conditions and no convection expected to impact the terminals
now through Thursday. There is an outside chance DEN`s southeast
gate may see thunderstorms after 4 PM this afternoon, mainly
across far eastern Colorado. Elsewhere it should remain free of
any convection. and for the most part we also don`t expect any
sudden wind shifts from gust fronts and outflow boundaries. Expect
winds to come around to northwest today by early afternoon and
continue into the evening. Should be in the 10-20 kt range, with
the strongest winds at DEN. SSW drainage winds tonight will again
be up around 15 kts at APA and DEN, around 10 at BJC out of the
west. By Thursday morning winds will go more west, and should
remain generally out of the west most of the afternoon in the 8-12
kt range.

The main impacts today through Friday at the terminals will be the
heat resulting in low density altitudes. Temperatures will be
95-100 from noon through early evening this afternoon, Thursday,
and Friday.

&&

.FIRE WEATHER...
Issued at 335 AM MDT Wed Jul 17 2019

Near critical fire weather conditions may occur today near the
northern foothills and adjacent plains due to low humidity
near 10 to 15 percent today and gusty westerly winds between 15
to 25 mph. However, fuels seem to be relatively green, but confidence
in fuel moisture is low. Will refrain from issuing any highlights
at this point, but will continue to evaluate fuel moisture.

Elevated fire danger will continue on Thursday given
the low humidities and breezy conditions in the mountains.
However, fuels moisture is still relatively high in the mountains
and foothills so no highlights are planned. Conditions expected
to improve slightly on Friday with a bit less wind.

&&

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

$$

UPDATE...Schlatter
SHORT TERM...Kriederman
LONG TERM...Entrekin
AVIATION...Schlatter
FIRE WEATHER...Kriederman/Entrekin


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