Special Weather Statement, NWS Denver/Boulder, CO

Area Forecast Discussion
National Weather Service Denver/Boulder CO
215 PM MDT Mon Jul 3 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 156 PM MDT Mon Jul 3 2017

Upper trough has moved to the SE portions of the state with
increasing moisture helping to fire off afternoon convection.
Portions of the higher terrain and and eastern plains will see
increased chances of slower moving, single cell linear storms
through the early afternoon. CAPE and shear along with increased
low level moisture exist over the far eastern plains near NE and
KS. Closer to the foothills areas could still see a slight chance
of storm development with the main hazard being gusty outflow
winds. Hail is less of a threat further west but some light rain
and occasional lightning will be possible. Further east the
hazards will be winds gusting up to 60 mph with possible pea to
quarter size hail and lightning with the stronger storms. Areas to
the south of the Metro and North will see increased coverage as a
low-level circulation has helped to usher in some low -level
moisture. This will help to fire off storms late this afternoon.

For Tuesday the upper ridge that is in place over the SW will
start to build in helping to increase stability over the region.
Tomorrow will be a transition day with a convection picture
similar to today. CAPE values ranges from 400-800 over the area
with weak shear and dry in the lower levels. Main hazards for
tomorrow be winds...lightning and some stronger storms to the
east. Temperatures will once again be above average with 700mb
temperatures slightly higher than today with highs in the lower to
mid 90s.

.LONG TERM...(Tuesday night through Monday)
Issued at 156 PM MDT Mon Jul 3 2017

A ridge of high pressure aloft will dominate the sensible weather
across our area over the extended time period. The center of the
ridge wobbles a bit Wednesday through early next week but in
general remains over western Colorado and the western half of the
Four Corners region. The persistence of this ridge, in both
location and strength, would mean monsoonal moisture should not
make a run at the state of Colorado for at least the next week or
more. The obvious impact of a strong ridge just upstream of our
area are above normal temperatures in the extended and
a suppression of thunderstorm development east of the Rockies.
Despite hot and dry conditions the winds near the surface should
be relatively weak area-wide lessening the potential spread of new
fires that develop. Specifics on the extended forecast below:

Wednesday and Thursday look to be the hottest afternoons in the
extended period because of the collocation of the 500 mb and 700
mb ridges over western Colorado. On Wednesday afternoon 700 mb
temps are forecast to be around 17-18 degC, while on Thursday
afternoon 18-19 degC. Quite toasty but not warm enough for record
high temperatures both days, but expect highs in the mid to upper
90s below 6000 feet elevation, and 70s to upper 80s across the
higher terrain. Thunderstorm chances are low across the Plains
with weak low-level moisture that quickly mixes out by midday, and
PWATs around climatological averages of 0.5-0.75". Chances of
diurnal convection are quite a bit higher in the mountains
especially the Front Range and Central mountains as enough mid-
level moisture is available for widely scattered storms. Any
storms that form should dissipate as they run into the dry air
east of the mountains.

On Friday the 598 dam ridge retrogrades slightly west across
southern Utah, putting northeast Colorado in weak northwest flow
aloft. The GFS and NAM both hint at a weak short wave trough
moving across southern Wyoming, which generates convection and
surface high pressure to the north, leading to the passage of a
cool front late in the day on Friday. The ECMWF does not show a
short wave and thus does not generate convection nor a surface
high pressure area. However, all models indicate a cooling around
700 mb across the northeast quarter of the state. This points to
cooler high temperatures 5-10 degrees most areas across the
Plains compared to Thursday. The mountains will not see a similar
drop in temperatures given proximity to the 500 mb ridge and 700
mb temps remaining close to Thursdays highs across central
Colorado. Thunderstorm chances Friday afternoon overall should increase
mountains and Plains but the low levels are still very dry that
chances are just 10-20% below 7000 feet elevation.

Saturday through Monday the mid and upper level ridge weakens a
bit and remains upstream of Colorado for continued weak northwest
flow aloft over our area. The GFS keeps the ridge over southern
Utah/northern Arizona, while the ECMWF moves it slowly east such
that by early next week it moves over eastern Colorado but
thankfully not as strong as this week, with a height of 593 dam.
With the ridge upstream for several days the GFS increases the
PWAT to over 1.0" from I-25 east to the Kansas border Friday,
Saturday, and Sunday afternoon, with 0.5-0.75" over the mountains.
If that were to occur the PWAT would exceed the 90th percentile
relative to climatology. Given model disagreement in that time
frame and a unfavorable trajectory for rich moisture to stream
into the state, feel the GFS is overdone in terms of convective
chances this weekend. The blended forecast approach yields 20-40
PoPs each afternoon for the mountains and 5-20 PoP for the Plains,
which at this point are reasonable. High temperatures will
continue above normal during this time frame with low 90s plains
and mid 70s to upper 80s across the mountains.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 156 PM MDT Mon Jul 3 2017

Main impacts to area airports for the TAF period will be gusty
outflow winds from area storms. Storms will generally produce
light rain and some lightning with winds up to 40 mph. Expect this
to impact mainly DEN and APA with some gusting up to 30 possible
over BJC through 23z. Ceilings will remain around 150 and above
with mostly FEW to SCT. Winds are expected to veer around as
storms move to the SE through the evening then around to drainage
by 05z. Expect tomorrow to be a similar set-up with VCSH in the
afternoon with the main impact being gusty outflow winds.

&&

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

SHORT TERM...Bowen
LONG TERM...Schlatter
AVIATION...Bowen

Hazardous Weather Outlook
National Weather Service Denver/Boulder CO
1035 AM MDT Mon Jul 3 2017

COZ030>051-041645-
Jackson County Below 9000 Feet-
West Jackson and West Grand Counties Above 9000 Feet-
Grand and Summit Counties Below 9000 Feet-
South and East Jackson/Larimer/North and Northeast Grand/
Northwest Boulder Counties Above 9000 Feet-
South and Southeast Grand/West Central and Southwest Boulder/
Gilpin/Clear Creek/Summit/North and West Park Counties Above
9000 Feet-Larimer and Boulder Counties Between 6000 and 9000 Feet-
Jefferson and West Douglas Counties Above 6000 Feet/Gilpin/Clear
Creek/Northeast Park Counties Below 9000 Feet-
Central and Southeast Park County-
Larimer County Below 6000 Feet/Northwest Weld County-
Boulder And Jefferson Counties Below 6000 Feet/West Broomfield
County-
North Douglas County Below 6000 Feet/Denver/West Adams and
Arapahoe Counties/East Broomfield County-
Elbert/Central and East Douglas Counties Above 6000 Feet-
Northeast Weld County-Central and South Weld County-Morgan County-
Central and East Adams and Arapahoe Counties-
North and Northeast Elbert County Below 6000 Feet/North Lincoln
County-
Southeast Elbert County Below 6000 Feet/South Lincoln County-
Logan County-Washington County-Sedgwick County-Phillips County-
1035 AM MDT Mon Jul 3 2017

This hazardous weather outlook is for northeast and north central
Colorado.

.DAY ONE...Today and Tonight

Isolated showers and thunderstorms will develop over the
mountains and adjacent plains by this afternoon before spreading
east on the plains through the early evening hours. Expect gusty
winds to 40 mph but only brief light rain for mountains and
speeds of up to 30 mph over the adjacent plains. Potential for a
few strong to severe storms increases over the far northeast
plains, generally east of a line from Akron to Sterling. Damaging
winds up to 60 mph and large hail are possible. Temperatures will
reach into the low 90s today with 50s to lower 60s expected
overnight.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

Temperatures will be warming Tuesday through Thursday, with mid
90s expected across the Plains. Isolated to scattered
thunderstorms are possible over the mountains each afternoon,
while the Plains will have a slight chance of thunderstorms
Tuesday, then again Friday through Sunday. The main impacts from
any storms that form will be lightning and gusty winds to 40 mph.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed over the far northeast plains of
Colorado after 2 pm.

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Special Weather Statement
National Weather Service Denver CO
230 PM MDT MON JUL 3 2017

COZ040-045-032115-
Central and East Adams and Arapahoe Counties CO-
North Douglas County Below 6000 Feet/Denver/West Adams and Arapahoe
Counties/East Broomfield County CO-
230 PM MDT MON JUL 3 2017

...SIGNIFICANT WEATHER ADVISORY FOR NORTH CENTRAL ARAPAHOE...
NORTHEASTERN DENVER AND CENTRAL ADAMS COUNTIES UNTIL 315 PM MDT...

At 226 PM MDT, Doppler radar was tracking a weakening high based
thunderstorm producing strong and gusty microburst winds. This storm
was 5 miles north of D.I.A Terminal and Concourses, or 21 miles
northeast of Denver, moving southeast at 30 mph.

Wind gusts in excess of 40 mph will be possible with this storm.

Locations impacted include...
Aurora, eastern Commerce City, Denver International Airport, Bennett,
Watkins, Manila Village, D.I.A Terminal and Concourses and Strasburg.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Prepare for sudden strong and gusty winds capable of producing
considerable blowing dust. Secure loose objects and move to a safe
shelter inside a building or vehicle.

&&

LAT...LON 3994 10424 3967 10431 3965 10449 3966 10450
      3965 10450 3965 10452 3964 10456 3963 10467
      3987 10479 3999 10453
TIME...MOT...LOC 2026Z 325DEG 26KT 3993 10464

BAKER