Special Weather Statement, NWS Denver/Boulder, CO

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1201 PM MDT Thu Aug 10 2017

.UPDATE...
Issued at 1200 PM MDT Thu Aug 10 2017

Local ACARS soundings still showing a healthy cap over the Denver
area up around 8500 feet. Will need some convection to fire off over
the higher terrain and push east to erode the cap and allow for
convection over the plains. Will need to delay PoPs slightly for
this.

UPDATE Issued at 1030 AM MDT Thu Aug 10 2017

Northeasterly push currently seen in observations and on satellite
over the northeast corner of the state. This will help provide
lift early this afternoon for storms that fire off the high
terrain. Ample sunshine over the rest of the plains will allow for
surface heating to help increase CAPE values this afternoon.
Several models continue to show 1000-1500 J/kg over the plains,
with areas over 2000 J/kg down over Lincoln County. Bulk shear of
upwards of 50-60 kts will provide storms an environment to become
strong to severe strength. Environment will be suitable for large
hail, up to golf balls with the strongest storms, heavy rain, and
strong damaging outflow winds near 60 mph. An isolated tornado
can't be completely ruled out due to the shear. The best area for
the strongest storms will be over far eastern Elbert and Lincoln
counties. Other areas over the plains could see smaller hail and
gusty winds this afternoon.

&&

.SHORT TERM...(Today and tonight)
Issued at 255 AM MDT Thu Aug 10 2017

A disturbance embedded in the westerly flow aloft is expected to
keep isolated/showers and thunderstorms going through mid morning.
Expect a lull in activity late this morning before convection
reforms this afternoon due to day time heating. A cold front will
push south across northeast Colorado early this afternoon.
Northeast upslope flow behind it will help produce lift and
trigger storms. Thunderstorms development is expected to spread
from north to south through the afternoon and early evening hours.
Chances for severe weather should be limited to the plains south
of I-70. Later arrival of the cold front will allow for more
heating and a little better instability. Hail up to one inch in
diameter and wind gusts to 60 mph will be the main severe threats.
High temperatures today are expected to be cooler due to cloud
cover and a cooler airmass moving in behind the cold front.

Convection is expected to end early to mid evening as the best
lift sinks into southern Colorado. Will have the mention of
showers and storms through most of the evening, then dry after
midnight. Clearing skies and a moist layer of air near the surface
should produce low clouds and patchy fog across northeast
Colorado tonight and into Friday morning.

.LONG TERM...(Friday through Wednesday)
Issued at 255 AM MDT Thu Aug 10 2017

The airmass over north central and northeast Colorado will remain
moist on Friday. In the morning, there will be a fair amount of
low clouds or fog backed up against the Front Range through the
mid morning hours. A moderate west to northwesterly flow aloft
will be in place over the state, with an east/southeasterly flow
at the surface. NAM12 forecast soundings still advertise high
CAPE values in the afternoon, 1500-1800 J/kg. Precipitable Water
values still around one inch. SPC has most of the northeast plains
of Colorado in a marginal risk for severe and this looks
reasonable at this time with large hail and damaging winds the
primary threat. However, plenty of cloud cover around could
hinder the heating which may limit the severe thunderstorm threat.
Storms will likely initiate off the higher terrain than spread
into the adjacent plains. The pattern will remain unsettled on
Saturday and Sunday. The lower levels are progged to dry out
across the Urban Corridor but deeper moisture still present
further east. The flow aloft will remain northwesterly through
Sunday, so will continue to keep sct pops in the grids for most of
the cwa in the aftn and evening hours. Lesser pops along the
urban corridor where it will be drier in the lower levels. The
ridge aloft is still progged to build some for early next week,
with the axis shifting over the state. Warmer temperatures with
more subsidence in the mid levels should help limit the
thunderstorm development to some degree in the aftn and evening.
For now will continue with the trend of warmer and drier next
week. In spite of the warming trend, the temperatures overall will
continue to run below average.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1030 AM MDT Thu Aug 10 2017

Mid and high clouds will stream across the area with ceilings
above 6000 feet. A front will cause winds to turn northeasterly
around 20Z. There will be a good chance for thunderstorms across
the Denver area from 19Z to 01Z. Added a tempo for the expected
thunderstorms. Ceilings could approach MVFR in stronger storms.
Low clouds and fog will form over northeast Colorado after 06Z
Friday. Ceilings are expected to drop below 1000 feet with a good
chance for fog and visibility of a quarter mile or less for Friday
morning.

&&

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

UPDATE...Kriederman
SHORT TERM...Meier
LONG TERM...Cooper
AVIATION...Meier

Hazardous Weather Outlook
National Weather Service Denver/Boulder CO
1051 AM MDT Thu Aug 10 2017

COZ030>051-111700-
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-
1051 AM MDT Thu Aug 10 2017

...Strong storms possible this afternoon into this evening south
of I-70 and east of the Palmer Divide...

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

.DAY ONE...Today and Tonight

Scattered showers and thunderstorms are expected to redevelop this
afternoon. A few storms could become severe, mainly over the
eastern plains south of Interstate 70 over eastern Elbert and
Lincoln counties. Hail up to golf balls, heavy rain and wind
gusts of 60 mph or stronger will be possible with the severe
storms. Elsewhere, storms will be capable of producing brief
heavy rain, small hail, and wind gust to 50 mph.

Storms should move out of the area by midnight, with clearing
skies allowing the formation of areas of fog over the plains into
the early morning hours.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

The pattern will remain unsettled on Friday, with sufficient
moisture coupled with afternoon heating to produce another round
of the thunderstorms. A few of the storms over the far northeast
plains as well as along and southeast of the Palmer Divide could
become severe. The main hazards would be hail up to one inch in
diameter, wind gusts to 60 mph and locally heavy rainfall.

A chance of weaker afternoon and evening showers and
thunderstorms will continue this weekend, with some drying by
early next week. Temperatures will continue to run slightly below
seasonal normals.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed after 2pm today, likely over
Elbert and Lincoln counties.

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DENVER CO
202 PM MDT THU AUG 10 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0200 PM     HAIL             5 NW RED FEATHER LAKES  40.85N 105.65W
08/10/2017  M0.50 INCH       LARIMER            CO   TRAINED SPOTTER

&&

RGD

Special Weather Statement
National Weather Service Denver CO
203 PM MDT THU AUG 10 2017

COZ035-038-102045-
Larimer and Boulder Counties Between 6000 and 9000 Feet CO-
Larimer County Below 6000 Feet/Northwest Weld County CO-
203 PM MDT THU AUG 10 2017

...SIGNIFICANT WEATHER ADVISORY FOR NORTHEASTERN LARIMER COUNTY UNTIL
245 PM MDT...

At 202 PM MDT, trained weather spotters reported a strong
thunderstorm near Red Feather Lakes, or 30 miles northwest of Fort
Collins, moving east at 35 mph.

Dime size hail and winds in excess of 40 mph will be possible with
this storm.

Locations impacted include...
Northeastern Fort Collins, Wellington, Mishawaka, Livermore, Red
Feather Lakes, Teds Place, Bellvue, Poudre Park and Virginia Dale.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

&&

LAT...LON 4061 10494 4061 10518 4077 10565 4079 10564
      4081 10567 4086 10568 4087 10566 4086 10565
      4087 10562 4089 10565 4089 10567 4093 10564
      4100 10494
TIME...MOT...LOC 2002Z 274DEG 29KT 4084 10553

GIMMESTAD