Flash Flood and Hail in Colorado, June 18 to 19, 2018

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1017 AM MDT Mon Jun 18 2018

.UPDATE...
Issued at 1016 AM MDT Mon Jun 18 2018

This morning began with low stratus and areas of drizzle which
could stick around until noon or later in some areas keeping
temperatures cool to start the day. Looking at model soundings of
the Denver metro and nearby plains, temperatures should heat up
to about 80 degrees by mid afternoon allowing for CAPE values to
increase above 1500-1700 J/kg. This would coincide with the HRRR
models which appear to be starting convection near the Denver area
between 2 and 3 PM in current runs. There will be a chance of
large hail and strong winds associated with any severe storm. With
a convergence line due to the Denver Cyclone providing lift and
shear present, a tornado or two cannot be ruled out.

The location of where storms will fire differs between the HRRR
and ESRL HRRR starting storms south of Denver and north of Denver
respectively. Will be keeping an eye on model trends through the
morning.

&&

.SHORT TERM...(Today and tonight)
Issued at 400 AM MDT Mon Jun 18 2018

Water vapor imagery showing tail end of upper trof moving
northward across north central colorado with drying aloft moving
into much of western and central Colorado. Radar imagery still
showing light rain showers shifting north of Denver and focused
closer to our border areas with Wyoming and Nebraska. Much of
these showers should mainly be north of the border by sun up.
Still expect a fair amount of low clouds and some fog covering
much of northeast Colorado with continued light north to northeast
upslope flow.

Main focus for today will be the severe potential across urban
corridor and northeast plains this afternoon and evening. Shallow
moisture and inversions expected to dissipate by this afternoon
while low level flow shifts southeast and increases. This is
expected to develop a developing Denver cyclone which could act as
a focus mechanism for storm initiating this afternoon. High
resolution models continue to focus convection around Denver and
locations north and east. Forecast CAPE values of 1200-1800j/kg
and drying aloft could spell a few severe storms with large hail.
Could also see a few weak tornadoes along the convergence
boundary. SPC already has this area for a marginal threat of
severe and looks reasonable. Could see lingering showers/storms
well into the evening and overnight as upper low shifts into the
Northern Rockies and increasing synoptic scale ascent as noted on
QG fields. Developing surface low pressure over southeast Colorado
and influx of moisture from storms likely to redevelop stratus
and keep low level moisture plentiful through tonight. West of the
continental divide will see much drier air and little if any
chances for storms in next 24 hours.

.LONG TERM...(Tuesday through Sunday)
Issued at 400 AM MDT Mon Jun 18 2018

The large upper trough and its unusually cool airmass hanging out
over the northwest U.S. the past few days will make slow eastward
progress across the northern and central Rocky Mtn region during the
next few days. This will keep temperatures below average in areas
east of the mtns, and near average on the west slope. A shortwave
disturbance rounding the bottom of this trough is progged to swing
acrs the CWA late on Tuesday. At the same time, models show a sfc
trough aligned north-south on the plains from about Ft. Morgan to
Karvel in lincoln County. East of this trough east-southeasterly
winds and sfc dewpts in the mid 50s to lower 60s prevail. West of
this boundary, slightly drier west-southwesterly winds. Along the
southern end of this boundary or convergence zone acrs Elbert,
Lincoln and srn Washington Counties, models indicate CAPES in the
1000-2500 j/kg range by the afternoon. Although, the GFS indicates
its higher CAPES closer to the Kansas border acrs Logan, Sedgwick,
Phillips and earn Washington Counties. Also indicated on model
soundings is the fair amount of CIN around, which may not entirely
erode even during max heating. SPC has essentially all of northeast
Colorado in marginal risk for severe weather Tuesday. Should an
updraft manage to punch through the stg bndry layer cap, it could
produce a storm cell capable of large hail, strong gusty winds and
perhaps a brief tornado. In addition, some storms could also produce
locally heavy rainfall. Don't believe heavy rainfall will be
widespread, so no flash flood watch at this time. But, storm precip
totals from 1 to 2.5 inches will be possible, which could result in
localized flooding of low-lying areas such as along drainages and
dry washes. It's also possible that one or two storms will be
capable of brief heavy rain and small hail over and along the Front
Range with a few hours of upslope flow late in the day. A slightly
drier sub-cloud environment here would favor stronger outflow winds.

On Wednesday, models show the upper trough passing by the forecast
area with the 500 mb low gaining strength over central Nebraska.
This places the CWA under a slightly drier northwest flow aloft. The
passage of a cold front Tuesday night also shifts sfc winds on the
plains to a slightly drier northerly direction. However, by
afternoon, could see winds turn easterly which would moisten the
boundary layer and thereby support scattered t-storm development
over and along the Front Range and Palmer Divide. The potential for
severe storms appears lower on Wednesday due in part to lower
daytime temps.

Thursday and Friday should be drier and warmer with a strong upper
ridge building over Utah and western Colorado. This ridge shifts
more over Colorado on Friday sending daytime temps up another 4-8
deg F. Lower humidities and strong warming aloft should
significantly reduce the chance for t-storms.

By Saturday, medium range models show this ridge moving out of the
picture and being replaced by a shortwave trough which they show
quickly dropping in from the Pacific Northwest. However, may not see
much of an impact on this quick moving feature other than a few deg
drop in temps and slight uptick in afternoon t-storm coverage. By
Sunday, the picture is not as clear as the GFS shows this wave
hanging around for one more day. The Canadian model does the same,
but keeps the cooler air and moisture up over Wyoming. Whereas, the
ECMWF shows this shortwave trough well east of the state by then and
a return to a drier and warmer northwest flow aloft. For now, will
trend towards drier and warmer conditions for Sunday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1016 AM MDT Mon Jun 18 2018

Models are indicating that the stratus may not clear until
18-20z. VFR ceilings are expected except near the foothills where
upslope winds are holding the clouds in. High resolution models
are still showing thunderstorms moving into Denver this afternoon
after 20z. Confidence is increasing with the potential of some
storms hitting severe criteria possibly producing large hail,
damaging winds, and a potential brief tornado.

Tonight there are indications of status reforming over the plains
which could lower ceilings at the terminals to MVFR or IFR
conditions. through early to mid-morning.

&&

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

UPDATE...Sullivan
SHORT TERM...Entrekin
LONG TERM...Baker
AVIATION...Sullivan

Hazardous Weather Outlook
National Weather Service Denver/Boulder CO
1024 AM MDT Mon Jun 18 2018

COZ030>051-191630-
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-
1024 AM MDT Mon Jun 18 2018

...Strong storms possible over Denver Metro Area and areas north
and east this afternoon...

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

.DAY ONE...Today and Tonight

Low clouds and patchy fog and drizzle will continue to diminish
late this morning across the area.

Attention switches to this afternoon when the clouds will burn
off, sufficient heating and moisture will combine with a
developing Denver Cyclone. Scattered thunderstorms will develop
near the convergence zone south of this cyclone as well as over
the Front Range foothills. As of now, this looks to occur by
2 pm before spreading northeast across the rest of the plains. A
few storms may become severe with large hail up to 2 inches in
diameter along with damaging winds and heavy rain. The convergence
zone and shear above this will also allow for a chance of an
isolated tornado or two. The best chance for severe storms will
stretch along and north of a line from south Denver to Akron.
Over the far northeast plains, the airmass will be more stable
with lesser chances of a severe storm, however the synoptic
situation and convective activity pushing east may allow the
storms to last through the evening. Expect another push of low
clouds and patchy fog towards Tuesday morning over the plains.

Further west of the Continental Divide, the airmass will be warm
and dry with no thunderstorms expected.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

Scattered showers and thunderstorms are expected on Tuesday, with
some late day storms on the plains capable of large hail, strong
winds and locally heavy rainfall. The heavy rainfall may produce
flooding in low lying areas such as along drainages and dry
washes. Storm chances will decrease on Wednesday, then dry and
warmer Thursday and Friday. Storms may return for the weekend
with the passage of a weak upper level weather disturbance.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed today, after 1pm.