Tornadoes and Hail in Colorado, September 11, 2019

Area Forecast Discussion
National Weather Service Denver/Boulder CO
238 PM MDT Wed Sep 11 2019

.SHORT TERM...(This evening through Thursday)
Issued at 238 PM MDT Wed Sep 11 2019

A strong upper level trough will move across the Central and
Northern Rockies tonight. It will strengthen into a closed low over
the Northern Plains Thursday. Scattered to numerous showers and
thunderstorms are expected to form and track northeast north of I-
70. The best chance for convective will be across far northern
Colorado. Surface based CAPE will be 1500-2500 J/kg. This combined
with good shear is expected to result in some severe storms. Hail up
to 2 inches in diameter, wind gusts to 70 mph, and an isolated
tornado or two will be possible. Best chance for severe storms will
be northeast of a line from Greeley to Akron. A cold front drops
south across eastern Colorado this evening bringing gusty north to
northwest winds. This could also trigger a few weak showers and
storms through the evening. Over the mountains, favorable orographic
flow and moisture will result in rain and snow showers over the
mountains. Snow level falls to around 10000 feet after midnight.
This could result in a few inches of snow above timberline.

Subsidence and dry air moves in behind the exiting system for
Thursday. Gusty northwest winds will prevail across the plains and
northern foothills. It will be cool and sunny across the area with
highs in the 70s across northeast Colorado.

.LONG TERM...(Thursday night through Wednesday)
Issued at 238 PM MDT Wed Sep 11 2019

Dry and stable weather will be the rule for much of the extended
period with temperatures near to above seasonal average. Will need
to monitor Saturday afternoon for potential critical fire weather
concerns across the higher northern mountains and North Park. The
next period of unsettled weather arrives in the late Tuesday and
Wednesday timeframe. No significant changes to the current
forecast.

For Thursday evening into Friday, widespread mid and upper level
subsidence will dominate the central Rockies in moderate westerly
zonal flow aloft behind the main trough axis which passes across
the region Thursday morning. As the upper closed low moves out of
the northern Rockies and into the northern Plains states, surface
return flow brings moist northward across eastern CO on the west
side of a weak surface high over the central US plains. This will
form an axis of ML Capes in the 500-1500 J/kg range over the far
northeastern counties for late Friday afternoon and evening. In an
environment of strong synoptic subsidence, forecast soundings
indicate the mid-upper level stability eroding by 06z Saturday,
however will likely be too late for any shower or storm initiation
east of I-76.

For Saturday, dry and stable zonal flow will persist across
Colorado as a weak and fast moving disturbance passes across the
US/Canadian border. This short wave will bring a brief tightening
of the mid-level gradient across WY and northern CO. With low
relative humidity in place, gusty west winds will be likely across
the northern high country lending to the potential for critical
fire weather conditions by Saturday afternoon.

The ridge/trough pattern will become more amplified across the US
starting Sunday as a strong Pacific trough begins to drop
southward into the Pacific northwest and the subtropical ridge
becomes re-established over the Gulf states. Sunday remains dry
and stable over NE CO under weak ridging, however a better chance
for afternoon shower and storms will be possible, mainly over the
high country as the subtropical moisture becomes drawn northward
on the west side of the summer subtropical ridge and ahead of the
approaching trough for Monday afternoon.

For Tuesday into Wednesday, this cold Pacific trough begins
movement into the northern and central Rockies placing moderate to
strong southwesterly flow aloft over the Colorado. At this time,
will keep scattered PoPs over the high country with isolated
showers and storms over the lower elevations. Will watch to see
how this trough progresses across the Rockies in later runs. It
may require higher PoPs as often times the region does well in the
precip category in unstable southwest flow aloft. This storm
system may also bring a good chance for snow in the northern high
country late Wednesday and a few days beyond.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 238 PM MDT Wed Sep 11 2019

There will be chance for thunderstorms through 03Z at the Denver
airports. Storms will be capable of small hail, brief heavy, and
wind gusts to 40 knots. The chances for storms are around 30 percent
for the Denver airports. Outflow wind gusts will likely produce a
wind shift or two. North to northeast winds should prevail through
the evening and then become a light southerly direction towards 09Z.
For Thursday, mostly clear skies will prevail with north to
northwest winds.

&&

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

SHORT TERM...Meier
LONG TERM...Fredin
AVIATION...Meier

Hazardous Weather Outlook
National Weather Service Denver/Boulder CO
1032 AM MDT Wed Sep 11 2019

COZ030>051-121645-
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-
1032 AM MDT Wed Sep 11 2019

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

.DAY ONE...Today and Tonight

Scattered showers and thunderstorms over western and north central
Colorado will spread east across the northeast plains by mid
afternoon. A few of the storms may become severe across the plains
along and north of a Greeley to Akron line. Wind gusts to 70 mph,
hail up to 2 inches in diameter and an isolated tornado will be
possible.

For tonight, showers and storms will continue this evening across
the plains. In the mountains, light snow will develop along and
north of I-70 by midnight, with snow levels down to 10000 feet. A
few inches of snow will be possible overnight.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

Cooler temperatures but dry conditions are expected Thursday as
an upper level disturbance leaves the state and high pressure
begins building in from the west. Summer will make a return over
the weekend and early next week as an upper ridge builds over the
state. Temperatures will be warmer with dry conditions.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will be needed after 2 pm across the northeast
plains.