Special Weather Statement, NWS Denver/Boulder, CO

Area Forecast Discussion
National Weather Service Denver/Boulder CO
305 PM MDT Fri Jul 26 2019

.SHORT TERM...(This evening through Saturday)
Issued at 250 PM MDT Fri Jul 26 2019

Storms have formed along a boundary that was roughly over Greeley
to Sterling this afternoon. These storms have formed two outflow
boundaries with one moving northward and another moving
southward. Each of these may create more storms this afternoon so
POPs were increased along and north of the I-76 corridor.

This evening, storms will push off the mountains onto the plains
with generally light precipitation amounts. The trend in the high
resolution models is for the storms to be weaker this evening
than previous model runs. This would make sense with the
relatively dry air below 500 mb. Recent runs of the HRRR showed
showers and weak storms continuing after midnight over the
northeast corner of the state. I added POPs to indicate a slight
chance of showers over this area between 06 and 12Z tonight.

Moisture will increase on Saturday as precipitable water values
will reach above an inch on the plains during the late afternoon.
A mid to upper level shortwave will be moving through northern
Colorado around midday which will provide enough forcing to
generate scattered to widespread showers and storms. There are
likely POPs in the mountains tomorrow and people planning on being
outside should be aware of the high threat for lightning during
the afternoon. These showers and storms will produce brief heavy
rainfall and gusty winds. If storms begin to train over the same
area tomorrow there could be some minor flooding but with mean
wind increasing due to the shortwave bringing in better flow
aloft, the flood threat will be low. A severe storm or two cannot
be ruled out near the Wyoming border but low to moderate
instability and deep layer shear indicates storms will struggle
to sustain strong updrafts. Otherwise, highs will be a couple of
degrees cooler.

.LONG TERM...(Saturday night through Friday)
Issued at 245 PM MDT Fri Jul 26 2019

Storms will continue through Saturday evening, both over the
higher terrain and over the eastern plains as the upper trough
traverses across the forecast area. A jet pushing into
southwestern Wyoming overnight will help sustain a slight chance
of mountains showers and thunderstorms through the night.

High surface pressure behind the upper trough will push down from
the lee of the Northern Rockies through the day on Sunday. This
will bring cooler, drier and more stable air, mostly to the
plains, with PWs about a third of an inch lower than Saturday.
Some upslope flow into the foothills and mountains will likely be
just enough to have isolated to scattered storms there in a low
CAPE and moderate shear environment. GFS shows a jet max pushing
into Nebraska Sunday night, putting our northeastern corner into
the RRQ of the jet. All the other models have the jet max
stretching further back into Wyoming, but if the GFS is right, we may
see a slight chance of elevated thunderstorms overnight Sunday
night.

The upper high will be just south of the Four Corners area Monday
morning, and will begin pushing east-northeast to be over far
southeastern Colorado by Thursday night. This will allow
subtropical moisture into the area, increasing diurnal
thunderstorm coverage each day, and keeping temperatures steady as
slight warming is counterbalanced with an increase in cloud cover.

On Friday, there are model discrepancies, but overall there will
be a shortwave trough passing to the north in Canada which will
push the upper high south. The only real affect should be cooler
temperatures but continued stormy afternoon and evenings.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 250 PM MDT Fri Jul 26 2019

An outflow boundary in southern Weld County is moving southward
toward the airport. This should turn winds to the north with gusts
up to 19 knots. Showers and storms may form along this boundary
with the best chance of storms being between 23Z and 03Z at DEN.
The primary threat from these storms will be gusty winds that
could reach up to 35 knots. Due to dry air in the low levels,
ceilings should remain in VFR thresholds. Winds will trend toward
drainage around midnight tonight. Easterly winds will take hold
tomorrow with a better chance for storms.

&&

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

SHORT TERM...Danielson
LONG TERM...Kriederman
AVIATION...Danielson