Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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504
FXUS65 KBOU 212045
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
245 PM MDT Wed Jun 21 2017

.SHORT TERM...(This evening through Thursday)
Issued at 245 PM MDT Wed Jun 21 2017

Isolated storms have formed already, mainly over southern
Colorado. Additional storms should develop later this afternoon
over northern Colorado when moisture upstream moves in. Like
yesterday, the showers and thunderstorms are expected to be high-
based and produce gusty outflow winds to 40-50 mph. Any rainfall
is expected to be light. Expect convection to end mid to late
evening as the airmass stabilizes. Because of the warm airmass in
place, expect overnight lows to be warm again tonight.

For Thursday, upper level ridge over the Desert Southwest will
sink south as a trough moves over the Northern Rockies. Over
Colorado, a west-northwest flow aloft will continue. At the
surface, a cold front will south push in northern Colorado early
Thursday morning. This cold front is expected to track across the
Denver area 9-10am. Gusty northeast winds behind the front will
usher in moisture and cooler air. Precipitable water values climb
to around an inch across much of the lower elevations by late
Thursday afternoon. CAPE will climb to 700-1300 J/kg. There will
be sufficient shear with this CAPE for a few severe thunderstorms
to form. Highs will be cooler Thursday with most locations
topping out in the mid to upper 80s. Southern areas may be a
little warmer and approach 90 degrees.

.LONG TERM...(Thursday night through Wednesday)
Issued at 245 PM MDT Wed Jun 21 2017

A cooling trend will start by Friday and continue through the
weekend.

A low pressure system over the North moving eastward into the
Great lakes region will bring an elongated trough over the Great
Basin region and Colorado. Thursday evening into Friday a cold
front will back in from the northeast bringing increasing NE to
easterly flow. CAPE values will increase Thursday along the
frontal boundary with values up to 1500 J/kg by Thursday evening.
Persistent upslope along the foothills will bring a chance of
convection that could spread over the plains into the evening
hours. Upward QG motion is progged over the area with low level
and deep shear values both registering in the 40s. With PW values
above 1 inch by Friday severe hail on the plains with period of
heavy rain and high winds will be possible. By Friday morning
conditions will stabilize gradually behind the front with models
showing stratus moving into the foothills through Friday late
morning. Cooler temperatures will prevail Friday with highs in
the 70s. Model upper air soundings showing conditions more stable
on the plains for convection but left a slight chance of some
showers for Saturday and Sunday. The mountains will see a chance
of convection through the weekend due to increased surface
pressure providing for ongoing orographic upslope conditions. Main
threats will be brief, heavy rain and winds with deadly
lightning.

Temperatures through the weekend will stay cool as another...more
aggressive cold front will move in Saturday morning. GFS is
running slightly warmer than the EC...which has run a tad cooler
over the last model run. Kept highs in the lower to mid 70s for
the weekend.

By Monday the upper ridge will build back in with increasing
temperatures through the middle of next week. There will be a
slight chance of storms for the afternoon for areas south of I-70
and closer to the KS border.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 245 PM MDT Wed Jun 21 2017

Isolated thunderstorms are expected across the Denver area
through about 04Z. Gusty winds to 40 knots will be possible with
these storms. Rainfall will be light. Winds outside of convection
are expected to be northerly through mid to late evening. Winds
are then expected to turn to a normal drainage direction. On
Thursday, a cold front will push through the Denver area around
15-16Z, gusty northeast winds will develop behind. Moisture will
increase behind the front and bring scattered thunderstorms
Thursday afternoon and evening.

&&

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

$$

SHORT TERM...Meier
LONG TERM...Bowen
AVIATION...Meier



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