Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 252101

Area Forecast Discussion
National Weather Service Denver/Boulder CO
301 PM MDT Tue Apr 25 2017

.SHORT TERM...(This evening through Wednesday)
Issued at 301 PM MDT Tue Apr 25 2017

Scattered showers and thunderstorms will continue through the
afternoon and into the early evening hours. Airmass is slightly
unstable with CAPE less than 500 J/kg. Because of the cool airmass
in place most of the thunderstorms will produce small hail. In
the mountains, brief heavy snow will accompany the showers and
storms where a quick inch or 2 will be possible in an hours.

A cold front will drop south across eastern Colorado late this
afternoon and early evening. This will help trigger additional
showers and thunderstorms. Behind the front, north-northeast
upslope winds will help produce lift and expect stratiform rain to
form in the upslope favored areas south and west of Denver.
Precipitation is expected to shift south across the area this
evening and exit the area around midnight. It becomes cold enough
for snow down to 5000 feet, so any precipitation that lingers
through mid evening will likely change to snow. Drier air and
subsidence moves in after midnight bringing clearing skies with
it. Could see fog in low lying areas along the Front Range if
enough radiational cooling takes places. Expect enough wind over
the eastern plains to keep fog from forming. Lows will be chilly
tonight with lower 30s expected over northeast Colorado.

For Wednesday, subsidence and dry air is expected to produce
partly cloudy to mostly sunny skies during the morning. Orographic
lift over the mountains due to northwest flow aloft may produce a
few snow showers during the morning. Moisture and instability
over the mountains and adjacent plains increases during the
afternoon. Expect scattered afternoon showers over the mountains
with isolated showers and storms possible over the plains. Like
today it will be cool with readings in the mid to upper 50s.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 301 PM MDT Tue Apr 25 2017

...Unsettled weather pattern through Saturday...

Wednesday night into Thursday QG energy will increase with a finger
of the 100kt jet moves over NE Colorado Wednesday evening through
early Thursday. Pacific moisture will increase combining with
decent lapse rates to bring continued snow showers to the high
country Wednesday night into Thursday. Snow level should drop to
around 8000 ft with 3 to 6 inches expected by Thursday afternoon.
On the plains there is a slight chance of rain showers with a
frontal boundary pushing south by the afternoon increasing
convergence over the northern plains. Some isolated thunderstorms
are possible by late Thursday afternoon but at this time CAPE
values are only showing in the 200 to 300 j/kg range with deep
shear in the 20s. Temperatures on Thursday will continue to show
cooling with highs in the 50s and lows overnight dropping to just
above freezing after cold frontal passage.

For Friday and Saturday models are showing more agreement with the
path of the storm minus the tail end on Saturday. The upper level
trough will flatten out as the closed low begins to move south by
Friday morning bringing upper level flow more westerly. Westerly
flow over the higher terrain combined with a deepening inverted
trough at the surface will help to increase downsloping at the
base of the foothills through Friday afternoon. This will keep
areas along the Urban Corridor dry while the mountains and
portions of the eastern plains continue to see increased chances
of precipitation. The upper low will continue to drop south during
the day Friday along with the surface low increasing easterly
flow on the plains by Friday evening into Saturday. This will
increase upslope and bring moderate precipitation to the foothills
and plains late Friday into Saturday. Snow levels will be low
enough for snow on the plains by Friday afternoon with
temperatures in the 40s on Friday decreasing to below freezing
overnight into Saturday. Snow amounts will vary on the plains with
anywhere from 2 to 8 inches possible by late Saturday. As
mentioned before the models start to diverge on Saturday afternoon
with the GFS having the low center moving faster to the SE then
the euro. If the euro solution comes to fruition then more snow
would be possible as favorable upslope would continue longer. Will
keep confidence low at this time given variations in the models.
Temperatures will stay in the 30s on Saturday with temperatures
dropping into the 20s overnight into Sunday.

By Sunday into Monday the upper low will transition to the south and
east with increasing subsidence over the region helping to clear out
precipitation. Temperatures will slowly rebound through Monday with
highs expected to return to the 60s.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 301 PM MDT Tue Apr 25 2017

Scattered showers and thunderstorms will continue in an unstable
airmass ahead of a cold front this afternoon. Brief small hail
will accompany the thunderstorms. Ceilings will lower to 3000 to
5000 feet MSL under the stronger showers and thunderstorms.

The cold front will drop south across the area late this
afternoon bringing northerly winds to 30 knots. This front is
expected to produce additional showers and thunderstorms. Best
chance for rain showers will be 22z to 02z. After 02Z,
precipitation is expected to sink south of the KDEN. If it
continues past 03Z, precipitation will likely be snow. Doesn`t
appear it will snow long enough for much if any snow
accumulation. Ceilings will lower to MVFR after 00z, but should
improve back to VFR after 09z. Winds will be decreasing below 10
kts by 06z. If it clears out enough, there will be a chance for
radiation fog around 12z.

Subsidence behind the exiting system will result in mostly sunny
to partly cloudy skies Wednesday morning. Clouds will increase
Wednesday afternoon as the airmass becomes unstable. Isolated
showers and thunderstorms will be possible after 21z.




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