Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 200922
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
322 AM MDT Thu Apr 20 2017

.SHORT TERM...(Today and tonight)
Issued at 322 AM MDT Thu Apr 20 2017

Elongated trough is moving onto the west coast at this time. At
the base of the trough, a strong short wave trough is tracking
across California. This wave will continue eastward into the Great
Basin this afternoon. Lift ahead of the trough is expect to
produce scattered showers after 18Z. Airmass becomes slightly
unstable with CAPE up to 400 J/kg. So will mention a slight chance
for thunderstorms this afternoon and early evening. Rain and
mountain snow will increase through the afternoon and is expected
to become widespread by 00Z. Convergence at 700mb is expected to
move across Colorado tonight and help focus the heaviest
precipitation over the plains. In addition to this, the front left
exit region of the jet will shift over the state. This will help
produce lift over the mountains and Front Range.

Looking at wet bulb zero heights, appears the snow level will fall
to around 7000 feet MSL by 12Z Friday in the foothills. It will
likely dip little lower to the north where snow will be possible
along the Wyoming border by 12Z.

As far as precipitation amounts go, a few locations may see an inch
of rain by 12Z Friday where a thunderstorm passes over. Much of the
area is expected to see around a half inch of precipitation through
tonight. Lesser amounts are expected over the eastern plains where
the rain will begin later. Higher amounts will be possible over
northern areas where the best lift will be located. Plan on issuing
a Winter Weather Advisory for the mountains where 4 to 10 inches of
snow will be possible by Friday morning. The heaviest amounts are
expected to be north of I-70.

.LONG TERM...(Friday through Wednesday)
Issued at 322 AM MDT Thu Apr 20 2017

An upper lvl low will be over ern CO Fri morning and then will move
quickly eastward into cntrl KS by late Fri aftn.  QG fields show
favorable mid lvl ascent across the plains in the morning which will
then move east of the area during the aftn.  Overall will have high
pops over the plains in the morning and then decrease them fm west
to east by aftn.  In the mtns pcpn will decrease by mid morning as
drier air spreads across along with downward forcing.  However by
mid aftn decent lapse rates combined with increasing moisture should
allow for showers to redevelop.  As for pcpn amounts, the plains
could see total amounts around an inch on average with 1.5 to 2
inches in a few areas. Across the mtns snow amounts will generally
be on the lower end of advisory criteria. Highs on Fri will be
cooler readings in the 45 to 50 degree range across nern CO.

For Fri night into early Sat another piece of energy will move se
across the area along with some wk mid lvl ascent.  Cross-sections
still show quite a bit of moisture so there could be period of
steady lighter pcpn overnight.  Buffer soundings show potential for
snow levels to drop down to 5000 ft after midnight.  On Sat the 2nd
piece of energy should move ese of the area by midday.  Thus should
see pcpn chances end except for some lingering showers in the mtns.
Highs on Sat will rise into the 55 to 60 degree range over nern
CO.

On Sun dry wly flow aloft will develop over the area with no pcpn.
With downslope low lvl flow in place aftn highs will wrm into the
lower to mid 70s across the plains.  By Mon the flow aloft will
become more wnw as an upper level trough moves across the nrn
Rockies.  Cross-sections show some mid lvl moisture affecting the
nrn mtns so will keep in low pops.  Across nern CO it will be dry
with gusty wnw winds.  Highs may end up a few degrees cooler
across the plains however readings will still be abv normal.

For Mon night into Tue there are some differences between the ECMWF
and GFS.  The ECMWF has WNW flow across the area with no well
defined features embedded in the flow.  Meanwhile the GFS has an
upper level trough moving across on Tue.  As one might expect this
leads to some substantial differences between the two.  As for the
mtns both solutions do show favorable moisture and lapse rates thus
this would lead to a good chc of orographic pcpn.  The GFS has
heavier pcpn due to the passage of the upper level trough.

As for nern CO both the ECMWF and GFS have a cool fnt moving across
the plains Mon night into early Tue.  With the position of the upper
level jet this could lead to a chc of banded pcpn Mon night into
early Tue based on the ECMWF solution.  Meanwhile the GFS solution
would lead to a higher chc of pcpn as the upper level trough and
associated mid lvl ascent move across.  At this point confidence is
not high as to what will happen so will just mention a slight chc of
showers across the plains.

On Wed the flow aloft will become more wly as surface low pres
develops fm cntrl WY into cntrl CO.  Both the ECWMF and GFS show
abundant moisture affecting the mtns with quite a bit of pcpn.
Although orographics are favorable pcpn amounts will likely depend
on mid lvl temps as too wrm of an environment would limit pcpn
amounts. Meanwhile as far as nern CO with sely low lvl flow
developing will see an increase in low lvl moisture and better
instability. Thus may see a chc of tstms across the plains in the
aftn.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 322 AM MDT Thu Apr 20 2017

Clouds will increase this morning with ceilings of 6000 to 8000
feet by 18Z. Clouds will lower through the afternoon and evening.
After 00Z, expect ceilings to be below 3000 feet with rain showers
across the Denver area. There will be a slight chance for a
thunderstorm from 20Z to 01Z. If a thunderstorm affects an
airport, it is expected to be short lived. After 00Z through 12Z,
periods of rain should prevail with ceilings mainly 1000 to 3000
feet and reduced visibility. A slight chance for fog with
visibility falling below a half mile after 06Z.

Light winds early this morning will turn southeast and increase with
gusts to 30 knots possible. As a surface low forms over the Denver
area, winds may turn more easterly around 00Z. Eventually the
surface low shifts east of the area and causes winds to turn to the
northwest around 12Z Friday.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 PM this evening to 9 AM MDT
Friday for COZ031-033-034.

&&

$$

SHORT TERM...Meier
LONG TERM...RPK
AVIATION...Meier


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