Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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742
FXUS65 KBOU 270207
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
807 PM MDT Wed Apr 26 2017

.UPDATE...
Issued at 807 PM MDT Wed Apr 26 2017

Isolated/scattered showers will decrease and end over eastern
Colorado in the next few hours as the airmass stabilizes. The 00Z
sounding at KDNR showed a dry sub cloud airmass. So any
precipitation making it to the surface will be light. Next short
wave trough is over the Great Basin and is bringing showers to
Utah and western Colorado. Snow will increase overnight in the
mountains as the wave moves eastward. Nudged up pops even more in
the mountains. Heaviest snow is expected to occur late tonight and
Thursday morning. Should be enough snow to produce snow and slush
covered roads. Rain showers will move onto the Front Range
Thursday, up north first during the morning. Only updates will be
to try and time the precipitation better. Will lower pops this
evening and then increase them a little more towards 12Z and into
Thursday.

&&

.SHORT TERM...(This evening through Thursday)
Issued at 327 PM MDT Wed Apr 26 2017

A 180 kt jet streak is making its way onshore this evening across
the northern California Coast. The jet streak is part of the
northwest jet that extends from the West Coast to the base of the
250 mb trough over the Southern Plains. Strong northwest flow
aloft will prevail across Colorado through Thursday morning. With
such a long fetch of strong northwest flow, ample Pacific moisture
is being brought into the state in the mid levels. Westerly
upslope combined with in an influx of moisture will lead to snow
showers in the mountains later today. GOES-16 WV imagery and VIS
channels show a fair amount of instability upstream of Colorado
this afternoon, associated with a weak short wave trough in the
northwest flow currently over northwest Utah. This short wave
trough looks to move across the state tonight into Thursday
morning. The mountains should see a increase in the snow rates
later this evening with the short wave trough and associated
dynamic lift and cold advection, and thus the winter weather
advisory for this evening through tomorrow evening for 4-8 inches
above 9000 ft looks on track. Travel impacts across the higher
mountain passes, including I-70, probably won`t begin late tonight
and continue through Thursday midday or so.
Instability is limited this evening and into the overnight hours
across the Plains so only slight chances of rain showers are
possible through Thursday morning, with the best chances of rain
reaching the ground occurring across the northern counties that
border Wyoming and Nebraska. Current radar trends have
precipitation across northern Weld County moving southeast into
Morgan County so the high res models seem to have this convection
reasonably well handled this evening and tonight. Virga and light
sprinkles are probably all that may occur across metro Denver and
points east.

On Thursday the trough across the Central Plains moves north and
east into Canada, turning the flow aloft to more westerly across
Colorado by Thursday afternoon, with our area located just north of
a strong westerly jet and under an area of persistent cold advection
throughout the day.  The air aloft is quite cold, with
-25C noted at 500 mb and 700 mb temps cool 3-4 degC relative to
 Wednesday. Snow will continue in the mountains throughout the day
 Thursday with favorable upslope and continued influx of mid and
 upper level moisture, along with cold advection aloft leading to
 decent instability across the high country. The northern mountain
 ranges look to have the best chance of accumulating snow on
 Thursday, generally along and north of I-70. Across the Plains,
 downslope flow along and west of I-25 will offset the cooling
 aloft, resulting in high temperatures in the mid 50s, similar to
 today. Rain chances Thursday afternoon for the I-25 urban
 corridor will be low with the downslope flow. However, high-res
 models develop surface based instability in the 500-800 J/kg
 range Thursday afternoon across the eastern quarter of the state,
 likely because of the advection of cold air aloft. We are
 reasonably confident that convergence along a surface trough will
 be enough to generate convective showers with isolated thunder
 Thursday afternoon east of a Fort Morgan to Limon line. These
 showers should move east into Kansas by midnight and the rest of
 the Plains will likely remain dry. Upslope snow showers across
 the West slope of the divide will continue late Thursday but
 additional accumulations will be light and thus travel impacts
 across the high mountain passes should be less of an issue
 relative to Thursday morning.

.LONG TERM...(Thursday night through Wednesday)
Issued at 327 PM MDT Wed Apr 26 2017

The pattern going into Friday will have the elongated upper trough
extending over the Great Basin with the nose of a 150kt jet moving
SE. Some drying will occur Thursday night into Friday on the
plains behind a shortwave that will move through on Thursday.
Lingering moisture in the mountains will allow for some light
snow showers. Westerly flow over the mountains will also increase
downsloping off the foothills further aiding in a dry period over
the Urban Corridor and the plains by the morning hours on Friday.
By Friday afternoon models have the upper trough transitioning
into a closed low over Utah and Colorado further filling in and
dropping to the SW over the four corners region by late Friday
evening. A cold push from the NE will move onto the plains
ushering in cooler air and more favorable easterly upslope
conditions late Friday. Model soundings show increased ENE flow up
to 500 mb through the late evening on Friday to early morning
hours on Saturday. Snow levels will start around 7000 ft over the
NE corner Friday evening and drop to around 5000 ft on the plains
by Saturday morning. This will bring rain to the plains late
Friday evening and through some of the overnight hours before
changing over early Saturday. The details with this storm and the
progression of the 500mb track will be very important in deciding
the amount of snow that is possible throughout the NE plains. The
current track of the low center has it dropping to the south by
Saturday afternoon and progressing eastward along the northern NM
border. This is helpful in keeping a source of moisture moving
around the low center into the plains through the day Saturday. At
the surface winds will be from the east through the morning, then
switch to a more NE then northerly direction by the afternoon
keeping accumulating snow over the southern foothills and Palmer
Divide. At this time amounts are still fluid but the possibility
of 2 to 6 inches for the NE plains with up to 8 possible over the
Metro area continue. For the foothills up to a foot is possible
over the higher elevations given the continued upslope
possibility. Expect the snow ratio to be somewhat low as it is a
warmer snow so some will melt initially before accumulating on
surfaces. Changes are possible with amounts but still could end up
being a high impact event for portions of the Denver area and
south.

After the storm moves out by late Saturday drying will occur
Sunday into Monday with slightly increasing temperatures. Some
light snow may continue for the mountains through Sunday. Another
shot of rain/snow for Tuesday as another disturbance makes its way
over the region.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 807 PM MDT Wed Apr 26 2017

Ceilings in the Denver area are expected to remain above 6000
feet through at least 18Z Thursday. A few showers will be possible
after 12Z Thursday with a better chance after 18Z. Ceilings may
fall below 6000 feet, especially under the showers.

Easterly winds will turn to a normal southerly drainage direction
by 05Z. A weak push from the north will be possible around 12Z as
a Denver Cyclone forms. Winds will remain north to northwest
Thursday with an increase of winds to 30 knots behind a cold front
around 18Z.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM MDT Thursday for COZ031-033-
034.

&&

$$

UPDATE...Meier
SHORT TERM...Schlatter
LONG TERM...Bowen
AVIATION...Meier



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