Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 282137
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
337 PM MDT Fri Apr 28 2017

.SHORT TERM...(This evening through Saturday)
Issued at 337 PM MDT Fri Apr 28 2017

Showers breaking out over northeastern Colorado this afternoon
while low clouds, lighter snow, and cold northeast winds are
moving in behind a cold front from the northeast. All of this
should be coming together over Denver during rush hour.
Temperatures should be relatively warm, and the ground is warm, so
the impact on the roads should be reduced. Expect that roads
should stay wet through rush hour, even with some decent snow
rates in some of the showers.

This area of showers is actually in between the areas of higher
level forcing over Wyoming and southern Colorado. This evening we
will see the low level cold/moisture/upslope at its best with the
instability aloft over it. The deeper lift over Wyoming will
rotate southward but weaken as it does so...leading to a period of
steady but lighter snow later tonight. There will still be pretty
good upslope though, so while the intensity should be lower,
there will be significant accumulation through the night.

Bigger questions about what happens on Saturday. Today`s model
runs are similar to those we saw earlier this morning, with
perhaps a little more slowing to the eastward progress of the
system. We made a shift in that direction this morning, so we are
sticking with those adjustments at this time. This forecast has
Denver in weak QG lift with northerly low level winds, so less
snow from Denver northward, and perhaps ending early in
Greeley/Fort Collins, but a continuing steady snow south of
Denver. The real wild card is the stronger QG lift and better
moisture wrapping around the deep slow moving low that will be
moving across eastern New Mexico. Current thinking is that the
stronger lift will remain SE of Denver for two reasons: The track
of the low is far enough south, and there is not enough time for
it to wrap all the way back to Denver. QG diagnostics and 700-500
mb temperature advection suggest that the strongest lift may get
as far up as Washington and Lincoln counties late Saturday into
Saturday night. So the slower motion could prolong snow out there,
but drier air should be working into Denver in that scenario.

Of course, this is too close to comfort and there is a chance that
there could be a lull over Denver during the day Saturday and then
more later in the day. Even so, think the risk of more heavy snow
in Denver is low, but higher across our eastern plains if that
were to happen.

.LONG TERM...(Saturday night through Friday)
Issued at 337 PM MDT Fri Apr 28 2017

An upper trough axis is over the forecast area at 00Z early
Saturday evening with a closed circulation over northeast and
east central New Mexico. By 12Z Sunday morning the trough axis is
just east of the eastern Colorado boarder and the upper closed
low is over the northeastern panhandle of Texas. There is north-
northwesterly flow aloft Sunday becoming just northwesterly at 90
knots on Sunday night. Weakening northwesterly flow aloft
continues over Colorado Monday and Monday evening, becoming more
zonal early Tuesday morning. There is still pretty decent upward
vertical motion on the QG Omega fields Saturday night into Sunday
morning certainly for the eastern half of the CWA. There is weak
downward motion after that well into Monday. The low level winds
still look to be northerly upsloping Saturday evening, then the
winds become more northwesterly overnight. North-northwesterlies
are progged Sunday and Sunday evening. Downsloping west and
northwesterlies are progged the rest of Sunday night and Monday
with more normal diurnal patterns by Monday night. For moisture,
models now show it to be quite deep Saturday evening, then slowly
decreasing across the CWA from the northwest to east well into
Sunday. The far east and southeastern CWA have wrap-around
moisture from the slow moving upper low through Sunday afternoon.
Some moisture returns from the west Sunday evening and sticks
around through Monday night. The QPF fields have minor amounts of
measurable snow Saturday evening, then is decreases from west to
east through Sunday afternoon. There is not much. There is some
measurable precipitation in the mountains Sunday night and Monday.
There is very little noted after 06Z Monday night. So will keep
"chance"s going for pops Saturday evening, decreasing them only
"slight chance"s over the far east by Sunday. Will go with 30-60%s
in the mountains Sunday night into Monday evening. For
temperatures, Sunday`s highs are 4-9 C warmer than Saturday`s
highs. Monday`s highs are similar to Sunday`s. For the later days,
Tuesday through Friday, models have zonal flow early on Tuesday,
then an open upper trough moves across later Tuesday into
Wednesday morning. Northerly flow aloft is in place the rest of
Wednesday and into Thursday morning. Then an upper ridge axis gets
into Colorado and stays well into next weekend. Will need pops
for all the CWA Tuesday through Wednesday, then and Tuesday night.
it will be dry.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 337 PM MDT Fri Apr 28 2017

Showers and a few thunderstorms will be around KDEN in the late
afternoon, then spread across the rest of the Denver area by 00z.
By that time, precipitation will likely be all snow with MVFR
conditions giving way to IFR conditions and areas of LIFR ceilings
by 03z. Those conditions are expected to last through the night,
though the heaviest snow will likely be between 00z and 06z. For
Saturday IFR conditions in light snow is expected through 18z,
then MVFR conditions in the afternoon.

Expected snow accumulations are 4-8 inches at KDEN and KBJC with
8-12 inches at KAPA.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM MDT Saturday for COZ033-034-
037.

Winter Storm Warning until 6 PM MDT Saturday for COZ035-036-041.

Winter Storm Warning from 6 PM this evening to 6 PM MDT Saturday
for COZ039-040-046-047.

Winter Weather Advisory from 6 PM this evening to 6 PM MDT
Saturday for COZ045.

&&

$$

SHORT TERM...Gimmestad
LONG TERM...RJK
AVIATION...Gimmestad


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