Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 091631
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
931 AM MST Mon Jan 9 2017

.UPDATE...
Issued at 930 AM MST Mon Jan 9 2017

Winds in the high country have eased some in the past hour or so
according to a number of observing sites such as the Berthoud
Pass...Niwot Ridge...Cameron and Loveland Pass wind sensors.
Chinook winds have also diminished some in the past hour on and
near the east face of the Front Range foothills. However Chinook
winds still howling in the northern Jefco and Boulder area with
gusts in the 50-65 mph range at this time. The reduction in wind
speeds probably only temporary as mesoscale model winds fields and
cross sections show strong w-swly momentum aloft mixing downward
again and even surfacing on the plains later this morning or early
afternoon. At this time...it appears sustain wind speeds and
gusts on the plains east of the I-25 corridor will remain below
high wind criteria. But will continue to monitor and issue any
wind high lights if necessary.

Now...upstream over west-central and southwest Colorado watching
the next surge of moisture rich air heading for our mountain
ranges. Could see it arrive in the next 1-3 hours. With its
arrival should see a marked increase in snowfall intensity and
rates...especially on the high mtn passes and higher west facing
slopes. Could see a return to 1-2 inch per snowfall rates...and
even locally up to 3 inches per hour with embedded convection.
Winds may also strengthen as well resulting in near Blizzard
conditions as is indicated in our Winter Storm Warning.

In the valleys west of the Divide...current temperatures warm
enough for rain as is occurring at Kremmling and a few other
valley locales. But with intensification of precip later this
morning...even the valley floors could see a change over to a
rain/snow mix or even even some wet snow.

All that said...will make only subtle adjustments to forecast
grids and text products at this time. Winter storm and high wind
warnings will stand.

&&

.SHORT TERM...(Today and tonight)
Issued at 532 AM MST Mon Jan 9 2017

Wind forecast is looking good as the mountain wave amplified
around 2 am as expected. Mainly a strong chinook wind pattern but
with the cold air gone on the plains the winds are spilling out
into areas downstream of the foothills hot spots, so places like
the western suburbs of Denver, Longmont, and Berthoud should be
pretty windy today. The mountain wave wind should be peaking this
morning, but areas depending more on mixing like Douglas county
and the Wyoming border areas may be windiest this afternoon.

As for the snow, there has been a significant break overnight that
will continue this morning. Still some light snow and lots of wind
though. Deeper moisture comes back in this afternoon, then the
flow becomes more westerly with some cooling and there is less
stability. This should result in a round of heavier snowfall
this evening. With the wind direction never really becoming
northwest, this will still favor areas north of Berthoud Pass, and
possibly also Vail Pass. With the timing of the wind shift, the
main threat to Interstate 70 is probably this evening, with a
steady light to moderate snowfall before then.

Not much time to tend to other details, but the temperature
forecast looks reasonable.

.LONG TERM...(Tuesday through Sunday)
Issued at 532 AM MST Mon Jan 9 2017

Models have west-northwesterly flow aloft for the forecast area
Tuesday. Jet level speeds are up to 150 knots. The flow aloft is
zonal Tuesday night, then west-southwesterly Wednesday and
Wednesday night. Speeds are still high; in the 90-130 knot range
those three periods. The QG Omega fields keep upward synoptic
scale energy over much of the forecast area all four periods. For
boundary layer flow, there are west-southwesterlies progged for
the CWA Tuesday, with more normal drainage patterns Tuesday
overnight. Models have a cold front push into the plains on
Wednesday bringing easterly upslope winds up to the lower
foothills. Downsloping west and northwesterly flow dominates
Wednesday night. Cross sections point to another decent mountain
wave set up for the foothills and maybe western plains Tuesday
night into Wednesday morning. Perhaps more highlights?  Moisture
wise, there is quite a bit progged for the mountains through
Wednesday night. For the plains, there will be some in the mid and
upper levels. The QPF fields have measurable snow in the
mountains all four periods. The amounts are light by later Tuesday
and Tuesday evening, but get a bit higher again on Wednesday,
then decrease again Wednesday night. There is little to no
measurable precipitation noted over the plains. For pops, will go
with 60-100%s in the mountains through Wednesday night. The on
going highlights look fine into mid day Tuesday for heavy snow.
Perhaps some highlights may be needed in the mountains for snow on
Wednesday again. For the plains, pops are negligible. For
temperatures, Tuesday`s highs will be 2.5 - 4.5 C cooler than
today`s. Wednesday`s highs will be close to Tuesday`s for much of
the CWA. The northeast corner looks about 10 C colder than
Tuesday`s highs. For the later days, Thursday through Sunday,
strong west-southwesterly flow aloft continues for the CWA
Thursday. Weaker southwesterly flow aloft is progged Friday well
into Saturday, then an upper trough moves across Colorado Saturday
night into early Monday morning. A cold front is supposed to move
into the CWA Thursday and there is even some upslope and perhaps
some light snow for the plains Thursday night into Friday. It is
pretty meager looking right now. Snow looks to continue in the
mountains into Friday evening.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 930 AM MST Mon Jan 9 2017

Chinook wind buffeting the denver metro area will likely continue
for the remainder of the day. Characteristic of this wind is how
it pulses. This is what is happening now with a drop off in speeds
across southern and eastern portions of the metro area.
However... west-southwest winds still howling across northwest
sections such as around KBJC and Boulder where gusts still in the
45 to 60 mph range. Look for wind speeds to cycle up and down
across all of the Denver metro area at least through mid-
afternoon. Strongest winds should remain west of Interstate 25. By
night fall...should see steady reduction in speeds from east-to-
west across the metro area with gradual collapse of the leeslope
Mtn Wave.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 5 PM MST this afternoon for COZ035-036-
038-039-041.

Winter Storm Warning until 11 AM MST Tuesday for COZ031-033-034.

&&

$$

UPDATE...Baker
SHORT TERM...Gimmestad
LONG TERM...RJK
AVIATION...Baker



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