Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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000
FXUS65 KBOU 140322
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
822 PM MST Sat Jan 13 2018

.UPDATE...
Issued at 820 PM MST Sat Jan 13 2018

Updated overnight low temperatures to warm them up a degree or
two. Temperatures over the past several hours have been trending
warmer than forecast, especially on the northeast plains. Blended
in some of the latest guidance to bump them up. Otherwise forecast
tracking along nicely, with partly cloudy skies and light winds.
Left in mention of late evening snow showers over the mountain
summits with decreasing chances through the night.

&&

.SHORT TERM...(This evening through Sunday)
Issued at 306 PM MST Sat Jan 13 2018

Northwest flow aloft will persist over Colorado through Sunday.
There is still enough moisture and orographic lift over the
mountains to produce clouds and areas of light snow/flurries.
Haven`t seen any accumulating snow yet today, though a light
accumulation can`t be ruled out over the ridges. The areas of
light snow and cloud cover are expected to continue into the
evening and then decrease tonight as drier air moves into the
mountains.

For northeast Colorado, mid and high clouds will continue to race
across the area through tonight. Clouds should decrease late
tonight/early Sunday with the arrival of drier air. Increased
sunshine and a slightly warmer airmass will lead to mild
temperatures Sunday with highs in the 50s over northeast Colorado.

.LONG TERM...(Sunday night through Saturday)
Issued at 306 PM MST Sat Jan 13 2018
Significant changes to the weather are coming to Colorado late
Sunday night and into Monday. A high amplitude ridge will be
across the Pacific Northwest, and a longwave trough will extend
from Ontario south to the lower Mississippi Valley. Strong
northwest flow aloft will be ongoing across the northeast quarter
of Colorado. A quick moving clipper system will arrive in
Colorado riding along the northwest flow by late Monday morning.
Some moisture aloft is available for clouds and mountain snow but
instability will be limited with just a few degC cooling in the
500-700mb layer occurring throughout Monday. WNW Upslope will
occur west of the Continental Divide and light snow is expected
much of Monday but amounts are expected to remain below advisory
criteria. 3-7 inches across the Front Range Mountains and 2-5
inches across the I-70 corridor. Across the Plains, the daily high
temperature across the I-25 urban corridor is likely to occur
late morning and then fall steadily throughout the day with strong
low-level cold advection. The initial cold front should arrive
late evening Sunday, and as the 1048 mb surface high moves south
across the northern Plains, the cold advection will increase
throughout the day. The source region for the air behind the front
is Arctic in nature, but for now looks like the bitterly cold air
will be northeast of our area with the surface high too far east
to push it into Colorado. Nevertheless, the air behind the front
pouring into the state throughout Monday will drop temps into the
low teens by Monday afternoon northeast Plains, and into the low
20s across the I-25 urban corridor. Flow behind the front will be
nearly saturated with a good but shallow low-level upslope
component, enough to support light snow throughout much of Monday.
Synoptic scale lift is best across the northeast corner, which
also coincides with the best mesoscale lift from the jet streak
given NE CO will be in the right entrance region of the jet. All
that said we think this will be a high PoP (i.e. good chance of
precip), low QPF (not much snow will fall) set up for the Plains.
Trace to 2 inches are possible across most of our Plains. With the
cold temperatures and cloudy skies, roads may become slick for
returning travelers across our area. The moisture moves south of
our area late Monday and surface winds across the Plains will
slowly turn southeast, shutting off the shallow upslope flow. Snow
should exit most areas by midnight other than the Palmer Divide
where light snow may linger into early morning Tuesday. Subsidence
aloft and dry southerly low- level flow will lead to a quick
clearing Tuesday morning and cold overnight low temperatures.
Single digits are expected mountains and plains, and it will
likely drop below zero far northeast plains given the expected
clearing and light winds.

Tuesday should be dry but chilly. Models, particularly the ECMWF,
have trended more pessimistic on precip chances for Wednesday. The
ECWMF has the next short-wave trough undercutting the long wave
ridge, resulting in weak flow aloft and advecting any moisture
well south and west of our area. The GFS still has a weak cut off
low moving across southwest Colorado which would lead to light
snow showers the last half of Wednesday if that scenario verifies.
Feel snow chances are looking poorer over time, and have just
isolated snow for the northern and central mountains on Wednesday.
With the Plains in the doldrums on Wednesday and WNW flow at
ridgetop inducing a surface lee trough along I-25/US-85, weak
surface winds from Ft. Morgan to the Palmer Divide should result
in only a modest warm up, as arctic airmasses are tough to
eliminate this time of year without low-level downslope. Temps
should rebound into the low to mid 40s across the Plains with 30s
to low 40s in the mountains.

Thursday and Friday will be dry with a ridge axis moving across the
state Thursday into Friday.  Temperatures will moderate quite a bit
with mid 50s both days across the Plains and mid 30s to low 40s in
the mountains as warm advection at 700 mb gets going once flow
goes more southwest.

Global models are in decent agreement that a promising West Coast
trough will move onshore California on Friday along with a 120 kt
jet. Moisture content looks decent at this point and the GEFS 500 mb
heights are in reasonable agreement on trough location, in line
with the deterministic ECMWF. Details hard to pin down that far
out but by overnight Friday into Saturday precipitation chances
should be increasing with a trough just upstream of our area.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 425 PM MST Sat Jan 13 2018

VFR conditions with no restrictions to visibility and any ceilings
above 8000 ft. Tricky wind forecast however for KDEN and KAPA in
light flow. Went with persistence and extended easterly flow at
KDEN for the next few hours, then bring around to south around 03z
when drainage flow kicks in. Generally southerly flow all night,
then continued previous forecast trend of easterly during the day
with weak Palmer divide eddy. KAPA seeing similar light east
flow, and brought them around to south as well after 03z. KBJC
under the influence of westerly downslope, and is forecast to
continue through the TAF period.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

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



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