Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 122149
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
249 PM MST Mon Feb 12 2018

.SHORT TERM...(This evening through Tuesday)
Issued at 249 PM MST Mon Feb 12 2018

A cut off low at 500 mb over the Great Basin will affect our
weather through Tuesday. Because of the omega block over the
Pacific Ocean the low will slowly retrograde to the southwest
toward the Southern California Coast. Southwest flow downstream of
the low is clobbering the San Juans in Southwest Colorado with
much needed snow. Across our area the moisture is a more limited
though snow chances are high through Tuesday and beyond for our
Mountains. A 110 kt jet streak will move across central Colorado
across northeast Colorado this evening into the early overnight
hours. Across North Central Colorado along the Wyoming border
will be in a sweet spot for mesoscale lift in the left exit
region of the jet as it lifts north. It`s a short window but the
Park Range, Medicine Bows, Northern Gore, and northern Front Range
across Larimer County could see a mesoscale snow band develop
this evening. If a snow band develops the southwest facing
mountains could see 1-2 inches of snow an hour for a couple hours.
For this reason will hoist a winter weather advisory for zone 31
through tomorrow morning in line with neighboring offices. The
same band(s) could affect the Plains of Larimer and Northern Weld
Counties with a quick inch or two after 8 PM this evening. The
high res models are the only guidance that has picked up on this
potential, and they`ve been consistent from run to run. With nice
convective bands across W. Colorado associated with the jet
ongoing this afternoon, feel it`s a decent bet but we needed to
smear the PoPs and QPF a bit across the Plains given the challenge
with nailing down the location of such a band. In addition to the
potential for a snow band across the northern mountains,
favorable orographics and convective instability should lead to
several inches of snow across the central mountains as well. 1-4"
for the I-70 mountain corridor through Tuesday evening are
possible, and maybe 2-6 inches for the Front Range mountains, with
the heaviest snow along and north of Rocky Mountain National
Park.

Across the Plains, stratus will continue to hold temperatures in
check the rest of today and most likely keep temps from dropping
too far tonight. Lows will be between 7-12 degF across much of the
eastern Plains, with teens across the I-25 urban corridor and
Palmer Divide. We`ve been looking for mechanisms to erode the
stratus deck, which roughly extends to around 8000 feet MSL
across the eastern Foothills and adjacent Plains, but nothing
sticks out until Tuesday morning or perhaps not until midday
Tuesday. Winds at 700 mb veer to more west by late morning but
stability profiles do not favor mixing down to the surface across
the adjacent Plains. During the day Tuesday the surface high
across the Central Plains shifts south, with a gradient favoring
light westerly winds across much of NE Colorado. The hope is that
those winds finally clear out the stratus deck and allow
temperatures to return to near seasonal normals Tuesday afternoon.
The cold air, as typical, will be slowest to clear out of the
lowlands of Weld/Larimer and Morgan Counties, where highs may only
reach the low 40s. Metro Denver should reach the upper 40s but
have stayed on the cool side of guidance given model tendency to
underestimate the strength of these shallow near-surface cold
layers. Winds should be relatively light throughout the period
with weak pressure gradients across eastern Colorado.

.LONG TERM...(Tuesday night through Monday)
Issued at 249 PM MST Mon Feb 12 2018

The upcoming week through Thursday will be characterized with an
occasional moist, west to southwest flow aloft moving across
Colorado. This will keep some light snow going in the mountains
but since moisture depth is rather shallow, any accumulations
will be on the light side. The higher impact looks to be the
strong winds likely to develop over the higher mountains and Front
Range Tuesday night and Wednesday. Cross barrier flow increasing
to 50kt by Wednesday morning which would result in gusts in excess
of 60 mph possible in this period. Forecast soundings do show
some mountain top stability developing but there is significant
shear aloft due to the strong jet streak overhead which will not
be as conducive for the amplification of the wave. For now, no
high wind watches but certainly it will get quite windy over
higher mountains and exposed east slopes. Have increased winds in
the forecasts to reflect the higher winds. Expect some of the
windy conditions to bleed out over the plains by Wednesday
afternoon with good mixing along with some substantial warming
temperatures.

The next upper level disturbance and cold front will move across
Colorado on Thursday with a better chance of snow in the mountains
along with colder temperatures and a slight chance of snow across
the plains. Moisture looks to be a bit scarce so will only have a
slight chance of snow on the plains. A drier airmass will move
into Colorado for Friday and Saturday with mainly dry and warmer
conditions for much of the state.

The next major change looks to approach Colorado in the late
Sunday and Monday time frame for a chance of snow and colder
temperatures. Both GFS and European are showing a similar general
solution but trof positions and amplitude are different. Certainly
cooler temperatures and at least a chance of precip is warranted
in the out periods.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 249 PM MST Mon Feb 12 2018

The threat of flurries has ended based on recent GOES-16 and
surface obs across the metro area terminals. The seeder-feeder
process was seen nicely with GOES-16, moving northeast away from
the metro area and ending the chances of flurries midday today.
Just enough dry air has worked it`s way into the area that CIGs
have improved to 010. The trend of raising CIGs should continue
this evening but will likely remain IFR or MVFR until Tuesday
morning before scattering out. After about 12Z VFR conditions
should prevail through Tuesday with no precipitation expected.
Winds should not be an issue over the next 30 hours with 10 kts or
less expected area-wide.

&&

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

$$

SHORT TERM...Schlatter
LONG TERM...Entrekin
AVIATION...Schlatter



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