Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 190414
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
914 PM MST Thu Jan 18 2018

.UPDATE...
Issued at 914 PM MST Thu Jan 18 2018

No changes to the forecast package this evening. High level
cloudiness continues streaming over the state in the anti-cyclonic
westerly flow pattern aloft. Temperatures are staying warmer due
to the lack of radiational cooling under the cloudy skies.
Pressure gradients across eastern Colorado are weak as lee trough
remains in place.

A quick look ahead at the evening run from the NAM shows this
weekend`s storm to still be on track. The upper level circulation
is forecast to follow a preferred track over the Four Corners
while surface cyclogenesis occurs over southeastern Colorado. A
surface cold front moving over northeast Colorado Saturday evening
will usher in a period of snowfall that continues through a good
part of Sunday. The amount of moisture available for the storm
system seems to be the last piece of the puzzle that need to fall
into place.

&&

.SHORT TERM...(This evening through Friday)
Issued at 255 PM MST Thu Jan 18 2018

Ahead of the incoming trough, increasing mid level westerly flow
is expected as the ridge axis moves east  of Colorado. This will
allow for the formation of a mountain wave this evening continuing
though much of Friday. Wind gusts are forecast to reach about 70
mph over the higher mountain passes. Lighter winds will likely be
present through the lower foothills due to the lack of mountain
top stability.

Clouds will increase as moisture continues moving over the ridge
into the state. Simulated satellite and model winds indicate a
mountain wave developing later this evening through the day
Friday. This slight warming tend is expected to continue through
Friday.

.LONG TERM...(Friday night through Thursday)
Issued at 255 PM MST Thu Jan 18 2018

Increasing cyclonic energy produced by deepening southern stream
upper trough still progged to track almost due east over the lower
Great Basin and Four Corners region Saturday and Saturday night,
then east-northeast across Colorado on Sunday. Models show the
trough closing off and intensifying over southeast Colorado Sunday
morning as per Q-G and instability fields. GFS, ECMWF and NAM all
indicate a 35-45 neg Q-G bulls eye over nern Colorado Sunday
morning. Models also indicate a southerly 700-500 mb flow over
eastern Colorado Saturday night through Sunday morning with the
longitudinal oriented upper low. This flow is not all that
favorable for generating heavy orographic precipitation along the
Front Range, though it continues to advect moisture up into the
area. Meanwhile, pre-frontal nely low-level flow will be well
under way by late Saturday night following the passage of a cold
front Saturday evening. NAM and GFS show the front reaching the
Denver area not long after 00z/Sunday. ECMWF and Canadian models
delay its arrival closer to midnight. When this cold front
arrives will determine when the rain and/or rain/snow mix shifts
over to all snow on the plains. Right now, believe precip within
the I-25 corridor will change over to snow between 02z-05z, and
not too long after that farther out across the plains.

Snowfall in the high country is expected to be most intense Saturday
night and Sunday morning with best Q-G ascent and deepest moisture
over the area. High country snow/water ratios in the 13:1 to 17:1
range to start out Saturday night and end around 20:1 on Sunday
with the colder air advecting in behind the trough. Certainly too
early to reliably predict snow amounts, but it`s not out of the
question many high mtn areas could see storm totals well over a
half foot.

On the plains, as the upper trough passes overhead on Sunday, low-
level flow is progged to shift from northeasterly early in the
morn to northwesterly by afternoon. The northeast flow would
favor snow production along the Front Range and western sections
of the Palmer Divide where we could see several inches of snow
accumulate by midday. That includes the greater Denver metro area.
The shift to a northwest sfc-700 mb flow will favor the elevated
rangeland to the east-southeast of the Denver metro area where we
could see a few hours of moderate to heavy snowfall late
morning/early afternoon. However, it`s in areas east of there
where strengthening circulation around a tightly wound 700-500 mb
cyclone could create the biggest weather concern. Still early to
hoist any winter weather highlights out there for snow and blowing
snow. But, certainly something to monitor closely over the next
24 hours especially if models move this system slower and track it
northeastward as they now indicate. That said, could see snowfall
lingering on the northeast plains through Sunday evening, while
conditions quickly improve in the mtns and along the Front Range
with a drier northwesterly/downslope flow.

On Monday, passage of a shortwave ridge should clear skies, warm
temperatures, and drop humidities. However, temps remain below
average. Monday night-Tuesday shortwave clipper system in moderate
northwest flow aloft is fcst to sweep across the region. Snowfall
and gusty winds produced by this system should remain confined to
higher elevations. Lower elevations should remain dry, except
maybe a few rain/snow showers across the far nern corner of the
state early Tuesday morning. Should see little change in temps on
Tuesday. Finally, dry and slightly warmer conditions return
Wednesday with shortwave ridging.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 914 PM MST Thu Jan 18 2018

No aviation impacts this evening or tomorrow. Winds will be
typical drainage overnight and west to southwesterly tomorrow.

&&

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

$$

UPDATE...Dankers
SHORT TERM...Sullivan
LONG TERM...Baker
AVIATION...Dankers



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