FXUS65 KBOU 170329

Area Forecast Discussion
National Weather Service Denver/Boulder CO
829 PM MST Fri Nov 16 2018

Issued at 810 PM MST Fri Nov 16 2018

Main upper trof is across Northern Rockies this evening along with
upper jet draped across northern Wyoming. Initial cold front has
pushed into the Front Range this evening with northeast winds
behind it. Still mainly mid level deck of clouds over northern
sections. The precipitation falling is still across central
Wyoming, mainly from around Lander to Douglas and points to the
north. Latest models bring precip down along the Front Range a bit
slower, mainly between 09-12z along the northern Front Range and
then into Denver and southward from 12z to 15z. Moisture depth is
rather shallow, initially only up to 700mb which would promote the
potential for freezing drizzle along with a dry area from 700mb
to 500mb. By Saturday afternoon, there is enough saturation aloft
and jet dynamics to result in all light snow for most of the area.

Main changes will be to delay the onset of precipitation until
later tonight/Saturday morning but overall snowfall amounts still
look on track and will keep current advisories in tact.


.SHORT TERM...(This evening through Saturday)
Issued at 320 PM MST Fri Nov 16 2018

Latest model iterations appear little changed from the previous
runs. Although the southward progression of cold air, low clouds
and precip associated with the mid-level open wave trough over
Wyoming appears a bit slower to move over the forecast area next
12-36 hours. Before arrival of the trough, a sfc cold front is
still forecast to slide south across northeast Colorado this
evening; its passage in the Denver area around 03z this evening.
Expect to see gusty north-northeast winds for a short time behind
the front, then a drop off in speeds as winds gradually transition
to an east-northeast component late tonight and tomorrow. Models
show wind speeds increasing again across the far eastern counties
during the late morning and afternoon. The 100kt jet max and
moderate QG ascent over southeast Wyoming tonight moves over
northeast Colorado on Saturday. Models show the strength of the
jet and QG ascent weakening along the way. Before the deeper
moisture and large scale ascent arrives during the day tomorrow, a
shallow, moist upslope flow on the plains and a favorable
temperature profile will set the stage for several hours of
freezing drizzle and areas of fog in the lower Front Range
foothills and adjacent plains. As temperatures cool aloft and mid-
levels saturate, should see the freezing drizzle change over a
mix of freezing drizzle and snow by mid to late morning, first up
around Fort Collins, Greeley and Sterling, then southward across
southern urban corridor and I-70 corridor by early afternoon.
Could see depth of ice accumulation up to a tenth of an inch on
the plains and lower foothills, heaviest southwest of Denver.
Bridges, overpasses and sidewalks will be especially susceptible
to icing up with pavement temperatures falling below freezing.
Heavy cloud cover will significantly limit the amount of melting
tomorrow, and with daytime temps only slightly warmer than low
temps tonight.

Anticipated snow totals by late tomorrow afternoon; 1-3 inches
with the i-25 urban corridor, inch or less east of there, 3-6
inches in the Front Range foothills, and 4-8 inches in the mtns
with the greater amounts in the northern mtns and east facing

Ice glazed road surfaces will most likely be the greatest hazard
on the plains with air temperatures expected to remain below
freezing from late tonight after frontal passage through all day
Saturday. Winter driving conditions will also a concern in the
high country on Saturday. Hence the winter weather advisory for
these areas.

.LONG TERM...(Saturday night through Friday)
Issued at 320 PM MST Fri Nov 16 2018

The disturbance that will be affecting our CWA on Saturday will
continue through much of Saturday night. A mid to upper level jet
streak will be near the Colorado-Wyoming border around 00Z Sunday
resulting in ascent over our CWA due to the right entrance region
being overhead. Consequently, the 00Z to roughly 04-05Z period
may have the heaviest snowfall rates from the I-70 corridor
mountains to the Denver metro because the jet dynamics will align
with the best snow ratios during this period. I have increased QPF
and snowfall amounts for the aforementioned areas to reflect the
recent forecast trend for this period. Around 06Z, the upslope
flow over the urban corridor and eastern plains will become much
lighter as the subsident region of the jet streak moves over the
CWA. Snow should begin to quickly diminish and will move out of
the CWA by 12Z Sunday.

By midday Sunday, ridging will develop in the mid to upper levels
with high pressure at the surface. This pattern will last through
Wednesday with no POPs in the forecast and temperatures gradually
increasing each day.

Models then begin to differ on Thanksgiving day. The operational
GFS shows a positively tilted, weak shortwave trough that keeps
almost the entire state of Colorado dry Thursday and Friday. The
Canadian and European models both show a much stronger, negatively
tilted shortwave trough that brings decent moisture to the
Colorado mountains. With this disagreement, confidence in the
forecast is low for Thursday and Friday but there does appear to
be a chance of snow in the mountains. Lower elevations including
the urban corridor look to remain dry for the holiday.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 810 PM MST Fri Nov 16 2018

Weak surface front has moved through local terminals as winds have
shifted to the northeast. Low level moisture will lag well behind
with ifr/mvfr ceilings not expected until after 09z with the
potential for freezing drizzle mixed with light snow early
Saturday morning. Overall confidence of freezing drizzle is a bit
on the low side. Expect weather to be all snow by Saturday
afternoon as airmass becomes more saturated. Total snowfall
expected at local terminals to be in the 1-2 inch range at
KDEN/KAPA with potential up to 3 inches at KBJC by late Saturday


Winter Weather Advisory from 3 AM Saturday to midnight MST
Saturday night for COZ035-036-038>046-048-049.

Winter Weather Advisory from 8 AM Saturday to midnight MST
Saturday night for COZ031-033.



LONG TERM...Danielson
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