Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 160334

Area Forecast Discussion
National Weather Service Denver/Boulder CO
834 PM MST Mon Jan 15 2018

Issued at 830 PM MST Mon Jan 15 2018

Current temperature and dew point readings across the area, as
well as in Wyoming has prompted me to drop minimum temperatures
across the area tonight. Some dew points in the -10 to -20 degrees
F already indicate not much moisture to hold temperatures up, as
well as the potential for the readings to drop more than forecast.
Have made most of the plains drop below zero overnight, with the
exception of the western and southwestern suburbs and foothills
where an inverted trough is expected to develop overnight to keep
just enough downslope wind to keep things mixed. Interesting that
both the HRRR and RAP have temperatures across the South Platte
Valley and south into Washington County drop as low as -15 to -24
degrees - at this point, think this is too low. Went ahead and
added northern Lincoln County to the Wind Chill Advisory with
colder min temperatures expected and with just enough wind to push
the wind chills to -18 degrees F or colder. Winds aren`t exactly
going to be strong, but the danger of exposed skin will still be

A band of clouds still exists over the foothills and adjacent
plains of Boulder, Jefferson and Douglas counties, showing up well
in the GOES-East nighttime microphysics RGB channel. Believe
these should dissipate over the next few hours, though there is a
small chance that the slight easterly surface component may keep
them longer. This would keep the temperatures warmer under the
clouds. Will watch the evolution on satellite imagery.


.SHORT TERM...(This evening through Tuesday)
Issued at 227 PM MST Mon Jan 15 2018

Deep northerly fetch of dry and bitter cold Arctic air on
the Pacific side of a broad Great Lakes cyclone will
continue to spread south over the forecast area during the
next 24 hours. The last of the jet induced/orographic snowfall
lingering over southern portions of the I-25 corridor and Palmer
Divide should exit to the south of the area by night fall.
Clearing skies, light winds and falling dewpoints will likely
result in some of the coldest minimum temperatures of the winter
season. Low temps in the 5 to 10 below zero range and wind speeds
around 5 kts will produce hazardous wind chills in the 15 below to
20 below zero range across the northeast corner of the CWA. A Wind
Chill Advisory has been issued for this area beginning at 11 PM
MST tonight and lasting to 9 AM MST tomorrow morning. Tuesday
high temperatures on the plains under clear/mostly clear skies as
much as 15 to 18 degs f warmer than those today. High country
temperatures will be little different under a stiff northerly
flow aloft.

.LONG TERM...(Tuesday night through Monday)
Issued at 227 PM MST Mon Jan 15 2018

Upper level ridge will build over the Great Basin to the Northern
Rockies Tuesday night and Wednesday. This will bring weak
northwest flow aloft to Colorado resulting in dry and mild
conditions. Surface high pressure will be over the region. This
will produce light winds and poor mixing, so low lying areas may
stay cooler under an inversion.

On Thursday, the ridge moves over the Central Rockies.
Temperatures are expected to climb into the upper 50s to lower
60s. This ridge shifts east of the region Friday as an upper level
trough moves onto the west coast. This will result in an
increasing southwest flow aloft. Temperatures will remain mild
Friday under this pattern. Increasing moisture in the southwest
flow aloft could bring mountain snow late Friday.

The upper level trough tracks across the Great Basin Saturday and
is centered near the Four Corners 00Z. Lift from a jet and the
upper level trough is expected to produce snow over the mountains
starting Friday night. Snow is expected to shift east across
Colorado on Saturday. There may be a brief period of rain
initially, but as the airmass cools precipitation is expected to
change to snow. Models have been very consistent with this trough
and showing snow for Saturday/Saturday night. Will increase pops
into the likely range from this system.

Northwest flow behind the exiting trough will bring cool and dry
conditions for Sunday. Light snow may linger into Sunday morning
over the eastern plains and in the mountains. Any additional snow
Sunday should be light. Ridging will quickly move across the state
Sunday/Sunday night bringing a dry period. Models have some
differences for Monday regarding flow aloft. The GFS shows a moist
westerly flow aloft, while the ECMWF indicates a dry southwest
flow aloft. Will have low pops in the mountains with temperatures
across the area near normal.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 830 PM MST Mon Jan 15 2018

A thin line of clouds up against the foothills of Boulder,
Jefferson and Douglas counties with ceilings of 020-030 (affecting
BJC) should dissipate by 05-06Z. Otherwise, clear skies and VFR
conditions are expected for the next 24-36 hours. Light east-
southeasterly winds should be turning south- southwesterly or
drainage over the next few hours at 5-12 kts. On Tuesday, clear
to mostly clear skies can be expected with light southerly breezes
early in the morning becoming east to southeasterly by late
morning and/or early afternoon at speeds generally under 12 kts.


Wind Chill Advisory until 9 AM MST Tuesday for COZ046-048>051.



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