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000 
FXUS65 KBOU 071041
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
341 AM MST Sat Dec 7 2019

.SHORT TERM...(Today and tonight)
Issued at 340 AM MST Sat Dec 7 2019

High level moisture rounding the upper ridge axis across the state
has created thin mountain wave clouds over and just downwind of 
the high mountains this morning. This was portrayed quite nicely 
in the CIRA WRF Simulated IR. Low level moisture continued to 
plague southern Weld County and surrounding areas with fog. Was 
thinking that the increasing high cloud cover was going to lessen 
the severity, but it got even thicker, so am issuing another Dense
Fog Advisory for this morning. The fog should become less dense 
and/or totally dissipate by 9 or 10 am. 

Viewing the CIRA WRF Simulated IR imagery through the day shows
the initial thicker and lower wave cloud lasting through the
morning, then a secondary but thinner wave cloud for the
afternoon. This will definitely make the temperature forecast for
the urban corridor have a high bust factor. We were originally
thinking most of the urban corridor would see temperatures closer
to 60, but if the high clouds are thick enough, they'll help keep
the inversion holding on and not allow for the significant warming
we were expecting. Viewing other model data hasn't been very
conclusive either. However, looking upwind at the water vapor
satellite imagery moving in from the west, will assume there will
be some sort of wave cloud to influence the temperatures,
especially over the northern half. Have decreased temperatures,
especially in the southern Weld County area were they've been
stuck under a strong inversion with fog to contend with as well. 

An upper trough off the CA coast will move inland later today to 
push the ridge east of the state, and switch winds aloft from the 
southwest and increase in speed. These winds will be reflected in 
the mountains, with gusts in the 35 to 50 mph range to create some
patchy blowing snow. Mid level moisture will move in from the west
to begin some light snow showers over the mountains mainly around
and after midnight with less than an inch expected by Sunday
morning.

.LONG TERM...(Sunday through Friday)
Issued at 340 AM MST Sat Dec 7 2019

Sunday will bring increasing moisture and snowfall to the high 
country. The deep upper level trough just off the California coast
today will begin to shear out as another disturbance drops in 
from British Columbia, and then pushes southeast across the 
Rockies. Due to overall shearing/weakening of the upper level 
trough, Q-G support will be relatively weak. Nonetheless, we'll 
have good orographics with flow turning more westerly by afternoon
and then northwest Sunday night. That's when the best snow should
reach the northern mountains and Summit County. Overall, we'll 
likely see the heavier snow totals accumulate in the northern Gore
and Park Ranges, as initial westerly flow there will support 
better orographics for a longer period. Our preliminary forecast 
accumulations for the high country looks to range from 3 to 7 
inches for the Front Range Mountains and Summit County, with 5 to 
10 inches for the northern Gore and Park Ranges. We'll take a look
at the latest data today before hoisting any Advisories, as the 
main period of accumulating snow doesn't start until late Sunday 
afternoon and evening for our mountains. Moisture depth may be the
one thing that holds down accumulations. Winds will be strong 
enough to support areas of blowing and drifting snow as well. 

On the plains, a cold front will push southward across the High
Plains and move across northeast Colorado toward midnight Sunday
night. Behind that, we'll see shallow anticyclonic upslope and
thus a chance of light snow showers. If we do see snow, moisture
depth and stability is quite poor so only a dusting would be
expected. Mild temperatures Sunday will be replaced by below
normal temperatures Monday. 

Temperatures should then moderate again and return to slightly 
above normal readings by Wednesday and Thursday. Mainly dry 
conditions will persist with flat upper level ridging in place
both days.

Models seem to be coming around to the idea that a stronger
Pacific jet stream will plow across the Pacific and into the
Northern and Central Rockies by Friday and Saturday. If this setup
occurs, we'll see the orographic snow machine turn on for the
northern mountains. That could also bring bouts of strong, gusty
winds to the plains by late in the week and next weekend. Still a
long ways out on that. 

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 340 AM MST Sat Dec 7 2019

High clouds will likely be over the area terminals through most 
of the day. Southerly drainage winds early this morning should
keep the fog over Weld County to remain north and northeast of 
DEN. Though some high resolution models have been wanting to let
it push slightly south and west into Boulder County, so this could
impact BJC, but for now am discounting it. Winds will veer to the
west and northwest this afternoon before returning to a drainage 
overnight tonight. 

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 9 AM MST this morning for COZ038-042-
043.

&&

$$

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