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FXUS65 KBOU 080250

Area Forecast Discussion
National Weather Service Denver/Boulder CO
750 PM MST Sat Dec 7 2019

Issued at 741 PM MST Sat Dec 7 2019

Quiet evening across the Central Rockies. Snow is still expected 
to move into the north central mountains late tonight, though the
snow may be a few hours slower coming in than in the forecast. 
Will decrease PoPs around and a little after midnight to reflect a
later start to the snow. Also temperatures and sky coverage to 
reflect current trends. 


.SHORT TERM...(This evening through Sunday)
Issued at 252 PM MST Sat Dec 7 2019

The axis of a fairly weak ridge of high pressure will continue to
weaken as it migrates over Colorado and onto the central US 
plains this afternoon and evening. Subsidence and dry conditions 
will dominate the plains and mountains for this evening ahead of 
increasing mid and upper level Pacific moisture which begins to 
stream into the high country late tonight and into early Sunday 
morning. For the lower elevations, with the low level inversion 
expected to set up again tonight, typical light drainage surface 
flow is expected. A few patches of fog may develop across the 
lowest areas of the Platte River valley.

Over the higher mountains late tonight, the next winter feature of 
concern begins to influence the northern and central high country
for late tonight and beyond as a broad and moist Pacific trough 
moves southeastward into Colorado. Latest runs have the dynamics 
with this disturbance coming in slightly weaker as widespread 
moderate strength QG ascent is somewhat disorganized. As moderate
depth moisture up to 600 mb rides in on the northeastern edge of 
70- 80kt jet level winds over southern and central Colorado, 
snowfall production for the duration of this system look to be 
primarily orographically driven. At this time, will begin a Winter
Weather Advisory for the Northern Gore Range and Rabbit Ears Pass
vicinity at Midnight tonight. By the 12-18z timeframe, deeper 
moisture enters the central and northern portion of the Rockies 
along with better lapse rates in the 7.0-8.5 C/km range and
persistent strong westerly winds at 700mb and higher. This period
of deepest moisture, orographics and best lapse rates should 
persist through 00z Monday and then continue on into early 
Monday. Best snowfall will land on west facing slopes above

Across the plains Sunday, after temps reach near seasonal highs, 
a weak initial northeasterly surge will move onto the northeast 
plains during the late afternoon ahead of cold air in later 
periods. Conditions are expected to remain dry across the plains 
through 00z Monday.

.LONG TERM...(Sunday night through Saturday)
Issued at 321 PM MST Sat Dec 7 2019

Moderately strong zonal flow is progged for the CWA Sunday night.
This decreases somewhat on Monday with a weakening upper trough 
to move across Monday evening. Strong north-northwesterly flow 
aloft is over us after midnight into Tuesday morning. The flow 
relaxes through the day Tuesday and Tuesday night, becoming zonal 
as upper ridging moves in. The synoptic scale energy is weak 
upward Sunday evening, then downward motion is place the rest of 
the Sunday night through Tuesday night. For the low level winds, 
there will be normal drainage patterns Sunday evening. The models 
are still showing a cold front with upslope flow behind it moving 
southward late evening Sunday. The upslope is short lived and done
by mid morning Monday. There are some northwesterlies over the 
eastern plains into Monday evening. The rest of the area should 
have drainage patterns Monday night. More normal wind patterns are
progged Tuesday and Tuesday night. Looking at moisture, it is 
fairly deep over the mountains Sunday evening, with some in the 
mid and upper levels of the plains. After midnight into mid Monday
morning, moisture decreases in the mountains and cross sections 
show some shallow low level moisture over the plains. It continues
to dry out for the rest of Monday in all areas. There is a bit of
upper level moisture around here and there for the CWA Monday 
night through Tuesday night. Considering highlights for snow in 
the mountains, there will still be fairly decent orographics in
place over the mountains Sunday night into early Monday. In 
collaboration with short term forecaster, will go with a snow 
advisory through Monday morning. For pops on the plains, will go 
with 10-30%s Sunday night into Monday morning. Snow accumulations
not expected. For temperatures, Monday's highs are 5-9 C colder 
than Sunday's highs. Tuesday's readings are 0.5-2.0 C warmer than 
Monday's highs. For the later days, Wednesday through Saturday, 
models have a weak, dry upper trough move across Colorado 
Wednesday night, with upper ridging Thursday. Strong northwesterly
flow aloft is progged Friday and strong, more due westerly flow 
aloft is indicated on Saturday. There looks to be some moisture in
the flow for orographic alpine snow Friday and Saturday. The 
plains look dry all four days of the extended period. The 850 MB, 
700 MB and 850-500 MB thickness fields never show temperatures to 
get much above seasonal normals all four days. 


.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 741 PM MST Sat Dec 7 2019

VFR conditions will prevail through Sunday with high clouds 
continuing to stream over the region. Winds will be a southerly 
drainage direction tonight at KDEN and KAPA. Winds will shift to the 
northwest to north a little after 18Z. A few rain showers will be 
possible towards 00Z. Ceilings could fall under 6000 feet beneath 
the showers late Sunday, otherwise ceilings should remain above 6000 


Winter Weather Advisory from 5 AM Sunday to 11 AM MST Monday for 

Winter Weather Advisory from 11 PM this evening to 11 AM MST 
Monday for COZ031.



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