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FXUS65 KBOU 182157

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

.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 

.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 18Z TAFS through 18Z Friday afternoon)
Issued at 255 PM MST Thu Jan 18 2018

VFR conditions through the period. The normal diurnal wind 
pattern is expected with wind speeds under 12 kts.




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