Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 182218

Area Forecast Discussion
National Weather Service Denver/Boulder CO
418 PM MDT Thu May 18 2017

.SHORT TERM...(This evening through Friday)
Issued at 410 PM MDT Thu May 18 2017

A very moist and slow moving storm system will continue to track
across Colorado tonight, weakening along the way. Models show the
upper low now over Montrose CO in southwest CO becoming less
organized and actually spinning into two circulation centers
overnight. Should this occur, the deep east-northeast upslope flow
over northeast CO for most of the day should weaken and as the
northern low passes over the area later tonight, wind fields show
850-600 mb winds become variable in direction. At this same
time...qpf fields indicate a dramatic drop off in precip over
sern portions of the fcst area apparently due to a shift to drier
nwly low and mid-level flow. As a result should see a steady drop
off in snowfall along the I-25 corridor as well as the rainfall
farther out across the plains. By midday Friday, expect to see
only scattered rain/snow showers on the plains, with the heavier
mostly rain showers across the far eastern sections of the CWA
during the afternoon on the back side of the exiting upper low.
Will leave detailing for additional snowfall in the Winter Storm
Warning updated at 3:28 PM today.

Bottom line...heavy snowfall in and along the Colorado Front
Range and the light to moderate rainfall of the northeast plains
should gradually decrease tonight through Friday morning. The
heaviest snowfall in and near the east slope of the Front Range
mountains will likely occur before midnight. On Friday, lingering
rain and snow showers will gradually diminish from west to east as
the storm system slowly moves away from the Front Range.
Additional snow accumulations will generally be light.
Temperatures on Friday will remain well below average on Friday.

.LONG TERM...(Friday night through Thursday)
Issued at 410 PM MDT Thu May 18 2017

The upper low/trough is over the eastern half of Colorado at 00Z
early Friday evening. By 12Z Saturday morning, the upper trough
axis is just east of the eastern Colorado border. Fairly weak
northwesterly flow aloft is expected on Saturday. West-
northwesterly flow aloft is progged Saturday night through Sunday
night. The is still some upward QG Omega progged for the eastern
CWA Friday evening, then all downward motion is progged the rest
of Friday night through Saturday night. Weak upward motion is
progged Sunday and Sunday night. The low level winds will be
northwesterly downsloping much of Friday night and Saturday.
Normal diurnal patterns look good for Saturday night through
Sunday. For moisture, there is still quite a bit around in the
lower levels Friday evening. Moisture decreases significantly
overnight into Saturday morning. There is some moisture around in
the mountains Saturday night. Moisture increases on Sunday into
Sunday evening, then decreases Sunday night late. The precipitable
water values get above 0.50 inch on Sunday for the eastern two-
thirds of the CWA. There is minor CAPE over the mountains late day
Saturday. There is a bit more over the western CWA late day
Sunday. There is a tad of measurable precipitation on the QPF
fields over the southern CWA Friday evening. There is a small
amount late day Saturday, mostly over the mountains. There is bit
more QPF over most of the CWA on Sunday, and even Sunday night.
For pops, will go with 30-50% Friday evening for light snow. Will
keep some minor pops going in the mountains Friday overnight and
Saturday. On Sunday and Sunday night will need 20-50% pops for all
the CWA. For temperatures, Saturday`s highs are 6-10 C warmer
than Friday`s. Sunday`s highs are close to Saturday`s. For the
later days, Monday through Thursday, there is upper troughing
around on Monday and Tuesday, then north-northeasterly flow aloft
Wednesday. There is weak north-northeasterly flow aloft on
Thursday with upper ridging moving in from the west. Will need
some pops Monday and Tuesday, then Wednesday and Thursday look
dry. Temperatures warm up to near seasonal normals by Thursday.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 410 PM MDT Thu May 18 2017

ILS to MVFR cigs and visibilities will alternate the rest of
this afternoon through 3 AM MDT tonight as bands of light to
moderate snowfall move northward over the metro area. The heavier
bands could deposit a quick inch or two of wet snow on paved
surfaces. But, once the snowfall stops, should see most of this
snow either melt or change to slush. On Friday, snowfall in the
Denver metro area should steadily diminish during the morning with
only scattered light rain showers or rain/snow showers in the
afternoon. Winds will be northerly tomorrow at 10-15 kts with
gusts approaching 25 kts on the east side of the metro area during
the afternoon.


Winter Storm Warning until 6 PM MDT Friday for COZ033>036.

Winter Storm Warning until 3 PM MDT Friday for COZ038>041.

Winter Weather Advisory until 6 AM MDT Friday for COZ032-037.



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