Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 272156
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
356 PM MDT Thu Apr 27 2017

...Strong spring storm to affect Colorado Friday evening into
Saturday...

.SHORT TERM...(This evening through Friday)
Issued at 227 PM MDT Thu Apr 27 2017

GOES-16 Water Vapor and Visible imagery continues to show in
great detail the short wave trough that continues to generate
convective snow showers across the high country and rain showers
along with a few thunderstorms, across the Plains this afternoon.
The main activity is currently well east of the I-25 corridor,
across Morgan, Lincoln, and Washington counties and will continue
to move east through this evening. A fair amount of convective
instability (300-600 J/kg ML CAPE) is available to the leading
line/cluster of showers and thunderstorms, which would be
favorable for sustaining the thunderstorms and generating small
hail. The rain showers and thunderstorms should exit the Plains
into Kansas by late this evening. Across the mountains, with the
departure of the shortwave trough and loss of diurnal heating, the
snow shower activity should diminish this evening. Until this
evening however, conditions are still favorable for brief,
moderate to heavy snow showers/squalls that adversely impact
travel across the higher mountain roads, especially along the I-70
along corridor across Summit county. Will keep the advisory in
place for the mountains above 9000 feet until 6 PM given the
expected potential travel disruptions with the snow squalls.

Tonight the lead short wave trough moves into W. Nebraska taking
most of the synoptic scale lift with it. The exception will be
far NE corner of Colorado where light rain may continue tonight as
the short wave trough stalls over W. Nebraska. Westerly upslope
flow will keep light snow chances going tonight across the
northern and central mountains but any additional accumulation
will be light.

Friday morning, in the wake of the short wave trough with a
surface reflection, a strong surface low pressure area across NW
Kansas, will drive a cold front across the Plains of Colorado
Friday morning. Strong and gusty north/northeast winds will
accompany the frontal passage and another round of precipitation
is possible across the northeast Plains. Cloud cover and the cold
front will keep high temps in check across the Plains, with mid
and upper 40s across the southern part of the area including the
Denver metro, and struggling to reach 40 across our northern
border with Wyoming and Nebraska. The front will also generate
upslope snow showers across the eastern mountains and foothills of
the Front range and Palmer Divide Friday afternoon. Snow levels
Friday afternoon behind the front will be around 6500 feet MSL but
drop below 6000 feet by Friday evening.

Focus then turns to what may happen Friday night and Saturday,
and even 30 hours prior to the potential start of the snow, the
global models differ quite a bit on the synoptic scale setup. This
of course affects snow amounts across the I-25 corridor. A 140 kt
northerly jet digs south out of the Pacific northwest, located
across the Great Basin by Friday evening. The strong trough
downstream of the jet will then dig south into the 4 corners
region by Friday evening, setting the stage for strong dynamic
lift, a developing surface low, and upslope flow across northeast
Colorado. However, how the models handle the lead short wave
trough impact how much lift can be generated and for how long
across our area. GFS/NAM have the short wave trough deepen and
cut off over the Nebraska panhandle at 00Z, resulting in a much
broader, elongated 500mb trough from the 4 corners into the
Nebraska panhandle. The EC and Canadian models weaken the lead
short wave trough and move it quickly northeast, resulting in a
stronger, much less elongated 500mb low across the 4 corners at
00Z. Implications on the sensible weather at the ground will be
covered below by the long term forecaster.

.LONG TERM...(Friday night through Thursday)
Issued at 227 PM MDT Thu Apr 27 2017

Minor adjustments to the Friday night/Saturday forecast based on
accepting the trend toward a more elongated low and nudging the
forecast in that direction. This keeps the best lift roughly along
and south of Interstate 76 and limits the duration of the strong
upslope in areas north of Denver. This could result in less
precipitation, but the more northerly winds behind the stronger
development aloft over northeastern Colorado will also bring
cooling faster Friday evening. The result of this would be more
snow along the Front Range during the most intense part of the
storm Friday night. This trade off results in not much change to
our snow forecast for the Denver area. With the faster cooling,
there is less chance of getting skunked altogether, but we should
still wind up with a significant gradient across the city due to
colder temperatures and stronger and more prolonged upslope over
the southern part of the metro area. Impact will also depend
greatly on temperature. In the cities the roads should be warm
enough to restrain accumulation, though there could be some slushy
times later Friday night and early Saturday morning. Bigger impact
for the cities could be snow accumulation on trees and resulting
tree/power line damage. The higher end of our accumulation range
could be a significant tree-breaker and power outage event. For
areas above 6000 feet there is much more chance of some
accumulation on the roads.

The system moves out Saturday afternoon, with the mountain showers
fading in the evening. One big question is the amount of clearing
and wind speeds to affect low temperatures Sunday morning. At this
point it looks like there should be pretty good clearing, though
there will likely still be some wind in areas that aren`t
sheltered. Guidance has slightly cooler temperatures Sunday
morning and this looks good. This makes for mid 20s in a lot of
places that would impact tender vegetation and sprinkler systems.

The rest of the forecast features cool northerly flow with a
couple embedded disturbances. Unstable enough for some low shower
chances, but the best chance will be Tuesday and Tuesday night.
The ridge eventually builds upstream with a little warming and
drying trend toward the end of the week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 227 PM MDT Thu Apr 27 2017

The rain showers have moved east of the terminals this afternoon
and in the wake of the showers a few clouds around 020 will linger
this afternoon. For the most part the BKN CIGS will be VFR. APA
could see showers through the late afternoon but chances are too
low to mention in the TAF. DEN and BJC should remain dry the
remainder of this afternoon and overnight. All terminals should
be VFR later this evening and tonight. Tonight as the surface low
shifts east expect southwest flow at the terminals. On Friday a
cold front will move across the terminals mid morning, shifting
winds to the north and eventually northeast by Friday evening.
Lower clouds will likely accompany the frontal passage but
borderline MVFR are expected at this time. Later shift will
monitor for the potential for possible precipitation and MVFR CIGs
during the Friday afternoon period behind the cold front.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM MDT this evening for COZ031-
033-034.

Winter Storm Watch from Friday evening through Saturday
afternoon for COZ035-036-041.

&&

$$

SHORT TERM...Schlatter
LONG TERM...Schlatter
AVIATION...Schlatter


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