Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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899
FXUS65 KBOU 202123
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
323 PM MDT Thu Apr 20 2017

.SHORT TERM...(This evening through Friday)
Issued at 323 PM MDT Thu Apr 20 2017

Showers and a few embedded thunderstorms are developing pretty
much as expected this afternoon. As Q-G lift increases through
the evening, they will continue to expand in coverage and also
spread northeast across the plains, finally reaching the northeast
corner toward midnight. The Q-G lift and persistent frontogenesis
will combine with a slightly unstable airmass this evening
(already noted by thunderstorms), and then a neutral airmass during
the overnight hours. As a result, precipitation production will
be efficient through Friday morning.

The various models are showing waves of precipitation spreading
across the forecast area, one this evening and another overnight
into Friday morning. The second wave later tonight still seems to
focus itself just east of the I-25 corridor as some
northwesterlies work off the Front range, effectively pushing the
best mid level convergence and rain coverage to the east.
Therefore, boosted the QPF amounts there just a bit more with 1-2
inch rain expected over Weld county and perhaps south/east into
Morgan and Washington/Eastern Adams counties. The band over the
plains is expected to gradually decrease later Friday morning into
the afternoon and deep upper level support and frontogenesis
finally weaken. It should be noted if the upper level trough and
eventual cutoff low occurs farther south, then the band would
focus closer to the I-25 corridor or foothills with heavier
precipitation amounts there. At this time, confidence is still
pretty high for the heavier amounts farther east on the plains.
Will continue to monitor and adjust accordingly. Short hydrology
section below.

In the mountains, will continue the idea of heavier convective
snow showers this evening followed by perhaps a break and then
another round of heavier/more persistent snow during the late
night/early Friday AM hours as increasing northwest flow improves
the orographic forcing. Will continue the advisory for 4-10 inches
over the mountains above 9000 feet. Lower amounts of 1-4 in the
valleys with the later changeover, and 1-6 inches across the
foothills favoring the northern foothills toward the Wyoming
border for the heavier amounts with south/southeast flow early in
the event. Overall confidence for the snow accumulation forecast
is moderate.

On Friday, expect a cool and moist day. Showers should generally
decrease in the morning, but then redevelop in the afternoon with
any surface heating and destabilization. The afternoon showers
should be generally scattered in nature.

.LONG TERM...(Friday night through Thursday)
Issued at 323 PM MDT Thu Apr 20 2017

The weather pattern across across the U.S. is expected to remain
progressive through the next week as westerly flow aloft keeps
short waves moving along nicely. The first storm system will be
out of Colorado by Friday evening, although cool and unsettled
northwesterly flow aloft will remain over the state. This will
keep showery activity going over the mountains and somewhat onto
the plains. Mountain areas may pick up an additional one or two
inches of snowfall Friday night, as the showers begin to wind
down. On the plains, showers in the slightly unstable airmass will
be diminishing as temperatures also cool down. Showers may mix
with or change over to snow before they come to an end Saturday
morning.

The unsettled, cyclonic northwesterly flow aloft will continue
through much of Saturday, supporting scattered mountain snow
showers and few rain showers across the plains as temperatures
only climb to the lower and mid 50s. Subsidence and warming are
expected to arrive Saturday night and Sunday as an upper ridge
builds over the Great Basin. Each of the mid-range forecast models
show agreement with the progression from the troughiness to upper
level ridging that continues through Sunday evening. On Monday,
warm temperatures are expected to continue as the upper ridge
moves to the Mississippi Valley and Colorado comes under dry
southwesterly flow aloft.

Cooling is expected to return through the middle of next week as
models show a broad upper trough extending across the western
two-thirds of the country. The westerly flow pattern will most
likely produce more snow in the mountains than rain on the plains,
but embedded short wave disturbances might be able to produce
rounds of diurnal convection if enough moisture and instability
are present.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 323 PM MDT Thu Apr 20 2017

Showers and a couple embedded thunderstorms (20-30% coverage of
thunder) are spreading over the airports and will continue
through 04Z. After that, there may be a few breaks but overall it
will be hard to time the waves of showers that move across
through about 15Z Friday. Clouds will continue to lower through
03Z with MVFR and local IFR ceiling and visibility through the
night. A slight chance for thicker fog after 06Z with visibility
falling below a mile. There should be a few breaks in the showers
and perhaps visibility improvement late tonight and Friday
morning. Ceilings are expected to lift to MVFR or a bit higher
toward 16Z-18Z Friday. However, scattered convective showers and
ILS conditions still expected through Friday afternoon.

Winds will likely become variable with all the showers around this
evening, and occasionally gust to around 15-25 knots especially
early in the evening. After that, winds should transition to more
of a east/northeast flow and then northerly through Friday.

&&

.HYDROLOGY...
Issued at 323 PM MDT Thu Apr 20 2017

Rainfall amounts and liquid equivalent are expected to range
between one half and one inch over most of the forecast area, with
an area of 1 to 2 inches mainly from Weld county into Morgan
Washington counties. With the dry conditions as of late, no
hydrology issues are expected at this time.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 PM this evening to 9 AM MDT
Friday for COZ031-033-034.

&&

$$

SHORT TERM...Barjenbruch
LONG TERM...Dankers
AVIATION...Barjenbruch
HYDROLOGY...Barjenbruch



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