Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 191635
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1035 AM MDT Thu Apr 19 2018

.UPDATE...
Issued at 1035 AM MDT Thu Apr 19 2018

Going forecast is in good shape, may make minor changes to
temperatures, winds, and relative humidities to reflect current
conditions. May need to increase dew points and relative humidities
over the eastern plains. Dew points are in the upper 20s to lower
30s over western Kansas and far eastern Colorado. The gusty south-
southeast winds will advect the moisture somewhat westward.

As far as the approaching storm goes, will replace the Winter
Storm Watch with either a Winter Storm Warning or a Winter
Weather Advisory early this afternoon once the 12Z models finish
coming in. There is still a good amount on uncertainty when rain
changes to snow in the foothills and where the heaviest
precipitation will fall.

&&

.SHORT TERM...(Today and tonight)
Issued at 445 AM MDT Thu Apr 19 2018

Deep southerly flow will develop today ahead of the approaching
trough. There seems to be a be a weak ripple in the mid level flow
(a bit of a warm front aloft, perhaps?) that passes in the late
afternoon. Behind this, the low/mid level winds really crank up.
Model soundings show south winds at 50 knots over the plains at
midnight tonight. Guidance continues to be a little windier and
drier this afternoon. Blend of guidance gives us lots of 30-40 mph
gusts this afternoon, with up to 50 mph possible. We could be
close to High Wind criteria on the north side of the Palmer Divide
this evening (mainly Elbert county). I added areas of blowing dust
to the forecast as that much wind should produce some. So much
wind down low suggests we will decouple less than usual after
sunset, with strong winds and slower increase to the humidities
this evening. MOS dew points are a little lower than forecast, and
may still be too high as the source air for our SSE low level
flow is coming from areas that have no low level moisture...dew
points in the single digits and teens over southeastern Colorado
at this time, and only lower to mid 20s over northern and eastern
Kansas. You have to get well into Texas to find richer moisture.
We will probably moisten from the top down this evening rather
than advecting much in horizontally--that won`t come until Friday
morning when the low level flow goes more easterly.

Forecast temperatures look good--there should be some high clouds
although we will largely be in the mid level dry slot during the
day. Current forecast on the warmer end of guidance for tonight,
but the wind may keep it even warmer. Some showers should work
their way into the mountains late tonight, but this is looking a
little slower so the serious snow may not begin until Friday
morning.

.LONG TERM...(Friday through Wednesday)
Issued at 445 AM MDT Thu Apr 19 2018

At 12z Friday, the models keep a closed 500/700 low over east
central UT with a southerly flow over the cwa in the morning. By
00z Saturday, the closed circulation, overall fairly weak, will be
over south central CO. Moderate to strong mid level qg ascent
will be in place mainly in the 18z-00z period Friday afternoon,
then weak mid level qg ascent will be around Friday night into
Saturday morning.

At the surface, the trough will be over western CO Friday morning,
with an an inverted trough along the Front Range by 00Z Saturday.
Gusty east/southeast winds will occur over the northeast plains
in the afternoon and evening which will usher in plenty of low
level moisture. Along the Front Range the low level winds will be
weaker north/northeasterly. Forecast soundings show the best moisture
on Friday will be over the northeast plains. The moisture does
not deepen over Denver until Friday evening.

NAM12/GFS20 Spatial cross-sections not overly impressive
regarding the upslope. Both show it, but not for a prolonged
period of time and and overall it appears to be fairly weak. This
system not very cold initially either so it may take some time to
lower overnight which would lessen the accumulations. Plenty of
moisture however, and snow amounts of 8-18 inches over the 24-hr
period starting Friday afternoon will be possible especially above
7500 feet. At this time, will issue a Winter Storm Watch for the
Front Range Mountains and Foothills. Not confident enough based on
the snow level to hoist a Winter Storm Watch for the Palmer
Divide. Snowfall amounts there maybe in the 3-6 inch range.

By Saturday afternoon, a broad 500 mb circulation will be over
the Central Plains of KS/OK with a northerly flow aloft over the
cwa. Some residual moisture wrapping around the low, but weakening
qg ascent as the system continues to move slowly east. Only some
light showers expected in the mountains Sunday afternoon. A weak
westerly flow aloft will be over Colorado on Monday then the next
system is expected to pass across MT/WY Monday night into Tuesday.
This will bring some light snow to the mountains with isolated to
scattered showers over the adjacent plains, mainly along the
WY/NE borders. The pattern will remain relatively unsettled
through the middle of next week. The ridge will shift to the east
by Wednesday, but the models still show enough moisture around for
some shower activity, especially over the mountains.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1035 AM MDT Thu Apr 19 2018

VFR conditions will prevail through tonight. Low clouds move in
Friday morning, after 12Z with ceilings falling to 3000 to 6000
feet. South-southeast winds will continue through today and
tonight. Wind speeds will increase late this afternoon and
evening, with gusts of 30-40 knots at KDEN/KAPA 22-08z. Winds will
be much weaker at KBJC.

&&

.FIRE WEATHER...
Issued at 445 AM MDT Thu Apr 19 2018

Strong south winds will develop on the plains this morning, with
30-40 mph gusts becoming common and humidities dropping to 15-20%
by noon. This afternoon even stronger gusts are possible, and
humidities will drop to 10-15%. In areas west of I-25 from
Jefferson county north to Fort Collins the winds will be lighter
most of the day, but could become gusty in the late afternoon and
early evening, approaching Red Flag conditions for a few hours. In
areas east of Fort Morgan and Limon the humidities will be
higher, but it will be very windy.

Strong winds will continue overnight and will slow the usual rise
in humidities this evening. In some areas, especially east and
southeast of Denver, the strongest winds may be this evening. Even
after the Red Flag Warning expires at 8 PM, strong south to
southeast winds may create enhanced fire behavior on any fires
that are burning.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 8 PM MDT this evening for COZ214-216-
240>247.

Winter Storm Watch from Friday afternoon through Saturday
morning for COZ033>036.

&&

$$

UPDATE...Meier
SHORT TERM...Gimmestad
LONG TERM...Cooper
AVIATION...Meier
FIRE WEATHER...Gimmestad



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