Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 200250
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
850 PM MDT Thu Apr 19 2018

.UPDATE...
Issued at 850 PM MDT Thu Apr 19 2018

Forecast is on track with gusty winds continuing across the plains
and the southern/eastern portions of the Denver area. Radar is
beginning to show a few echoes but even mountain observations
would suggest most of this is virga. Moisture will increase as
deeper Q-G lift and nose of upper level jet arrives later tonight
through Friday morning. We expect snow showers to become more
numerous over the mountains and Park county, with rain showers
developing across the plains. The I-25 Corridor may remain in the
gap of best shower coverage, but that will change Friday afternoon
as flow turns more easterly and upslope forcing combines with a
slightly unstable airmass to produce more widespread rain.
Highlights for snow in the mountains and foothills look to be on
track, with heavy snow showers and even some thunder developing
Friday afternoon. On the plains and I-25 Corridor, only light if
any snow accumulation can be expected on vegetated/elevated
surfaces as the entire column gradually cools Friday night into
early Saturday morning. Palmer Divide would certainly fair better
with a few inches possible there.

&&

.SHORT TERM...(This evening through Friday)
Issued at 237 PM MDT Thu Apr 19 2018

Upper level low over California and Nevada will track east the
next 24 hours and be centered over Colorado Friday afternoon.
Surface low pressure over the Great Basin will continue to bring
gusty south- southeast winds to the eastern plains tonight with
gusts to 45 mph. The gusty winds will result in mild overnight
lows for many places. Models are in good agreement showing the
left exit region of the jet producing an area of showers over far
eastern Colorado late tonight and Friday morning. A second of area
of precipitation is expected over the Central Colorado tonight
due to diffluence aloft ahead of the approaching upper level low.

South-southeast surface winds continue into Friday as the surface
low and upper level low near the region. This will pump moisture
into the area. Clouds will thicken and lower through the day due
to the increasing moisture and lift. Expect QG ascent to bring
rain and snow to much of the area Friday afternoon. Will upgrade
the Winter Storm Watch to a Warning for the mountains. Snowfall
amounts are marginal for warning criteria. However models indicate
airmass over the mountains becomes unstable with CAPE up to 500
J/kg. Because of this, will go with the warning with some areas
seeing a quick round of heavy snow. Tougher call on how much snow
will fall in the foothills. Snow level starts off high, around
8500 feet and lowers during the afternoon. Period of northeast
upslope winds is short and not that strong, so will go with a
Winter Weather Advisory for the the foothills.

.LONG TERM...(Friday night through Thursday)
Issued at 237 PM MDT Thu Apr 19 2018

For Friday night into Saturday the low pressure system will be in
place over Colorado and stacked through 300 mb. At this time
confidence is high that the mountains and higher foothills will see
decent snow accumulations into Saturday evening. However,
temperature profiles on the plains continue to be tricky. Latest GFS
continues to be on the warmer side with the NAM as the outlier on
the cold side. The 00z EC is in the middle so will most likely lean
to this solution. A surface low to the south over NM will move
eastward increasing NE flow over the plains and keeping much of the
NE region under the deformation zone. This set-up combined with the
stacked and slower low progression could keep precipitation falling
into Saturday evening. At this time QPF totals through 18z Saturday
for the Urban corridor and east are between three quarters of a inch
up to an inch on the EC with less over the east side of DIA and
increasing as you go toward the KS border with the help of increased
lift. Wet bulb zero levels show a decreasing trend after 00z Saturday
with the EC dropping between 5800 and 6000 by 12z Saturday. Between
03 and 06z the cold push from the NE will reach back to the
foothills that will help to cool and saturate the column through 300
mb. Models are indicating cold air damming against the northern
foothills that could significantly add to snow amounts in this area
through Saturday morning. By 18z Saturday the low pressure system
will start to transition to the SE over the TX panhandle while
maintaining the def zone over the east and NE portions of the state.
This will continue light to moderate snowfall into the afternoon
hours. As the storm transitions east-southeastward increasing
subsidence behind it with WAA increasing. This will help to clear
out conditions by the later afternoon hours on Saturday helping to
push daytime temperatures on the plains into the upper 40s to low
50s.

For Sunday into Monday upper level ridging will increase helping to
warm areas back into the 60s with 70s possible by Monday. By Monday
night a shortwave will skirt over the northern portion of the state
bringing snow to the mountains with a slight chance of rain to the
NE plains by early Tuesday morning. The active NW pattern will
continue through mid week bringing additional chances of
precipitation to the region.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 850 PM MDT Thu Apr 19 2018

Southeast gusting to around 30 knots should slowly weaken through
the night. There is a chance that winds go much lighter and even
variable as a Denver convergence zone develops near or just north
of KDEN-KAPA.

VFR conditions will prevail through tonight, and probably most of
Friday. Ceilings should gradually lower with developing showers
after 20Z Friday, with MVFR conditions possible and ILS approaches
at a minimum. There may be a brief sprinkle or light shower
Friday morning, but ceilings should stay above 8000 feet in this
deep southerly flow. MVFR and eventually IFR conditions will
become more widespread Friday evening with the onset of more
significant showers and easterly upslope flow.

&&

.FIRE WEATHER...
Issued at 850 PM MDT Thu Apr 19 2018

South-southeast winds gusting to 30-40 mph will continue tonight
for areas east of I-25 and south of I-76. Relative humidity
recovery will be slow due to the gusty winds, but the Red Flag
conditions of earlier today have improved as humidity recovery is
now underway. Showers will increase in coverage across the
mountains late tonight, with rain showers spreading onto the
plains by Friday morning and becoming more numerous in the
afternoon, easing fire danger.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 AM Friday to noon MDT Saturday
for COZ031-035-036.

Winter Storm Warning from 6 AM Friday to noon MDT Saturday for
COZ033-034.

&&

$$

UPDATE...Barjenbruch
SHORT TERM...Meier
LONG TERM...Bowen
AVIATION...Barjenbruch
FIRE WEATHER...Barjenbruch


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