Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 151659
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1059 AM MDT Thu Apr 15 2021

.UPDATE...
Issued at 1100 AM MDT Thu Apr 15 2021

Overall the current forecast looks on track with only a few minor
details added based on the latest high res data. First, The
clouds are holding strong and the latest satellite model trends do
not provide much clearing. Thus, temperatures west of I-25 and
along the Palmer Divide will remain chilly with Tw remaining 30-34
deg in those areas, and actual temps probably not breaking out of
the 30s today. This also means that the p-type should be snow or
at least only an hour of rain before changing over west of I-25.
East of I-25 Tw should allow for rain initially after 2 or 3 PM,
but a change over between 5-7 PM. There is still quite a bit of
uncertainty across Washington and Lincoln Counties in terms of QPF
and snow amounts, but the latest ensemble guidance suggests
advisory amount snow even there. HRRR, EC ensembles, GFS ensembles
all point to advisory level snow for all of the I-25 corridor and
eastern plains. Around Greeley stands to receive the least amount
of snow, but still 1-4" is possible for them. Slush/slick and
snow covered roads are certain in the mountains later today into
Friday. Across metro Denver, roads may not become slushy/slick
until after 7 PM. Tomorrow`s rush hour is also likely to be
impacted with any untreated roads freezing up. With models coming
more into alignment for this event, and potential for travel
issues Friday morning, we have issued an advisory for most of our
CWA east of the Divide other than southern Lincoln County and Park
County.

Updated snow amount forecasts will be available early this
afternoon.

&&

.SHORT TERM...(Today and tonight)
Issued at 330 AM MDT Thu Apr 15 2021

A broad trough continues to position itself over the Western US
with southwesterly mid to upper level flow over Colorado. There is
some QG ascent on the east side of this trough centered over
northern Utah this morning. This QG ascent has not been strong
enough to overcome the dry air and produce precipitation and that
problem has plagued this system throughout this week. Until the
trough begins to enter Colorado late this afternoon, the forcing
will not be stronger enough to produce precipitation with an
exception over the far northern mountains where upslope flow is
better. PoPs and QPF were lowered or taken out of the forecast for
much of the area as the message should be plenty of dry hours
today for people to go about their business.

There is considerable cloud cover over Colorado this morning and
much of it should remain in place during the day. However, there
will be breaks in the clouds in the mountains which will allow for
decent instability to build in with mid level lapse rates around
8 C/km in the northern mountains. Whether or not the instability
builds in across urban corridor and centrals plains is a big
question mark. Models appear to be clearing out clouds too much
and over-mixing the boundary layer. Nonetheless, showers and
storms will form over the north-central mountains by the early
afternoon with intense snowfall rates and will move across the
urban corridor by 3-6pm. The HRRR seems to be overdoing the
thunderstorms that develop over the urban corridor during the late
afternoon and evening but brief heavy rain switching over to snow
can be expected. With the warm surface and pavement temperatures
as well as a mid April sun angle, it will take a couple hours for
the snow to accumulate on roads and sidewalks. This should limit
the impact on the evening rush hour although the end of the rush
hour could have slick, slushy roads especially across the northern
I-25 corridor. By the evening hours, strong QG ascent with
increasing upslope flow and mid level frontogenesis will produce
widespread moderate snowfall. Due to the warm low level
temperatures along with initially melting, snow ratios may start
out around 8:1 and then trend toward 10:1 overnight as
temperatures cool. Generally 2-4 inches of snow are expected
across the urban corridor on grassy surfaces with less on
pavement. Snowfall across the eastern plains was a tougher
forecast with the GFS and NAM keeping the highest values of QPF
farther north than the GFS and Canadian. The forecast ended up
being a blend of these models with a couple inches taken off snow
amounts and that result in a 3-7 inch forecast. This forecast
still favors the northernmost tier of counties rather than areas
along I-70. The Winter Weather Advisories in place across the
northeast plains were left unchanged in an effort to keep the
same messaging although the start times are likely too early.
Across the mountains, travel may be very difficult under the
possible thundersnow late this afternoon/early evening. Amounts by
sunrise Friday morning are still expected to be in the 4-8 inch
range.

.LONG TERM...(Friday through Wednesday)
Issued at 206 AM MDT Thu Apr 15 2021

Sharp ridge of high pressure along the Pacific coast through the
extended period will keep northwest flow across Colorado, with
periodic shortwave troughs and surface cold fronts dropping in
from the north. Temperatures will trend 10 to 20 degrees below
normal through the period.

Departing low pressure Friday morning will have snow showers
lingering across the area, decreasing in coverage and intensity
during the day. Best chances in the morning, when northeast plains
and mountains could pick up another inch or so of snow. Given
time of year and strength of April sun the snow ratios were
lowered a bit on the plains, closer to a 10:1 from higher guidance
values. As surface and upper low moves out of eastern CO into KS
Friday will be breezy on the plains, and blended in some high
winds mainly Friday afternoon. Strongest winds in
Washington/Lincoln counties where gusts to near 30 mph possible in
the afternoon. Wind should drop off quickly in the evening as
pressure gradient relaxes and surface based stable layer sets up.

Snow showers will continue Saturday mainly in the mountains and
adjacent plains with a second weak shortwave trough at 500 mb and
reinforcing surface front. Less moisture with the Saturday system
than tonight/Friday, and drier air moves in from the north late
Saturday afternoon. Lapse rates increase during the day Saturday
and we even reach some positive surface based CAPE below 600 mb to
produce scattered snow showers. Surface high temps into the 30s
mountains and around 40 on the plains. Between systems on Sunday,
and although cool northerly surface flow will continue, dry air
and plenty of sunshine will warm temps about 10 degrees over
Saturday.

On Monday another NW flow shortwave trough and surface front will
move through. ECMWF is faster and sharper with the trough than
GFS, with chc pops breaking out Monday afternoon. Precip starting
Monday afternoon will be rain or rain/snow mix, then turning cold
enough for snow Monday night. Little moisture to work with, coming
in on NW flow and so any precipitation will be light. Cold front
will drop max temps back into the 40s Tuesday, followed by a
slight warming trend Wednesday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 859 AM MDT Thu Apr 15 2021

MVFR/ILS conditions will be the norm through the period, with a
few periods of IFR this evening in moderate snow. Winds at DEN
should continue strong out of the SE and E until 5 or 6 PM, when a
surface cyclone moves east of the airfield. Winds will shift
rapidly to the north and gust between 20-25 kts. Rain will change
to snow by 6 PM as well at DEN, and IFR conditions are possible if
not likely from 6-10 PM, and maybe a little longer. APA will have
similar conditions and timing with wind, CIGs, and snow. At both
APA and DEN snow accumulation is expected on the runways late this
evening, with a couple inches of slush. At BJC winds should remain
steady out of the north, and chances are precip will start as snow
around 3 PM. The moderate snow moves east of the terminals after
midnight but light snow and IFR/MVFR CIGs should continue through
midday Friday. Not much additional accumulation is expected after
midnight but could get another inch on the runways through early
Friday morning. Winds should remain out of the north 8-14 kts all
night into Friday morning. CIGs around 020 should continue through
much of Friday but after about noon no additional snow should be
falling.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 3 PM this afternoon to noon MDT
Friday for COZ042>046-048>051.

Winter Weather Advisory from 3 PM this afternoon to 6 AM MDT
Friday for COZ033>035.

Winter Weather Advisory from 3 PM this afternoon to 9 AM MDT
Friday for COZ036-038>041.

&&

$$

UPDATE...Schlatter
SHORT TERM...Danielson
LONG TERM...Hanson
AVIATION...Schlatter


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