Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 231916
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1216 PM MST Fri Feb 23 2018

.UPDATE...
Issued at 1210 PM MST Fri Feb 23 2018

A quick update to freshen temperatures and wind, blended in latest
lave grids. Fog and low clouds have generally lifted, with the
rest of the day partly to mostly cloudy. Some concerns for higher
snow totals in a band to develop from Boulder Co NNE into Larimer
& Weld counties. Will evaluate more for next forecast update.

UPDATE Issued at 839 AM MST Fri Feb 23 2018

Just updated this morning forecast grids and text product, mainly
to account for the areas of fog on the plains, some of which has
become dense (VSBYS 1/2 mile or less) in the past hour or so
across eastern portions of the Denver metro. Past few minutes VSBY
dropped to a 1/4 mile in dense fog at KDEN. Counting on VSBYS
rising in the metro area in the next hour or so now that sfc winds
have gone drainage, albeit a light southerly wind. To the north
through east of the Denver, should also witness a gradual
dissipation in the fog coverage, from south to north-northeast
over the next 1-3 hours. Low lying areas along the South Platte
River north of the metro area will be the last to see the fog mix
out later this morning. Otherwise, rest of the forecast pretty
much on track. May still end up making adjustments to sky cover
and snow shower coverage through the day with the approaching
storm system moving closer to the fcst area.

&&

.SHORT TERM...(Today and tonight)
Issued at 233 AM MST Fri Feb 23 2018

Current synoptic pattern has an upper trough digging into the
Great Basin with the upper jet pushing out of the SW. The
anticyclonic side of the jet is moving over the state helping to
bring some stability to the NE region of the state. This will
slightly diminish precipitation in the mountains and bring
clearing conditions to the plains. With the increased drying low
level moisture will help to create fog on the plains into the
early morning hours. Expect areas over the Urban Corridor and
eastward to the Kansas border to see areas of fog development.
With the southerly flow to the south of I-70 fog will be less with
the help of drying off the Palmer Divide. Visibility could get
down to a mile or less in some areas so caution is advised for
early morning travelers.

The next batch of energy provided by the cyclonic side of the
approaching jet will increase snow in the mountains starting this
afternoon and continue through Saturday morning. The upper jet
will once again provide the instability needed for banding snow
potential. QG ascent numbers are good with -40 in the mid levels
and upwards of -7 at the surface. Convective snow bands are always
a challenge to pin down but model guidance is in decent agreement
with some setting up from a line moving NE from Boulder into
Larimer county later this evening. Went ahead and increased pops
for these areas. Most areas will see some light snow potential but
under the bands a quick inch of snow accumulation will be
possible. Further east on the plains ahead of the main convective
band there will be enough low level moisture to allow freezing
drizzle for a couple of hours late this evening. As the jet
pushes ESE the column will cool changing the freezing drizzle
over to snow where periods of heavy snow will be possible into
the early morning hours on Saturday. A winter weather advisory
has been issued for various counties in the NE plains due to local
impacts from the freezing drizzle and possible heavy snow. As
mentioned before, with this type of convective set-up advisory
criteria can be reached quickly so it is possible that areas on
the plains under the bands will see quick accumulation. The
situation will be monitored closely through the day but caution
is advised for travelers if you are caught under one of these
bands due to greatly reduced visibility and slick roadways.

Temperatures today will stay on the cool side with highs in the
mid to upper 20s. Overnight lows will drop into the teens.

.LONG TERM...(Saturday through Thursday)
Issued at 233 AM MST Fri Feb 23 2018

Storm system will be exiting to the east Saturday morning, with
snow diminishing and ending from west to east across the northeast
plains. We`ll keep the Winter Weather Advisory in effect until 9
AM for now as most snow should be over by then. Along the I-25
Corridor, downslope flow will be kicking in with gusty
west/northwest winds expected as stronger subsidence develops
behind the trough axis. Despite the winds and added mixing,
temperatures will remain quite cold with strong cold advection
occurring. Highs will barely make it to freezing on the plains,
and with gusty northwest winds it will feel more like the teens.
In the mountains, orographic light snow showers will also be
decreasing and coming to an end during the day as dry, subsident
air moves in. Highs there will only be in the single
digits/teens, with wind chill readings holding below zero.

By Saturday night, the next short wave will begin to approach the
region so should see snow showers redevelop across the high
country as mid level moisture increases in an unstable
environment. Those should linger into Sunday morning with
generally light to moderate accumulations expected, as best
synoptic scale lift will be passing by to our south. The plains
should remain under dry, downslope flow during this period.
Temperatures will remain well below normal under the influence of
the sweeping cold trough across the northern Rockies/High Plains.

Some moderation in temperatures will finally occur by Monday
as a deeper trough digs down the west coast of the U.S.  That
should lead to short wave ridging and warm advection across
Colorado, while lee troughing will aid downslope warming. We
still might have to watch for weak backdoor cold fronts given
short waves riding across the northern U.S. into Tuesday. Then a
piece of that upper level trough may push across the forecast area
by Wednesday with slight cooling. This could also bring a return
of light snow showers. Temperatures should stay near or just
below normal for the middle of next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1210 PM MST Fri Feb 23 2018

CIGS/Vis IFR at TAF issuance time, forecast to improve to VFR by
19-20z. VFR through the afternoon with light southeast wind
turning northeast in advance of approaching low pressure system.
VCSH beginning 21-22z, then snow begins in earnest and categorical
00z onward with generally VFR cigs/vis in snow. Conditions improve
to VFR after 12z Saturday.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 5 PM this afternoon to 9 AM MST
Saturday for COZ048>051.

&&

$$

UPDATE...Hanson
SHORT TERM...Bowen
LONG TERM...Barjenbruch
AVIATION...Hanson



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