Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 121738
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1038 AM MST Mon Feb 12 2018

.UPDATE...
Issued at 938 AM MST Mon Feb 12 2018

Made minor changes to today`s high temperatures across Weld County
and west and south into metro Denver as the cloud cover and lack
of mechanism to erode the arctic air will likely hold temps in the
upper 10s to near 20 across Weld and low 20s across metro Denver.
Recent surface pressure trends have shown a building high
pressure area over Weld County and the north side of metro Denver,
reinforced by surface warming south and east of the Palmer Divide
and thus lower pressure there. This should act to continue the
flow of cool north and northeast winds across the I-25/US-85
corridor and keep the clouds and cold air in place through today.
The airmass is cold enough that flurries are possible/likely but
little to no accumulation is expected. Across the mountains the
forecast was left alone, as latest HRRR agrees with the going
forecast for light snow with 1-4" through this evening.

&&

.SHORT TERM...(Today and tonight)
Issued at 508 AM MST Mon Feb 12 2018

Once again, colder air is bringing low clouds into Denver just
before sunrise. Stratus deck looks to be mainly off the ground, so
not expecting much in the way of thick fog and freezing
drizzle/flurries, but there could be some spots. This would be
most likely in upslope areas south and southeast of Denver. Not
expecting anything of consequence.

Today`s developing shortwave is modeled to generate more moisture
as the lift moves across Colorado today. So far there is little
mid level moisture, though there are signs that the cloud deck is
starting to develop with some mid level ceilings just recently
appearing at a few stations. Some of the models generate some
light precipitation from this developing moisture overrunning the
cold air over northeastern Colorado during the day. Others keep
this further north and east. This is a reasonable concept, but the
moisture availability both in the overrunning airmass or in the
boundary layer all seems quite marginal. We`re showing an
increased chance of light snow during the middle of the day as
this lift passes over and the atmosphere tries to saturate.
Interesting struggle in the models as the southwest flow aloft
descends and there`s drying aloft, but still shallow moisture.
Again, the cold layer will probably be a bit more persistent then
what the models are showing, but the low level gradient should
also be going a bit more southerly again by morning. This should
at least start the exodus of the low clouds and cold air. Areas
south of Denver more likely to have clearing and some warming by
morning, while there will probably still be some clouds with the
cold air hanging in the South Platte valley north and northeast of
Denver all night.

.LONG TERM...(Tuesday through Sunday)
Issued at 508 AM MST Mon Feb 12 2018

On Tuesday and Tuesday night, the mid levels will remain
subsident with weak to moderate downward qg descent over the
region. A moderate southwesterly flow aloft will be over Colorado
with an elongated trough axis associated with a splitting systems
stretching from the Upper Midwest to the central CA coast. The
models generate a little bit of qpf in the mountains on Tuesday.
Coverage isolated to scattered and primarily over southwest facing
aspects. Over the northeast CO, dry and warmer with a surface lee
side trough developing over the eastern plains. Wednesday and
Wednesday night, the zonal flow aloft increases, as two troughs
over the west coast appear to merge into one. One drops out of
British Columbia, while the closed upper low over southern CA
coast starts to move onshore. As these systems consolidate into
one large trough Wednesday night, weak to moderate qg ascent will
develop over CO ahead of this system. This will result in an
increase in snow west of the Continental Divide. By 12z Thursday,
the trough axis stretches from western WY into the Four Corners
region with moderate qg ascent over western CO. Timing issues
start to crop up between the ECMWF and GFS Thursday into Friday,
with the GFS the faster of the two. Both however track the trough
axis east across CO through Thursday night, with the mid level
winds transitioning from southwesterly to to northwesterly with
the passage of the trough axis on Friday. A cold front on Thursday
will move across the area in the afternoon, with north to
northeast post-frontal winds Thursday night. In addition, a strong
140-150 kt upper jet is forecast to stretch across southern and
eastern CO around 00Z Friday, which then shifts eastward Thursday
night. On the cold side of this jet max, the potential exists for
enhanced CSI snowfall bands Thursday night over the eastern plains
of CO. The best chance of precipitation across the cwa appears to
be Thursday afternoon and Thursday night. On Friday, a drier and
subsident northwesterly flow would spread over northern CO. There
may still be some lingering orographic snowfall in the mountains,
dry and continued cool over the northeast plains until Saturday.
By the weekend, the flow aloft will transition from northwest to
westerly. Generally dry and warmer across the eastern plains, with
a chance of snow possibly slipping back in the high country on
Sunday ahead of another system that could impact north central and
northeast Colorado early next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 938 AM MST Mon Feb 12 2018

IFR conditions expected at the metro area terminals today and into
tonight. Flurries have been reported at BJC and DEN, but
accumulating snow is unlikely with the shallow nature of the cold
air and lack of any mid/upper level support. APA recently went
down to 1/2SM VIS as the upslope favors fog and lower CIGs there. Any
flurries should end by 01Z. Have handled the occasional flurries
and lower VIS with TEMPO groups through 21/22Z for now. Current
north winds at the terminals will gradually shift to northeast
this afternoon, but remain less than 10 kts. Some uncertainty as
to when the CIGs will improve. Late tonight looks possible for
scattering out but some of the guidance has the IFR CIGs holding
tough until around 17/18Z Tuesday. Will keep the more pessimistic
CIG forecast for all metro area terminals as the shallow upslope
typically holds tight for low clouds without anything obvious to
scour it out until late morning Tuesday. The bulk of Tuesday
should be VFR with light winds.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

UPDATE...Schlatter
SHORT TERM...Gimmestad
LONG TERM...Cooper
AVIATION...Schlatter



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