Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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189
FXUS65 KBOU 211755
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1055 AM MST Mon Jan 21 2019

.UPDATE...
Issued at 1055 AM MST Mon Jan 21 2019

...Blizzard Conditions for Parts of Northeast Colorado Tonight and
Tuesday Morning...

Winter storm and blizzard conditions to hit parts of northeast
Colorado tonight and Tuesday. Models still having a hard time
determining where this storm will reform east of the mountains.
Water vapor satellite imagery showing the main circulation tracking
east across southern Nevada at this time. This is more in line with
the southern solution that the 00Z ECMWF is showing. The 12Z NAM
shifted south, not as far as the ECMWF though, but the trend south
is noteworthy. Will shift the forecast towards the more southern and
slower track, which will result in heavier snow and more snow
farther west and south. The 12Z GFS has continued with its quicker
and northern track with this system, which isn`t helping with the
confidence on the storm track. Confident there will be a swath of 6
to 10 inches of snow with 40-50 mph winds, but location remains highly
uncertain. Will base warnings and advisories on the stronger and
southern track. Therefore will upgrade much of the Winter Storm
Watch to a Blizzard Warning. Will also add a winter weather advisory
westward into the Denver metro area and Greeley area. Some areas of
this advisory may not receive any snow if the quicker and
northern track prevails.

&&

.SHORT TERM...(Today and tonight)
Issued at 420 AM MST Mon Jan 21 2019

There is a storm system which is moving into central Nevada early
this morning which will move into Utah by midday.  After that there
is no consensus on exactly where an upper level low will move by
tonight.  The majority of models move an upper level low across srn
Wyoming/nrn CO and into wrn NE/Nern CO by late tonight.  The ECMWF
is the only model that has an upper level low taking a more southern
track and into east central CO by late tonight.  Analysis of height
falls and 250 mb warm pool would suggest a more southern track is
certainly possible much like the ECMWF shows.  Thus can`t completely
rule out its solution.

Thus this leads to a very complicated forecast.  If I were to
believe the majority of models the only potential for decent snow
across nern CO would be near the WY-NE border area with limited
snowfall over the rest of the plains. Meanwhile the ECMWF
solution would match up well with the current watch that is in
place over portions of nern CO and would also lead to higher
snowfall including portions of the Denver metro area. So the
question is what do I do at this point?  Considering overall
confidence along with the timing of impacts over the plains (late
this evening and overnight) will leave watch in place and add
areas near the WY-NE border. Meanwhile there is still a potential
for blizzard conditions late tonight over the plains and Palmer
Divide as winds increase behind the system.

As for the mtns as the system moves across later this aftn and
tonight snowfall will increase as well with advisory amounts in most
areas.

Finally in advance of the storm system for today a sfc low will
intensify over nern CO with gusty southwest winds over the Palmer
Divide and possibly over the Denver area.  As for highs there is a
front over the far nern plains which will remain nearly stationary
much of the day.  Thus highs over the far nern corner may not get
out of the upper 30s while temps further west and south reach to 55
to 60 degree range.

.LONG TERM...(Tuesday through Sunday)
Issued at 420 AM MST Mon Jan 21 2019

At 12Z Tuesday morning, models show an upper closed low
circulation over northwestern Kansas/south central Nebraska with
an upper trough stretching southwestward across the CWA down into
southwestern New Mexico. The upper trough is progged to move
slowly eastward across the eastern half of the CWA through Tuesday
with weak flow aloft. Strong northerly flow aloft is progged
Tuesday night, the then strong northwesterly flow aloft is in
place Wednesday and Wednesday night. The QG Omega fields have some
upward vertical velocity in place Tuesday morning, then weak
downward motion the rest of Tuesday into Wednesday morning. Weak
upward motion moves in Wednesday afternoon and night. Strong
north- northwesterly low level winds will continue over the plains
on Tuesday into Tuesday evening. Normal drainage wind patterns
are likely overnight Tuesday. Downsloping west and northwesterly
winds are progged on Wednesday into the evening. Models have a
cold front and low level upslope flow moving into the CWA around
09-12Z Thursday morning. For moisture, cross sections show Tuesday
to be cloudy through about 18Z, then some partial clearing in the
afternoon. It is fairly dry Tuesday night. Moisture increases
again Wednesday and Wednesday night. The QPF fields have some
measurable snow continuing over much of the CWA Tuesday morning.
After 18Z, there is a mere tad remaining over the far east and in
the high mountains through the day. The QPF fields show a return
of measurable snowfall in the high mountains after 15Z on
Wednesday continuing Wednesday night. There is also some over the
plains after 06Z Wednesday night. For pops, will go with "likely"s
Tuesday morning, then "chance"s in the mountains Tuesday
afternoon. No pops Tuesday night, then I will increase them in the
mountains on Wednesday, with "likely"s by afternoon and Wednesday
night. Will go with "slight chance"s over the plains Wednesday
night. Will also make sure blowing snow is mentioned over most of
the plains on Tuesday. For temperatures, Tuesday`s highs will be
5-11 C colder than today`s highs. Wednesday`s highs will be 3-6 C
warmer than Tuesday`s. For the later days, Thursday through
Sunday, models continue to have a significant mean trough over
much of North America all four days. The flow aloft will be strong
and mainly northwesterly all four days. There will be moisture
enough for some pops Thursday for much of the CWA and just over
the mountains Thursday night though Saturday evening. Snowfall
amounts will not be significant. Temperatures now look to be at or
below seasonal normals through Saturday, then above normals by
Sunday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1055 AM MST Mon Jan 21 2019

Heavy snow will be possible at KDEN and KAPA from 03Z through 18Z
Tuesday. Forecasted snow amounts remain highly uncertain, but a
period of heavy snow is likely for KAPA and KDEN. In addition to the
snow, strong north winds will result in near blizzard conditions,
mainly at KDEN. Visibility is expected to fall to a quarter mile
under the heavier snow and blowing snow. So there could be an
extended period of very low visibilities if the snow sets up over
the airports. Expect snowfall of 4 to 8 inches, though confidence
is low due to the uncertain storm track.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 8 PM this evening to 9 AM MST
Tuesday for COZ036-039-040-042-043-048-050-051.

Winter Weather Advisory from 3 PM this afternoon to 9 AM MST
Tuesday for COZ031-033-034.

Blizzard Warning from 8 PM this evening to 11 AM MST Tuesday for
COZ041-044>046-049.

&&

$$

UPDATE...Meier
SHORT TERM...RPK
LONG TERM....RJK
AVIATION...Meier



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