National Weather Service Forecast Discussion

512
FXUS65 KBOU 282120
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
220 PM MST Sat Nov 28 2020

.SHORT TERM...(This evening through Sunday)
Issued at 200 PM MST Sat Nov 28 2020

A cut off trough is departing the region this afternoon and is
headed eastward over the Southern Plains. Water vapor imagery
shows a shortwave trough moving across the Pacific Northwest with
an underwhelming warm conveyor belt out ahead of it. This
shortwave, which will move southeastward and graze northern
Colorado tomorrow, lacks substantial moisture and will fail to
bring precipitation to Colorado. The only things this system will
bring is cooler temperatures and increased winds tomorrow.

A cold front seen in observations currently over Montana will
push through our forecast area around midnight tonight shifting
winds to the north. The cold air behind this front is
unimpressive as temperatures in southern Canada are in the low 30s
to upper 20s. Highs tomorrow will certainly be cooler than today
but not all that cold as temperatures will top out in the upper
30s to low 40s across the plains. Winds may gust to 35 mph across
the eastern plains at times during the day tomorrow and will
weaken after sunset. Otherwise, partly cloudy skies and dry
conditions are expected.

.LONG TERM...(Sunday night through Saturday)
Issued at 153 PM MST Sat Nov 28 2020

Sunday night will be clear and cold with lows in teens for most of
the plains and foothills and near zero in much of the mountains. The
mountains valleys and low-lying areas of Lincoln County will be
especially cold due to cold air pooling in these areas overnight.
The area will warm again quickly Monday with ridging and a return to
northwest flow aloft. Under clear skies, highs will reach the 50s
across most of the plains and foothills and 30s across much of the
mountains. Monday night, flow aloft will begin to turn more westerly
then southwesterly as another trough will approaches the area from
the Pacific northwest.

There is less disagreement between models on the progression of
this next system Tuesday in their latest runs. The ECMWF is now
predicting a trough through the Northern Great Plains by Tuesday
night while the GFS is closing a cutoff low by this time. Either
way, the passage of the system will be accompanied by a surface
cold front which will bring northerly winds and high temperatures
back down near normals. Both solutions are showing similarly low
precipitable water values, between 0.1 and 0.3 inches, across the
area. However, the ECMWF solution has much stronger synoptic
forcing. A brief quasi-geostrophic analysis shows 10 to 20 mb/hr
of ascent from the surface to 500 mb in the ECMWF and very little
forcing at all in the GFS. Of note, the NAM and GDPS show forcing
much more similar to the ECMWF than the GFS. I think it is likely
that the GFS is simply missing some of the dynamics of this event
as it keeps some of the stronger forcing north of Colorado for
longer. Also, an approaching jet is positioned slightly better in
the ECMWF than GFS to provide some additional lift Tuesday
afternoon. For event impacts, the main difference is the that the
GFS is predicting less widespread snowfall than the ECMWF. We will
keep slight chance of light snow across the mountains and
foothills with some light snow spreading out to the nearby plains,
which may end up being just flurries. This light snow will
continue overnight Tuesday into Wednesday.

Models are now in better agreement about the synoptic situation
Wednesday as well, with the ECMWF setting up a large cutoff low over
central CONUS and the GFS keeping a similar area as part of an
elongated, positively tilted trough. The low might span from the
Dakotas to Northern Mexico and from the Rocky Mountains to nearly
the foot of the Appalachian Mountains, centered somewhere over the
Midwest, or the trough from the Southwest to the Great Lakes.
Northeast Colorado will still suffer from a general lack of moisture
and only a slight chance of light precipitation in the high terrain
seems possible at this time. Northwest flow aloft early in the day
will shift more northerly during the day then northeasterly by the
evening.

This low or trough should advance slowly east across the Midwest
Thursday and Friday. No precipitation is expected at this time and
continuing near normal high and low temperatures Thursday, while
clear skies and may allow for warming back to the low 50s on the
plains and 30s in the mountains on Friday. Flow aloft should turn
more northerly Friday as the low or trough departs. Dry and warm
conditions are expected to persist Saturday under mostly clear
skies and decreasing northerly flow aloft.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 200 PM MST Sat Nov 28 2020

VFR through tomorrow. Light southeast or east winds should
continue into this evening. Winds will then transition toward
drainage before a cold front arrives shortly after midnight.
Winds will increase and become northerly behind the front for a
few hours and then will weaken with the direction becoming more
variable during the mid morning hours.

&&

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

$$

SHORT TERM...Danielson
LONG TERM...Direnzo
AVIATION...Danielson

NWS BOU Office Area Forecast Discussion