National Weather Service Forecast Discussion

000
FXUS65 KBOU 222130
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
230 PM MST Fri Jan 22 2021

.SHORT TERM...(This evening through Saturday)
Issued at 230 PM MST Fri Jan 22 2021

Snowfall will increase slightly in intensity and areal coverage
this evening and overnight as WNW flow aloft shifts more W then
WSW ahead of the incoming trough, but should remain confined to
the mountains and some parts of the northern foothills. Mild
subsidence aloft will also be replaced by weak ascent ahead of the
trough, though models are in good consensus about the best
forcing missing northeast Colorado. The forecast area will be
under the left exit region of a roughly 100 kt jet streak on
Saturday night, which should also provide a dynamic boost by
increasing mid level divergence and orographic flow. We have kept
a Winter Weather Advisory in effect for the the Park Range and
part of the Gore Range in Grand County from 5 PM this afternoon
through 11 PM Saturday, expecting the greatest snow accumulations
of 4 to 10 inches with gusts up to 35 mph in this area. Continue
to expect less accumulations of 1 to 5 inches elsewhere above 10
kft. Expect lows tonight between 1 and 5 degrees warmer than last
night across most of the area, putting much of the plains in the
low to mid 20s F, foothills mid to upper 20s F, and mountains mid
teens.

Light snowfall will continue Saturday morning and probably
through the day, intermittent except in the highest terrain.
A weak front is expected in the early evening across the plains,
which will likely not impact temperatures much being after the
high but should bring some moderately gusty northwesterly winds. Highs
Saturday are forecast to be similar to highs today though a few
degrees warmer across much of the plains, reaching the low 50s F
in much of the area adjacent to the Palmer Divide. Behind the
front there is a chance of some light snowfall during the
afternoon/evening hours spreading east from the foothills over
the urban corridor to about I-25, north of I-70, and across the
plains of northern Weld County as well, but looks to be only
flurries at this time.

.LONG TERM...(Saturday night through Friday)
Issued at 230 PM MST Fri Jan 22 2021

The short wave cutoff trough is across SoCal late Saturday into
Sunday.  On Sunday the low becomes vertically stacked and then
absorbed into the westerly flow as a weak open wave overnight into
Monday.  Besides being vertically stacked, it begins to get absorbed
into a broader upstream trough as a stronger west coast trough
digs southeast across Oregon and California. What that means is
synoptic forcing and moisture advection across our area with the
first tough will be weak. There will continue to be snow showers
in the mountains into Sunday morning given SW flow and really good
lapse rates, but without another source of lift the amounts will
remain on the light side and only minor travel impacts are
expected. There will be a lull in snow across most of the area
during the day on Sunday until the weakening trough gets kicked
east/absorbed and the first trough axis moves across Colorado
late Sunday into early Monday. Snow showers should arrive mainly
along and south of I-70 overnight into Monday morning, but again
amounts will be very light, T-2" or so. The plains should remain
dry other than maybe the Palmer Divide and some flurries for the
eastern foothills and west/south suburbs of Denver late Sunday
into early Monday.

On Monday the stronger trough digs south across CA and builds a
progressive ridge that moves across Colorado throughout the day.
Thus, our area should remain dry on Monday during the daylight
hours.  Despite SW flow aloft afternoon, cold advection will occur
and will keep temperatures in check.  Across the plains highs should
be in the 30s, but in and around Denver metro may top 40 degrees.
The mountains will be in the 20s. Winds will be on the light side
across the area with the ridge moving overhead.

On Tuesday models in general are still having trouble agreeing on
the evolution of the trough. The EC is more progressive and
brings it ENE across southern Colorado, while the GFS is slower
bringing it across central Colorado. The GFS solution would be a
wetter solution for us with slower movement and better lift across
our area. Neither solution would favor deep east/northeast
upslope component, and QG lift is meager. Even the jet would be
too far south as a source of lift. Orographics will be the primary
mechanism for lift across the area with shallow upslope across
the plains and WSW flow across the mountains. Looking at the
ensemble output, the GEFS would favor snow across the plains, but
the ensemble mean liquid equivalent is about 0.1-0.3" depending on
location relative to terrain. The faster EC is about half that
for the plains and I- 25 urban corridor. Both ensembles have
little spread indicating confidence (for now) of light snow across
the plains. In the mountains, relying again on just orographics
and lapse rates means light snow as well.

Models agree that Wednesday will be dry across the area with yet
another west coast trough building a ridge that moves across
Colorado throughout the day.  700 mb temps warm 10 degC by early
Thursday but not soon enough for Wednesday`s high temperatures.
It looks to remain a little below normal across our area for late
January. Highs across the plains should be in the low 40s, 30s in
the eastern foothills, and 20s in the mountain valleys. Thursday
also looks dry as another storm system digs south across
California and generates increasing southwest flow aloft. Should
be enough warm advection in the flow to result in highs above
normal.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 230 PM MST Fri Jan 22 2021

Expect VFR conditions at all terminals through the TAF period.
Mid to high level cloudiness should continue to gradually increase
this afternoon and evening with broken to overcast skies
overnight. No precipitation is expected east of the foothills
today or tonight, and tomorrow only flurries are expected on the
adjacent plains. Northeasterly winds this afternoon should persist
through the evening until being replaced by drainage winds
overnight. Tomorrow, expect a period of gusty northwesterly winds
in the late morning or early afternoon preceding stronger, gusty
northeasterly winds behind a cold front.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 5 PM this afternoon to 11 PM MST
Saturday for COZ031.

&&

$$

SHORT TERM...Direnzo
LONG TERM...Schlatter
AVIATION...Direnzo

NWS BOU Office Area Forecast Discussion