Winter Storm Warning 8, NWS Denver/Boulder, CO

Area Forecast Discussion
National Weather Service Denver/Boulder CO
313 PM MST Wed Mar 8 2023

.SHORT TERM...(This evening through Thursday)
Issued at 312 PM MST Wed Mar 8 2023

A couple of issues for tonight. A weak shortwave will move through
overnight, bringing some QG lift especially to areas north of
Denver. However, this doesn't have much moisture to work with.
There's nothing but high clouds associated with it at this hour,
with quite dry air beneath it. The models have backed off on the
idea of snow production with this, though most still produce some
light snow over the mountains, the plains may just get a mid level
cloud deck or some virga falling into the dry air below it. We'll
hang on to some low PoPs, with better odds towards the northeast
corner where the developing cloud deck will be better developed
and the surface cool air deeper. Even there, this probably
wouldn't be more than a dusting of snow.

Meanwhile at low levels, moisture will continue to advect in on
east to southeast winds. The boundary layer closer to the
foothills has dried out a bit, but it's still close to saturation
over the eastern part of the plains. There shouldn't be much more
increase in the moisture, but a little cooling could bring the
clouds down to the surface to produce some fog and freezing
drizzle. A negative factor is the winds that could keep enough
mixing and try to keep the fog from getting thick. That could
limit the fog/drizzle to upslope areas in southeast flow, like the
northern border.

Thursday morning, subsident dry westerly flow kicks in. Strong
winds should surface pretty well for a while, then in the
afternoon we'll start to get a bit of anticyclonic turning with
more northerly and lighter winds into Denver. The post-trough warm
advection aloft kicks in pretty fast and there may be a narrow
window of opportunity for an amplified mountain wave in the
morning/midday, but the flow is still pretty sheared. If there was
decent amplification, this would be a short-lived high wind threat
for the foothills, but the odds look too low for a watch at this
time. We'll have lots of 40 mph gusts for the plains and lower
foothills, but limit stronger winds to the higher elevations for
now. With the mid level dry air being forced downward, this will
drop humidities enough for fire weather concerns in the limited
areas where the fuels are dry enough. See below for more on that.

.LONG TERM...(Thursday night through Wednesday)
Issued at 312 PM MST Wed Mar 8 2023

Thursday night, there will be a flat ridge over the western states
and Colorado will be warm, dry and mostly clear. That ridge will
advance early Friday and the axis should be over Colorado by
afternoon, as a trough comes on shore over Pacific Northwest. There
will be increasing zonal flow aloft and a significant increase in
moisture, especially over the mountains. Models are showing
generally about 150 to 300 percent of normal percipitable water
across northeast Colorado by Friday evening. It looks to get breezy
across most of the area early Friday, but not nearly so much as
Thursday. Peak gusts will likely be to around 55 mph across the
mountains and 40 mph on the eastern plains. The increasing westerly
orgographic flow into the mountains with good moisture and some
synoptic ascent ahead of the next wave should mean widespread snow
showers in mountains, beginning in the morning and gradually
increasing through Friday night. Snowfall amounts may warrant winter
highlights for the mountains late Friday and much of Saturday,
considering breezy winds and travel impacts. Late Friday, early
Saturday, that shortwave should advance and pass mostly to our
north, but push a cold front over the plains Saturday. This would
bring some light rainfall to the plains and back to near normal or
slightly cool temperatures for Sunday. Precipitation should diminish
Saturday after the front and with synoptic subsidence behind the
passing wave.

Sunday, models have a larger ridge building over the Rockies with
rising pressure heights over Colorado, and northwestelry flow aloft
ahead of the ridge axis. Early next week, they have that ridge
advancing with drier weather and warming for Colorado, the axis
overhead and peak heights Tuesday as the next trough comes on shore
over the Pacific Northwest. The northwesterly to westerly flow aloft
with some moisture out ahead should support isolated to scattered,
light snow showers mountains.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1104 AM MST Wed Mar 8 2023

Winds will stay mostly E/SE through the day unless the Denver
cyclone sets up. Tonight fog and lower CIGS around 1000ft between
04z-12z, Thursday morning CIGS will lift and VFR conditions will
prevail. Thursday winds will increase from the W/NW with gusts up
to 30kt beginning around 15z.

&&

.FIRE WEATHER...
Issued at 312 PM MST Wed Mar 8 2023

Strong west to northwest winds will develop behind the trough axis
Thursday morning, diminishing late in the day. Humidities should
drop to at least the 15-20% range in the afternoon for the lower
foothills and adjacent plains, and possibly as low as 10%. We'll
issue a Red Flag Warning for zones 239, 240, and 241, the lower
elevations from Boulder and Broomfield counties south through
eastern Douglas/western Elbert counties. Further north and east
humidities look a little higher and the fuels are likely not as
dry due to the winter snowpack that is still disappearing in some
areas. We checked on foothills/South Park fuels and they're not
considered ready for a Red Flag warning yet. Still some snow in
most areas, or there was recently. The fuels contacts did note
that the warmer/drier areas are quite dry and would carry fire
well, but only in those limited areas.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 11 AM to 5 PM MST Thursday for COZ239>241.

&&

SHORT TERM...Gimmestad
LONG TERM...EJD
AVIATION...Chavez/Gimmestad
FIRE WEATHER...Gimmestad