Four Square Mile, Colorado 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

599
FXUS65 KBOU 231048
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
348 AM MST Sat Nov 23 2024

.KEY MESSAGES...

- Dry with well above normal temperatures today.

- Mountain snow and light low-elevation rain/snow mix late Sunday
  and Sunday night.

- Heavier mountain snow late Tuesday through Thanksgiving
  morning...widespread and significant travel impacts likely.

- Increasing potential for snow across the lower elevations
  Tuesday night and Wednesday.

- Chilly but mostly dry Thanksgiving Day.

&&

.SHORT TERM /Through Tonight/...
Issued at 342 AM MST Sat Nov 23 2024

Water vapor imagery shows ample high level moisture over Colorado
coming from the Pacific as a jet streak moves over the Intermountain
West. Our forecast area will be under the right exit region of this
jet which will provide subsident flow and will keep conditions dry.
This setup will also lead to well above normal temperatures. With
that being said, it will be tricky to forecast high temperatures
today since there will be high levels clouds that persist throughout
the daylight hours. The thinking is that highs will be close to
yesterday`s highs if not a couple of degrees warmer. That means the
low 60s are expected in Denver again while Greeley, Longmont, and
Erie will have temperatures in the 50s. Areas that have snowpack
will also see cooler temperatures. Across the higher terrain, the
approaching jet streak will increase wind gusts and since there
will be low humidity in the foothills, there will be pockets of
elevated fire weather conditions.

A shortwave trough will continue to move closer to Colorado by
the overnight hours tonight. Mid level Pacific moisture will make
its way to the Park Range mountains and a few light snow showers
may begin before sunrise. Since this initial snowfall will likely
be very light, a Winter Weather Advisory for the Park Range
mountains was not started until 6AM Sunday morning. Otherwise,
tonight will be quite mild as clouds and winds keep temperatures
elevated.

&&

.LONG TERM /Sunday through Friday/...
Issued at 202 AM MST Sat Nov 23 2024

Mild temperatures will take a hiatus Sunday into Monday as a shortwave
clips the state, bringing an influx of low/mid-level moisture
below 400mb and light precipitation focused predominantly across
our higher terrain. Snow showers will first initiate in the
northern mountains Sunday morning before expanding towards the
I-70 mountain corridor in the afternoon and evening, gradually
dissipating as we near sunrise Monday. Accumulations should
remain below 1-2" for most of the high valleys, with 2-6" for the
higher elevations. Mountain passes will be the most susceptible
to minor travel impacts. Any precipitation for the lower
elevations will hold off until Sunday evening/night as a cold
front brings a shift to northeast winds and gradual moistening of
the boundary layer. Note the use of the term "any" - as not all
guidance is on board with the shallow moisture being sufficient
for measurable precipitation in the urban corridor (~30-40% of
ensemble members keep most locations dry). That said, with some
dynamic support in play stemming from the left exit region of the
jet, it`s prudent to maintain an initial wintry mix and subsequent
transition to snow showers in the forecast for the overnight
period. Accumulations, if any, would be around 1" or less, with
locally slick roads a possibility early Monday morning, albeit
shortlived.

Daytime conditions Monday will be cooler, with 40`s in the plains,
but largely dry as a transient ridge axis traverses overhead. Come
Tuesday, robust westerly flow aloft will replace the weakening
high pressure as a positively tilted trough inches onshore,
advecting a healthy plume of Pacific moisture into the Rocky
Mountain region. Confidence remains high in the rapid development
of snow in the high country during the latter half of the day,
leading to deteriorating road conditions. Subsident flow should
keep the lower elevations dry during daytime hours.

The upper-level trough accelerates eastward over Colorado
Wednesday. Colder air aloft will help to steepen lapse rates
moderately beginning Tuesday night and boost snowfall rates in the
high country, with increasing potential for significant travel
impacts on most mountain roadways. Storm-total snowfall amounts
exceeding 1 foot appear increasingly likely (>80% chance) for the
higher mountain elevations, with a possibility of higher amounts
if wetter solutions verify. Deepening moisture and lift will
support the spread of precipitation into the lower elevations
during the day Wednesday, with an average of ~80% of ensemble
members now supporting a few tenths of an inch of precipitation
for much of the urban corridor. Although drier solutions remain in
play, the two "wetter" scenarios (relative to the multi-model
mean) identified through cluster analyses represent roughly 65% of
total members, compared to approximately 50% some 24 hours ago,
suggestive of a slight trend towards the moderately wetter
solutions. The wettest members (~5%) indicate an upper bound near
1" of QPF for the urban corridor, with the driest few (also ~5%)
being almost dry, mainly correlating to the fusing of the low with
broader longwave troughing to the north. The current forecast
falls mostly in the middle, albeit with some downward adjustments
to temperatures (and thus quicker rain to snow transition) under
the assumption of fairly efficient evaporative cooling, which NBM
guidance seems reluctant to capture. The Probabilistic WSSI
(Winter Storm Severity Index), with the caveat that it`s one
forecast cycle behind, suggests ~40% probability of minor to
moderate overall impacts for the lower elevations Wednesday, and a
less than 5% chance of major impacts, which appears quite
reasonable.

This system is then slated to quickly eject eastward on
Thanksgiving, with precipitation probabilities dropping
significantly through the morning, and all areas likely drying out
by the afternoon. Regardless, temperatures will be chillier with
highs in the 20`s and 30`s for mountains and lowlands
respectively, possibly colder if there`s abundant snow cover. Cool
conditions look to continue into Friday under northwest flow
aloft.

&&

.AVIATION /06Z TAFS through 06Z Sunday/...
Issued at 1127 PM MST Fri Nov 22 2024

VFR through the TAF period. Winds will be drainage tonight and
will become light during the day Saturday. The most direction
winds will take at KDEN is some form of easterly in the afternoon.
Winds will return to drainage Saturday night.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 AM Sunday to 6 AM MST Monday for
COZ031.

&&

$$

SHORT TERM...Danielson
LONG TERM...Rodriguez
AVIATION...Danielson