Johnstown, Colorado 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

676
FXUS65 KBOU 232341
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
441 PM MST Sat Nov 23 2024

.KEY MESSAGES...

- Mountain snow and light low-elevation snow late Sunday and Sunday
  night.

- Heavier mountain snow late Monday night through Wednesday night with
  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 Sunday/...
Issued at 146 PM MST Sat Nov 23 2024

Current observations across METARs display afternoon temperatures
on track to reach above normal temperatures through the early
evening hours across the region. Gusty southwest winds produce
wind gusts up 35 mph continue over the higher terrain this
evening due to an upper level jet. Cross sections display incoming
upper level moisture tonight thus partly cloud skies are expected
for most the of the lower elevations and foothills. Additionally,
tonight`s temperatures slightly above normal as the plains reach
25-36F and mountains reach 14-27F.

Our upper level pattern quickly changes by Sunday afternoon as a
shortwave trough brings Pacific moisture through northwest flow.
Orographic flow should lead to 2-5 inches mainly for areas above 9
thousand feet north of the I-70 corridor. A few mountain valleys in
Jackson and Grand counties could also receive 1-2 inches of snow
through Sunday. There is potential for the incoming cold front to
arrive late Sunday afternoon shifting surface winds northwest. With
mid to upper level cloud cover increasing by Sunday afternoon, it is
possible a few areas across the plains may struggle to reach the mid
50s. Thus, this update includes a decrease in afternoon highs across
the eastern plains. A few areas along the I-25 corridor will be able
to peak through the cloud cover but expect upper 40s to mid 50s for
the lower elevations. High temperatures for the high country reach
the low 30s to low 40s.

&&

.LONG TERM /Sunday night through Saturday/...
Issued at 146 PM MST Sat Nov 23 2024

Widespread snow is expected in the mountains Sunday evening as
the tail end of an upper level trough and left exit region provide
lift. Cross sections show saturated air up to roughly 450mb.
However, lapse rates will only be 5-7 C/km and mountain top winds
end up being relatively weak (less than 20 knots). Add this up and
the orographic lift component of this system is weak. Going
snowfall forecast of 2-6 inches and up to 3 inches in the valleys
looks on track.

For areas east of the mountains...the foothills, urban corridor
and plains, there seems to be decent model agreement between the
GFS and ECMWF models and their ensembles with a period of light
snowfall Sunday night. However, the NAM/RAP/HRRR models don`t have
a clue. NAM cross sections for Sunday night show barely enough
moisture for clouds, let alone snowfall. The NAM/RAP/HRRR keep
primarily northwest (downslope) winds for most of Sunday night.
With surface high pressure building in from the north, we should
see a period of northeast to east winds as the GFS and ECMWF are
advertising. These east to northeast winds lead to convergence
along the foothills and upslope flow. Temperatures appear cold
enough for all snow, though can`t rule out a little rain at first.
Plan to nudge snowfall amounts up, especially on the west side of
the urban corridor, in and near the foothills. Snowfall amounts
still look to be on the light side, less than 2 inches, but a
better chance for slick roads for the Monday morning commute
across western portions of the metro area. It quickly clears
Monday morning as upper level ridging and subsidence moves
bringing mostly sunny skies. Temperatures will be a little cool
with highs in the mid to upper 40s across northeast Colorado.

For Monday night through Wednesday night, westerly flow aloft
will transport Pacific moisture across the intermountain west
resulting in snowfall for the Colorado mountains. For the first
part of the storm, orographic lift will produce most of the
snowfall across the mountains through late Tuesday. Then an upper
level trough moves into the region providing lifting across the
mountains and also for the lower elevations Tuesday night through
Wednesday night. Still plenty of model differences out there
regarding this storm. The latest models move up the timing of the
trough passage. The GFS and its ensemble members are on the lower
end for snowfall with less than a foot of snow for the mountains.
The ECMWF and Canadian ensemble members favor 1-2 feet of snow for
the mountains. Will trend the forecast towards the heavier
amounts given the amount of moisture being transported into the
region and the duration of snowfall being 48-72 hours for the
mountains. For the lower elevations east of mountains, the upper
level trough should bring a decent shot of snow late Tuesday
through Wednesday night. Amounts are still quite unclear this far
out, but it`s very likely we will see snow covered roads Wednesday
and Wednesday night, leading up to Thanksgiving.

For Thanksgiving, the upper level trough and snowfall associated
with it shifts southeast of the area, with only a few ensemble
members hanging onto snow in the morning. However, even though the
snow is expected to be finished, roads will likely be slippery
Thanksgiving morning. Northwest flow aloft will prevail behind the
exiting system Thanksgiving. This combined with fresh snow
covered will produce cool temperatures with highs in the 30s
across northeast Colorado.

For Friday and next weekend, a longwave trough will reside over
northeast North America with a high over low set up along the west
coast of North America. This pattern will bring northwest flow
aloft to Colorado resulting in cool and dry conditions.
Temperatures are expected to be below normal with highs in the 30s
to lower 40s during this period.

&&

.AVIATION /00Z TAFS through 00Z Monday/...
Issued at 436 PM MST Sat Nov 23 2024

Winds will gradually shift to the south by 01z and then more SSW
by 05z. On Sun winds will become SW by 13z. Meanwhile, confidence
in wind direction by midday is low at best. HRRR has winds
becoming gusty from the west while other models have a Denver
Cyclone SW of DIA which leads to a more E or NE direction in the
aftn. For now have maintained continuty with previous fcst and
kept them from the west thru 22z. After 22z the winds may swicth
to a more NW component.  Meanwhile VFR conditions will continue
tonight thru Sun aftn.

&&

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

&&

$$

SHORT TERM...AD
LONG TERM...Meier
AVIATION...RPK