892
FXUS65 KBOU 211135
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
535 AM MDT Sat Sep 21 2024

.KEY MESSAGES...

- Cooler today, with highs near or below normal.

- Storm system will impact the area this evening through Sunday.

- The first real mountain snow is likely, with >70% probabilities
  of accumulating snow above 9,000ft. Minor travel impacts over
  high mountain passes Saturday evening - Sunday morning.

- Well below normal temperatures for Sunday, but likely a short-
  lived cooldown before a return to near or above normal
  temperatures next week.

&&

.SHORT TERM /Through Tonight/...
Issued at 344 AM MDT Sat Sep 21 2024

A few changes to precipitation chances and temperatures the short-
term forecast. Overall, model guidance has a good handle on the
location of the upper level low compared to current GOES-18 water
vapor satellite imagery. As this system continues to slowly push
into western Colorado, there will be a delay in precipitation as
majority of forcing will arrive late this afternoon.

Before the arrival of our storm system, a cold front should drop
temperatures along the forecast area. Afternoon temperatures will
peak between the upper 60s to low 70s for the plains.
Additionally, the Front Range mountains and valleys should drop
between the mid 40s to low 60s. As the trough approaches, mid
level to surface winds increase shifting to northeast this
afternoon. Lower elevations could experience wind gusts up to 40
mph before any precipitation develops. Any heating and instability
remains limited, thus the flash flood threat over the burn areas
has significantly decreased given the isolated thunderstorm
threat. There is a low chance of early showers developing this
afternoon but will likely fight drier air near surface. Stratiform
rainfall should begin this evening across the lower elevations.
Although majority of ensemble members for both the GFS and ECMWF
have decreased, QPF fields between 0.30-0.90 inches seem
reasonable for areas below I-76 corridor. There is some
uncertainty for areas such as Fort Collins and northern Weld
county due to this upper level low becoming weaker. The only
advantage these areas have are periods of upslope flow possibly
tonight. It is evident majority of the forecast areas will receive
much need precipitation especially for areas in severe drought.
Snowfall amounts remain unchanged in the forecast with areas
above 9,000 feet receiving 3-8 inches therefore the Winter Weather
Advisory continues starting this evening. Tonight, low
temperatures drop below normal for the entire forecast areas and
rainfall continues overnight.

&&

.LONG TERM /Sunday through Friday/...
Issued at 344 AM MDT Sat Sep 21 2024

The 500mb vort max should be pretty close to the Denver metro by
Sunday morning, and there should be a narrow band of showers
arcing around this vort max at the start of the forecast period.
Guidance still isn`t in great agreement on where this sets up, but
the multi-model mean would place this somewhere near Denver and
off to the east-northeast. A couple slower solutions keep some
rain around through around noon, but most of the area should by
dry as we get into the afternoon hours. Gradually clearing skies
are expected through the day, but this will do little to warm up
temperatures during the day. 700mb temperatures do manage to climb
back to around +2-4C by the afternoon, and high temperatures
should reach the upper 50s to perhaps the low 60s across the
plains.

Northwesterly flow aloft is expected on Monday, with temperatures
warming back up into the 70s. Moisture aloft gradually increases
by the late afternoon hours ahead of an approaching shortwave and
it may be enough for a few rain or snow showers across the higher
mountains by the evening hours.

Guidance begins to quickly diverge by Monday night/Tuesday. A
shortwave tracking out of the northern Rockies should dive
southeastward towards the Central Great Plains, with an
accompanying cold front pushing towards the region late Monday or
early Tuesday. The GFS/UKMET keep this feature much closer to the
parent trough over the upper Midwest, while most other models have
the s/w detached from the longwave trough (and eventually cuts it
off into a closed low), which ushers in cooler weather into our
area again. There isn`t any particularly clear reason to go with
one solution over the other, and for now the NBM is a reasonable
middle ground.

Forecast uncertainty grows as we get into the latter half of the
week, with the GFS developing a strong ridge over the region,
while other models still have lingering effects from the weakening
upper low over the Central/Southern Great Plains. At this point
the GFS and its ensemble are outliers compared to the rest of the
long range guidance, and our forecast is closer to the non
GFS/GEFS mean. Despite the uncertainty in temperatures, there is
rather high confidence that this period will be dry with no
meaningful PoPs.

&&

.AVIATION /12Z TAFS through 12Z Sunday/...
Issued at 521 AM MDT Sat Sep 21 2024

VFR conditions are possible through Saturday afternoon. Rain
showers and lower ceilings will lead to MVFR conditions as early
as this evening through early Sunday.

There is uncertainty in showers occurring before the storm system
arrives. The greatest time period falls between 22Z-24Z. By 00Z,
widespread rainfall should arrive to all terminals. It is possible
MVFR conditions could occur as early as 02Z. These conditions
could continue through Sunday morning. Northerly winds should
shift northeast this afternoon increasing in speed. By this
evening easterly winds are likely.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 PM this evening to noon MDT
Sunday for COZ033-034.

&&

$$

SHORT TERM...AD
LONG TERM...Hiris
AVIATION...AD