991
FXUS65 KBOU 170953
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
353 AM MDT Tue Sep 17 2024

.KEY MESSAGES...

- A line of thunderstorms will move across the area today bringing
  winds of 40 to 60 mph along with brief rain.

- Over the northeast plains, severe storms with high and
  potentially damaging winds will be likely this afternoon with
  gusts of 70 to 80 mph possible.

- Cooler temperatures this weekend with good chances (30-60%) for
  rain and high mountain snow.

&&

.SHORT TERM /Through Tonight/...
Issued at 352 AM MDT Tue Sep 17 2024

Water vapor satellite imagery showing a broad upper level low
spinning over Nevada. Ahead of the low, lift is producing isolated
showers over western Colorado already. The center of the low
lifts into Wyoming by this evening, but as the trough axis races
northeast across Colorado it will provide plenty of lift. By mid
morning expect scattered thunderstorms across western Colorado.
Daytime heating will destabilize the airmass ahead of the trough
over central and eastern Colorado. Still good model agreement that
a line of storms form by late morning over western/central
Colorado. Mid level flow increases in the afternoon with 700 mb
winds increasing to 35-40 kts by 21Z-00Z. Storm movement looks to
be 35-40 knots as well and as mentioned in the last discussion a
20 knot forward outflow out of a storm moving 35 knots yields 55
knot winds. So it won`t take much to produce strong winds today.
Greatest instability will be over the eastern plains with SBCAPE
of 500-1000 J/kg. The greater instability should produce more
organized storm segments, leading to winds of 70 mph or locally
stronger. The enhanced severe risk over the northeast plains looks
on track. No big changes to the forecast for today. Will continue
to highlight the strong wind threat from the thunderstorms this
afternoon.

For this evening and tonight, storms continue to race
northeastward and exit the state by early to mid evening.
Subsidence quickly moves in and brings mostly clear skies
overnight. This may keep windy conditions going across the higher
foothills and Front Range Mountains this evening. Expect a chilly
night compared to recent nights due to the clear skies and dry
airmass. Lows are expected to fall into the 40s overnight across
northeast Colorado. The mountain valleys, likely fall below
freezing with lows in the mid 20s to lower 30s. A little moisture
hangs on overnight over the northeast plains where a little fog
will be possible towards sunrise Wednesday morning.

&&

.LONG TERM /Wednesday through Monday/...
Issued at 352 AM MDT Tue Sep 17 2024

A few small changes were made this update to long-term forecast
package. Strong subsidence values are displayed by QG fields
along northeastern Colorado Wednesday. Strong southwesterly flow
continues under high pressure creating mostly sunny skies and dry
conditions. 700mb temperatures between 11-13C will lead to above
normal temperatures Wednesday and Thursday. Additionally, low
relative humidities are possible. Although wind gusts remain below
25 mph for the lower elevations, there is an increasing chance
southwesterly winds could increase mid- afternoon given the upper
level stationed in southeastern Wyoming especially for the
foothills areas. This may lead to elevated fire weather conditions
for the foothills briefly but mainly for Jackson and Larimer
counties Wednesday afternoon. Additionally, Thursday similar
conditions could occur with slightly stronger winds for the lower
elevations leading to near critical conditions for the plains,
Palmer Divide, and Lincoln county.

There is good agreement for a weak shortwave increasing mid-level
moisture starting Friday late afternoon. While MLCAPE values are
low near 100 J/kg, isolated high based showers and storms with
little rainfall are possible through late evening for both the
mountains and plains. DCAPE values approach 1000-1300 J/kg
therefore expect gusty winds associated with storms and showers.
This forecast package included a decrease to PoPs Friday.

Our next upper level trough pack a punch which could arrive this
weekend. Majority of ensemble members are favoring precipitation
arriving late Saturday morning first for the higher elevations. QPF
fields range between 0.5-2 inches for the region. There is some
uncertainty if this system hovers in the northeastern corner
through Sunday afternoon. This resulted in NBM isolated PoPs for
northeast Colorado through Sunday evening. ECMWF also keeps
showers and storms lingering through Sunday evening. Either way,
there is high confidence in cooler afternoon temperatures this
weekend through early next week especially Saturday and Sunday.
Saturday compared to Friday, temperatures are almost 10-15 degrees
cooler for both the mountains and plains. Maybe even a chance for
sweater weather?

&&

.AVIATION /06Z TAFS through 06Z Wednesday/...
Issued at 1129 PM MDT Mon Sep 16 2024

VFR conditions will continue through Tuesday morning. Strong
southwest flow aloft will bring south-southwest winds through
Tuesday afternoon. South winds increase to 30 knots around 18Z.
Forecast guidance is in good agreement with a line of convection
moving across the mountains and into the Denver area Tuesday
afternoon (20-23Z). This is expected to bring a brief period of
strong outflow winds as the storms move through, with the
potential for 40-50kt gusts for a brief period. Best timing of
this generally looks to be 20-23z. Additional convection beyond
that seems less likely and winds should diminish beyond 00z as
daytime heating ends.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

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