889
FXUS65 KBOU 192345
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
545 PM MDT Thu Sep 19 2024

.KEY MESSAGES...

- Dry on Friday with above normal temperatures and areas of
  elevated fire weather conditions.

- Storm system will impact the area this weekend. 70-80% chance of
  precipitation, but considerable uncertainty regarding how much.
  The chance of significant rainfall has decreased somewhat, but
  several inches of mountain snow possible depending on
  track/evolution.

- Turning much cooler by late Saturday through Sunday. Below
  normal temperatures early next week, then likely (60% chance)
  back to drier and warmer weather toward the end of next week.

&&

.SHORT TERM /Through Friday/...
Issued at 306 PM MDT Thu Sep 19 2024

Still very little cloudiness over the forecast area this
afternoon. Temperatures across the plains currently stand in the
mid 70s to lower 80s. Winds are light and variable.

Models continue to show fairly strong southwesterly flow aloft for
the CWA tonight and Friday. Jet level speeds are progged in the
75 to 90 knot range for both periods. The synoptic scale energy
will be benign overnight then weak upward vertical velocity moves
in by Friday afternoon with the approaching upper trough/closed
low. The moisture will remain sparse over the CWA tonight and
Friday with Precipitable water values in the 0.2 to 0.6 inch
range. Precipitation is not expected. Temperatures will remain
above seasonal normals for one more day on Friday with afternoon
highs 1.5 to 3.0 C warmer than today`s readings.

&&

.LONG TERM /Friday night through Thursday/...
Issued at 306 PM MDT Thu Sep 19 2024

The main concern for this forecast period continues to be this
weekend`s storm system and resultant weather. As stated yesterday,
confidence is only expected to slowly grow as these compact upper
lows ejecting to the northeast are always tricky. It`s almost a
winter-like setup with upslope and moderate synoptic scale QG
lift, but with the initial warmth and potential for convection on
the front end.

Regarding the storm track, there is general agreement that an upper
low lifts northeast from the CA/AZ border to the Four Corners area
by Saturday evening, and then continues northeast across the
Colorado Front Range Saturday night.  That`s where this forecast
gets tricky, as there are some slight differences in how the upper
low ejects across the forecast area.  But, as we know, those slight
differences can spell large differences in the resultant upslope
component and precipitation amounts.

This uncertainty is still shown in the ensemble output, with only
slight narrowing in the 25th-75th percentiles (as an example
yesterday Denver was 0.25-1.5 inches and has now narrowed to ~0.3-
1.2 inches). Meanwhile, the extremes are still out there with
anywhere from 0 to 3.5", although there has been been meaningful
thinning, or lower probabilities of the higher end precipitation
amounts. Looking at the means, QPF was near 1" for the Front Range
and plains, with lesser amounts farther west (due to the
upslope/downslope components). There has not been any meaningful
north/south shifts in this output over the last 24 hours, although
there are some unique north/south shifts noted within the 12 hour
data (i.e. 12Z-00Z- 12Z).

Temperatures will be turning much cooler with more widespread
precipitation developing, and continued cold advection.  We`ll trend
our temperatures colder considering the expected precipitation,
clouds, and low level moisture. We`ll also be blending the NBM
(National Blend of Models) toward the colder raw model output. Thus,
we think there`s a high probability (>70%) that we only stay in the
50s for highs across the plains.

Colder temperatures also means lower snow levels. It still appears
we`ll see several inches of high mountain snowfall (above 9,500-
10,000 feet), but some accumulation down to 8,500-9,000 feet if the
heaviest/coldest solution verifies. We considered a highlight, but
it`s still very early, a lot of uncertainty with regard to QPF, and
impacts could be limited by relatively warm temps.

The main message is still this...Considerable uncertainty exists,
but there is a very good chance (70-80% chance) of precipitation
from late Saturday into early Sunday morning. We have a good
chance (50-60%) of seeing a moderate amounts of precipitation, and
low (25%-35% chance) of seeing light precipitation, and now a
smaller 10-15% chance of seeing a high end precipitation event.

By early next week, there is decent agreement of northwesterly flow
aloft developing as a ridge amplifies over the West Coast. This
would allow additional shortwaves toward Monday and Tuesday with at
least a small chance of showers and storms, and temperatures
remaining below normal on average. Models seem to be pointing at a
pretty strong cold front toward late Monday.

A warmup is still expected toward the middle to end of next week
with a return of upper level ridging. It`s just the timing of the
ridge building in and thus quickness of warmup that`s somewhat
uncertain (delayed slightly).

&&

.AVIATION /00Z TAFS through 00Z Saturday/...
Issued at 543 PM MDT Thu Sep 19 2024

VFR conditions will continue thru the period. Gusty SE winds
should decrease by 01z. By 05Z winds will have a SSW component
and continue overnight. By 14Z Fri winds will become light
and variable.  A Denver cyclone will be in place Thu aftn and
should be SW of DIA. This will allow for ESE winds thru the aftn.

&&

.FIRE WEATHER...
Issued at 306 PM MDT Thu Sep 19 2024

Humidity levels will drop into the teens across much of the
forecast area on Friday afternoon for a few hours. Southwest
winds gusting from 15 to 25 mph over the Palmer Divide divide and
Lincoln County will created elevated fire weather conditons.

&&

.HYDROLOGY...
Issued at 306 PM MDT Thu Sep 19 2024

We are seeing average precipitation amounts decrease slightly over
the last couple model runs, with the potential for a high end
precipitation event decreasing. Also, models are a bit slower with
any convective precipitation with limited instability on the front
end of this system. Thus, the chance of burn scar flooding are
somewhat diminished, but still some threat for late Saturday
afternoon through Saturday evening as our thresholds are quite low
for the new burn scars.

&&

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

&&

$$

SHORT TERM......RJK
LONG TERM.......Barjenbruch
AVIATION...RPK
FIRE WEATHER....RJK
HYDROLOGY.......Barjenbruch