568
FXUS65 KBOU 200928
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
328 AM MDT Sat Jul 20 2024

.KEY MESSAGES...

- More scattered showers and thunderstorms today with cooler
temperatures.

- Widespread rain, heavy at times, on Sunday with a significant cool
down.

- There will be a limited threat of flash flooding, mainly
  on the burn areas.

- Gradually warmer/drier through most of next week

&&

.SHORT TERM /Through Sunday/...
Issued at 143 AM MDT Sat Jul 20 2024

You know what is pretty cool? I mean literally cool. The
temperatures the next 2-days across the region. Add in some deeper
moisture, easterly surface flow and a weak shortwave(s) trough
aloft, you have just made the recipe for some free water.  We`d love
to say that the entire region will get a good dousing but it is
summer-time and widespread events are hard to come by. It is
definitely a case of the have and have-nots still, but

Expectations are for slightly less coverage in showers and
thunderstorms this afternoon and evening with a slower moving
shortwave today. Ample moisture and a subtle shortwave passage will
help provide some focus for showers and storms today, mainly tied to
the foothills and adjacent urban corridor. Given the pattern aloft
is north-northwest flow, which seems to always support a surprise
(anecdotal to say the least), gusty thunderstorms would be the main
impact if anything trends towards the severe side. With the cooler
temperatures and the better moisture in the area, it`s hard to see
much in the way of a stronger virga burst, so the stronger gusts
could be tied directly to the upright convection and the ensuing
outflow boundaries.

Speaking of outflow, it is interesting to see some late evening
convection try to fire up along the foothills today, but would have
to assume that is the main driver of this more localized pocket of
higher QPF.

Sunday looks glorious with the much cooler temperatures and higher
rain/thunderstorm chances (70-90%) in a lot of places outside of the
far eastern plains. The biggest change from Saturday to Sunday is
the proximity of the mid-level low pressure spinning away to our
east-northeast the direct north-south passage of an embedded
shortwave trough of low pressure over the region. The east-northeast
surface flow will help aide in better upslope flow tapping into the
deeper moisture across the region for the storms to initiate off.

As mentioned by the previous long term forecaster, Sunday is our
best look at a decent rainfall event that we`ve seen in quite a
while. Steering flow is weaker on Sunday so storms will slowly drift
north to south throughout the day so there is a slight concern for
heavy training rains, mainly over the burn scars...more below in the
hydrology section about our burn scars.

Ensemble data is showing the better chances, 50-60%, of over a half-
inch of rain over the Palmer Divide in Sunday. The HFEF 24-hour LPMM
also supports the higher QPF amounts over the Palmer Divide on
Sunday.


&&

.LONG TERM /Sunday night through Friday/...
Issued at 143 AM MDT Sat Jul 20 2024

The synoptic pattern for Monday will be relatively unchanged, with
northeast Colorado remaining wedged between a slowly amplifying
ridge to our west, and a positively tilted trough to our east.
However, an intrusion of notably drier air at upper levels will
knock moisture availability down several notches. Together with
slight warming aloft and tapered instability, afternoon convection
will be considerably reduced across our lower terrain in
particular, with fairly limited rainfall outside of locally
favored spots in the foothills and Park County. Temperatures will
rebound slightly but remain a few degrees below normal. Will also
need to keep an eye on smoke trends, as after a couple of days of
relatively persistent northerly flow aloft, HRRR/RAP guidance
indicates potential for increased advection of primarily Canadian
wildfire smoke into northeast Colorado by Sunday night.

Amplification of the ridge will accelerate Tuesday, together with
some eastward expansion of the high pressure center - a trend that
will continue into Wednesday and Thursday. Dry northerly flow
aloft and increased subsidence will restrict the higher chances of
afternoon showers to the higher elevations for mid-week, and
temperatures will steadily return to the low to mid 90`s beginning
Wednesday.

The upper-level ridge looks to lose some of its structure by
Friday, as a longwave trough starts to exert a greater influence
on the PacNW. This could eventually throw some waves our way, but
at the very least will provide for more moist southwesterly flow
as we near the weekend, sufficient to increase coverage of
afternoon convection in the mountains to begin with, and possibly
the lower elevations as well as long as we can tap into better
low-level moisture.

&&

.AVIATION /06Z TAFS through 06Z Sunday/...
Issued at 1131 PM MDT Fri Jul 19 2024

Saturday - Saturday Night...VFR. A little less coverage in
afternoon and evening showers and thunderstorms but we continue
with the VCTS for our terminals. Winds generally back to the east-
northeast during the afternoon hours, not accounting for any
outflow boundaries that may push them to the north-northwest as
storms roll off the foothills during the afternoon hours. We push
the VCTS through the late evening hours as the aforementioned
outflows may be enough to continue the TSRA potential along the
foothills well after sunset. Medium confidence overall.

&&

.HYDROLOGY...
Issued at 143 AM MDT Sat Jul 20 2024

While we have heavier rains the forecast the next two-days across
the region, concerns for burn scar flash flooding remain on the
lower side. The uncertainty in the intensity of thunderstorms,
even with all the moisture we`ll have to work with, will likely be
tempered by the cooler than normal temperatures across the region.
A cooler/more stable airmass would favor more stratiform
precipitation with longer duration, lighter rainfall rates.

No significant hydrological concerns for the Monday through
Thursday time period, as drier air will lead to notably less
coverage and intensity of afternoon showers/thunderstorms, with
limited heavy rainfall potential. The high country could see an
injection of higher moisture by Friday/Saturday.

&&

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

&&

$$

SHORT TERM...Heavener
LONG TERM...Rodriguez
AVIATION...Heavener
HYDROLOGY...Heavener/Rodriguez