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000
FXUS65 KBOU 210113
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
713 PM MDT Sat Apr 20 2024

.KEY MESSAGES...

- Areas of dense fog across the lower foothills, I-25 corridor,
  and adjacent plains through Sunday morning.

- Scattered snow showers diminishing through this
  afternoon/evening, with little to no accumulation expected.

- Warming and drying trend begins on Sunday, with highs back in
  the 50s or 60s

- Eyeing stormier weather pattern for late this coming week and
  next weekend.

&&

.UPDATE...
Issued at 710 PM MDT Sat Apr 20 2024

Have made a few minor adjustments to precip chances this evening,
mainly to better reflect some stubborn snow showers over the
northern foothills. Otherwise the forecast remains largely on
track. Some question as to just how extensive the dense fog will
be overnight, and how quickly it will clear Sunday morning
(leaning towards a slower clearing), but for now the current
expanse of Dense Fog Advisories looks appropriate.

&&

.SHORT TERM /Through Sunday/...
Issued at 303 PM MDT Sat Apr 20 2024

Quiet weather is not too far away, but we`ve still got quite a few
things to discuss from now through midday Sunday. Let`s dive in...

A couple of stronger snow showers developed across the high
country earlier in the afternoon, which produced pea sized
graupel/hail/snow pellets near Kremmling in one of the stronger
cores. These showers have since drifted south across the I-70
mountain corridor into the foothills and Denver metro, though
impacts have been limited. This activity should persist through
the afternoon hours and gradually weaken through the evening as
instability diminishes.

Meanwhile, hidden underneath the extensive stratus deck, some
areas of dense fog have developed this afternoon... mainly across
the northwest metro. This has largely followed the solutions
offered by the HRRR/RAP today, which develop much more widespread
fog this evening. We mentioned the potential need for Dense Fog
Advisories... and well, here we are. Given that several webcams
and a few ASOS/AWOS sites have already reported 1/4SM visibility
(including a 0SM from KBJC), we`ve gone ahead and went straight to
a blanket advisory across a large portion of our forecast area,
where fog is either (1) already locally very dense, or (2)
expected to develop over the next several hours. Time will tell if
any zone refinements are needed tonight. While not in the grids,
it`s also worth noting there could be some brief light
drizzle/freezing drizzle potential outside of the fog, but the
main impact will be fog.

The weather will turn warmer and drier tomorrow. Fog is expected
to linger into the morning hours, eventually lifting and burning
off as a warm/dry air advection regime takes over. By Sunday
afternoon, mostly sunny skies should return to the forecast area.
Temperatures should warm up quite a bit compared to today, with
highs in the 50s to 60s. A little uncertainty remains in the
forecast temperatures as most guidance isn`t resolving the current
snowpack well, but either way it will feel much more like spring
than today.

&&

.LONG TERM /Sunday night through Saturday/...
Issued at 303 PM MDT Sat Apr 20 2024

Northwesterly flow aloft will transition to zonal as a trough passes
along the northern border of the U.S. late Sunday into Monday. This
westerly flow will aid in the development of a mountain wave early
Monday morning. Winds will increase over the higher mountain
elevations Monday morning and wind gusts could reach up to 50 kts at
the highest elevations. Some gusty winds could make their way into
the foothills reaching up to 30 kts. Downslope winds and ample
mixing will help temperatures climb into the 60s across the
foothills and widespread 70s across the plains. This will be about
10 to 15 degrees above normal for this time of year. The mountains
will be 3 to 5 degrees above normal as they make it to the high 40s
to low 60s. Dry conditions and gusty winds will cause concern for
critical fire weather conditions to develop in southern Lincoln
County Monday afternoon. We will hold off on issuing any
highlights for now but will continue to monitor relative humidity
and winds closely. A cold front may influence high temperatures
along the northern plains depending on its time of arrival,
therefore they may stay a few degrees below what was forecast.
With the frontal passage, a slight chance of thunderstorms can`t
be ruled out.

A shortwave will traverse the area Tuesday bringing another chance
for afternoon storms across the forecast area. Temperatures will be
slightly milder than Monday and return to more on par with the norm.

Ridging continues to build on Wednesday bringing temperatures up to
the high 70s across the plains. Elevated fire weather conditions may
be a concern for parts of Lincoln and Elbert Counties as relative
humidity drops to below 20%. This will continue to be monitored in
the coming days.

All guidance agree that a trough will dig into the western CONUS by
late week, however, they have yet to agree on specifics. Solutions
vary on timing and intensity of the shortwave arrival into the
Central Rockies. As of now, it may be anytime between Thursday
morning and Friday morning. This will bring increased chances for
showers and thunderstorms across the forecast area, with some snow
showers possible at the highest elevations. More details will be
provided as things become more clear.

&&

.AVIATION /00Z TAFS through 00Z Sunday/...
Issued at 525 PM MDT Sat Apr 20 2024

Outlook for tonight is becoming slightly higher confidence given
current trends and increased consistency across model guidance.
Reasonably confident in dense fog persisting for majority of
Denver metro area, including KDEN, through the overnight period
and into early Sun AM. Current robust northerly flow will
gradually weaken over the next few hours, with flip to southerly
drainage generally favored early morning. Whatever the true wind
direction, speeds will be very light and likely insufficient to
scour out the fog all that early.

The trickiest forecast component, therefore, remains the timing
of improving VIS and CIGS Sun AM. Have pushed this back slightly
given latest model trends and climatology. Some very marginal
improvement looks possible ~13-15Z, but currently have a hard time
seeing fog dissipation any earlier than that. There`s potential
(~30% change) of low CIGS <005 and FG persisting as late as 18Z,
but for now will go for middle of the road scenario, with gradual
scattering and improved CIGS in the 16-18Z (1000-1200 MT) timeframe.

As far as winds Sunday, expecting SE/ESE winds for most of the
afternoon period with speeds 8-11 kts before a return to S/SW
drainage early evening.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 9 AM MDT Sunday for COZ036-038>043-045-
046.

&&

$$

UPDATE...Rodriguez
SHORT TERM...Hiris
LONG TERM...Bonner
AVIATION...Rodriguez