Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 201747
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1147 AM MDT Sat Apr 20 2024

.KEY MESSAGES...

- Snow ending this morning... but fog developing this evening? Low
  confidence, but the chances of dense fog across the metro
  appears to be increasing...

- Cold through Saturday, then drier and warmer for Sunday through
  much of next week.

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

&&

.UPDATE...
Issued at 1029 AM MDT Sat Apr 20 2024

Another spring, banded-snow storm is in the books today. Morning
snowfall reports indicate a narrow band of 4-9 inch snow amounts
stretching from Golden/Rocky Flats towards DIA, with generally 1-4
inch totals elsewhere.

While the snow has just about wrapped up everywhere (except for
portions of the northeast plains), the remaining forecast is still
challenging. Current satellite shows a stratus deck draped across
the eastern half of the state, with some patchy drizzle/fog at the
moment. A weak Denver cyclone has also been noted this morning,
which has kept light northerly flow in place across most locations
west of I-25.

The only consistently modeled part of the forecast today is some
high mountain showers. There is a consistent signal for a few
weak showers to develop across the higher elevations, with a
couple drifting into the urban corridor this afternoon... enough
to keep at least chance PoPs in the grids through the evening. On
the other hand, guidance almost immediately disagrees on how
robust this stratus deck stays today. The HRRR/RAP are notably
cooler than most other guidance this AM... keeping temperatures in
the mid/upper 30s. As a result, these solutions are also much more
aggressive on bringing in fog to the metro this evening. Some
soundings would also imply the potential for some drizzle/freezing
drizzle, but surface temperatures hovering near or just above
freezing would likely limit any impacts there.

In terms fo the grids, we`ve made a few changes to the T/RH grids,
as well as updates to PoPs. Furthermore, some fog has been added
into the grids for this evening (after 00z), with drizzle added
during the day today for most of the lower elevations. We`ll re-
evaluate the need for any fog headlines and finish up the rest of
the short term forecast package this afternoon.

&&

.SHORT TERM /Through tonight/...
Issued at 252 AM MDT Sat Apr 20 2024

Current radar shows several snow bands extending from the Front
Range eastward onto the plains this morning. Some of these bands
have contained heavier snowfall rates with Denver International
Airport reporting periods of 1/4 visibility and heavy snowfall.
The general batch of these bands has shifted northward into
Boulder and even as north as Fort Collins. Over the next few
hours, high-resolution model guidance shows a slight southward
shift back toward the Denver area. In that timeframe, localized
heavier snow rates are possible resulting in slick roads and
reduced visibility. Toward mid morning, there will be a downward
trend in coverage and intensity of snow showers. This puts the end
time of the Winter Weather Advisory on track for the Front Range
and urban corridor(expires 9AM). The Winter Weather Advisory was
ended early for the Palmer Divide as the bulk of the snow is now
to the north. Areas could see an additional inch or two, locally
up to 3 inches in the heavier bands when all is done.

Clouds will likely stick around for day today even after the snow
diminishes. This combined with a cooler airmass will keep
temperatures cool in the upper 30s to low 40s for most of the
lower elevations and 30s for the mountains/foothills. High
mountain valleys will be warmer in the 40s to near 50. Marginal
instability develops in the afternoon over the high country
bringing shower chances in the afternoon. There is a low chance
(<30%)for a few to stray onto the plains. Tonight, the main
concern for impacts will be the potential for fog. Model soundings
show clearing of the upper level cloud decks overnight with a
shallow moist layer staying intact. This supports either low
stratus or even areas of fog overnight from the east plains up to
the lower Foothills.

&&

.LONG TERM /Sunday through Friday/...
Issued at 252 AM MDT Sat Apr 20 2024

Areas of stratus and fog will likely remain across the plains
into early Sunday morning with weak boundary layer flow and narrow
T/Td spreads. However, we will be under the influence of large
scale subsidence, and a developing downslope component will help
erode any low clouds by mid morning. Then, sunshine, warm
advection, and downslope will all aid warming. We`re still on
track to reach the lower to mid 60s over most of the plains,
although we did taper back the warming a couple degrees due to
recent precip and wet ground/melting snow.

Monday will be even warmer as stronger downslope flow and breezy
conditions develop. There does seem to be a trend toward a
faster/stronger shortwave across the Northern Plains, and that is
giving us some pause about the amount of warming. Temperatures may
actually fall late in the day as a stronger cold front arrives
from the north, but most of the plains should still be able to
make it into the 70s. The northern border area would have a
reasonable (>50% chance) of only seeing upper 60s before the front
arrives.

With the arrival of the front and slight cooling aloft,
we`ll have a slight chance of showers and storms Monday, and then
again Tuesday. We`ll see high temperatures drop back closer to
normal for Tuesday with high pressure building over the Central
Plains.

There`s good agreement the warmest day of the week will be
Wednesday as an upper ridge builds over the Central Rockies. Upper
70s will be a good bet in the lower elevations, with a 50-60%
potential of exceeding 80F despite what the ensembles show
(slightly cooler).

By Thursday, ensembles are still all over the place whether a
faster lead shortwave arrives late in the day (mostly GEFS
members) or whether the shortwave is deeper and slower (mostly
EPS). At this point, we`d lean a bit toward the slower solutions
but the predictability is quite low. Whatever the case, it appears
we`ll be in for not just one but two potential disturbances late
next week into next weekend. Both of these will support increased
chances for showers and thunderstorms along with cooling
temperatures.

&&

.AVIATION /18Z TAFS through 18Z Sunday/...
Issued at 1143 AM MDT Sat Apr 20 2024

Complicated TAF forecast through Sunday AM. Weak Denver Cyclone is
in place, with winds finally transitioning to more of a NW
component at DEN over the past couple of hours. Winds should
remain light and generally out of the north or northwest through
this evening. Ceilings continue to bounce back and forth between a
BKN/OVC005-015, but still think this will gradually rise a bit...
closer to 015-025 later today. A few mountain showers will also
attempt to reach the terminals after 20-21z but impacts from this
will be limited to a brief, very light snow shower or drizzle.

As we get into the evening hours... guidance has gradually become
more bullish on the FG potential. Several runs of recent hi-
resolution guidance bring in dense fog to the terminals near or
just after 00z, continuing through most of the overnight. Have at
least hedged in that direction but want to see how the afternoon
temperature/moisture trends evolve before committing to a longer
prevailing group. Fog/mist/stratus will persist into Sunday
morning but then slowly break up from 12-18z. VFR conditions are
expected by late Sunday morning and will continue into Sunday
afternoon.

&&

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

&&

$$

UPDATE...Hiris
SHORT TERM...Mensch
LONG TERM...Barjenbruch
AVIATION...Hiris


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