000
FXUS65 KBOU 270532
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1132 PM MDT Fri Apr 26 2024

.KEY MESSAGES...

- Cooler on today, with scattered showers continuing.

- Significant snowfall for the mountains and foothills tonight
  through Saturday night.

- Snow level 6000 to 7500 feet MSL.

- Rain Saturday/Saturday night lower foothills, Urban Corridor,
  and eastern plains with 1 to 3 inches of rainfall.

&&

.UPDATE...
Issued at 826 PM MDT Fri Apr 26 2024

A couple severe storms popped up earlier this evening with one
inch hail reported in the Denver area. These storms have weaken
and precipitation is becoming more stratiform. There may be a
brief lull in the rain overnight, but by early Saturday morning
widespread moderate to heavy rain and snow is expected across the
Front Range. Models continue to trend cooler, so we lowered snow
levels a little more, 6000-7500 feet by late Saturday morning.
This should lead to at least 4 to 8 inches of snow over the Palmer
Divide and perhaps 5 to 10 for South Park where we just hoisted
Winter Weather Advisories. Observations and web cameras are
already showing snow along U.S. Highway 285 through South Park.
Even with warm road temperatures, snow is expected to be falling
hard enough to overcome the melting rate and produce slippery
conditions due to snow and slush on the roadways.

&&

.SHORT TERM /Through Saturday night/...
Issued at 149 PM MDT Fri Apr 26 2024

A busy day or two lie ahead...

Before discussing the main course, let`s focus on the aperitif. We
cleared up nicely today east of the Front Range, nudged between
the departing surface low over Nebraska, and the incoming trough
to our southwest. Relatively warm surface conditions have helped
destabilize us enough to tap into a stream of mid-level moisture,
which is producing a few lines of high-based showers and even a
weak thunderstorm or two this afternoon.

That moisture aloft will see a steady deepening this evening as
the upper-level trough begins to approach, with showers and a few
thunderstorms becoming more numerous. Some of these could be
rather productive in the foothills and urban corridor.

An increase in QG ascent and isentropic lift becomes evident
overnight, combining with a steady moistening of the column. This
will translate into increasing coverage of mountain/foothill snow,
and lower elevation rain along the I-25 corridor.

As the low pressure system shifts northeast across southern
Colorado early Saturday, low to mid-level flow will start to carry
a more substantial easterly component, cranking up the upslope
flow machine and pushing precipitable water values to over 0.60"
along the urban corridor and lower foothills per forecast
soundings - above the 90th percentile for the date per SPC
climatology. Persistent moderate to locally heavy precipitation
will result, with a focus on the lee of the Front Range mountains
(including foothills) and the urban corridor. The latest guidance
has shown some continued cooling trends and, anticipating very
efficient evaporative cooling, we`ll have no trouble closing in on
our wet bulb temperatures. Many of the higher resolution models
have shown potential for wet-bulb zero heights between 6,500 and
7,000 ft MSL. With all this in mind, have continued to force the
forecast temperatures and snow levels downward. Suspect these may
need to be tweaked farther.

The peak intensity of the rain/snow looks on track for Saturday
morning, with fairly steady but slightly less intense
precipitation continuing throughout the day. Throughout Saturday
morning, consistent snowfall rates of ~2"/hr for 6+ hours looks
well within the realm of possibilities for the upper foothills and
eastern mountains. As such, have upgraded all of the Winter Storm
Watches to a Warning, with some minor adjustments to start and
end times. Areas above 8,500 ft will see the heaviest
accumulations, but impactful accumulations will likely occur below
that as well.

An additional aspect of concern will be the potential for
impactful accumulations over the Palmer Divide. If temperatures
trend even 1-2 degrees cooler for tonight into Saturday, it could
certainly have an impact on I-25 travel between Denver and
Colorado Springs. Will hold off on highlights for now given lack
of confidence, but this will be a case where observational trends
will need to be monitored closely. Finally, some flakes could
definitely mix in across other portions of the Denver metro and
urban corridor, especially later in the day, although we`ll be
more hard-pressed to get any accumulations.

The upslope flow machine will begin to break down tomorrow evening
and particularly by Saturday night, as drier downslope flow begins
to fill back is as the trough pushes east. A widespread 2"+ looks
quite reasonable for a majority of the I-25 corridor, and 2-3"
liquid equivalent for a wide swatch of our foothills. There are
still a handful of solutions that depict potential for localized
higher totals. Regardless, this is a hefty late April storm, and
one that will add a considerable few steps to our already
impressive year-to-date precipitation across the Front Range.
Although the event will more closely resemble steady (albeit
moderate) stratiform precipitation, there could be some embedded
convective elements at times on Saturday, so some minor street
flooding/ponding and creek rises will be possible.

&&

.LONG TERM /Sunday through Friday/...
Issued at 149 PM MDT Fri Apr 26 2024

As the upper level low ejects out of Colorado on Sunday, enough
moisture will remain to allow for some lingering afternoon mountain
snow showers and rain showers across the plains. There should be
enough instability present as models indicate MLCAPE values of 100
J/kg - 800 J/kg present across the forecast area, that thunderstorms
can`t be ruled out through the evening. Accumulations are expected
to be light.

For Monday, weak ridging moves in behind the exiting low bringing
drier and warmer conditions. Cross sections indicate a mountain wave
signal developing early Monday morning with westerly flow aloft.
Temperatures will be a few degrees above normal reaching the 70s
across the plains, high 60s for the foothills, and 50s for the
mountains. A shortwave passing to the north will bring some weak PVA
into northern Colorado as well as a strengthening jet just to our
north. This could bring a chance for some light snow showers and
thunderstorms across the northern mountains and a slight chance
for thunderstorms to develop across portions of the plains. 700 mb
temperatures indicate a cold front will pass through the forecast
area Monday night, however, temperatures are still expected to
make it back up to above normal highs for Tuesday. Two more
disturbances will pass to our north that may bring some weak
forcings to the northern portion of the state Tuesday through
Thursday. Light snow showers will be possible for the northern
mountains late Tuesday into Wednesday and rain
showers/thunderstorms across the plains on Wednesday. A second
cold front will pass through sometime Wednesday that will drop
temperatures back down to more seasonal conditions for Wednesday
and Thursday.

Ridging will begin building in over the area Friday through next
weekend.

&&

.AVIATION /06Z TAFS through 06Z Sunday/...
Issued at 1122 PM MDT Fri Apr 26 2024

Persistent rainfall and low cloud decks through the period. MFVR
conditions early in the period will deteriorate to IFR shortly
(after 08z) and continue through the period. There is a low
probability of a rain/snow mix at APA or BJC, which could lower
the ceilings/vsbys further. Surface winds will transition from NNW
to NE after 08z and continue that way through the rest of the
period. Sustained winds in the 8-12kt range can be expected. Once
the precipitation settles down by the end of the period, fog may
also need to be addressed in the outlook period for DEN (06-12Z
Sunday).

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until midnight MDT Saturday night for
COZ033-034.

Winter Storm Warning from 3 AM Saturday to 6 AM MDT Sunday for
COZ035-036.

Winter Weather Advisory until midnight MDT Saturday night for
COZ037.

Winter Weather Advisory from 6 AM Saturday to midnight MDT
Saturday night for COZ041.

&&

$$

UPDATE...Meier
SHORT TERM...Rodriguez
LONG TERM...Bonner
AVIATION...Cooper

NWS BOU Office Area Forecast Discussion