National Weather Service Forecast Discussion

Thornton Weather Forecast   
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000
FXUS65 KBOU 290232
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
832 PM MDT Thu Mar 28 2024

.KEY MESSAGES...

- Accumulating snow and minor travel impacts across the northern
  mountains tonight through Friday night. More significant
  accumulations in the Rabbit Ears Pass and Park Range, and
  lighter accumulations in the I-70 Mountain Corridor.

- A few showers spill onto the plains Friday afternoon and Friday
  night.

- Next storm system late Sunday into Monday night with mountain
  snow, and a mix of rain and snow likely for the plains. Mostly
  light but there could be some impact for the mountains and
  foothills.

&&

.UPDATE...
Issued at 817 PM MDT Thu Mar 28 2024

Current satellite water vapor imagery shows moisture entering
western Colorado and starting to make its way into the forecast
area. Aside from a few adjustments to the sky grids, the current
forecast looks to be in good standing so will let it ride.

&&

.SHORT TERM /Through Friday/...
Issued at 300 PM MDT Thu Mar 28 2024

Satellite shows mid and high clouds increasing across Colorado in
advance of the next shortwave. This disturbance is now hitting
northern Utah as seen in enhanced radar returns, and coincident
with lift and frontogenesis in the left exit region of the upper
jet. This feature will shift eastward into Colorado this evening,
with snow spreading into the northern mountains. There should be a
short period of healthy snowfall with snowfall rates of an
inch/hour or more. That heavier snow would be due to the
ingredients, such as instability (7-8 C/km lapse rates), forcing,
and relatively high water content (700-500 mb specific humidity
near 2.5 g/kg and Integrated Vapor Transport near 2 standardized
anomalies). That band will gradually shift southward overnight
into the I-70 Mountain Corridor as the upper level jet does, but
intensity is expected to decrease somewhat as frontogenesis
gradually weakens. While that area is also not favored by
west/southwest flow aloft, this would be the season for more
convective related snowfall, and the location of the upper level
speed max is concerning for perhaps heavier snow than forecast.
For now, we`ve beefed up the I-70 Corridor snow forecast a little,
while issuing a Winter Weather Advisory farther north where the
best combination of orographics and dynamics come together. We`ll
run that Advisory into Friday night as the jet axis pushes back
northward taking the bulk of snow with it. Whatever the case, be
prepared for snowy travel conditions in the high country and slick
roads especially at night/early morning and over the higher
mountain passes.

For the plains, we can`t rule out an isolated shower drifting out
of the mountains given the jet proximity and mid level
instability tonight. However, most of these should hold off until
we further destabilize on Friday afternoon. It`s getting to be
that time of year again, so an isolated rumble of thunder or two
is certainly possible as we head into Friday afternoon in/near
the mountains. Temperatures on Friday will cool a few degrees and
be near or just below normal.

&&

.LONG TERM /Friday night through Thursday/...
Issued at 300 PM MDT Thu Mar 28 2024

A few changes for the long-term forecast package mainly on Saturday
through Monday night. A shortwave continues Friday night ushering
light to moderate snow across the mountains and valleys.
Southwest flow could hinder a few spots from receiving additional
amounts but overall, snowfall should continue through Saturday
morning. Highest snowfall totals and impacts of snow covered roads
would likely be Rabbit Ears Pass, and parts of Middle Park. Thus,
a Winter Weather Advisory continues until 6 AM MDT Saturday
morning.

The last round of embedded shortwaves arrives Saturday across our
higher peaks. Southwesterly flow should bring Pacific moisture
cut off from the main trough located in on the western US coast.
This will lead to light snow showers for area above I-70 for the
mountains and valleys. This forecast introduced a slight decrease
in QPF values along the Park Range Saturday afternoon due to the
potential for warming. NAM indicates warm air advection especially
across the urban corridor and plains. A few foothill areas such as
Larimer and Boulder counties may experience a rain snow mix but
will keep a slight chance (20-30%) of scattered rain showers for
the plains along the Wyoming border due to the assistance of the
upper jet.

Uncertainty lingers for the next upcoming system Sunday through
Tuesday morning. Given this upper level system has now switched to
a closed low instead of an open trough, there could lead to
increasing chances of snow along the northeastern plains and
Palmer Divide but that will depend a few mesoscale factors such as
how cold are surface temperatures Monday afternoon. Considering
the uncertain, will focus this discussion on key points that have
higher forecast confidence.

Sunday, as the low approaches, winds shift west and orographic lift
will bring light to moderate snow for areas above 8k ft and the
mountain valleys. Rain showers are possible for the urban
corridor and plains through Sunday evening. Generally, widespread
rain showers seem likely for the urban corridor and plains Monday
afternoon. The backside of cyclone will likely shift our winds
northerly which may allow for a low level jet to lead to upslope
flow during Monday late afternoon through Monday night. This may
result in high snowfall amounts for the southern foothills Monday
night.

By Monday night, ECMWF and GFS are extremely different solutions as
GFS keeps lingering widespread showers through Tuesday morning.
Without much forcing, this outcome seems unlikely thus this forecast
update keeps NBM PoPs ending Monday night at midnight. By the middle
of next week, above normal temperatures are expected to return.

&&

.AVIATION /00Z TAFS through 00Z Friday/...
Issued at 547 PM MDT Thu Mar 28 2024

Winds have been tricky this afternoon but it looks like they have
settled in to a northerly component for KDEN for the moment.
There is a boundary just to the north that could shift KDEN to a
gusty westerly if it makes it a little further south, if not, the
northerly should hang on before transitioning to a southwesterly
drainage overnight. Winds will become northeasterly in the early
morning as a weak cold front passes through. All airports will
have a chance for showers in the afternoon that could produce some
gusty northerly outflow beginning around 19Z.

&&

.FIRE WEATHER...
Issued at 300 PM MDT Thu Mar 28 2024

Near critical fire weather conditions are possible mainly across the
Palmer Divide and adjacent plains Saturday afternoon. Relative
humidities drop near 16-20 percent. Southwest winds could produce
wind gusts up to 25-35 mph leading to fire spread if one were to
occur.

With the potential for wetting rain late Sunday afternoon, this
could ease fire weather concerns for the same areas through Sunday
evening.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 AM MDT Saturday for COZ031.

&&

$$

UPDATE...Bonner
SHORT TERM...Barjenbruch
LONG TERM...AD
AVIATION...Bonner
FIRE WEATHER...AD

NWS BOU Office Area Forecast Discussion