839
FXUS65 KGJT 041136
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
536 AM MDT Sat May 4 2024

.KEY MESSAGES...

- Scattered showers and isolated thunderstorms are possible
  across the central and southern Continental Divide mountains
  this afternoon and evening. Warmer and dry conditions are
  expected elsewhere.

- Very strong winds are likely on Sunday with gusts upwards of
  65 mph in the valleys and 80 mph in the mountains. A High
  Wind Watch remains in effect for much of eastern Utah and far
  western Colorado Sunday morning through Sunday evening.

- A strong storm system and cold front will bring widespread
  precipitation and much cooler temperatures Sunday night into
  Monday with the potential for accumulating mountain snow.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 320 AM MDT Sat May 4 2024

The cold front that dropped southward through most of the
forecast area yesterday will quickly return northward as a warm
front today. This is in response to strengthening southwest flow
ahead of a large storm system off the coast of northern
California. Strong warm advection within the southwest flow will
boost surface high temperatures well into the 70s across the
lower valleys this afternoon with 80s looking like a solid bet
across canyon country of southeast Utah. Also embedded within
the southwest flow will be a weak shortwave that will pass near
the central and southern Continental Divide mountains. Just
enough forcing, moisture, and instability will be present with
this wave to spark a few scattered showers and storms this
afternoon and evening. Elsewhere, warm and dry weather will be
the rule for the first Saturday of May under mostly sunny skies.
Like the past few days, afternoon breezes are likely to kick up
a bit again today, but weaker winds aloft should keep gusts
under 30 mph for the most part. The same cannot be said on
Sunday, however, as the potential for a high end wind event
awaits to round out the weekend.

As the strong storm system off the coast of northern California
tracks inland across the Great Basin Saturday night into
Sunday, the nose of a potent mid and upper level jet will
overspread eastern Utah and western Colorado from the southwest.
Wind speeds at 700mb will be approaching 50 to 60 kts while
speeds at 250mb will likely exceed 100 kts. In fact, the 700mb
wind speeds being advertised by ensemble model guidance from a
climatological standpoint are near the 99th percentile for this
time of year, with return intervals of 1 day every 10 years or
more. In other words, those kinds of speeds at that pressure
surface are highly anomalous and rare. So as those mid and upper
level jets nose in, the pressure gradient in between the deep
closed low to our west and a ridge to the east will
significantly tighten. Combined with strong diabatic heating and
the resulting deep boundary layer mixing, the lower atmosphere
will be very efficient at transporting those winds aloft down to
the ground. What`s also concerning is the potential for virga
to enhance the downward momentum transfer of those winds given
increasing moisture aloft but very dry air in the subcloud layer
down to the surface. Latest hi-res soundings show this very
well with a nearly saturated layer above 600mb and the classic
inverted V signature on downward. Taking this altogether, the
ingredients are there for a potentially significant and high end
wind event on Sunday with gusts upwards of 65 mph possible in
the valleys and up to 80 mph in the mountains. Interestingly
enough (but maybe not all that surprising given the above), the
EPS Extreme Forecast Index (EFI) is showing a very strong signal
that a very unusual wind event is becoming increasingly
likely... with at least 10 percent of ensemble members pointing
towards an extreme outcome. That`s something worth paying
attention to. However, despite these concerning trends and
signals, opted to leave the High Wind Watch unchanged with this
package to let the day shift take a look at more data. Should
these trends continue, upgrades to High Wind Warnings and Wind
Advisories would almost certainly follow. In the meantime, start
getting prepared for high winds by securing loose outdoor
objects or furniture, making plans to postpone any outdoor
burning, and staying up to date with the latest forecasts.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Issued at 320 AM MDT Sat May 4 2024

Models were in good agreement in shifting the strong spring
storm eastward across the forecast area Sunday night through
Monday morning with wrap around moisture impacting northern
Colorado into the afternoon. Strong south/southwest winds ahead
of the cold front associated with the system will shift to the
west/northwest and decrease as the boundary moves rapidly to the
east, exiting to eastern Colorado by sunrise Monday. Impressive
dynamic lift indicated from the surface to the jet level Sunday
night which should be the most intense phase of this storm. On
Monday, dynamic forcing decreases, but favorable westerly flow
working on deep moisture in an environment featuring steep lapse
rates will continue to generate orographic showers across the
eastern Uinta Mountains, the northwest Colorado plateau, and the
northern and central Colorado mountains and their adjacent
valleys. In fact, orography is expected to continue generating
showers Monday night and into Tuesday. Meanwhile, the influx of
cold air is expected to bring snow levels below mountain bases
in the north with some light accumulations possible for the
lower elevations of northwest Colorado. All told, latest
guidance suggested snowfall amounts of 6 to 12 inches across the
Elkhead, Park and Flat Top Mountains, mainly above 8 Kft from
Sunday night through Tuesday afternoon. Snow amounts will be
progressively lighter and confined to higher elevations to the
south.

The storm center moves to the northern High Plains where it
stalls on Tuesday. A short wave trough on the flanks of the deep
northern low is expected to push southeastward through the area
from Tuesday night into Wednesday. This disturbance will
continue to generate shower activity, mainly across the north,
while bringing another shot of colder air to the forecast area.
Only light additional accumulations are expected during this
period.

The latter part of the week appeared to be less certain with
model agreement trending down. In general, 00Z/Sat operational
runs depicted a secondary low pressure system developing over
the Great Basin on Thursday which becomes quasi-stationary
through Friday. This leaves the forecast area under a broad col
between the low to the west and the northerly flow to the east.
Consequently, showers remain in the forecast with the Colorado
mountains favored.

Temperatures cool by 10 to 15 degrees on Monday with highs
expected to run close to 10 degrees below normal for most
locations. Temperatures moderate slightly Tuesday, then dip
again Wednesday before trending milder during the latter half of
the week. Wednesday morning appears quite cold and there is
some potential for freezing temperatures in Grand Valley and
Uncompahgre basin where vegetation is susceptible to damage.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 536 AM MDT Sat May 4 2024

VFR conditions will prevail for the next 24 hours. Partly to
mostly sunny skies are in store today with generally light and
terrain driven winds aside from a few gusts here and there. More
clouds and a spotty shower or two can`t be ruled out near KASE,
KGUC, and KTEX this afternoon and evening as a disturbance
passes by to the south and east. Quiet weather returns tonight,
though clouds and winds will start to increase towards daybreak
Sunday morning.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...High Wind Watch from Sunday morning through Sunday evening for
     COZ001>003-006-011-017-020-021.
UT...High Wind Watch from Sunday morning through Sunday evening for
     UTZ022-024-025-027>029.

&&

$$

SHORT TERM...MDM
LONG TERM...NL
AVIATION...TGJT