206
FXUS65 KGJT 131121
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
521 AM MDT Mon May 13 2024

.KEY MESSAGES...

- Afternoon showers and thunderstorms will once again form over
  the higher terrain.

- An approaching disturbance will bring more widespread shower
  and storm activity Tuesday and Wednesday, with the San Juans
  most favored Thursday.

- Temperatures will continue to warm through the week.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 230 AM MDT Mon May 13 2024

Aside from a few lingering showers, mostly clear skies and quiet
weather has prevailed over eastern Utah and western Colorado over
the last few hours. The showers will continue to taper off as
subsidence associated with high pressure aloft builds in, bringing a
quiet start to the workweek. Warm air associated with the ridging
will allow temperatures to climb a few more degrees, running 4-8
degrees above normal for mid-May. This means low to mid 80s across
the desert valleys and 70s to the higher valleys. Mountain towns may
see highs running closer to normal this afternoon, as daytime
heating works on moisture still pooled over the higher terrain and
leads to another round of isolated to widely scattered showers and
thunderstorms. This activity is expected to remain over the terrain
and adjacent high valleys, with gusty winds, light to moderate
rainfall, and maybe some small hail being the main threats. Activity
will taper off with the loss of daytime heating, with Monday night
once again being mostly quiet.

In the meantime, a trough will be moving through the Pacific
Northwest, and a cutoff low is forecast to move into Southern
California. These two features will be the next weather makers for
eastern Utah and western Colorado starting Tuesday afternoon.
Previous guidance had the trough and cutoff low in phase as they
moved into the region, leading to a broad area of showers and
thunderstorms. The latest guidance has backed off a bit on this,
tracking the trough through first on Tuesday before stalling out
just to our east, while the low remains somewhere over the border
between SoCal and Baja California. This will have an impact in the
weather into the long term period, but keeping to the short term,
this will bring unsettled weather Tuesday afternoon.  Isolated to
scattered showers and thunderstorms will once again pop over the
higher terrain as daytime heating once again acts on residual
moisture. A weak cold front associated with the trough will drop
south through the CWA during the afternoon, bringing a band of more
organized precipitation along with it. Behind the front, cooler and
drier air will lead to a downturn in activity through the evening
hours, along with lows Tuesday night into Wednesday approaching more
normal values after a milder Monday night.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Issued at 230 AM MDT Mon May 13 2024

Any lingering convection looks to come to an end Tuesday night with
a relatively quiet start to the day on Wednesday. However, Wednesday
appears to be the most active day of the week in terms of showers
and thunderstorms as the low pressure trough drops southward through
the area, resulting in more widespread showers and thunderstorms.
Storms may initially favor the high terrain Wednesday afternoon but
will quickly fill in among the valleys as sufficient dynamics and
forcing exist to allow storms to move off the terrain. There is
somewhat of a split flow pattern here as the polar jet actually
remains north of the region despite this trough moving southward
through the area. The descending trough will actually meet up with
and eventually merge with a cut off low that is parked over Arizona.
The subtropical jet is rounding the base of this cut off low with
fairly good jet support as a 120+ kt jet streak rounds its base
through northern Mexico. So, it looks like this southern system is
more of a player in this game than previously thought. Drawing more
influence from the subtropical jet and our CWA stuck in the middle
between the polar jet to the north and subtropical jet to the south,
temperatures will remain milder with slight CAA with trough passage,
but snow levels look to remain high. In fact, H7 temperatures peak
in the 4C to 10C range during peak heating Wednesday afternoon and
drop to as low as maybe 2C to 4C Wednesday night following trough
passage. Therefore, not expecting much in the way of snow in the
higher elevations with this system...maybe a couple inches at best
at elevations above 11kft. So temperatures will still be relatively
mild but cooler than Tuesday by about 5 degrees, bringing highs
closer to normal.

The trough is projected to be south of the Four Corners with the H7
and H5 low over eastern New Mexico and the Texas Panhandle by
Thursday afternoon. H7 temperatures will quickly bounce back,
returning to 8C to 10C by Thursday afternoon as WAA returns the
above normal temperatures despite the northerly flow following
trough passage. Enough moisture and instability will still linger in
the trough`s wake to result in scattered thunderstorms Thursday
afternoon, favoring the central and southern mountains, potentially
drifting southward into adjacent valleys late in the day.

Models seem to be in better agreement that a large ridge of high
pressure will build in from the west and over the Great Basin on
Friday, ushering in a much drier airmass and pushing the moisture
out, resulting in mostly sunny skies and even warmer temperatures
about 5 to 10 degrees above normal. The ridge axis moves east of the
Divide by Saturday with southwest flow returning as a low pressure
trough moves into the PacNW with a cutoff low forming off the SoCal
coast. This will allow some moisture to advect in and result in some
afternoon convection on Saturday, focused over the high terrain.
Given the drier air mass, low levels will be dry so breezy
conditions are possible with virga from any storm activity. There
appears to be disagreement between models on how to handle these
systems for the weekend and beyond so won`t put much stock in that
yet. The better bet is to keep a drier and warmer trend with
potential for isolated to scattered storms each afternoon over the
high terrain.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 518 AM MDT Mon May 13 2024

Some mid level clouds and maybe a sprinkle or two are currently
passing over central and northern portions of western Colorado.
Eastern Utah remains under clear skies, as does northwest
Colorado. This clearing will continue to expand over the next 6
or so hours, before afternoon showers and thunderstorms begin
over the higher terrain. These showers and storms may impact
higher elevation terminals, but confidence is not high. Any
shower that moves over a terminal will be capable of briefly
lowering visibility and ceilings with moderate rainfall, and
producing gusty and erratic winds. Otherwise, conditions will be
quiet, with VFR prevailing. Afternoon gusts up to 20 knots will
be possible at most terminals.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...None.
UT...None.

&&

$$

SHORT TERM...BGB
LONG TERM...MDA
AVIATION...TGJT