487
FXUS65 KGJT 161110
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
510 AM MDT Mon Sep 16 2024

.KEY MESSAGES...

- Widespread showers and storms are expected today and tomorrow
  as a system moves through the region.

- Some storms may be capable of producing large hail, damaging
  winds and heavy rainfall. Therefore there is a chance for
  localized flooding.

- Temperatures dip to near normal today in response to clouds
  and precipitation. A cold front moving through the region
  tomorrow is expected to bring cooler than normal readings for
  the latter part of the week.

- Expect a break in the action Wednesday and Thursday before
  another system impacts the region Friday into the weekend.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 233 AM MDT Mon Sep 16 2024

Deep moisture from the remnants of post-tropical storm Ileana is
beginning to advect into the forecast thanks to a strong low
pressure currently located over northern California. The jet
associated with that system is also spreading over the region. The
combination of that along with the moisture advection has resulted
in scattered convection, which will spread northward through the
morning hours. The mositure is expected to increase even more later
this morning so expect an increase in coverage of showers. As that
precipitation moves to the northeast some clearing takes places in
the Four Corners region. This surface heating coupled with steep
lapse rates will result in CAPE upwards of 1000 j/kg. The shear
provided from the California/Nevada low pressure in combination with
this instability could support a round of more intense convection
this afternoon. A few of these storms could become strong to severe
and capable of producing large hail and damaging winds. Given the
quality of the moisture heavy rainfall is also possible from strong
cells and perhaps multiple rounds of storms. This means there is the
potential for localized excessive rainfall, which could lead to
debris/mud flows in our susceptible spots. The best location for
these storms will be across the southwestern half of the forecast
area. As the CAPE decreases after sunset the storm intensity should
reduce. Although given the continued moisture advection isolated to
scattered convection is possible tonight for a majority of the area.
Highs today will be cooler than what we have seen the past week or
so due to cooler midlevels, clouds and precip.

Tomorrow the deep low pressure lifts northeastward and tracks over
Idaho, northern Utah and Wyoming. The trailing cold front will pass
through our area during the day. Forcing from the system increases
and the front should provide lift as well. With the moisture in
place we expect another round of widespread showers and storms ahead
and along the front. The eastern two thirds of the area will be
favored for showers and storms. Behind the front a dry slot
pushes in from the southwest and modest subsidence so conditions
should clear quickly from west to east. The main uncertainty
tomorrow is how much instability will be present given pre-
frontal clouds and precip. With shear in place any CAPE that can
develop could result in a few strong storms capable of damaging
winds. Most locations will clear out by sunset. The peaks could
pick up some snow as freezing levels drop from convection and
cold air advection.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Issued at 233 AM MDT Mon Sep 16 2024

Drier air sweeps in behind Tuesday`s cold front, ushering in a much
cooler air mass as overnight lows Tuesday night drop about 10
degrees from the previous night to around 5 to 10 degrees below
normal. Rather crisp fall morning in store Wednesday with efficient
radiational cooling under clearing skies. Highs on Wednesday will
also be around 4 to 8 degrees or so below normal but a shade milder
than Tuesday afternoon due to strengthening southwest flow ahead of
another deep trough moving southward from the Pacific Northwest into
the Great Basin. Models diverge in their solutions beyond Wednesday
with not much agreement on the timing of this trough. The GFS is
quicker with the trough moving across the area as an open wave
Thursday night through Friday night, while the ECMWF is slower with
this trough moving it across Friday night through Saturday night.
The one thing models agree on though is this trough weakening
significantly as it tracks eastward across the area...but the
difference in timing is significant by about 24 hours difference.
The blended guidance keeps scattered PoPs in the forecast Thursday
night through Saturday afternoon, so essentially splits the
difference which seems reasonable given the uncertainty. For what
it`s worth, the ECMWF is a bit more robust with this trough than the
GFS but time will tell how much moisture comes with it and what this
means for precipitation amounts. It is cool enough that we could be
talking snow at the higher peaks with snow levels expected to be
around 11kft at this moment in time with the passage of this open
wave trough late in the week.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 507 AM MDT Mon Sep 16 2024

Scattered to widespread showers and storms are expected today
therefore kept VCTS at most TAF sites this afternoon into early
evening with showers becoming more isolated tonight. Periods of
light to moderate rain is possible this morning, while hail,
strong gusty winds and heavy rainfall is possible this afternoon.
VFR to MVFR expected with brief periods of IFR/ILS conditions
possible in stronger storms.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

SHORT TERM...KJS
LONG TERM...MDA
AVIATION...TGJT