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000
FXUS65 KGJT 231115
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
515 AM MDT Sat Sep 23 2023

.SHORT TERM...(Today through Sunday)
Issued at 302 AM MDT Sat Sep 23 2023

Split flow has returned to the Pacific Coast with the stronger
northern stream system pushing a cold front through the CWA prior
to midnight. Some impressive ducted waves are showing up on the
88D in the wake of this front. Cooler air filling behind is
dropping temperatures into the 30s across the middle Yampa River
Valley but clouds are so far hampering the temperatures in the
upper valley. If the freeze warning does not quite work out
tonight there will be another chance tonight as clear skies and
light winds will once again allow for efficient radiational
cooling in the high valleys and basins. Meanwhile the southern
stream is pumping Pacific moisture across the Desert SW into the 4
Corners region...showing up well on the 310K theta surface. The
moisture however remain quite elevated and not much is reaching
the ground per upstream obs. The dropping front will likely help
add some low level ascent and will have some minimal PoPs near
the AZ/NM border this morning as a result...but measurable
precipitation seem like a low probability scenario in most areas.
Temperatures will be generally be some 5 to 8 degrees cooler
behind the front today. A closed southern stream circulation is
moving onshore in central Cali early this morning and this
weakening/opening system will be translating across southern Utah
by late afternoon. Again mositure transport vectors on the same
isentropic surface show another surge of mositure ahead of this
system arriving to our far southern CWA and when combined with
modest ascent with the wave...should lead to widely scattered to
isolated convection through the mid to late PM hours. Again can`t
see much measurable precipitation reaching the ground but surface
dew points look to push back into the mid to upper 40s...so less
than a tenth of an inch is forecast attm. This wave will move East
of the Divide by sunrise with ridging building in for the
afternoon. This should push temperatures back in the other
direction by 5 to 8 degrees under plenty of sunshine. A dry
forecast is in the books and this should continue well into the
upcoming week.

.LONG TERM...(Sunday night through Friday)
Issued at 302 AM MDT Sat Sep 23 2023

The synoptic pattern opens Sunday evening with a Rex block high over
the Hudson bay and troughing from the Ohio Valley south to
Mississippi. To the west is a deep low pressure system in the Gulf
of Alaska with troughing extending south over the Eastern Pacific,
and ridging along the Rocky Mountains. The models are struggling to
forecast the Rex block in this transition season as indicated in the
wide dispersion among the models run-to-run beyond a few days.
General consensus is for the ridging over eastern Utah and Western
Colorado to continue through mid week, yet even on the latest run
this solution is brought into question with a shortwave knocking
down the ridge Tuesday, shifting it out onto the Plains and allowing
the longwave trough along the Pacific Coast to slide into the Great
Basin (very low confidence). In these situations, persistence it a
good forecasting tool. Hence, the forecast holds with the Rex block
remaining in place through mid week, possibly beyond into next
weekend and even longer (a Rex block is a "blocking pattern" that
tends to hang around for an extended period). The Rex block will
tend to hold the longwave pattern in place keeping the Gulf of
Alaska low in place and ridging generally over the Rockies. This
will bring us warming temperatures reaching eight to ten degrees
above the seasonal normal by Wednesday across the region under
mostly sunny skies. Model guidance says the the blocking pattern
breaks down and the longwave starts shifting east brining a cooling
trend and possible unsettled weather to the region next weekend, but
persistence says no, and persistence is often hard to beat. Thus,
for this discussion, the extended forecast through the end of the
week is for continued warm dry conditions (with low confidence of
course, but not as low as seeing unsettled weather by next weekend).
Of course, we`ll just have to wait and see how this plays out, so
stay tuned.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 515 AM MDT Sat Sep 23 2023

Mid to high level cloudiness is working through early this
morning with isolated showers possible near the NM/AZ border
though 17Z. Another batch of moisture arrives this afternoon and
could again lead to shower and thunderstorm activity over the
South through the PM hours. VFR should hold over the next 24
hours with only low probability of impacts from the passing
storms.


&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Freeze Warning until 9 AM MDT this morning for COZ002-005.

UT...None.
&&

$$

SHORT TERM...15
LONG TERM...DB
AVIATION...TGJT