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FXUS65 KGJT 231514 AFDGJT

Area Forecast Discussion National Weather Service Grand Junction CO 914 AM MDT Sat Sep 23 2023

UPDATE
Issued at 911 AM MDT Sat Sep 23 2023

After a brisk start to the first day of fall, temperatures have risen above freezing for zones 2 and 5 so the freeze warnings have been allowed to expire. Rest of the forecast looks good.

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...None.
UT...None.

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