Sweater Weather?

It has been a marginally active end to September, with continued unsettled weather expected to start off October. Over the weekend, spotty showers and thunderstorms flared up each afternoon, mainly focused of the higher terrain across Utah but at times drifting over the valleys. For those that went up into the mountains to do some leaf peeping, you likely noticed some nice billowing cumulus clouds or might of even got caught under a shower. The fall colors are peaking right now for many areas, but unfortunately they are also fleeting. I don't imagine the leaves hanging on much longer. Give it till the end of the week or weekend and most of those vibrant yellows and oranges will be nothing but bare branches. 

Utah is currently located on the downstream side of a broad upper-level trough, digging south along the U.S Pacific coast. Today - Tuesday 9/30 - a piece of energy rotating along the base of the trough will progress through the state, bringing showers and thunderstorms mid morning to late afternoon. 

Left: 500 mb gph, winds, and temperature; Right: 400-250 mb potential vorticity

The bulk and most widespread precip looks to focus across the northern half of Utah, but storms and showers are possible over the southern portions of the state as well. The sounding from KSLC this morning reveals a bit of elevated instability and some decent moisture. Some upper level support from a jet streak and cooler temps should aid in shower and storm development. Showers are already moving in along the Wasatch Front, and will likely fill in further in the next hour or two. Hopefully, the liquid will help quash the fire up Dry Creek behind the UoU and Avenues.

Left: Composite radar reflectivity 9/30 @ 0940 MDT; Right: Forecasted composite reflectivity for 1200 MDT

Precipitation should largely be out of the state by late today, however, a few isolated showers/storms remain possible through tonight. Wednesday and potentially Thursday looks to be dry and pleasant. It will be a good day or two to get out between systems. 

Going into late this workweek and into the weekend the upper-level trough off the U.S Pacific coast will move into the Great Basin region then Utah. Details on impacts and timing from this system are still uncertain as the trough may become pinched off and become a closed low for a short time. However, there is the potential for the low to combine with a reinforcing shot of energy and colder air diving down along the Continental Divide, which could spill west into Utah. 

ECMWF 00Z 9/30 four-panel forecast for 10/5/2025 @ 00Z: (Top left to bottom right) MSLP/precip and 1000-500 mb thickness; 300 mb gph and winds; 500 mb gph, vorticity, and winds; 700 mb gph, RH, temp, and winds 

Left: Forecasted 700 mb temps (C) 12Z 10/5/2025

There are likely many scenarios that can play out over this weekend, with forecast details changing with each new model run. Nonetheless, it's likely that sometime between Fri-Sun there there will be periods of rain and even some high elevation mountain snow (> 8000') as the 700 mb 0 C isotherm inches south over Utah. This will be the first time for the 700 mb 0 C isotherm crosses back into Utah since the spring. The valleys along the Wasatch will likely see much more fall-like weather, with cool and wet conditions Saturday or Sunday. Hard to say how long the cool temps will stick around, but there are indications that the fall temps will stick around at least through early next week. A cooler more fall like start off October. Bring out the pumpkin spiced lattes and sweaters... 

ECMWF forecasted KSLC high/low temps