Cut-off Low Woes

It's officially fall but we have yet to really get a good early season fall storm. Usually by this point in September, the high summits of the Wasatch have been noticeably dusted once or twice by some wintry precipitation. Not the case this year, and I don't think it will be the case before the month closes out. The first half of this past workweek saw some pleasant September days. Thursday was considerably warmer, topping out at 88 F at KSLC which is 12 degrees above average (76 F) for this time of year. 

KSLC MET obs 9/25/2025

500 mb gph, winds, temp, and NEXRAD radar composite: Closed low SW CONUS

So will we seen any change up? Kind of... At the moment a closed upper-level low is slowly rotating across the SW CONUS. Closed lows and their proceeding evolved form - cut-off lows - are notoriously difficult to forecast. Closed lows form when an upper-level trough/circulation becomes largely detached or pinched off from the main steering flow in the westerlies (jet stream). As the closed low circulation becomes fully separated from the jet stream, a cut-off low is born. Now the orphan child of the upper-levels, a cut-off low spins without guidance or direction. For days on end, a cut-off low can spin nearly stationary without east or westward progress. This feature is somewhat analogous to a whirlpool in a stream or river.

Development of a cut-off low

Cut-off lows are occlusions in nature and areas of weakened baroclinicity, which means they don't have strong frontal boundaries. However, the cooler mid to upper-level air under the center of the cut-off low can create extended periods of cool and cloudy conditions. For all the reasons mentioned, cut-off lows become a thorn in the side of numerical weather models and forecasters. Difficult to predict their movement as well as placement and timing of clouds and precipitation. The forecast for the next 24-hrs is given below for the 500 mb level. The forecast periods reveal the evolution of the cut-off low across the SW CONUS over the next day.  

ECMWF 500 mb gph, vorticity, and winds: Forecasted development of a cut-off low across the SW CONUS

What does this mean for Utah weather? Well, going forward Friday into Saturday moisture will be advected northward into Utah from Baja and Arizona. Some of that moisture has already made it's way north into the state as observed by the precipitable water (PW) value of 0.86" from this morning's sounding at KSLC. PW just measures the total amount of water content in the atmospheric column above a location. For this time of year, a PW value of 0.86" is abnormally high (> 90th percentile). 

KSLC 9/26/2025 12Z sounding

The combination of moisture and daytime heating means there will be moderate instability and the potential for scattered showers and storms for areas of the state, mainly in the afternoon. The greatest threat for showers/storms Fri-Sat looks to be focused across the southern 2/3rds of the state, with the mountains having the best chance. By Sunday, a wider swath of moisture moves into Utah as the cut-off low slowly ambles E-NE. Northern Utah looks to have a better opportunity for showers/storms during this period, and once again best potential will be in the afternoon. I don't expect any one day to be a washout and temperatures will continue to be warm 70-80s for the valleys and 60s for the mid-elevations in the mountains. In reality, this will likely play out like most unorganized rain events in Utah, which means we will still have a good amount of nice weather but may catch a quick spitting shower or even a snappy "downpour" before the sun comes back out. A typical Utah rain one might say...

ECMWF four-panel 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 

Things get a little more interesting late Sunday going into Monday and early next week as a long-wave trough approaches the Pacific coast and the cut-off low becomes integrated once again into the main westerlies. As of now, some combination of moisture and energy look to impact Utah Mon-Wed of next week, but difficult to say exact details at present. A likely scenario is that Utah will see some sort of precipitation activity during this period. However, the most probable scenario is forecast details will continue to change until that time, so we'll have to wait and see as the timing gets closer.