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 over 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 til 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

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. 

The 22nd of September

The 22nd of September... Not quite the 21st, but close enough. Fall colors have already filled in across the Wasatch foothills and mountains, but technically in the eyes of the sun it hasn't been fall. Today at 12:19 PM MDT the autumnal equinox will arrive. The sun will be directly overhead at the equator, day and night will be nearly equal in length, and the Northern Hemisphere will no longer be the sun's preferred child. From this day until March 20, 2026, the sun will radiate more energy across the southern tier of the earth than the north. The incremental warming to peak heating we have experienced from the previous spring into summer will now gradually dissipate as the solar surplus we have had for the past six months turns to a solar deficient. But why..? 

It's often misunderstood why we have seasons. One incorrect explanation for the seasons is the earth is closer to the sun in spring/summer and further away from the sun in fall/winter. Although the Earth's distance to the sun does vary slightly throughout the year, this is not the reason for the seasons. Let's take it back to earth science 101. 

The Earth is actually tilted on it's axis (straight line going from pole to pole) at a 23.5 degree angle, which stays consistent through time and space. Due to the Earth's tilt, the Northern and Southern Hemispheres are rarely ever angled  - only at the fall and spring equinoxes - the same towards the sun throughout the year. When one hemisphere is angled toward the sun (spring/summer) the other is angled away from the sun (fall/winter). The effect of this is unequal heating of the globe and the changing of the seasons. The fall and spring equinoxes mark the point at which the hemispheres are beginning to become angled toward or away from the sun. However, at the exact instance of the equinox there is a very brief and fleeting moment that the hemispheres are equals in terms of the sun rays. 

But why is the earth tilted? Well it's hypothesized that billions of year ago, when the Earth was a newly born planet, earth collided with another smaller planet (Theia) which knocked the Earth off it's axis. The impact of this collision is still observed today as the Earth's tilt of 23.5 degrees and changing of the seasons. One other still observable impact from the collision is our moon, which was formed from debris ejected into space when Earth closed-lined the smaller planet. 

The equinox isn't a switch that causes weather conditions to instantaneously change. In Utah, it may be technically fall but it's still relatively warm and the storm activity over the weekend was more summer than fall like. Expect a few scattered storms today across the northern part of the state as a punchy little system/trough and cool front quickly passes over the state. According to this morning's sounding at KSLC, there is a 'decent' amount of moisture and CAPE to help initiate some storms/showers. With the cooler mid to upper-level temps associated with this trough, there's a good opportunity for some graupel/hail in heavier showers. There will be a brief cool down the beginning of this week from a weak intrusion of cold air behind the cool front, but beyond that things will gradually warm up again as some ridging develops. In fact, besides some potential showers and storms later this weekend in some areas of Utah, there doesn't seem to be a ton of activity weather-wise going forward into next week. That's not to say things can change, but there aren't any indications as of now. 

If you feel it, chase it

Over the weekend, Utah had not one but two tornadoes touchdown in far SE reaches of the state. The tornadoes developed near the UT-CO border SE of the Monticello and Blanding area on the afternoon of 9/13, touching down and dissipating multiple times. It seems mostly scrub and sagebrush were causalities of these storms, but there were some reports of damage to homes. I am not sure if these were true supercell tornadoes or the estranged sibling of supercell tornadoes - landspouts. 

Many of the tornadoes in the western U.S can be classified as landspouts, which differ from the typical type of supercell tornadoes in one major way. Landspouts form from the ground up when an area of rotation/spin at the surface is stretched and pulled into the updraft of a developing non-supercell thunderstorm. Conditions favorable for landspout formation include: developing storms in proximity of a surface boundary with low-level instability (high CAPE and steep lapse rates). Supercell tornadoes develop when the the rotating updraft of a storm extends to the surface. Conditions favorable for supercell formation include: deep layer shear, unstable environment, and low-level jet/strong inflow. Typically, landspouts are weaker than supercells, topping out at an EF2 in terms of strength. 

Credit: John Davies 

Credit: National Weather Service 

So were the tornadoes in SE Utah on 9/13 supercell or landspout tornadoes? There was a surface boundary and increased surface spin noted in the area along with relatively steep low-level lapse rates and instability present in the vicinity of storm formation, which is just the type of environment capable of producing a landspout. 

On the flip side, there was a bit of deep layer shear (~25-30 kts), helicity, CAPE, and unobstructed southwesterly inflow (SW surface winds), which were all within the low-end ranges for supercell development. Additionally, the storm cell that produced the tornado/landspout was to the SE of the main surface boundary and not aligned with the region of surface spin. This lone cell could have tapped into the right environment for supercell development, or tapped into another smaller surface boundary created by outflow interactions or terrain.

Since, that area of the state is rugged and rural, radar coverage and surface observations are sparse. A composite and velocity radar image from KGJX is shown. Looking at the Grand Junction radar on the afternoon of 9/13, the cell that produced the tornado is located on the edge of the radar's range and in an area where the radar beam is scanning high into the atmosphere. Confidence in reflectivities is decreased at this range and noise and radar artifacts become a problem. Nonetheless, the radar imagery reveals a well defined cell with what looks like a notch-ish/kinked shape, as well as small but present velocity couplet coinciding with the kink. It's hard to say if this is a true velocity couplet/storm rotation or just noise, so I can't say with certainty if the radar was actually seeing the rotation associated with a supercell storm. 

So was this just another landspout or a big daddy tornado? Even with all this analysis, I have a difficult time deciding with confidence, but I do feel pieces of information align more closely with a supercell tornado. In summary, my best informed guess was this was an actual supercell tornado or potentially some hybrid... 

Do you smell that smell

I woke up last night to a strange smell. It took me a minute before I realized where the funk was permeating from... the Great Salt Lake. I had kept a northwesterly window open overnight to let in the cooler air, but in through that window wafted the gases of the GSL. For any resident along the Wasatch Front, the GSL stink is a reminder of home and also a reminder of the giant salt water puddle sitting in the middle of a desert. 

So why was I smelling the GSL funk last night, well an indecisive cool front/boundary is to blame. I noticed yesterday evening winds across the northern SL valley shifted W-NW after having consistently breezy winds out the the south all day. Winds largely stayed W-NW all night at KSLC, which is contrary to the usual southerly nighttime valley winds. It looks like the front waxed and waned across the northern half of the SL Valley overnight, before retrograding a bit to the north near sunrise. A quick HYSPLIT trajectory analysis, to see how air sourced from the GSL wetlands was transported between 1800 MDT to midnight, shows that it's likely that GSL air was advected through the northern tier of the SL valley last night... hence the smell.

Estimated frontal boundary overnight 9/11-9/12

Left: temp, rh, and dwpt; Right: wind speed and direction

HYSPLIT

500 mb gph, winds, and temp w/ composite radar

It's not uncommon for us valley folk to get a good drag of GSL gases after cold front pass over and shift winds out of the north, which just so happens to be the direction to transport GSL gases into the SL valley. So why was a cool front moving back and fort across the valley? Looking at the broader picture, we see that a slow moving upper level low, spinning across the Great Basin region. 

This upper level feature will bring a bit of a change up to the weather in through the next couple of days. Northern Utah will have the opportunity for a few scattered shower this afternoon/evening, but the main focus of precip will be in eastern/southeastern areas of the state where more moisture and forcing is available. There could even be a few stronger thunderstorms that develop. The SPC has a marginal risk for far eastern areas.

Later today this low will make some more easterly progress and the cool front will once again drift south and east across the SL valley. Will we smell that smell again? I'm not sure, but I will be welcoming in the cooler temps behind the cool front. Residual moisture hangs around Saturday, with a spotty afternoon showers/thunderstorms possible. May even see some snow above 9000-1000' in the Uintas under heavier rates. Things largely dry out and warm a tad by Sunday.

Soft launch into Fall

850 mb temp anomalies 9/1-9/7

We have finally crossed into meteorological fall and are only two weeks away from the astronomical equivalent. After a rather dry and moderately hot June-August, the last tendrils of summer have finally loosed their grip. Large portions of the U.S have already gotten a taste or at least a Lacroix flavor of fall the past couple of weeks, particularly east of the Continental Divide (right figure). But for the most part, Utah has only seen a very gradual shift away from blasting heat and a sputtering monsoon to what I like to call Utah "window weather". 

Now to avoid confusion, there is the Icelandic definition of "window weather" which is when weather looks pleasant from your window but is actually unpleasant when experienced outside. However, I am not referring to this definition. I define "window weather" as when the weather looks nice from your window and is actually nice. We have been dipping our toes "window weather" here in northern Utah, but should see an even more pronounced shift to these conditions by the end of the workweek. 

Over this past weekend, a bit of heat and floundering monsoonal pushes of moisture kept conditions mildly summer like. We saw temperatures largely stay below 90F at KSLC (figure left/bottom) and had a few days of scattered/hit or miss showers/storms. I think the most noticeable yet subtle changes were overnight lows consistently dipping well in the 60s in the Salt Lake Valley, which was a nice respite from 70+ F lows observed during July and August. There were also a few scattered showers and storms across the state, with mountainous areas of southern and central Utah picking up a bit of liquid (figure left/top). A few storms even made there way into SLC, but with not much measurable precipitation.

By this time of year, the sun angle is decreasing at a significant rate with solar heating following suite. This means that with every bit of cooling it is increasingly more difficult for temperatures to warm back to previous level. With conditions expected to change going this weekend, we are one step closer to consistent "window weather".

At the moment, a slow moving closed upper level low is spinning westward into the west coast of the CONUS. Utah now sits to the east of the trough axis (figure below/right), with increasing SW upper level winds and some high clouds associated with mid to upper level moisture.

This low will progress across Utah in the next few days, bringing some gusty southerly winds Wednesday and the opportunity for a few scattered showers and thunderstorms through Saturday (figure below). There will be some upper level lift associated with a jet streak passing over the intermountain region, but in large the energy and moisture from this system is expected be be largely broad and unfocused. This means much of the precipitation development will likely be aided by topography or daytime heating. The forecast ECMWF Sat 00Z forecast period (6pm MDT) is shown below for upper and mid levels.   

500 mb heights, winds, and temp 9/9

GOES 18 true color satellite imagery Sept. 9 

Forecasted Sat 00Z 500 mb heights, winds, and vorticity (left) and 700 mb heights, winds, and moisture

Temperatures will likely top out by Wednesday afternoon before cooler air, associated with the upper level low, moves in and knocks temps down 10-15 F (figure right). More importantly, overnight lows look to drop down into the 50s, which means shutting off the AC and opening the windows overnight. 

How long will this spurt of milder weather last? Probably at least through the weekend. There are hints of some marginally active conditions the third week of September, but that period is still out in forecasting fantasy land and nothing could be said with confidence.

Left: Forecasted 6hr precip at 00Z 9/13; Right: High/Low temps at KSLC 

Summer 2025 Postmortem

Mean 500 mb geopotential height July, 2025

It's been a long and abnormally dry summer across Utah and the West. There have been a number of light wildfire smoke days, but nothing like the fire and brimstone smoke season of 2021. The monsoon was largely MIA in Utah this year due to an unfavorable pattern (right figure). If you compare the average pattern this summer to different monsoon regimes, you can see that the summer 2025 upper levels match most closely to analog years where conditions are more favorable for the monsoon to focus over eastern areas of the Desert Southwest (figure below/top).

Typically, we want to see the broad Four Corners high establish itself and a heat low form over the Mohave Desert to facilitate moisture transport northward from the Sea of Cortez/Baja area into Utah (figure below/bottom). In general, this setup did not develop favorably for a good Utah monsoon year. Some could argue other regions of the west, including NM and far southeastern areas of AZ, got a decent monsoon but in large it was meager pickings across the west.

Less ideal 500 mb height pattern for a good Utah monsoon

More ideal July 500 mb height pattern for a good Utah monsoon

Depending on which angle you view it from, records were set in Salt Lake City for having nearly the driest summer season on the books. June-mid August only totaled 0.36" of rainfall at KSLC when on average those month receive a combined 1.44". We had some weak monsoonal pushes in late August that brought three days of measurable rainfall totaling 0.76" for the month, which is surprisingly above the monthly normal of 0.58". Unfortunately though, Salt Lake Valley is still about 5" liquid below where we should this time of year, sitting at an accumulated 10.35" for the year. To put it in perspective, SLC gets about 16" of water a year, so 5" is nearly a third of the normal yearly precipitation.

The water year ends at the end of September, meaning there is no real chance for recovering 5" of liquid to come out near normal. If you look at the chart above, the whitespace between the black and blue line represents the difference between observed precip and normal precip. The whitespace gap has grown substantially over the past few months. For a bit of reference, SLC had a accumulated a little over 16" for the water year at this time last year. In summary, 2025 has been a dry summer.

PRISM temp anomalies Jun-Aug 2025

While temperatures weren't exceptionally hot this summer, compared to the past few summers, the average high temperature for June-August 2025 was 91.8 F. The normal average high for June-August is 90 F, so contrary to anecdotal feelings of how hot or not hot this summer was, this summer was in fact warmer than normal. A total of seven days of temperatures >= 100 F were observed in Salt Lake City. It's always nice to know how these numbers fit into the broader picture, so for comparison the summer of 2024 saw 20 day >= 100 F and had an average temperature of 93.3 F. However, 2024 summer was wetter. 

I have heard many water cooler conversations mention that the 2025 summer was "not that hot", however, I think this exhibits how weather and temperature can be highly relative and subjective. In a large part, we have gotten used to extreme heat as the new norm and therefore when people don't experience that extreme heat they qualify conditions as cooler when in fact conditions are still abnormally warm. The table below outlines some simple stats characterizing this past summer. In simplistic terms, it was a significantly dry and slightly hotter than normal summer. 

I can't extrapolate how others "feel" about this summer, but for me the heat was mostly bearable but the addition of the extreme drought really exacerbated the unpleasantness. If I had to rank this summer out of 10, I would give it a 5.5. Below I have shown my rankings for the past six summers. I mainly take into account temperature, number of wildfire smoke days, and amount of precipitation in my ranking logic. Being originally from the east, I appreciate a certain amount of rainfall and cloud cover. Now that the summer is finished, all I can hope for is a nice transition into fall, with a mix of some wet day and bright, clear, and mild days.