An Uncertain Sunday

Forecast uncertainty doesn't always grow continuously with forecast lead time.  Sometimes there is an issue with the intensity and timing of a weather system that can cause uncertainty to increase temporarily and then decrease at longer lead times.

A good example is the forecast for northern Utah this weekend.   The models all bring in a strong cold front, but there are variations in the timing and intensity.

The GFS forecast valid 1200 UTC 13 September (0600 MDT Sunday) shows one scenario with the cold front through northern Utah, a deep trough at 500 mb centered over the central Idaho Mountains, and cold post-frontal flow over northern Utah.  700-mb temperatures in far northwest Utah are below -2°C.  

The ECMWF AIFS has different ideas.  The front is much slower and for the same time valid above (0600 MDT Sunday), the front is just getting to Salt Lake City and the upper-level trough is centered over northeast Oregon.  The 700-mb temperatures in northwest Utah are around +6°C, 8°C warmer than the GFS.  

Put these and a gazillion other forecasts into the National Weather Service blend of models and you get a lot of forecast spread for temperatures on Sunday.  Note how the spread of the box-and whisker plot of forecast maximum temperatures (red box-and-whisker with annotated temperatures) gets really large on Sunday.  The middle 50% of the forecasts like within the colored red box which has a range from 68° to 84°F!  

The middle 80% of the forecasts lie between the ends of the whiskers (the white lines that extend from the red box).  The numbers aren't annotated, but the range is between 64°F and 93°F.  And to think your TV meteorologist will have to pick one number to put in their forecast for Sunday afternoon!

Max temperatures on Sunday are as much of a crap shoot as you can imagine and a prime reason not to look at only one model.  This is a classic case of what meteorologists call anticyclonic Rossby wave breaking over the north Pacific and western North America, which introduces uncertainty in the strength and movement of the upper-level trough as it amplifies along the Pacific coast.  I'd check the forecasts in a couple of days if you are planning a weekend backpack to high elevations.  

Flies in the Labor Day Ointment

Weather for the start of the Labor Day weekend today (Friday) looks pretty good.  The large-scale pattern hasn't really changed dramatically for some time.  There's a trough centered over the Pacific Northwest coast and a ridge over the central US.  These features have been around for some time, shifting in strength and position.  Right now, the southwesterly to southerly flow between these features is relatively dry, so shower and thunderstorm activity is suppressed or limited.  Today should be quite nice across most of Utah other than some gusty winds, as indicated by the ECMWF HRES forecast for 0000 UTC 5 September (1800 MDT Friday).  

However, lurking in that forecast is Hurricane Marie off the coast of Baja (Hurricane Karina is also visible in the plot farther to the west).  Marie is expected to move northwestward over the next few days, and therein likes the forecast problem.  Although Marie will weaken, her moisture is going to move into the southwesterly flow ahead of the upper level trough, leading to another surge of moisture into Utah.  The ECMWF HRES forecast for 2100 UTC 6 September (1500 MDT Sunday) shows this well.  Marie (yellow L) has drifted ever so slightly northwestward and with high relative humidity air and strong vapor transport extending to her northeast and into Utah (red arrow).  

Right now Saturday looks good, but the specter or showers and thunderstorms will be on the increase on Sunday as this moisture surge moves into Utah beginning in the St. George area Sunday Morning but then expanding northward.

Although we developed the Utah Snow Ensemble for powder, it does include downscale precipitation forecasts.  Below is the forecast of 6-h accumulated precipitation for Sunday afternoon [1800 UTC 6 Sep to 0000 UTC 7 Sep (1200-1800 MDT Sunday)].  The upper left is the ECMWF HRES showing a band of precipitation extending across central Utah from St. George to the Uintas (upper left), although there is also some precipitation in other areas.  The ensemble mean also keys in on that area (upper right), again with some members producing precipitation in other areas.  

We'll see where this surge ends up along with its associated precipitation, thunder, and potentially flash floods.  Best to check forecasts carefully if you will be recreating on Sunday and Monday.  

Summer 2026 Is in the Books

Meteorological summer (June, July, and August) is now in the books and as usual it is great to have it in the rear view mirror.  

Summer was a scorcher by long-term standards at the Salt Lake City International Airport.  The average temperature was 80.2°F, the 7th highest all time (the record is 81.4°F set in 2022).  A look the long term trends in summer temperatures shows we are no-longer operating in the climate that prevailed during the 20th century.  The 10 hottest summers on record were all in the 21st century when the trend is clearly upward.  

Mean Average Temperature June-August at the Salt Lake City International Airport.  Source: https://xmacis.rcc-acis.org/.

We also set an all time maximum temperature record of 109°F on July 12th, tied the all time high minimum temperature record of 82°F on July 27 (set previously in 2021 and 2023), and saw 20 days with a maximum temperature of 100°F or higher (the record for summer is 27 in 2022 and for the year is 34 also in 2022).  

Precipitation at the airport provides a good example of the devil being in the details.  For the whole summer, the airport recorded 2.63" of precipitation, which is above average (2.02"), but June was well below average (.17" vs. .95), July just a shade above average (.62" vs. .49"), and no measurable rain fell the first 12 days of August.  Thus the summer featured a precipitation deficit through about mid August, when we got into a wetter pattern.  Percent of normal precipitation over northern Utah shows many areas above 300% of normal, although there are a few bands below average.  Many of our storms in the latter half of August moved from southwest to northeast across the region, and this is quite apparent in the map below.  

For me, the summer of 2026 was one in which I fully experienced the ugly face of global warming.  I was in Europe until July 8, experiencing the early end of snow cover in the Alps and the June heat wave.  Then I came home for our hot summer and the 109°F all-time high at the airport.  

I miss the climate of the 20th century.  I suspect I'll miss it more in the future. 

Thunderstruck!

It sure can get exciting around here when monsoon (and tropical storm remnants) can interact with a mid-latitude trough.  This morning in the Salt Lake Valley has been incredible with round after round of heavy rain and thunder.  These are not those high-based "virga storms" that we sometimes get around here, but the real deal with pretty good downpours and loud, crashing thunder.  Love it.

We discussed the ingredients for today's rain in the post from a couple of days ago entitled An Unusual Moisture Pathway.  Those ingredients included the remnants of tropical storm Iselle and a front that was expected to develop over central Nevada.  Indeed these ingredients came together ahead of a deepening trough along the Pacific Northwest coast.  The front is especially apparent at 700 mb (lower left panel below) where there is temperature contrast and wind shift between southern Utah and northern Nevada and Utah.  The large-scale wind pattern is one that meteorologists call confluent and in this case the confluence and temperature contrast are setup to favor precipitation development along the front.  

MRMS precipitation estimates from 0600–1800 UTC (0000–1200 MDT) shows a band of accumulated precipitation aligned along the front with embedded bands of heavier precipitation associated with thunderstorms that moved along the front and from southwest to northeast across northern Utah.  The switch from green to yellow shading occurs at 0.5".  A good fraction fo the Salt Lake Valley saw at least that much.  There are a few areas above 1.0" including portions of the Avenues. 

Source; https://mrms.nssl.noaa.gov/qvs/product_viewer/

Those numbers may be a bit high compared to gauges, but a neighbor reported 0.69" and our lower University of Utah measurement stations are at 0.57" and 0.60".  

It's not over yet.  Showers and thunderstorms remain possible through at least tonight.  Some of those storms could be severe (e.g., wind gusts up to 70 mph, hail > 1 inch, heavy rain, lightning) as indicated by the National Weather Service Storm Prediction Center and Salt Lake City Forecast office.  

Source: NWS.  Downloaded ~1245 MDT 29 August 2026

Snow!

I'll just put these here for some high stoke, low probability snow possibilities.  There are some ensemble members producing snow in northern Utah late next week and weekend.    

OK, truth be told it's three ensemble members out of 82.  

I told you it was a low probability possibility.

An Unusual Moisture Pathway

The remnants of Tropical Storm Iselle are taking an unusual pathway into the western US interior and will be visiting northern Utah this weekend.  

Yesterday morning (1200 UTC 26 August), Iselle's remnants were to the west of the Baja Peninsula.  The color contours below are precipitable water in millimeters with warmer colors indicating higher values.  Precipitable water is a measure of the total water vapor in the atmosphere expressed as the depth of water if all of that water vapor were to condense and fall out.  Iselle's remnants were associated with a maximum in precipitable water above 65 mm.  

Iselle's remnants moved northwestward yesterday and last night.  As this happened, they began to interact with the southwest flow ahead of an upper-level trough off the Pacific Coast, resulting in a plume of high precipitable water movign northeastward toward California. 

That plume spreads eastward and ultimately serves as the locus for precipitation development over Nevada tomorrow afternoon (0000 UTC 29 August).  A relatively strong front also forms in that area during the day tomorrow due in part to daytime surface heating in southern Nevada, which is more cloud free, and northern Nevada which is cloudier and experiencing cooling beneath the precipitation.  This leads to a 700-mb temperature difference across Nevada of almost 10°C at 700 mb.  

That system then slides into northern Utah Friday night and Saturday.  At 1800 UTC 29 August (1200 MDT Saturday), the front is just to the south and east of the Salt Lake Valley with the GFS producing scattered precipitation (probably showers and thunderstorms) over northern Utah.  

A look at the ECMWF AIFS forecast shows it is much more bullish on precipitation Saturday morning.  The plot below (upper right) is a 6-hour accumulation whereas the one above for the GFS is a 3-hour accumulation, but still, the precipitation is clearly heavier and more widespread.  

Neither of these models is able to resolve the details of convective storms, so for now I have a wait and see attitude about how this will turn out.  Saturday though does look cooler and we can all be thankful for that.