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Impacts of Tropical Storm Rachel

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Tropical Storm Rachel is currently well off the coast of Baja California, but it and its remnants are expected to affect the southwest United States this weekend and Monday.  The latest track and intensity forecast shows Rachel moving on a northeastward path, ultimately making landfall somewhere on the northwest coast of Baja or the California Coast near San Diego as a Tropical Storm.  By 5 PM Sunday, Rachel is forecast to be a post-tropical cyclone as it moves into Utah.   Source: NHC A post-tropical cyclone has lost its tropical characteristics, including its warm core and symmetrical storm structure, but maintains a low-level circulation and in some cases evolves into a mid-latitude frontal cyclone through a process known as extratropical transition .  In this case, Rachel is forecast to maintain a low-level circulation all the way to Utah because it is gaining the assistance of a strong upper-level trough that is digging down the Pacific Coast, as evident i...
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Rogue Shower, October Storm

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Yesterday (Tuesday) morning, the Salt Lake City International Airport observing site, which is located at the south boundary of the airport just north of I-80, observed as rogue of a rogue shower as you will ever see.  The storm produced rain at the observing site from about 0725–0755 MDT, generating 0.16" according to the rain gauge.   KSLC Time Series 0725–0755 MDT 6 Oct 2026. Source: NWS. The radar at 1317 UTC (0717 MDT) showed the rogue cell moving over the airport as it intensified.  The cell was perhaps 5 km across and developed pretty much over or just upstream of the southeast corner of the airport.     The weather cam image at the top of this post, taken looking west from the University of Utah, shows just the narrow area of falling precipitation associated with the cell.  What conspired to make such an isolated precipitation shaft is known only to Mother Nature.   Radar-estimated precipitation from the National Severe Storms La...
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Provocative Public Lectures

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There are three provocative public lectures coming up this month at the University of Utah.   Two are this week, are hosted by The Wilkes Center for Climate Science and Policy, and will be given by Myles Allen, who is visiting from the University of Oxford.  The first explores the role of climate change in extreme weather events.   The second explores what it would take to stop global warming.   Each of these is being given in the L.S. Skaggs Applies Science Building, which is a new building (and my home) just off of University Street.  I don't think it shows up on Google Maps yet, but it is immediately west (and connected to) the Stewart Building (see  https://map.utah.edu/?buildingnumber=18 ). Do not confuse it with the Skaggs Biology Building which is a bit farther to the east.   The third talk is an evening Frontiers of Science lecture being given by Nobel Laureate Martin Chalfie on October 22.  Parking and RSVP information...
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How to Break the Jet Stream

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Two very powerful cyclones, one from the tropics and one from the mid to high latitudes, are going to conspire over the next several days to break the jet stream.  The tropical cyclone is Typhoon Choi-Wan, currently southeast of Japan with maximum sustained winds of 110 knots.  Source: Joint Typhoon Warning Center The mid to high latitude cyclone hasn't formed yet, but is expected to develop over the Sea of Okhotsk on Wednesday.  The ECMWF HRES forecast below from Tropical Tidbits is valid 1800 UTC Wed Oct 7 and includes sea level pressure contours (black lines) and 500-mb height (color fill).  The big circle identifies the mid to high latitude system, the little circle Choi-Wan, which at this point is transitioning into an extratropical system, and the black line the upper level ridge downstream of these two systems.   Source: Tropical Tidbits By 1800 UTC Saturday 10 October, those two cyclones have merged into a massive 976 mb low over the Bering Sea....
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Alexis Kelner 1938–2026

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I never met Alexis Kelner, but I was saddened to see that he passed on September 23rd at the age of 87.   If there was a Mount Rushmore of Wasatch Mountain backcountry skiers, mountaineers, historians, and conservationists, Kelner would surely be on it.  His obituary summarizes many but surely not all of his contributions.   I first encountered his name as one of the authors of the Wasatch Tours series, which was essential reading when I moved to Utah in the 1990s.  By that time, the series, co-authored with another Utah ski legend Dave Hanscom, had expanded to three volumes.  The original, published in 1976, was the first backcountry ski guidebook for the Wasatch.   For the youngsters out there, there were no cell phones or backcountry ski apps.  There was no Utah Avalanche Center (founded in 1980).  Avalanche transceivers were in their infancy and not available for Kelner's early ski touring adventures.  Skis were skinny and...
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The Updated weather.utah.edu

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Updates to weather.utah.edu went in today.  In addition to being on a new server, the site has a new look and feel matching current University of Utah branding and has better fonts, more descriptive alternative text, keyboard navigation, etc.  Bookmarks from the old site have a good chance of working, but if they are broke, you may need to update them.   Much thanks to our partners at the University of Utah Center for High Performance Computing (CHPC) for making this possible.  Thanks as well to data and software providers (NCEP, ECMWF, Penn State, Iowa State, AWS, Unidata), not to mention developers of the many python packages we use, students and other collaborators who have helped with code development, and OpenAI Codex which I've been using for code updates.  It takes a village to build a site like this.   We hope to eventually have a 1-2 month archive available through the "View a Different Run" option at the top for several models.  Rig...
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A Pattern Change to Start October

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The models are advertising a significant pattern change to start October, one that will curl the smiles of snow-safety teams in the Wasatch Range.   Over the past several weeks, the large-scale pattern has been characterized by persistent troughing along the Pacific coast and ridging over the south central United States, as shown by the mean 30-day 500-mb heights for the period from 31 August to September 29.  Source: PSD Maproom This general pattern is consistent with the North American Monsoon, but these two major circulation features were stronger than on average.  In other words, the Pacific trough was deeper and the ridge over the south central United States were stronger than observed.  This "opened the door" for frequent monsoon surges into northern Utah and occasional severe convective storms due to the stronger flow and wind shear between the two circulation features.   The pattern is changing, however, for October.  The ECMWF AIFS analys...
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