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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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The Updated Wasatch Weather Weenies

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After 16 years, the Wasatch Weather Weenies has a new look and feel as I was getting tired of the increasingly dated look of the old site.  Let me know what you think.  It should also be improved on mobile devices.   The Wasatch Weather Weenies began on October 1, 2010 as an invitation only blog for discussing the weather and climate of the Wasatch Front and Mountains, western United States, and mountainous regions in general.  It started out as invitation only because I didn't want discussions to descend to the levels widely seen in social media.  However, I quickly had so many people asking to read it that I decided to make it publicly viewable and accessible.  Thanks to you, comments have remained mostly substantive and approved for all audiences, and I'm grateful for that.  I'd actually like to see more questions, comments, and suggestions, so feel free to do so liberally.  Total page views for the blog recently eclipsed 10 million, but t...
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First Snow of the Season

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I spent the weekend in Seattle and got to spy my first fresh snow of the season while hiking south of Snoqualmie Pass on Saturday.  The higher elevations of the Alpine Lakes Wilderness, including Chikamin Peak had a fresh coat of white above 6500 feet.   To the northeast, the Stuart Range was also freshly coated.   Winter is coming....
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Monsoon Breaks and Monsoon Surges

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A monsoon is a seasonal shift in the winds that produce pronounced changes in precipitation.  In North America, the North American Monsoon System is a circulation system characterized by mid-and-upper level high pressure centered over the southwest United States that brings easterly flow across much of northern Mexico and showers and thunderstorms to that region and portions of the southwest United States.   Precipitation associated with the North American Monsoon waxes and wanes.  These variations are called monsoon breaks (little to no precipitation) and monsoon surges (more frequent precipitation).   Precipitation associated with the North American Monsoon is heaviest and most frequent in northwest Mexico and decreases to the west and north.  In northwest Mexico, monsoon precipitation is more frequent and may occur daily except during monsoon breaks.   Source: https://www.climate.gov/news-features/blogs/enso/north-american-monsoon In ...
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The Mt. Timpanogos Lightning Fatality

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Friday was a remarkably active thunderstorm day in northern Utah, including a verified tornado in the Wasatch Mountains east of Fruit Heights.   Sadly, a 19 year old man was killed by lightning on the summit of Mt. Timpanogos Friday morning.  Media reports suggest the victim was taking shelter during the storm in the shack on the summit, with authorities called around 6 AM.  Lightningmaps.org provides access to strike data for 24-hour periods.  The map below is for 0600 UTC 18 September to 0600 UTC 19 September (Midnight to Midnight MDT Friday) showing the high density of strikes in the Mt. Timpanogos area and surrounding region.  The prevailing flow was such that the storms on Friday were moving from southwest to northeast and you can see a strong clustering extending from southwest of Utah Lake over Mt. Timpanogos.   Source: https://www.lightningmaps.org/ It's unclear precisely when the lightning strike occurred, but at the time of the rep...
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Northern Utah Mesocyclone

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The NWS issued a tornado warning this afternoon for an area including portions of Weber, Davis, and Morgan Counties. Of concern is one of the best developed mesocyclones (a rotating vortex about 10 km across) I've seen in northern Utah.  Radar reflectivity showed a classic "hook echo" associated with the storm as it impinged on the northern Wasatch near Farmington.  NWS Doppler radars measure the radial velocity toward or away from the radar, so mesocyclones produce a very clear signature of rotation with a couplet of inbound (green and blue) and outbound (red) velocities colocated with the hook.  This is indicative of the small scale rotation associated with the storm.  The mesoscale rotation does not necessarily mean that there is a tornado.  About 25% of mesocyclones produce tornadoes, but that's certainly enough to warrant a warning in this case.   Let's hope this was just a well-developed mesocyclone that didn't produce a tornado.  
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Latest CPC Seasonal Outlooks

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CPC released their latest seasonal outlooks yesterday.  These are based on overlapping 3-month periods.  For November to January and December to February, all or nearly all of Utah is in "equal chances" for precipitation, which means there is no significant shift in the odds for below normal, near normal, or above normal compared to climatology.   Source: https://www.cpc.ncep.noaa.gov/products/predictions/90day/ In other words, that period could go any way.  There's no useful skill.   Later in the winter, during January to March and February to April, most of Utah is in the first level of "above normal" or, in the case of southern Utah, especially later in the winter, in the second level.  For southern Utah, this means there is an increase in the odds of above normal precipitation to 40-50%.  For northern Utah, that shift is much smaller and perhaps to around 35% or so for the Wasatch Range.   Source: https://www.cpc.ncep.noaa....
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