Daniel Swain: Another prolonged heatwave for southern & interior CA; hottest July on record statewide?

July 23, 2018 – One of the most impressive heatwaves in recorded history brought truly blistering heat to Southern California earlier this month. Nearly every kind of of temperature record was broken in at least a few locations, from all-time daily maximum to all-time overnight minimums, and a countless daily/monthly records. This heatwave was especially notable at least three distinct ways. First, extreme temperatures made it very close to the coast (for example, UCLA hit 111 degrees–an all-time record in any month). Second, extreme warmth persisted into the nighttime hours: temperatures near 100 degrees occurred after midnight in some places during the peak of the event, with lows failing to drop below 80 degrees at any point for several days in some spots (and setting all-time record overnight temperature records in the process). Finally, the “downsloping” winds that brought extraordinary compressional warming to the coastal plain in SoCal occurred out of season: those conditions are much more common in the autumn, when the overall airmass is somewhat cooler. The 120 degree reading in Chino, CA (about 20 miles east of Los Angeles) during this event was not only an all-time record at that site, but most likely the highest verified temperature ever recorded within 100 miles of the Southern California coast.

Interestingly, Northern California has generally not experienced the same kind of searing heat that has occurred in SoCal. While temperatures have generally been running well above average even up north (with the exception of portions of the SF Bay Area, where a persistent marine layer has acted as a natural air conditioner much of the time), there have not been a great number of extremely hot days so far this season. This type of subtle warmth is perhaps best reflected just across the state line in Reno, Nevada–which is presently on track to experience its warmest July on record despite not having experienced any daily records for the entire month to date! In fact, it is possible that California may experience its warmest July on record on a statewide basis, bolstered by both the extreme heat in the south and the more subtle but still anomalous warmth up north.

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