Many stations in southern California had already topped 100°F as of 1 pm PDT Tuesday, 24 October 2017, as shown in this WU WunderMap. Graphic: Weather Underground

By Bob Henson 
24 October 2017

(Weather Underground) – It’s not every October 23 or 24 that millions of Americans are swathed in temperatures above 100°F. This week has done just that, bringing some of the toastiest weather ever observed in the United States during late October, and more pre-Halloween heat is on the way. By far the most scorching weather has been in Southern California, although it’s also been exceptionally mild this month in settings as far-flung as Michigan, Florida, and New England.

A multi-day summer-like heat wave kicked into high gear on Monday and continued Tuesday along and well inland from the California coast, from Santa Barbara through Los Angeles to San Diego. Dozens of locations record highs for the date and all-time highs for this late in the year, and Santa Ana winds kept the temperatures amazingly warm throughout Monday night. In Orange County, the city of Fullerton soared to 107°F on Monday. According to WU weather historian Christopher Burt, this is likely the hottest single temperature recorded anywhere in the United States so late in the year. Even Death Valley has never recorded a temperature this high after October 16 in any year! For comparison, the national U.S. record high for November is 105°F, most recently at Tustin Irvine Ranch, California, in 1997.

Another impressive mark: downtown Los Angeles (the University of Southern California campus) hit 102°F. Prior to Monday, the downtown station had never topped 100°F after October 17, in records going all the way back to 1877. Incredibly, the USC downtown station got even hotter on Tuesday, reaching 103°F at 1 pm.

Here are some of the many records set on Monday, with the old daily record and year and the year that observations began. Asterisks denote that the high was an all-time record for so late in the year.

CityNew recordOld daily recordYear observations began
LOS ANGELES DOWNTOWN* 102 98 IN 1965 1877
LOS ANGELES INTL AP 101 97 IN 1965 1944
LONG BEACH AP* 105 99 IN 1965 1958
BURBANK AP* 102 98 IN 1965 1939
UCLA* 100 98 IN 1939 1933
SANDBERG (tie) 84 84 IN 1959 1948
CAMARILLO AP* 106 97 IN 2007 1923
OXNARD NWS* 104 96 IN 2007 1923
SANTA MARIA AP* 102 98 IN 1965 1948
RAMONA AP* 101 97 IN 2003 1974
RIVERSIDE* 102 101 IN 1959 1893
PALM SPRINGS (tie) 104 104 IN 2003 1893
SANTA ANA* 102 101 IN 1939 1906
VISTA* 100 99 IN 1965 1957
ALPINE* 99 98 IN 1959 1951
EL CAJON* 104 100 IN 2003 1979
BIG BEAR (tie) 74 74 IN 2003 1960

The heat wave and Santa Ana winds are being caused by a large, near-record-strength dome of high pressure that’s settled in over the Great Basin, a few hundred miles northeast of Los Angeles. The difference in pressure between this high-pressure system and lower pressure over Southern California has driven gusty northeast winds over Southern California. Since these originated over desert areas, they are hot and dry. As the air descends from the mountains to the coast, the air gets even hotter and drier due to adiabatic compression—the process whereby the pressure on a parcel of air increases as it descends, decreasing its volume, and thus increasing its temperature as work is done on it. These downslope winds are why the most impressive records on Monday occurred along and just inland from the coastline.

In locations where the winds kept up on Monday night, there was little relief from the heat. Overnight lows were as high as 90°F (Fillmore, in Ventura County) and 86°F (Van Nuys). Some locations stayed near 90°F all night except for just an hour or two when the winds slackened and the temperatures dropped to around 80°F. Even a weather station at 5655 feet, near the summit of Mt. Wilson, failed to dip below 65°F. [more]

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