October Temperature Extremes in a Climate of Change

Another round of record-setting weather patterns swept across California as September yielded to October, 2024. In page one of this story, we follow these atmospheric anomalies and investigate their causes. Then, in page two, we explore (highlighting a recent National Weather Service workshop) how climate change looms in the background, creating a challenging atmosphere of uncertainty. (In a future story, we will explore Northern California’s November atmospheric river, southern California’s destructive wildfires, and other autumn, 2024 weather whiplashes that will live in the history books.)

Breaking October Records

Autumn is usually the season when dense high pressure builds over cooling land surfaces in the Great Basin. This annual reversal in temperature gradients also helps to reverse the onshore pressure gradients that pushed sea breezes and marine layers inland during the long high sun season. In contrast to summer, autumn brings those offshore winds that howl off the continent and toward the coast, occasionally shoving summer’s moist ocean air masses far out to sea. As the continent cools further, these descending offshore winds are often compressed to produce some of the warmest and driest days of the year for immediate coastal strips from Oregon to Mexico. You will find detailed accounts of these seasonal turnabouts in previous stories (such as our Offshore Autumn) on this website. But in 2024, some unprecedented early autumn glitches developed as September progressed into October.

This 500 mb map shows an unusually deep upper-level low-pressure trough digging south over California. It sent a cold front with high surface winds that churned up cold waters from the depths all the way along the Southern California coast. The early low pressure would eventually be followed by strong high pressure that orchestrated record-breaking heat waves across the state well into October. Source: National Centers for Environmental Prediction, Weather Prediction Center. 

The first surprise was an early season Pacific Northwest storm that dragged a dry cold front down the coast in mid-September. High winds following that system churned up coastal currents, quickly dropping Golden State water temperatures by several degrees. For instance, Santa Monica Bay water temperatures that had just spiked near 72° F plummeted at least 7 degrees, down below 65° in just two days. Water temperatures wouldn’t recover, forcing an early end to my comfortable summer swims without a wetsuit. A premature Southern California coastal chill was suddenly noticeable in the daily sea breezes, which often ebbed below their dewpoints, causing the air to remain saturated for longer periods spanning several straight days. (Some NWS forecasters have postulated that the threshold water temperature, helping to determine whether or not summer marine layers are saturated with fog and low clouds as they drift onshore, is around 68°F.) Along the Southern California Bight, stubborn fog and low clouds condensed over the cold water and hugged the immediate coast into October as if May gray or June gloom conditions forgot to check the calendar, while the expected seasonal offshore wind trends were not strong enough to shove the shallow misty muck all the way out to sea. 

Dried yucca stalks pepper the chaparral and coastal sage scrub during autumn. They were blooming through the traditional May gray and June gloom seasons. This one looks down toward the ocean from the Santa Monica Mountains, into the strip of coastal fog that is more typical of late spring and early summer. Descending air masses out of upper-level high pressure clamped a lid on the shallow fog, but weren’t strong enough to push the marine layer out to sea.     
As inland regions (behind us) cook in record heat, the shallow, cold fog is trapped below descending air from strong high pressure. Surface air remains saturated above ocean temperatures that prematurely plummeted in mid-September, resulting in these scenes more typical of May and June along the immediate coastal strips. The cascading fog can only attempt to penetrate farther inland. 

Just several miles inland, and throughout parts of Northern and Central California, it was a radically different weather story. Some of the strongest high-pressure domes on record for the dates anchored directly over the state and into other southwestern states. As the calendar flipped from September to October, a prolonged and dangerous heat wave intensified. Air columns were compressed and superheated to break numerous daily records until several inland stations recorded their all-time highest temperatures for the month of October. As is often the case in autumn, the offshore trends occasionally swept all the way to the beaches in parts of Northern and Central California. Intense heating of the brilliantly clear dry air pushed temperatures over 90° F in San Francisco and 100° F slightly farther inland, from Santa Rosa to San Jose. Exemplifying those rare warm San Franciscan nights, surrounding Bay Area hills recorded overnight low temperatures well over 80° F beneath the descending air columns. More high temperature records were broken in the Central Valley. Back down in Southern California, the stage was set with conditions that produced some of the most extreme temperature gradients (across such short distances) on the planet.    

This 500mb map shows a massive, powerful dome of upper-level high pressure over California and the southwest on October 1. Descending air out of this dominant system was responsible for consecutive days of record high temperatures. Source: National Centers for Environmental Prediction, Weather Prediction Center.
By October 2, surface high pressure continued to dominate across southwestern states, supported by that upper-level high illustrated on the previous map. Thermal lows were locally forming in the super-heated air near the surface. Source: National Centers for Environmental Prediction, Weather Prediction Center.
By October 3, a sprawling upper-level high continued to dominate across California and the southwest. Descending air masses baked inland regions, extending a brutal summer well into October. Source: National Centers for Environmental Prediction, Weather Prediction Center.

Epic Southern California Temperature Gradients

As high pressure strengthened directly overhead, numerous daily records (and some all-time October records) were also broken in Southern California’s deserts and inland valleys (see the list near the end of this story). Indio and Palm Springs made it to 117° F, their hottest temperatures ever recorded for October. On October 6, it got up to 116° in Indio, which is the hottest it’s ever been so late in the year anywhere in California. Exceptional high-desert temperatures repeatedly exceeded 100° F in early October and inland valleys were even hotter. Paso Robles hit 107° and Woodland Hills (in the western San Fernando Valley) topped out at 113° F. And in contrast to the annual cooler Great Basin High common to autumn, these hot high-pressure domes formed right overhead. So, instead of generating strong offshore winds typical of October, they compressed gentler breezes that couldn’t make it to the surface along the immediate Southern California coast.

This resulted in unseasonal temperature gradients that would have been remarkable even for June and July. For instance, on the afternoon of October 1, it was 67° F in Oceanside when it was 117° in Palm Springs, less than 70 straight line miles and a few mountain barriers inland. On October 2, when it was 71° F along PCH in Malibu, it was a sizzling 111° in Woodland Hills, less than 10 miles and a small mountain range away. Imagine such a temperature gradient of more than 4 degrees/mile! Folks who headed to the beach to escape record inland heat were astounded when they were met with a wall of shallow cold fog that drifted along the beaches and only several blocks inland during the day, and then crept only a few miles farther inland at night: June- and July-style heat waves (as featured in an earlier website story) trending into October. By late October, inland temperatures were mercifully moderating, though persistent high pressure continued to clamp a lid on the stubborn shallow marine layer that was attempting to creep in below it. By the time the Dodgers were beating the Yankees in the first two games of the World Series, a packed Dodger Stadium crowd had a clear view looking inland toward the warm mountains, while a distant hazy marine layer loomed toward the coast. It was the kind of perfect Southern California baseball weather that can make national audiences envious, another interesting topic in my California Sky Watcher book.    

On this October 3 satellite image, notice how seasonal offshore flow was plenty strong enough to push the marine layer out to sea in Northern California, so that hot and dry conditions extended all the way down the Central Coast. But the land breezes weren’t strong enough to scour out the shallow fog and low clouds that hovered at the surface over unseasonably cold waters in the Southern California Bight … all the way down the Baja coast. An eddy of shallow cold fog enshrouded Southern California beaches while inland locations continued setting high temperature records. Source: NOAA/National Weather Service.
By October 5, a surge of misty cold marine layer fog and low clouds crept north up the Central Coast, more reminiscent of May gray and June gloom. But the string of high temperature records continued inland across the southwest states, as they were stuck below that high pressure dome. Source: NOAA/National Weather Service. 
On October 6, Huntington Beach was smothered in the shallow coastal fog while inland valleys and deserts continued to sizzle. This disappointed crowd came to observe the annual Pacific Airshow, which is almost guaranteed clear and sunny afternoons this time of year, after summer marine layers have burned off and before winter’s rainy season turbulence. October 2024 was a strange and chilly exception to the seasonal rule at the storied pier.     
Most visitors from afar had escaped record inland heat to see the Pacific Airshow in Huntington Beach in 2024. Instead of normally sunny early October afternoons, they were surprised to meet the enshrouding fog here at Huntington Beach Pier. There were no refunds for those who invested in tickets ahead of time to get their close-up narrations. The event evolved into an unseasonably awkward no-fly zone no show.

Though every heat wave has its own personality, these recent record busters followed a trend that has become all too familiar in California and the Southwest U.S.: while all weather stations are showing signs of warming, inland locations are heating up faster. This is likely due to the modifying effects of ocean breezes that dominate our coastal regions but don’t penetrate far enough inland, resulting in slower warming along the immediate coast and enhanced temperature gradients toward super-heated inland regions. And this leads us into Part Two of this heated discussion. If you are interested in what lurks behind all the wild weather whiplash and record temperature extremes that are threatening and remaking our Golden State, click on to Page Two of this story. You might first want to surf down the list (below) of some remarkably high temperatures recorded during our historic 2024 October heatwave and then see what weather patterns finally broke the heat spell.                 

Afternoon of Oct 1:
Oceanside: 67°F versus 117°F less than 70 miles east in Palm Springs (record for October).

Afternoon of Oct 2:
Topanga/Malibu coast: 71°F versus 111°F less than 10 miles inland in Woodland Hills (record).

Here are examples of early October, 2024 high temperatures (in F degrees) recorded at selected weather stations in California, with some comparisons to previous records:

  • Campo: 105 degrees.
  • Hanford: 100, breaking the prior record of 98.
  • Idyllwild: 98, breaking the prior record of 93.
  • Indio: 117, breaking the prior record of 111.
  • Kentfield: 100, breaking the prior record of 97.
  • Lake Cuyamaca: 94, breaking the prior record of 89.
  • Lancaster: 103, breaking the prior record of 100.
  • Madera, 100, breaking the prior record of 99.
  • Palmdale Airport: 104, breaking the prior record of 100.
  • Palm Springs: 117, highest ever for October (116 in 1980).
  • Palomar Mountain: 93.
  • Paso Robles Airport: 107, breaking the prior record of 106.
  • San Jacinto: 106, breaking the prior record of 105.
  • San Rafael: 105, breaking the prior record of 104.
  • San Jose: 100, breaking the prior record of 97.
  • Sandberg: 95, breaking the prior record of 92.
  • Stockton Airport: 101.
  • Woodland Hills: 113, breaking the prior record of 110.

Before clicking on to Page 2 to review climate change trends (from that National Weather Service workshop), you might check out the following weather patterns that finally broke through the high-pressure domes and heat waves as we made our way through October, 2024.

Breaking the Heat Wave

Even the most stubborn and historic weather patterns must come to an end, and so it was for the unprecedented summer that bled into October 2024. The following images illustrate how California weather finally shifted to what we might expect in autumn.

This October 16, 2024 water vapor image shows an early-season low pressure system finally moving through Northern California and just clipping Southern California. High pressure domes have been shoved aside, marking an end to the historic prolonged California and southwestern states heat waves of 2024. Source: NOAA/National Weather Service.
The October 16 visible image reveals a weak storm spinning across Northern California and a very thick marine layer pushing into Southern California’s inland valleys. Record high pressure and heat retreated into the rear-view mirror. Source: NOAA/National Weather Service.
By October 18, a deep upper-level cutoff low/inside slider (as shown on this 500mb map) had dropped just east of California. This upper-level pattern may be more characteristic of autumn and is diametrically opposite from the anomalous high pressure and inland heat earlier in the month. Also note how such amplified upper-level waves often produce contrasting weather conditions between eastern versus western sides of the continent. Source: National Centers for Environmental Prediction, Weather Prediction Center.
By October 27, remnant moisture (seen in this water vapor image) from a dissipated tropical storm was being pulled from southwest to northeast into California. This is a familiar pattern during this time of year, when the warm-season’s last Pacific tropical storms might interact with low pressure troughs that begin to drop south, circulating the moisture ahead, and announcing that winter’s rainy season is not far away. Source: NOAA/National Weather Service.  
This October 29, 2024 surface map shows a weak low pressure and frontal system drifting east after sweeping through California. The big highs and heat waves are memories. Source: National Centers for Environmental Prediction, Weather Prediction Center.
Here is what remains from a dead tropical storm on Oct 27. Various forms of cirrus, altocumulus, and altostratus clouds decorated the skies as they were drawn across the Pacific and onshore by low pressure sagging down from our north. They were a welcome sight in a state seared by those summer heat waves and plagued by SoCal coastal fog that dragged on through early October. But an unstable trigger in the upper levels never got far enough south to wring this moisture out. Southern California would remain mostly dry well into December.
Though early rains didn’t make it this far south, the skies over California were looking more changeable by Oct 27. Weak low-pressure troughs and eye-catching cloud formations (such as the high streaks of cirrus and darker altocumulus here) streamed overhead in the cooler air masses, confirming that the big heat domes were memories in the history books.
Natural and cultural signs of the changing seasons. High and middle-level clouds decorated late October skies at the big annual Dia de los Muertos Festival in Downey. Could the spooky Halloween season cloud backgrounds be harbingers of the historic atmospheric river that would hit Northern California in November?  
By October 28, weak low-pressure troughs and weather fronts had swept through the state, leaving stratocumulus to drift up and over the mountains with the cool breezes off the Pacific. What a contrast to the powerful persistent high pressure that dominated earlier in the month!
Another deep, cold upper-level low pressure trough dug east of California and over the southwest states by the end of October. It looked a lot like the system from just 11 days earlier (and what we might expect in October), bringing more cold air down from the north. Inland and desert regions that had baked at the start of the month were now freezing in nature’s autumn refrigerator. Once again, note how high pressure and warm weather dominated eastern states that were stuck beneath a ridge within such high-amplitude upper-level waves. Source: National Centers for Environmental Prediction, Weather Prediction Center.
By October 30, strong middle latitude cyclones, more typical of winter’s stormy season, were already bearing down on the Pacific Northwest. The first storms barely clipped Northern California (notice the cold front just approaching the coast) and weakened as they moved south. They eventually strengthened and inched farther south until a powerful atmospheric river made landfall north of the Bay Area in November. Southern California missed out on all the early storm drama only to suffer devastating wildfires into December. Such extreme precipitation events will be highlighted in our next story on this website. Source: NOAA/National Weather Service.

If you are interested in reviewing some of the trends (with help from a National Weather Service workshop) that might help to explain these autumn records, click on to Page 2 of this story. At the end, you will also find a long list of links that dive into climate change trends in California and how we are measuring and dealing with extreme heat events. You can start with the following image:

Here are historical and projected California temperature trends as shown in PPIC’s Priorities for California’s Water at https://www.ppic.org/publication/priorities-for-californias-water/
SOURCES: Statewide temperature raster data for the period 2024–2100 from downscaled general circulation models (GCM) were published by the Scripps Institution of Oceanography and obtained from Cal-Adapt. Historical observed temperature data for the period 1950–2023 were obtained from the Western Regional Climate Center.
NOTES: Historical observed temperature data are shown for the period 1950–2023. Data derived from the output of downscaled GCMs (e.g., climate model data) are shown for the period 2024–2100. The moderate emissions scenario is RCP4.5 and the high emissions scenario is RCP8.5. These downscaled GCM data were originally published in California’s Fourth Climate Change Assessment as annual maximum and minimum temperature time series. For further details, see the source notes to this brief.

Click to Page 2 for Researching Heat Risks and Heat Wave Trends with the National Weather Service.

   

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