As the calendar flips to October, summer is still raging in the western United States. Parts of the region are experiencing their most extreme heat wave on record for this time of year, with temperatures well into the triple digits.
Phoenix and Las Vegas both posted their hottest temperatures ever recorded so late into September over the weekend, and this heat wave is still in its early days. Phoenix soared to an astonishing 117 degrees Saturday — a temperature so high that it would set records on some days in July and August.
Temperatures are forecast to remain 15 to 25 degrees above normal for days to come, sending highs to at least 100 to 110 degrees in the Desert Southwest. Record-challenging heat will also expand as far west and north as the California Bay Area, where highs are poised to top 90.
Excessive-heat warnings not only cover Phoenix, but also a large chunk of western Arizona and adjacent Southern California and Nevada.
Heat advisories blanket the California coast, from east of San Diego to north of San Francisco. Some locations near the coast will probably witness their hottest day of the year this week.
The National Weather Service’s HeatRisk index is expected to hover at the extreme level in Phoenix and climb to major to extreme levels as far north as the Bay Area and Central Valley in California, while also extending into parts of interior Southern California.
The late-season heat is also boosting fire potential. One subdued blaze east of Los Angeles, the Line Fire, exploded in intensity amid heat and gusty winds Sunday.
How hot it’s already been
Phoenix’s high of 117 degrees Saturday, a record for September, marked its highest temperatures so late in the season by a month — the previous latest high of 117 occurred Aug. 28 last year. It represented a radical outlier from anything previously observed.
The monthly record occurred amid a six-day streak of calendar day record highs that began last week. The ongoing heat wave comes after the city posted a record 113 consecutive days at or above 100 degrees between May and September, shattering the old mark of 76 days in 1993.
In Las Vegas, it hit 104 both Saturday and Sunday — the highest temperatures so late in the season there as well. The previous high mark this late was 103.
Since Tuesday last week, more than 1,000 heat records have been set in the United States, mostly concentrated in the Southwest but extending at times as far away as the Upper Midwest. Temperatures into the 90s even reached the Dakotas.
How hot it’s going to be
At a minimum, dozens of record highs and record warm lows are forecast through the week from California to Colorado.
In Phoenix, the forecast calls for a continuation of record highs every day this week. The city’s October high mark of 107 will probably fall on the month’s first day, when it’s forecast to reach 109.
In Southern and Central California, temperatures will also soar toward record territory. Tuesday and Wednesday could feature widespread highs of 20 to 25 degrees above normal near the coast.
San Francisco and Oakland are both forecast to reach the 90s while inland areas climb above 100. San Jose is forecast to reach record-setting highs of 98 both Tuesday and Wednesday.
In California’s Central Valley, which experienced record-crushing heat in the peak summer months, highs are forecast to reach 100 to 105 through midweek.
The hottest desert areas in southwest California, southern Nevada and Arizona should see highs of 110 to 120.
Here are predicted highs for several locations in the region between Monday and Wednesday:
- San Francisco: Heat advisory through Wednesday. Sunny and hot: 83 (Monday), 92 (Tuesday), 87 (Wednesday).
- Burbank: Excessive heat watch Tuesday through Thursday. Sunny and hot: 95, 97, 100.
- Las Vegas: Excessive heat warning through Tuesday. Sunny and hot: 103, 103, 102.
- Phoenix: Excessive-heat warning through Friday. Sunny and hot: 105, 109, 106.
When will it end?
The expansive and intense heat over Central and Southern California will ease by Thursday or Friday, first relenting along the coast. Even so, temperatures are only forecast to dip from 15 to 25 degrees above normal to about 10 degrees above normal.
The large and intense heat dome over the Southwest that is expanding to the north and west this week should start to retreat some by the weekend. But the excessive heat in Southern Arizona may prove difficult to totally dislodge. The most reliable long-term forecasts suggest warmer to much warmer than normal weather is probable across the western United States through at least midmonth.
The summerlike stretch comes after extraordinary heat this summer in many parts of California and the West that observed their hottest summer on record. Temperatures averaged over the planet have been at or near record levels for the past 15 months as human-caused climate change continues to drive temperatures higher.
Jason Samenow contributed to this report.
