Recent West Coast heat wave was perhaps most remarkable because it happened in June

Temperatures surged to record levels earlier this week in the West, obliterating daily records in some places. For me, the most notable thing about the heat wave was not necessarily how hot it got, but that it happened in June.

The mercury hit 100 degrees at San Francisco International Airport on Monday (97 downtown), which was the first time the airport had reached triple digits outside of the month of September.

For those of you that don’t live on the West Coast, this may seem strange since many parts of the country see their hottest days in July and August, not September. The West Coast is sort of on an island when it comes to the hottest time of year.

Take a look at this map from the NCDC (National Climatic Data Center), which shows the warmest day (using the average temperature) of the year. The West has some notable differences from the rest of the country.

The West Coast is a prime example of that, as there is a thin sliver right along the coast that sees the warmest weather (on average) in September. There is one reason for that - the Pacific Ocean.

The cold water in the Pacific and the marine layer that results from it (the “June Gloom”) typically help to hold down temperatures in coastal areas.

The water is difficult to warm in the Pacific because of the clockwise flow of the North Pacific Gyre in the central Pacific. The flow brings the cold waters of the North Pacific southeastward along the U.S. West Coast, which makes it far more difficult to warm the waters compared to similar latitudes along the East Coast.

This chilly water of the Pacific allow places like San Francisco and San Diego to have very agreeable climates… as long as the wind is blowing in off the water.

By September, the water is somewhat warmer, which allows warmer days to happen more easily with less of a marine layer.

Also heading into the fall, the weather pattern typically becomes more amplified. Large ridges can develop, and any high pressure across the interior West can kick up the offshore winds, which can bring surges of heat all the way to the normally cooler coastal areas.

Santa Ana wind events also start becoming more common in September and through the autumn months, which is why the Los Angeles area can have triple-digit heat well into October.

As we learned this week, though, it can get pretty hot even though it's only June.

Early this week, a large ridge developed over the Western U.S., and that allowed an offshore flow to develop in much of the lower atmosphere. This ultimately kept the marine layer at bay and allowed the sun to do its work, while pulling in some of that hotter air from inland areas.

The result was a very hot couple of days across much of the West Coast. Temperatures were near 100 degrees in the San Francisco area on Monday, while in Portland (98) and Seattle (95), temperatures soared to record levels on Wednesday.

The exception was the Southern California coast, where LAX didn’t get any higher than 80 degrees and San Diego remained in the 70s. There was still enough of an onshore flow to keep these areas lower.

Areas just inland from the coast, though, did get in on the heat. Ontario, California, which is about 40 miles to the east of LAX, topped 105 degrees for three straight days, including a high of 109 on Tuesday. Even Downtown Los Angeles hit 90 degrees both Monday and Tuesday.

Near the coast, daily records were falling by 5-15 degrees. Quillayute, Washington, and Monterey, California, both broke their daily records by 14 degrees on Tuesday, and Sea-Tac Airport near Seattle beat its record by 8 degrees on Wednesday.

Many cities tied or broke monthly records as well. It just goes to show how impressive this setup was for June, and how difficult it can be to overcome the effects of the cold Pacific in coastal cities.

What may also catch your eye on the map of warmest days is the orange and red area in Arizona, New Mexico and West Texas, which indicates that the hottest day of the year is in June or the first half of July.

Take Phoenix in 2017, where a stretch from June 17 through July 8 netted 18 days with high temperatures at or above 110 degrees, which included a temperature of 119 degrees.

After that stretch, the temperature reached 110 degrees only seven more times over the rest of the summer.

The reason for the heat peaking so early in this area is the monsoon season that develops over the course of the summer. The sun is at its strongest in June and early July, which promotes some very hot days. When the monsoon kicks up, there are more clouds and thunderstorms, which can prevent maximum heating from happening.

Also when the monsoon kicks up, there’s more humidity in the air (so no, it’s not always a dry heat in Las Vegas and Phoenix), which also plays a role. The sun is more efficient at heating drier air versus moist air, which can make it more difficult to get those super hot days later in the season during the monsoon.

Wildfire Threat Update:
The main threat in the short term will be across eastern Arizona and western New Mexico on Friday due to gusty winds. This threat will lessen into the weekend.

The Pacific Northwest will need to be watched later next week as a trough dives in. This will likely lead to thunderstorms, and the lightning could start fires, especially east of the Cascades.

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