Weather Blog

First 75 degree day of the year to come at California's expense

First 75 degree day of the year to come at California's expense

It's been a strange March (top 10 wettest), a strange April (coldest on record) and a strange May (Top 10 coldest and wettest).

Why should June be any different?

Seattle and the Northwest are finally set to have their first true warm weekend of the year as we head into the first of the other three months when we joke: "Seattle, it rains 9 months of the year here."

Under sunny skies Saturday and partly sunny skies Sunday, high temperatures are expected to climb well into the 70s both days, with even some of the warmer spots out toward the Cascade foothills even getting a decent shot to hit 80 degrees.

It will be the second consecutive year that Seattle had to wait until June to receive it's first day in the mid 70s. Last year, we smashed the record, waiting until June 23rd to hit 75 degrees. This year, it looks like we'll make the list again in fourth place:

1) 2010: June 23
2) 1991: June 9
3) 1955: June 8
4) 2011: June 4
5) 2000: June 3

But if getting not only two consecutive reasonably sunny days to fall on a weekend *and* reach the mid-upper 70s isn't strange enough this year, how about the fact that it's coming at the expense of stormy weather in California?

The reason our area has mild summers is because the air flow predominantly comes off the cool Pacific Ocean. About the only way we get hot is to hold those ocean breezes back and get a wind coming from the east -- an "offshore" wind.

Due to the Cascade Mountains, that east wind as an additional warming effect as air warms and dries out when it sinks down the western slopes of the mountains. So that is why when it does get hot here, it's a desert-like dry heat and much more comfortable than the muggy mess the southeast deals with.

To get an east wind, the usual culprit is a thermal trough (also known as a "heat low") -- a mass of hot air that makes its way north out of the Desert Southwest along the Pacific Coast. Since heat rises, it creates low pressure along the ground and when that heat low comes up along or just off the coast, it draws in that warming east wind.

In this scenario, the entire West Coast bakes in 80-90+ degree heat with hotter weather in Oregon -- and even hotter weather in Northern California -- until that low eventually moves inland and the ocean breezes return to cool us off.

But the forecast for this weekend in Northern California is straight out of Seattle's spring playbook: Pouring rain and low-mid 60s. So obviously there is no thermal trough coming up from there -- quite the opposite.

So how are we getting such nice weather when they're sure to be writing angry letters to their weather forecasters about how they didn't move to California to have Seattle's weather?

We're getting the benefit of their stormy weather. Air circulates counter-clockwise around a low pressure center. As that low moves into California, the air flow will first be out of the southeast at first, pulling in some heat from Nevada and southeastern Oregon and making a "mini thermal trough" just over western Oregon and Washington. Then the flow will turn to the east -- giving us a little temperature boost from reasons I described above.

This chart shows the upper air flow at roughly 18,00 feet (500 mb of pressure) for Saturday. Air flows counter-clockwise around the big low pressure area. Note the east winds over the Northwest)

Now, it's not gang-busters heat -- temperatures can easily get into the 90s in early June around here (I know, hard to believe, but ancient hieroglyphics in Seattle weather record books indicate it has happened before). The easterly flow is not that strong and it won't last long. But it should be easily enough to get well into the 70s around here, even without much in the way of heat reinforcements from the Golden State.

As the low continues to move inland into Sunday, that will pull the flow around here back to the west and our ocean breezes return for the start of the workweek, making for cloudier skies and crashing temperatures back down into the 60s.

Meanwhile, parts of California will slowly climb back into the 70s, so no need for those residents to panic.