Weather Blog

Seattle ties for hottest November day in city's history

Seattle ties for hottest November day in city's history

Summer is back!

With a balmy 74 degree reading, Seattle has smashed the record high for the day of 70 degrees. But of more interesting note -- the 74 ties for the warmest November day on record, tying Nov. 4, 1949's 74 degree reading.

Here are the top 5:

  1    74.0  11/3/2010
  1    74.0  11/4/1949
  3    72.0  11/2/1970
  4    71.0  11/4/1980
  5    70.0  11/3/1970

And it's not just Sea-Tac Airport: The NOAA station at Sand Point reported 75 degrees, as did Renton and Kelso. Arlington, Everett, Bremerton, Shelton and and Puyallup were at 72, North Bend, 71 and Tacoma hit 70.

Yes, the irony is clear -- that November 3 will go down as a much nicer day than several others in July and August (And it would have tied for the second-warmest day in June). Welcome to the Northwest.

The heat has been a side-effect of blustery winds in the foothills and a warm air mass moving north from California. But the winds have been quite variable. To wit: it's a crystal clear, calm day in Seattle -- the mountains are out, and the trees are showing off the last bits of their fall colors, and what leaves are left are barely moving -- at least around the Seattle Center.

But over in the foothills, it's a blustery day.

"My gutters vibrated so you know the gusts are good," said Danna McCall who lives in Snoqualmie and and writes a nice blog up there.

Wind speeds have been in the 20-30 mph range with some gusts to 40-50 mph! A Wind Advisory was in effect for most of the afternoon.

And you can thank those breezes for ditching the heavy coats today as Seattle warms well into the 60s.

So what's going on? It's actually a modified thermal trough, November edition. Warm air pushing up from the south creating lower pressure to our southwest.

In addition, we've got a really big storm well offshore in the Pacific, and very strong ridge of high pressure (around 1040 mb) perched in southeastern B.C. just north of Omak. Put the three together, and we've got a decent difference in pressure between the higher pressure in Eastern Washington and the lower pressure off the Washington coast and to our southwest.

That set up is pushing the wind through the mountain gaps as the wind races to the west to equalize the pressure. So for Snoqualmie and North Bend and the like, that explains the wind.

Here is a good map showing the current pressure set up. Note the isobars (black lines) packed close together over the mountains. The closer the bars are together, the stronger the pressure difference and the greater the wind.

This UW forecast model map, featured first on Cliff Mass' excellent weather blog, has a pretty good representation of where the wind is blowing (the oranges and yellows -- Mass also noted the strong winds flowing out the western end of the Strait).

If you want to see the actual numbers check out this pressure gradient chart:

These tell you the difference in pressure between two cities. SEA is Seattle, YKM is Yakima, EAT is Wenathcee. When those go negative and the number is greater than 9-10 or so, that's a pretty good wind. Get well into the negative teens and you could have serious damaging winds.

Why so toasty out there?

The east winds have a secondary benefit of warming the air and drying it out. That's because of "compressional heating" -- as air sinks, its pressure rises, which increases its temperature and decreases its humidity. And as the easterly winds climb over the Cascades from the east, then sink down the western slopes, they get warmer and drier.

That's why the foothills are so warm today -- North Bend reported 70 degrees at noon -- and that warm air spreads gradually spreads across the rest of Western Washington. The coast gets this warming effect too off the Olympic Mountains and coastal rage -- the Washington coast was into the 60s today too and farther south, Astoria was at 73 and Tillamook at 75! -- All thanks to that east wind.

Why is it not so windy in July?

If this talk about thermal troughs and "compressional heating" sounds familiar, that's because we deal with this frequently in the summer and it's what causes our heat waves. But North Bend and Carnation people know that while the big east winds are usually common in fall and winter, it's usually not a problem in the summer even on the easterly wind days. Why?

It's because Eastern Washington typically doesn't have much higher pressure in the summer to create the large pressure differences. In fact, in the summer, it's so hot over there and since warm air is less dense, they're pressure usually it's too great. No, the east winds are more a function of the "heat low" over the coast pulling air through the mountains, and the pressure differences are not that large. (They don't need to be. A 15-20 kt east wind can do wonders to boost summer temperatures.)

But in the fall and winter, that's when we start to get our first arctic air masses in the Rockies and Eastern Washington. Cold air is very dense and is normally associated with very high pressure. So when Eastern Washington is stuck in their winter bowl of cold fog, it's at a very high pressure. All we need is a big low pressure approaching the coast and voila, you've got the big pressure differences.

In this particular case, it's a mix. There is higher pressure in Eastern Washington today than usually in summer -- it's a little chilly over there today but not to terribly cold. On the other hand, we have the lower pressure coming offshore from the distant storm and weak thermal trough to "grease the skids" on our side of the hills -- just enough for a decent east wind today, and just enough for a very sunny and warm November day.

Hey, it's not so toasty at my house!

Not everyone is basking in the 60s. Some spots in valleys had some fog this morning that was keeping a lid on temperatures. Since cold air is heavier than warm air, it can pool in valleys and keep fog around longer. UW Research Meteorologist Mark Albright found that Carnation this morning was stuck around 40 degrees in the fog, while just up the hill a bit where the warm, dry east wind was blowing in North Bend and it was a balmy 63.

Once the fog burns off, temperatures will quickly warm.

So get out and enjoy the November sunshine! As you are well aware, it probably won't last too long :)