Friday, July 17, 2009

When is the windiest time of the day?

This time of the year there are real differences in wind speed during various times of the day and in virtually all locations it is windiest during the late afternoon. Take a look at the above trace of weather observations at my department at the UW (you can click on it to see a bigger version). Note: times are in GMT/UTC... 00Z is 5 PM, 18Z is 10 AM, etc. Winds are light during the late evening and early morning, then pick up during the morning hours. You will also notice the wind shift...typically southwesterly in the morning and northwesterly during the higher wind period.

How about Hoquiam or Ellensburg over the past two weeks? See the attached graphics. You will see a strong daily (diurnal) variation in wind speed.
So why do winds vary during the day so much? A few reasons. The first is that winds generally increase with height in the atmosphere (since friction or drag is greatest near the rough surface). During the night when the surface cools, the atmosphere is more stable in the vertical (less mixing when cooler, dense air is low down). During the day, the surface warms and the air tends to mix in the vertical, as less dense, heated air rises. More mixing brings the winds down from aloft and thus the winds tend to be stronger.

But there is another reason...local, diurnal circulations like the sea breeze. At night land and water are similar in temperature around here, but during day, and particularly warm days, the land heats and pressure tends to fall over land compared to water. The result are sea breeze circulations that increase winds. On the coast near Hoquiam there is a classic sea breeze across the coast. Over Puget Sound there is the Sound breeze between the water-filled NW interior and central Puget Sound, which is mostly land. For Ellensburg, there is a huge "sea breeze" between the strongly heated eastern side of the state and the cooler west. If you want to learn more about these diurnal circulation, my book has a whole chapter on it.

Talking of weather, today (Saturday) is slightly cooler than Friday since there was more marine air moving in last night. Even more marine air will move into the western interior on Sunday. But temps will remain in the 70s and no precipitation is forecast. We have serious dry conditions, particularly east of the Cascade crest. The threat of major fires is quite real.


  1. Cliff, speaking of chance of major fires, do you think that chance will increase even more next weekend with absolutely no rain in sight, when temperatures appear as they will soar into the 90s here and over 100 in Portland and eastern Washington?

  2. It seemed awfully muggy out yesterday, yet I see on the NOAA site that in Olympia there was only a 30% humidity with the temperature at 90. Nothing like in the midwest I know, but still the air felt pretty thick. At 5:05 pm in Tumwater, there is a pretty good breeze blowing.

  3. I'm in Ellensburg and it is constantly windy over here. I mean, all the trees in Ellensburg face only one way because the wind blows every day in one direction. I have always wondered why this was? This blog gave me a little more info on that, thanks a lot Cliff!

  4. Chris..ellensburg is east of the major low level gap in the Cascades...that is why it is so

  5. Cliff,

    As Weather Is My Life mentioned...what do you see for next weekend in terms of "heat wave" Saturday the 25th forward...?

    Is it like Winter, where the models really aren't that accurate until 72 hrs out?

  6. And to follow up on Joseph's comment... the last four runs of the GFS have strengthened the cutoff low in the Pacific as it moves northeast and hangs it around western Washington through the weekend, instead of weakening it and merging it in with the flow around the SW Alaska 500mb low as previous runs had done. This of course translates to a little more in the way of cloud cover and temperatures about 20 degrees cooler than it had previously appeared for next weekend. At the same time, a cutoff high develops to the west over the Pacific before the low retrogrades back west and the high merges back into the high over the western US the week after for hotter conditions in the NW then.

    All this looks rather strange. What are your thoughts?

  7. We live right along the Hood Canal, but at 500 feet elevation, and my husband calls the afternoon breeze the "Mighty Wurlitzer" or, if it's really blowin', the "Chap Flapper". We are refugees from the CA Bay Area and we love how fresh the air is here. We won't be complaining about the wind, ever. I learned about using more clothespins when I watched our bedspread go flying off over the trees one afternoon!! ;~}

  8. NOAA released a report today that indicates that El Nino will continue through the winter, bringing us a much milder winter. I couldn't tell from the article if that means less rain as well?? What has typically happened here in the northwest during a strong El Nino~ pattern?

  9. Corie, it usually means a warmer and drier winter. However, there are other global weather patterns in play right now that could offset some of the effects that an El NiƱo brings to our region.

  10. If we have a drier fall...and it sounds like it will be, almost like an eastern fall, it could make for nice sky-watching, leaf-kicking etc. But for snow pack...hmm. Very interesting how it will pan out.

  11. Joseph, as suspected, the odd-looking cutoff low (particularly for late July) the GFS was hanging around our region for the weekend is now absent from the GFS again, which makes a lot more sense. The models are usually quite a bit better a week or so in advance in the summer than they are in the winter, at least across this region where rainfall is very sparse in July. Between the ECMWF, GFS, Canadian, and even the NOGAPS of all models, all showing the 500 hPa high building right over the Northwest by the second part of the weekend, there is increasingly significant evidence of intense heat (for the Northwest, that is) headed here for Sunday into next week.

    I'd say probably upper 80s Saturday, and mid to upper 90s could be possible next Sunday and Monday, hotter in the eastern highlands and closer to the foothills.

  12. Thanks Weather Is My Life :)

    I appreciate you staying on top of that...I have to say my model interpretation skills aren't on that level.

    I'm in Lacey, WA and looking at Wunderground, it's showing 95 each of Sat and Sun for highs. Sheesh.

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  14. Joseph, actually, yes Saturday could be quite hot too - it's looking again like that ridge is going to build by late Friday/Saturday morning. From what I'm looking at, I wouldn't be surprised to see temperatures in the upper 90s to near 100 in some spots, particularly away from the Sound and Lake Washington, south of Bellevue, near the foothills, on Sunday.

    Here are images from yesterday's 00z GFS-WRF run of temperatures at 5pm Saturday and 5pm Sunday:

  15. I've been sailing my engineless boat between Seattle and the San Juan Islands nonstop for about a month and can attest to the late afternoon winds. They've been pretty reliable.

    Today (Wednesday, July 22nd) I noticed that the wind is coming out of the south here in Seattle and that it began early, like 8 or 9am. The marine synopsis says there is still the typical high surface pressure offshore with low pressure east of the Cascades creating onshore flow pattern. I'm trying to square that synopsis with the early/southerly wind patterns today.

    Did the center of the NPH shift south and the onshore flow is finding its way through a gap south of the Olympics?

    Any insight would immensely help this sailor!