August 20, 2017

How Will the Eclipse Change U.S. Weather on August 21?

The total eclipse of the sun will profoundly change the amount of solar radiation reaching the surface on Monday, August 21st.

So how will this alter the weather across the nation?  Most of the usual weather prediction models  (such as the UW WRF) will not include the loss of solar energy in their forecasts.  But fortunately, my colleagues at NOAA ESRL have for the first time included the eclipse effects in a weather prediction this case the HRRR--the high resolution rapid refresh model.

To test the impacts, they ran the eclipse radiation code in a test run, using the weather situation of August 4.    Let me show you what they found--simulating for only the core hours of the event:  17, 18 and 19 UTC (9, 10, 11 AM PDT) on August 21st .

The radiation reaching the surface is shown below...the top line is using the normal (non-eclipse) radiation code.  The second line is with the case.  The bottom is the difference.  You can see a round region of profoundly reduced global radiation moving eastward across the nation over two hours.

But what about temperature?   Major cooling, with some areas cooled as much as 6°C (11°F)...but not over the Northwest, where the signal is about half as strong.   One major reason is that the eclipse hits earlier on the West Coast, when solar radiation is weaker, compared to the mid-day eclipse to the east.
What about wind?  Yes, there are effects, including effects on wind energy.  Here is the change of wind speed at 80m....near hub height of many wind turbines.  Interestingly, the wind speed effect is delayed a bit, with roughly a 1 meter per second (around 2-3 mph) reduction over the western U.S.
There have been a number of eclipse-weather studies, with documenting temperature declines at much as 15F--although in the Northwest I expect more like 3-6F in the region of totality.   So if you can, try taking temperature reading during the event and let us know what you find.

Eclipse Forecast Update

Everything looks good for the region of totality.  Best conditions around Salem (no clouds or smoke), with clear skies, with some smoke to the east.  Here is the cloud forecast for 10 AM. Great for Oregon.  Some residual clouds around the Sound.  Clouds along the coast.


  1. I'm getting up at 4 am and driving to Salem. If it's socked in by clouds, I'm gonna be mad.

  2. Edmonds temp:

    Sensor placement isn't perfect, but pretty good.

  3. Still 20 RV dry-camping spots available $30 at Cascade High Schoolin Turner 200 feet from centerline. I5 exit 248

  4. Drove from Bellingham
    hotel in Portland up at 4 AM to get to Salem by the event
    Hot Diggity hot dawg!!

  5. Clear star lit skies near Salem Thanks university blogger dude !!!

  6. 0751 Clear as a bell in Olympia

    Over at "Seattle's only Newspaper" the Stranger published this this morning

    Yeah, that's right. And it's not just the headline. 2nd sentence reads in part "Annie Dillard on witnessing a total solar eclipse in Washington State, near Yakima, in 1982." To his credit Charles Mudede didn't write it. The ignorance hurts.

  7. There was about a 3/4ºF drop on Lake Union during the eclipse:

  8. In Langley on Whidbey Island [48.026 -122.406] we dropped 2 degrees F from 59.5 degrees at 9:39 AM to 57.7 degrees at 10:47 AM. We're 1 mile from the shore and 262 ft in elevation. Our solar energy dropped to 32 watts/m2

  9. Wow! The weather in the Pacific Northwest sure cooperated! Two Thumbs Up!

  10. Cliff you didn't tell us about the traffic !!
    The eclipse weather was great but I've moved 5 miles in the last hour

  11. Chiloquin, Oregon saw a 10 degree drop in temperature during the Eclipse today.

  12. Solar eclipse day 08/21/17 temperature reading report at location 3 miles north of Hwy 2 at Monroe:
    9:08am-- 57.4
    9:55am-- 59.9
    10:04am-- 59.9
    10:16am-- 59.7
    10:43am-- 59.0
    11:18am-- 60.1
    11:38am-- 62.1

  13. Almost made it to Madras had to pull over to see it. No glasses. Traffic wasn't bad on the way back.

    Now that the eclipse is over can we talk about the drought and the fires?

    This is the driest summer ever in many areas, and vulnerable trees are dying because of it.

    Nobody ever talks about weather and racism either, but it exists.

  14. Cliff, consulting your advice we went to Salem, and it was fantastic!
    (Had made and paid for reservations early in Madras for best chance,
    then recently added Salem since you suggested Salem would be great.)
    And it was !!! Traffic on way down on Sunday was great,
    but coming back a bit over 8 hours. For parts in Oregon we followed
    some traffic suggested alternates off I-5.

    Just a great thank you for all the wonderful info you provided!

  15. Cliff, I was on the summit of Dixie Butte (7600 ft) in Eastern OR about 4 miles north of the centerline. I did not take any temperature measurements but a guy from the The Citizen Continental-America Telescopic Eclipse Experiment did. He told us the air temperature dropped 7 C. He also took soil measurements on the south facing slope and said the soil temperature dropped from 40 to 20 C. You and your readers may want to look up their website as they plan to piece together a video of the corona taken from 68 telescopes spanning the continental eclipse path. Amazingly it looks like there was clear weather along nearly the entire length of the eclipse path.

    The whole experience was absolutely incredible. The changing light was surreal and dreamlike. Given our elevation we could see the shadow coming and going for about a minute. During the eclipse we could see the edge of the shadow all around us and the corona was just amazing. Except for some smoke on the western horizon we had perfect weather. Thanks so much for the weather updates. Hopefully I can see another one. April 8, 2024 is penciled in on the calendar.

  16. We were on a hill on the path centerline in Malheur National Forest... air temps seem to drop around 10F or so; and the experienced temp swing due to the sun going away was even more dramatic; we were baking hot before, and during totality people were putting jackets on. Soil went from so hot that I couldn't walk on it barefoot (with tough feet) to cool.

    And it was amazingly beautiful. :-)


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