September 27, 2010

Incredible Humidity

Today the humidity has been extraordinary around western Washington and Oregon. I mean sticky. Feels like the southeast U.S. during the summer--and for good reason: we are experiencing very, very high humidities with dew points in the mid to upper 60s in many locations.

(Remember dew point is a very good measure of humidity...high dew points mean lots of water vapor in the atmosphere. Typically this time of the year dew points would be around 50F. )

Here are some dew point values at 5 PM:

UW 67F
Olympia 69F
Tacoma 67F
Portland 63F
Cascade Locks 72F

There is a satellite that senses water vapor from space...take a look below:

Pretty amazing...there is a plume of moisture streaming out of the subtropics from just north of atmospheric river. You can call it a pineapple express--but it is missing one thing...enough upward motion to give us heavy rain. Want to see a great video of this moisture plume? Check this out:

Here is a simulation of vertically integrated water vapor from the UW forecasting system valid 11 PM...the blues are the highest values and you can see it streaming from the southwest.

The humidity is so large that there has been some condensation in the form of shallow fog over parts of the Sound...which is cold enough to cause this moisture to condense. Some wind was also helpful, since it mixed the water vapor towards the cold surface. Look at this picture sent to me today by Greg Johnson of You see the grey haze near the surface? That is the shallow fog I was talking about.

Now I know for sure that this was the most humid day over the past year so far in terms of dew point ---and here is the proof for Sea Tac:

At Sea Tac the highest dew point of the day was degree shy of the ALL TIME RECORD HIGHEST SEPTEMBER DEWPOINT for that location.

And want more extreme weather...head to the LA basin for heat. A number of locations have had their ALL TIME RECORD TEMPERATURES, with observations in the LA area of 113-119F!! Be thankful you are living in the Northwest.


  1. Just moved from Philly (in fact the humidity feels like Philly in Seattle today). I used to get my weather info here and I was glad to find your blog about Northwest weather today.

  2. Dew point of 69 and humidity at 77 here in Eatonville with a temp. of 76. It was pretty breezy earlier today but that didn't help much! Hard to believe that it will be October 1st on Friday!

  3. So the 101 lesson of the day is even with abundant moisture, you still need a lifting mechanism like a front?

    Makes sense, guess it doesn't rain in Florida everyday.

    That fancy intregrated water column map you show, thats moisture more near the surface right? The WV satellite today is very dark meaning dry air?

  4. So, the next question: When will it end?

  5. "Be thankful you are living in the Northwest."
    Yah, sure, you betcha!

  6. I have never felt such horrible humidity here in chehalis before. Is it going to end any time soon?

  7. Yes, incredible humidity for this area, but "feels like the Southeast during the summer"? I'll take this weather any day over that swamp weather I lived in for soooo (too) long - 98F+100%RH (that should explain why Iced Tea is so popular in the south!) Amazing video of that moisture plume and why it's not raining. But...why is there no up lift to cause it to condense?

  8. I wonder what the all time record high dew point temperature for Seattle is? Can't be much higher than 70 I would assume...

  9. The fog WAS amazing this morning. I saw it daintily cling to the banks of the southern Duwamish near Tukwila. Also saw lots of fog gathering is small pockets up in the trees along W Marginal, etc. Very beautiful! I'm over the humidity for sure though. I am one of the few really looking forward to true fall and winter weather. As always, THANKS!

  10. I love it. It feels tropical, and I find it quite comfortable.

  11. Hi Cliff,

    What was the highest dew point of all-time?

    My station had 67.7 Dew point today, higher than even the muggy 103 degree day in 2009 of 67.0

    I am 3 miles SSW of Seatac.

  12. Fun weather tonight. One good example of the effect of the dew point being so high was that as I drove this evening with air conditioning on cold blowing in defrost mode at the windshield, water droplets began to collect on the outside of the car windshield where the temperature was 68°F with probably like you said dew point in the low/mid 60s, but the air conditioning cooled the surface of the window to below the dew point and thus dew formed.

  13. Avalanche: "That fancy intregrated water column map you show, thats moisture more near the surface right? "

    By "integrated water column" Cliff means the sum (the integral) of all the water (vapor) in the atmosphere looking vertically down on a point at the surface (i.e. in the "column" from top to bottom of the atmosphere).

    In an atmospheric river though it seems most of the humid air is at low level (under 3000 feet or say below 850mb or so). But these maps shows you all the water vapor in the atmosphere. See some of the links I posted on a previous post about atmospheric rivers.

    Quiet an amazing day for Seattle.

    Is there a "reason" behind the increasing dew points as the year has worn on. Is this typical and I've not noticed it before or is this something special?

    For those thinking about atmospheric rivers the animation of the measured integrated column water or the mesoscale prediction (on the UW site) shows you how the features move. The "river" is trapped between on the Gulf of Alaska low and California high. THe fronts drifting through turning the river on and off. Interesting to see the motion being "turbulent" with another river being dragged into place then merging with the current one (the California high really blocking the movement). I can see why the movement of these rivers (when they have both the air and the lift to generate rain) is an interesting topic. And if they should become stationary it's like getting sprayed with a fire hose. A metaphor I noticed when comparing the weather here in the pNW to the weather I was used to in the UK (more strong temperate cyclones drifting over the country).

  14. As I just planted some shrubs and a crop of chard starts, I am ever so grateful for this wonderfully balmy weather with a little bit of rainfall now and then, just enough to help the plants settle in! And how nice it was to sit on the front steps in the evening, with only the early-onset darkness tipping me off that it was actually autumn.

  15. Very interesting Shallow fog this morning on Bellingham bay.

    It started very low near Lummi island, and soon spread across the bay. I have been watching the bay for decades, and this was very unusual.

  16. Ok, here's a question not directly related to weather.
    Since our climate here has a lot of moisture, why don't we have any Fireflies?
    I would think they would survive quite well here in the Pacific Northwest.

  17. The foghorns were going all night in the Haro Strait.

  18. Cliff, ill informed question, but where is that moisture "going?" It's still in the atmosphere and somewhere between here and Atlantic States there has to be lift, particulate, and cooling. Rain has to be falling somewhere that can be associated with that deluge of atmospheric water pouring into North America.


  19. A few were asking what the highest humidity has been on record....the NWS posted this with their 9:00 am posting....

    Climatology...daily average dew point temperatures have been
    calculated at Seattle-Tacoma Airport since 1965. In the 46 years
    Worth of records there have only been 3 days with an average dew
    point in the 60's in September ( 9/20/1967 61 degrees...9/18/2000
    ..61 degrees and 9/27/2003 60 degrees ). The average dew point for
    Monday at Seattle-Tacoma Airport was 63 degrees. The 1971-2000
    normal dew point for September 27th is 49 degrees and the normal
    monthly average dew point for September is 50.5 degrees. Felton

  20. Imagine how much it would have rained if there was any kind of forcing or lift.

    For example, even with the radar and satellite being rather meek, the slope of Mt. Baker caused 2 1/2inches last 24 hrs.

  21. I have a question about Saturday's weather. During the summer, I noticed that clear days were associated with northerly sort of winds. On Saturday, it was blowing from the SW. Can someone help me out with this? Thanks.

  22. Cliff, related question: where is all that moisture falling as precipitation? Is the moisture streaming in to North America on the west coast related to the flooding in the upper Mid West? I'd think not much would make it over the Rockies, but....

  23. ok so checking out the different weather outlets and for Thur Sep 30 in seattle here is what I got...
    MSN 69
    Seattle times 72 & UW Probcast is at 72.
    National Weather Service has 73,
    Weather Underground has 74 (Up against the wall you MFers is optional---- sorry 60’s reference),, Yahoo,, Kiro & 75,
    King5 76
    KOMO 77

    so we'll have to see who is closest to right tomorrow.

  24. My Davis weather station recorded its highest dew point of the year that day. I sat amongst my tomatoes in my shorts and t-shirt, felt the sun, breathed in the smell, closed my eyes and pretended it was summer that could have been.

  25. Is your program going to survive these UW budget cuts we're reading about?


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