While much of the West is set to broil this weekend and into early next week, Seattle will dodge the intense heat, but instead be left with a much muggier pattern.
Warm, humid days are rare in the Puget Sound region, despite living next to the Pacific Ocean, because it takes a very specific wind pattern to get the combination of both warm and humid air.
When the winds blow off the ocean, as is the case a majority of the time around here, the air is relatively moist but also much cooler. Cooler air doesn't hold as much moisture as warmer air, and plus when it's cooler outside, your body doesn't notice the higher humidity as much because you're not trying to sweat and cool yourself off. (Higher humidity makes it more difficult for your sweat to evaporate and cool you off.)
But usually when it does get hot around here, it's due to an east wind that holds back those cool, ocean breezes. But those east winds come from the desert-like Eastern Washington. As an added bonus, that air sinks down the western slopes of the Cascade Mountains and as air sinks, it gets warmer and drier. So our hot days here are typically very dry -- just like the desert heat of southern California and Arizona.
A north upper level wind is fairly rare but it's usually pretty pleasant as it's drawing in cooler air from Canada that is usually fairly dry.
But a southerly wind? That's the Achilles' Heel of our muggy defense. It's usually due to low pressure offshore pulling up air from the warmer reaches of the Pacific Ocean to our south -- sometimes even tapping into tropical fetches of moisture around Hawaii. Since a low rotates counter-clockwise, that air will move east from the ocean waters toward the Pacific Coast, then turn to move essentially up the I-5 corridor through the hot interiors of Northern California and Oregon into Washington.
In this case, the air doesn't get the cooling effects of the near-offshore ocean waters, nor the drying effect of sinking down the Cascade Mountains. It stays warm and humid.
And low and behold, here is the general set up for this weekend through the middle of next week:
A good measure of how humid it'll be is to look at the dew point -- officially, the dew point is the temperature at which the air becomes saturated (100% humidity), but it can informally be used as a comfort meter. Remember, the warmer the air, the more moisture it can hold.
Around here, our summer dew points are generally in the low 50s, while in the Midwest and back East, they can get into the 60s and even 70s. Jonesboro, Arkansas on Thursday had a temperature of 100 degrees with a 75 dew point which is so ridiculous it should be illegal.
You can use this handy dew point guide for Seattle summer heat:
<45 degrees: Hot and dusty (likely a thermal trough situation with that dry, easterly wind)
46-53 degrees: Typical and comfortable
54-56: A hint of mugginess, but still not bad.
57-58: Starting to feel pretty muggy
>64: Break out the plastic flamingos
Forecast models for this week hold dew points in that upper 50s to low 60s range through the middle to latter part of the week -- about as high as they get around the Puget Sound region. High dew points and southerly winds are also ripe conditions for thunderstorm development since you have the required ingredients of warm and moist air, plus a bunch of mountains around to provide the necessary lift.
In these patterns thunderstorms will usually form over the Cascades and, if there is any kind of hint of an easterly component to the south winds (as in a South-Southeast wind or straight southeast wind), those storms can blow west into the Puget Sound lowlands. Just another "benefit" of muggy air around here.
The high dew points also mean that if you use a "swamp cooler" type air conditioning system, which are normally effective on our hot and dry days, it won't be as effective in a humid pattern because just like your skin uses sweat for the process of evaporation cooling to cool you off, a swamp cooler uses evaporation to create cooling as well. But in humid air, evaporation is limited so it won't produce as much cool air. You're stuck with a fan or cold drinks...