SEATTLE - As we get set to bake in perhaps a historic heat wave this weekend, you might be amazed at some of the high temperature forecasts being tossed around. 100? 102? 106? 109? Some places in Eastern Washington even … over 115?
Indeed, this is a rather big deal. But aside from the unusual heat, meteorologists have been abuzz with the rather unusual atmospheric setup. This is not shaping up to be your traditional Northwest heat wave!
And that is why there's been a fair amount of frowny faces in the weather community. Our forecast models are showing high temperatures at levels never before seen around here.
Experience tells us that those numbers shouldn’t be reachable in a mild, marine climate that is perched next to massive bodies of cold water and thus there is understandable skepticism that those forecasts are realistic. But countering that theme is the consistent and adamant nature of all the forecast charts that those numbers are what their calculations deem possible. And the models are usually right the rest of the time.
What's so unusual?
First of all, we have a massive -- and I mean MASSIVE -- ridge of high pressure building into the region. Big ridges are not unusual in themselves, but this one is unusual in its strength, location and time of year.
Strength wise, this has a chance to be the strongest heat ridge ever measured in the Pacific Northwest region. We can measure its strength by finding how high in the atmosphere the 500 millibar pressure level is -- it's usually around 17,000-18,000 feet or so in summer. But warmer air is less dense than colder air, which means a hot air mass will push the 500 millibar level higher into the atmosphere. Hot ridges will get into the 580 decameters range (we use metric system and decameters -- or "dam" for its measurement). Super hot ridges will push 590.
This one is coming in at a predicted strength of 597 dam over the weekend which may be the highest value ever seen. That's quite hot by Desert Southwest standards.
But part two: it's not sitting over the Desert SW. It's going to park in Southern B.C. just on the other side of the Washington border -- an exceedingly rare location for such a large ridge. Intense upper level high pressure systems push air downward and that sinking process dries out the air and heats it up. In this case, we'll get a supercharged push of sinking air, in turn supercharging the heat. Putting a ridge that strong, that close is the big 1-2 heat punch that's cooking us this weekend.
What's more, the position of the ridge will allow for some easterly winds to push through the region as well which as they sink down the mountains cause even *more* heating. Though in this case the east winds are not a strong component as they are in other heat events.
Hey, there's places to escape!
In traditional heat waves, the ridge is centered farther away but we get a strong easterly wind that boosts temperatures across the region by the double whammy of the aforementioned sinking down the mountains and holding back the cooling seabreezes. That means even the coast bakes in the 90s (sometimes Forks is even hotter than Seattle!) and there is nowhere to run to find cool air aside from Mt. Rainier.
However in this case, while there is a bit of east wind, there is not a strong east wind. Forecast charts have been pretty consistent in thinking that could allow a localized seabreeze right along the immediate waterfronts that could overtake the east wind and provide some cooling to the shores.
Thus spots like Alki Beach and Golden Gardens might offer some unexpected relief in the city (from the heat, not the traffic once word gets out). Also those north of Seattle along Puget Sound and along the coast and Strait of Juan de Fuca, while they will still be quite warm due to the super-heated air mass, may stay in the 80s or low 90s instead of triple digits with the sea breeze. That will be a big wild card of the event is how strong the seabreeze is and how far inland it can penetrate. Yet it does present a place to join 5 million of your neighbors in finding a place to avoid the brunt of the sizzle.
(Also, Mt. Rainier works too, but it won't be all that cold. Freezing levels are expected to be 18,000 feet with temperatures at the summit at 40 degrees!)
But those of you say, east of I-5 have little if any hope of this cooling seabreeze and are very likely to bake in triple digits once or twice in this event.
Will it be humid?
One of the tradeoffs for the lack of a powerful east wind is its drying capabilities. Sure it might add 3-7 degrees to the temp but it sucks the humidity right out.
With the east wind weaker, it does look like humidity levels will climb higher than usual, especially on Sunday and could lead to making it feel more miserable that it already would on temperature alone. It'll also have the second unwelcome effect of keeping temperatures warmer overnight by not allowing as much cooler air to radiate back despite clear skies.
Bottom line is it's shaping up to be a rather uncomfortable three days with the peak heat Sunday and Monday and while Monday might be a couple degrees hotter, Sunday could feel worse with the humidity.
When does it end?
There is some relief in the forecast from the bonkers heat but hot weather is here to stay for quite some time. A weak to moderate marine breeze is looking possible Monday night that would provide at least a little natural air conditioning to Western Washington. But the very hot air mass would remain as the ridge is still hanging around the neighborhood – though a bit weaker and farther away – and highs are still expected to reach well into the 80s the rest of the week.
Eastern Washington gets no such help from the Pacific Ocean and is expected to remain well over 100 if not 110 at times through next week.
The long range models show continued hot and dry weather holding into July. It may be a long while before we see a day in the low-mid 70s again.
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