Our hottest forecast ever: Seattle to hit 101

Our hottest forecast ever: Seattle to hit 101 »Play Video
SEATTLE -- Well, never thought I'd actually type this, but here goes:

The forecast high for Seattle on Wednesday is 101 degrees.

I'm not exactly sure if we need to set off balloons or confetti -- the balloons would probably pop anyway -- but yes, the meteorological evidence is there that Wednesday will go down as the hottest day in Seattle history.

In case you didn't hear, Seattle has hit 100 just once at Sea-Tac Airport -- July 20, 1994. It also hit 100 two other times when records were kept at the Downtown Federal Building between 1891 and 1972: on June 9, 1955 and July 16, 1941.

In fact, triple digits are so rare, it presented a Y2K-esque graphical challenge to our weather computer. Our extended forecast map is only built for two-digit temperatures. Had to do a little tweaking to squeeze in that third one, lest we have a forecasted high of "10".

(Incidentally, we didn't predict 100 the day we actually hit 100 in 1994. That was on pace to be about a 95-degree day when we had a shot of warming right at the end that boosted us six degrees in a short period. So that one snuck up on us.)

It has never reached 101, but could that change as well? If you believe some of the forecast models, 100 is too conservative, with one from the University of Washington showing 102, while another shows a potential high of 104!

How about temperatures Tuesday? Still gearing up for the 100-degree run. Highs around Seattle will be in the upper 90s, with 100s starting to pop-up in the South Sound and Cascade foothills. There's no real place to escape the heat without getting on a plane as temperatures on the typically-cooler coast and north interior will also be well into the 90s.

Then Wednesday comes 100-degree watch day, but for many other places, 100 might seem cool. I'll bet the many cities along the Cascade foothills go over 100 and might even close in on 105, as could Olympia (forecasted high: 102) and the southwestern Washington interior down into the greater Portland/Vancouver region.

There's even a chance that Thursday could also reach 100 in Seattle if an expected hot, east wind keeps blowing.

There may be some slight cooling as we get into Friday and the weekend, but we don't see a marked cool down now toward anything remotely close to normal highs until the middle of next week.

What's the deal with this heat?!?

We have the perfect heat scenario of an incredibly strong ridge of high pressure. That alone has been baking the Northwest into the 90s of late, but now we're adding in a second ingredient -- a thermal trough that will create a hot, dry east wind. Put the two together, and it's like mixing fire and oxygen.

Locally, the east wind makes it hotter for a few reasons. One, that air is coming from Eastern Washington, where is hot to begin with. Second, as that air crosses over the Cascades and then sinks down, it warms further. For those living along the foothills, this is akin to living at the end of a blow dryer and why your highs are among the hottest.

Now, as to why it's sticking around so long, the weather pattern over North America has two big features -- a big, big ridge of high pressure anchored along the western third (stretching from Baja to almost the Arctic Circle) and a big, big area of low pressure anchored over Hudson Bay.

Not only has that ridge baked the West Coast, but on the other end of the scale, that low has made life miserable for the rest of the nation east of Denver. There, summer has gone into hiding, with relentless rain and thunderstorms. New York City is on pace for one of their coldest July's ever.

With such exaggerated patterns, it's hard for them to budge because they are so strong they get stubborn. Incoming weather systems, typically weaker around here in summer anyway, are no match to move a ridge of this size, and then in turn, this ridge doesn't move to push the eastern low out of the way. It's like having a disabled semi jackknifed on the 520 bridge -- there's just not much room to move.

That ridge, in turn, keeps the thermal trough over our area. Heat waves usually don't go longer than two or three days because the ridge gets nudged east by the westerly flow of the planet, and once the thermal trough moves east of the Cascades, it opens the door for the cool west wind to kick up. But with the ridge so strong, it's able to hold back the ocean breezes and maintain the thermal trough right over Western Washington.

The last time we saw this pattern was 1977 and 1981, our two current heat wave champs. 1981 is notable for 5 days in a row over 90, including a 99 and 98, while 1977 had an 18 day period where it was over 79 every day (15 in a row over 80), 13 days over 85 (9 consecutive) and six days over 90 (4 consecutive).

What's the deal with this humidity?!?

Amidst all the other jaw-dropping events (at least, for meteorologists) with this heat wave, it's also uncharacteristically humid. We Northwesterners pride ourselves on not having the heat and humidity at the same time like the Midwest and East Coast.

But not only is it historically hot, the humidity is much higher than normal. This too can be chalked up to a very unique situation.

The humidity is leftover from the area of low pressure that stalled in Eastern Washington over the weekend and brought the storms to the Cascades and a few spots of the lowlands. Wandering lows like that are rare inside such strong ridges of high pressure, but this one managed to sneak through the door.

It added some moisture to the air that we haven't had a chance to wring out yet, and being trapped under such a big ridge of high pressure also leaves us with virtually no air flow to scour it out.

It's another item to chalk up to the growing list of unusual traits to this pattern. A typical heat wave here has that dry, east wind I mentioned earlier, which eats away the humidity. A 90-degree day around here is usually accompanied by humidities in the 20 percent range.

But we haven't had the east wind yet -- which makes this ridge of high pressure all the more impressive that we could reach a high of 94 without it Monday. Once the east wind starts going later today into the middle of the week, the humidity should drop, but since the east wind isn't expected to be overly powerful, it might not get as dry as normal. (Shudder to think what would happen with a 20-30 knot east wind in this set-up. Wonder if we'd get to 105 or more?)

Other Records in Jeopardy:

With all the talk of the 100 degree record possibly going down, there's another all-time record in jeopardy: The all time warmest low temperature. As of right now, there has never been a day in Seattle's recorded history where it has failed to drop below 70 degrees at some point. In other words, the all-time record warmest low temperature is 69 degrees, set Sept. 2, 1974.

We came oh-so-close to doing it Tuesday, where the preliminary low was 70 degrees, but later the thermometer briefly dipped to 69 at 6:16 a.m. So Tuesday could tie the record, but this still has a chance to fall on Wednesday and/or Thursday.

A quick list of other records that might fall this week:

  • Consecutive days at or over 90: 5 (Aug. 7-11, 1981)
  • Consecutive days at or over 85: 9 (Aug. 5-13, 1977)
  • Consecutive days at or over 80: 15 (July 30-Aug 13, 1977) -- incidentally, the 14th was 79, there were three more 80s afterward.
  • Number of 90 degree days in a month: 7 (July 1958)
  • Number of 90 degree days in a year: 9 (1958)
  • Hottest July on record (high temperature): 81.4 degrees in 1958. (If current 7 day forecast verifies exactly, our avg. this month will be 81.13)
  • Seattle daily records: Tuesday: 97, Wednesday: 95, Thursday: 94.

When Does It End?!?

As I mentioned earlier, this pattern has the makings of the 1977 heat wave that stretched 18 days. We should begin some gradual cooling as we get into Friday, and by the weekend, highs should be into the upper 80s as this ridge slowly weakens. But a new area of low pressure developing off the California coast, it will keep pressures lower offshore and could keep the surge of marine air from rolling in until the middle of next week, meaning several more days of above normal temperatures, although not to these extreme levels.

It will be interesting to see how the heat wave finally meets its end. Normally, the bigger they are, the harder they fall. With a heat wave this strong, I half expect it to snow sometime in August!