Wacky weather day features fog, summer heat, thunderstorms... and then a 20° drop

Fog burning off on a warm Seattle May 4th (Photo via Twitter user @KANtext)

SEATTLE -- Thursday might just go down as one of the wackiest weather days Seattle has seen in ages -- this coming in a city that has turned simultaneous rain and sunshine into an art form. It's a day that has already featured a stable layer of dense fog just hours before what was expected to become the most unstable and stormy air mass the region has seen in a very long time -- right before the fog rolls back in overnight.

Already the morning fog has made for some dramatic photos, including this Fog Bow spotted in the San Juan Islands:

The morning fog is courtesy of the warm, tropical air that moved into the region Wednesday. High dew points near 60 degrees made it easy for the air to saturate into fog during the nighttime cooling.

The fog will give way to a sunny and summer-like late morning and early afternoon with temperatures rapidly zooming well into the 70s, if not low 80s in spots. But it'll still have a tropical feel to it with high humidity.

Then, the real weather adventures get under way in the afternoon and last through the evening commute. An approaching trough of low pressure will start to cool off the upper atmosphere first, creating an extremely large difference in temperature with the summer-like heat on the surface. That, in turn, creates a very unstable atmosphere. And with all the heat and humidity on the surface providing plenty of fuel, and the incoming trough providing some lift, it's a recipe for towering thunderstorms to levels we usually don't see in the Northwest.

The result: A period of strong to maybe even severe thunderstorms late this afternoon into the evening hours. That means frequent lightning, large hail -- perhaps as large as 1" in diameter or more, torrential downpours bringing as much as 0.50-1.00" of rain in less than an hour, and a burst of gusty winds in the storm's outflow.

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NOAA's Storm Prediction Center still maintains the I-5 corridor in Oregon and Washington (including the greater Seattle and Portland areas), plus the Cascades and their foothills are in a "marginal" risk of severe weather meaning isolated severe storms are possible. A thunderstorm is considered "severe" if it has 1" diameter hail or more, has 58 mph winds or more in storm outflow, or is tornadic. It's the hail that we have the best chance of hitting that criteria -- tornadoes very unlikely.

But even if storms don't reach "severe" levels, they are still expected to be plenty strong by our standards. We're still expecting numerous storms with torrential rain, frequent lightning and hail Thursday between 3-9 pm. The national forecasters suggest there may be areas within western Washington and Oregon that have a higher chance of severe storms but too uncertain to pinpoint.

The good news is that this period of active weather is expected to be short-lived -- just the next item on our checklist of wackiness Thursday. While the cooler air comes in aloft this afternoon to trigger the storms, the trough will also trigger a very strong marine push during the late evening -- you might remember those from summertime when we get that blast of cool ocean air and marine fog that suddenly sweeps in during the evening when heat waves break. The marine push may create it's own front as it marches into this warm, unstable air mass, triggering even more thunderstorms along its initial boundary.

But once this boundary passes you, the weather will take another dramatic turn. Strong west/southwest winds will develop, the temperatures will begin to rapidly cool, and eventually a layer of low marine clouds will roll overhead. What could be near 80 degrees at dinner time could be in the 50s or 60s by bed time.

On the other hand, this marine push will act like a bowling ball to our huge thunderstorm set up, cutting off the heat at the surface, and in turn, cutting off fuel for thunderstorms. The storms should push east of I-5 and rapidly fizzle even as we get toward and just past sunset, leaving us with a much quieter night. There is a risk of thunderstorms continuing in the mountains and foothills into the night as some of the higher-based storms will still have some lingering momentum until they eventually die -- sort of like how your car will continue to drift down the road a bit even after it runs out of gas. But really, the area of thunderstorm concern is about 3-9 p.m.

The rest of the night will feature breezy southwest winds, much cooler temperatures, the death of the muggy feel -- and eventually some good old fashioned Seattle rain showers.

That's the story for Friday as that trough finally swings through with light scattered showers and highs back only in the 50s.

So to recap timing:

  • 8-10 a.m.: Area of Fog
  • 10 a.m. - 1 p.m.: Sunny, warm and muggy
  • 1-3 p.m.: Towering clouds begin to form
  • 3-9 p.m.: Scattered strong thunderstorms with hail, heavy rain and frequent lightning.
  • 9-midnight: Gusty southwest winds, rapidly cooling temperatures, increasing low clouds. Thunderstorm threat rapidly wanes.
  • Midnight-Friday: Scattered light showers.

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