Strange weather afoot: Warm, muggy air could lead to unusually strong thunderstorms
SEATTLE -- It seems if Seattle is indeed going to get some warm weather this year, it has to pay a price. As temperatures get set to rise to their warmest levels in several months, it'll come at the cost of muggy conditions and potentially strong thunderstorms -- stronger than we usually see around here.
We already hit 73 muggy degrees in Seattle on Wednesday -- part of the stormy setup pattern for Friday. Even warmer air is set to push in Thursday with temperatures climbing into the upper 70s -- perhaps even a few low 80s in the typically warm spots.
But with the really warm air comes a REALLY unstable atmosphere. Some forecast charts are predicting levels of instability that pushes the edges of what forecasters in Western Washington and Oregon can remember ever seeing.
An unstable atmosphere means conditions are ripe for warm air to rise to very high levels, creating towering thunderstorm clouds. The more instability, the taller and more intense the storms can be. And with warm, humid air at the surface, we have a lot of energy to fuel those storms.
Bottom line, all the ingredients are together for not just random thunderstorms, but rather strong thunderstorms, especially by Pacific Northwest standards where the term "thunderstorm" usually gets to be in quotes compared to their more intense counterparts in...just about anywhere else in the world.
Whereas a typical thunderstorm around here might have a few strikes of lightning (we sometimes call them "one strike wonders") with some small "pling-pling" hail and a nice drenching rain shower, these thunderstorms have potential for frequent lightning, rather large-ish hail and some real torrential showers for possibly some localized urban flooding. And maybe some wind too in the storm's outflow.
But there are three counter-balancing factors that work in our favor to keep this from being too much of a headache. One: The storms look like they will be scattered in nature and random where they strike. We're not looking at a massive severe weather outbreak with widespread storms commonly seen in Midwest springs. Many areas may miss out on the fireworks.
Two: The period of extreme atmospheric instability is rather short and transitory -- mainly from mid afternoon Thursday through the late evening/early nighttime frames. So we're probably in a 6-hour window of thunderstorm possibilities, then it tapers off.
And three, the models may be overdoing it, because as mentioned before, the levels forecast seem quite out of the ordinary for what a Pacific Northwest atmosphere can typically dish out.
But suffice to say there is risk of strong thunderstorms and should warrant attention. Right now the best bet for the strongest-of-the strong is along the I-5 corridor in Oregon and Washington east into the Washington Cascades and their foothills. NOAA's Storm Prediction Center, which is in charge of putting out severe weather outlooks for the nation and so rarely has to worry about the I-5 corridor their forecasters might not even be able to locate it on a map, actually have painted parts of Washington and Oregon in their "marginal" risk for severe weather -- including the greater Seattle and Portland areas.
I can't remember the last time Western Washington had anything beside the light-green general "thunderstorm" risk but goes to show a bit of the potential. Their forecasters note that the storms' greatest risk is from large hail and possibly some strong winds in the storms' outflow (the gusts created by torrential rains dropping from the cloud base).
The storms will be moving from south/southwest to the north/northeast so keeping an eye on the southern horizon and your radar apps will be key. And also give approaching ominous clouds some greater attention Thursday than perhaps we normally would on typical Seattle rainy/stormy days. It might not just be a harmless shower whose penalty for getting caught is just a bad hair day; these could be more intense than you'd expect.
With lightning -- especially frequent lightning -- being a rarity here, the National Weather Service has tweeted these handy tips for lightning safety:
Any thunderstorms will quickly die off by or just before midnight Thursday night, done in by a strong marine push (hey, remember those? A sign of summer!) that will shove the warm, unstable air mass off to the east and replace it with a low overcast and much cooler temperatures. It'll also likely trigger its own showers in the transition, but these would be the harmless Seattle type rain showers.
Speaking of harmless Seattle type rain showers-- that's the story for Friday as we get back to some semblance of normal. A cold front will gradually sweep through during the day for continued light rain showers and highs only in the 50s -- a 20 or so degree drop from Thursday's shenanigans. Showers continue into Saturday morning, then fizzle out leaving us with a... rather dry and pleasant weather pattern stretching from Sunday into next week? Not sure which is more unusual -- that forecast or the thunderstorm one?
Avalanche danger also quite high
With the "hot" weather this week comes the highest freezing levels the mountains have seen in a while. The Northwest Avalanche Center is warning of potential avalanches in the Cascades and Olympics. "Travel in avalanche terrain is not recommended," the agency said.