As I write this post, it feels a bit like autumn in Seattle with a light rain falling and a bit of a chill in the air. One might be tempted to think that summer is coming to an end, but a quick check at the weather forecast tells a different story.
Yikes. The 10-day ECMWF ensemble high temperature forecast for Seattle shows a steady climb for the next week with temperatures approaching the upper 80s-90s by Labor Day and the following week. Most ensemble members are on board with the heat.
A heat wave like this often happens in summer, but this is September. Last year, Seattle only hit 80 once in September, a high of 80 on the 3rd. In fact, Seattle has only experienced 17 days with 90+ degree highs in September since 1894, or once every 7.4 years.
So when the forecast is showing the potential for a couple of 90+ degree days in a row, there must be something interesting about to happen. Lets start with the current upper-level (500 hPa) pattern.
There’s already a ridge in place over the NE Pacific, flanked by a pair of troughs over Alaska and central Canada. It’s a nice ridge, but not nearly strong enough to bring 90+ degree temps west of the Cascades.
However, there is another feature of note on the above map, and it’s the reason I chose the Pacific view. See the little blue and purple bullseye in the Philippine Sea to the east of Taiwan? That is Typhoon Maysak, which currently has max sustained winds of 95 kt, equivalent to a category 2 hurricane.
In the short term, Maysak poses a serious threat to South Korea. In the longer range Maysak is also expected to have a major impact on the Pacific jet stream, resulting in significant weather impacts in the mid-latitudes, including over western North America.
The process by which this happens is called extratropical transition (ET) and it turns out that an appreciable fraction of significant autumn weather events in the PNW (both storms and heat waves) are at least partially triggered by storms that began as typhoons in the western Pacific. Lets step through the forecast and see how this is likely to unfold.
The key to getting a “supercharged” jet stream from an ET event is for a tropical cyclone to find itself in a favorable position as it moves into the mid-latitudes, ideally downstream of a developing trough. When this happens, energy from the tropical cyclone is injected into the jet stream. Models expect this process to occur later this week, as seen in the following sequence of forecast 250 hPa wind speeds (the upper troposphere, where the jet stream resides).
On Tuesday, the GFS model shows a stronger Malak in the East China Sea, with a trough (U-shape in the blue shaded jet stream) to its north.
Just 36 hours later, a massive increase in wind speed within the jet stream develops to the north and east of Malak.
What happens to all this energy? It causes a massive amplification of the downstream ridges and troughs in the jet stream.
We can easily see this happening in the forecast map for Saturday below. As a further consequence of the increased “waviness” in the jet stream, a big mid-latitude cyclone is predicted to form in association with the deep trough over the central Pacific. And closer to home, the ridge over the NE Pacific is expected to rapidly build.
By Labor Day, the model shows the Aleutian Islands getting slammed with a massive storm while a truly gigantic ridge is depicted off the west coast of the US and Canada.
Look at these height anomalies!!!!
How unprecedented is this ridge? The National Weather Service produces some useful graphics that show how the model forecast (the GFS ensemble) compares with the 1979-2009 climatology for the 3 week period centered on the forecast time. In the image below, the bright red areas show where the ridge is outside of climatology.
I have looked at these maps for many significant events in the past and I can not remember ever seeing such a large area outside of climatology.
Putting this all together, it is looking quite likely that numerous record high temperatures will fall along the west coast of the US and British Columbia in the September 6-9ish timeframe. However, it remains to be seen whether this will be a significant historic event (i.e. multiple days of 90+ in western Washington) or not. The extratropical transition process is very sensitive to details in the “phasing” between the typhoon and the jet stream so we will likely see some adjustments.
Finally, another concern that bears watching is smoke from the ongoing fires in California. As the ridge builds this week, an upper-level low off the coast of California may funnel some smoke north along the coast by the weekend, as depicted below.
However, combining a tropical cyclone, an upper-level low, and a wildfire smoke model is basically a trifecta of forecast uncertainty. So for now I’ll keep monitoring the forecasts before sounding the alarm on any smoke threat.