I've been quiet on the blogging front for the past several months as I've been transitioning out of graduate school life and into the postdoctoral world. I've moved from the University of Washington out to NCAR in Boulder, Colorado, which will give me any entirely new wealth of weather phenomena to talk about. But, I could not resist putting up a blog about the forecasted major wind events expected to impact the Pacific Northwest this weekend...
This looks like it's going to be a significant series of events throughout the Pacific Northwest. We can start by reviewing the large-scale synoptic pattern Here's this morning's GFS-WRF model analysis at 500 hPa over the region from my OLYMPEX model page:
We can see the multi-lobed threat heading towards the Pacific Northwest. There is broad-scale troughing across the entire northern Pacific. Embedded in that longwave trough are what we call "shortwave" troughs--little wiggles along the edges of the main trough. These wiggles may be little, but they represent enough instability aloft to generate some powerful storms. Note that one lobe has already moved up into northern Vancouver Island and the Canadian coast. That was associated with the rain and front from last night in the area. The next shortwave is still out to sea, but rapidly approaching the coast. By late this evening, it will have deepened considerably off the coast:
This is going to be followed by another shortwave that is so rapidly-developing that you can't even clearly see it yet in the above image. By late Friday evening, it's starting to show up---you can see a little ripple in the central Pacific associated with a little maximum in the colored vorticity field:
This trough is associated with the remnants of Typhoon Songda, as Cliff Mass discusses on his blog. This rapidly deepens throughout the night and into Saturday morning, reaching the coast by Saturday evening:
At the surface, these deepening troughs lead to strong cyclones with powerful winds (and a lot of rain too!). The local WRF model currently (as of the latest 12Z run, but this could change!!!) brings 25-30 knot winds to Seattle with the Friday storm:
But over 50 knot winds to Seattle with the Saturday storm:
This morning's WRF run, as seen above, takes the Saturday storm on an almost perfect track to produce a major windstorm in Seattle. This is a scary possibility, and concerning that this is the latest model development. But, should we believe just one model?
In modern weather forecasting, we like to look at our forecasts in terms of probabilities, as there remain great uncertainties in our forecasts. We often do this by considering not just one model forecast like I showed above, but several, often in the context of what we call "ensemble" forecasts. One ensemble used is the Global Ensemble Forecasting System (GEFS), run by the US National Weather Service. We can look at these ensemble forecasts and compare them to previous ensemble forecasts to see just how unusual or significant a particular forecast may be. NOAA has an (experimental?) product called the "Ensemble Situational Awareness" table (http://ssd.wrh.noaa.gov/satable/). You can go through time and different forecast variables over a particular region and at a glance see if there is anything significant going on in the forecast. Here's the table from last night's GEFS run over the Pacific Northwest:
Each row is a different forecast time and each column is a different forecast variable (e.g., SLP is sea-level pressure, WSP is wind speed). See all those "MAX" and "MIN" values? That means that somewhere in the Pacific Northwest the forecast at that time, for that variable, is greater than (or less than) any ensemble member's forecast ever for that location. This is a long period of extremes coming up for the Pacific Northwest and hits at the potential historic nature of this storm.
Let's dive more into the ensemble forecasts. Here's an image from Brian Colle's extratropical cyclone tracking page (http://smokey.somas.stonybrook.edu/cyclonetracks/Wcoast_Tprob.html) showing the forecasted positions of the Saturday low pressure center from all the members in two ensemble systems (SREF and GEFS).
You can see that the main swath of probabilities goes into central to northern Vancouver Island; this seems to be the most likely track if we consider all our model forecasts together. Such a path would really bring strong winds to the Strait of Juan de Fuca and surrounding areas of Vancouver Island. However, it would not be as bad over Seattle as the WRF run we saw above is suggesting. But even in the ensembles there is still a possibility it could cross the northern Olympic Peninsula...there are still respectable probabilities (20-30% in the blues) that the low could move over the northwestern Olympic Peninsula and into southern Vancouver Island. There is also still is a lot of uncertainty in the timing of the low since...as we saw...it's not even well-analyzed yet over the Pacific.
As noted by Cliff Mass in his blog, it's the path with the low coming closer to the Washington coast and across the northern Olympic Peninsula that tends to be the "worst case" for the Puget Sound region, as it sets up a strong north-south pressure gradient in the channel between the Olympic Mountains to the west and the Cascades to the east. This leads to the strongest wind events in the central Puget Sound region.
Speaking of strongest wind events, I was curious to see what the CIPS Analog system was saying for this storm (http://www.eas.slu.edu/CIPS/ANALOG/arch.php). This is a somewhat different method of forecasting from the raw numerical models we typically digest. In analog forecasting, we accept the idea that numerical weather models are often wrong, but assume that, given similar weather situations, they are wrong in the same way every time. So how does that help us?
To make an analog forecast, we take a model (here, the GFS) and look at its forecast for a certain time (I'm showing below the 72 hour forecast from last evening's 0000 UTC run, so this is the forecast valid on Saturday evening local time). We then go back in the records of all of the GFS forecasts (or reforecasts when the system is updated) ever made over the years and find all of the 72 hour forecasts that look most similar to the current one. We then look at what actually happened during those events and use that to make a guess as to what will actually happen this time. Remember, the key to analog forecasting is the assumption that, given similar weather scenarios, the models will be wrong in the same ways.
Anyhow, as part of this analog forecasting process, we can look at what historical events the system thought were "most similar" to the current one being forecast. Remember, this is limited to the time periods when we actually had GFS forecasts, so nothing past the 1980s is included. Here are the top 15 analogs, showing the sea-level pressure forecasts.
You can see that most of them have some sort of low off the northwest coast...a good sign that the analog is working. I tried cross-referencing these dates with the list of significant Pacific Northwest windstorms maintained by Wolf Read at (http://www.climate.washington.edu/stormking/). I was actually surprised to see that most of these dates did not correspond to particularly noteworthy windstorms (at least, what he has documented).
Interestingly, the one big match I found was the "Two windstorms in three days: November 13-15, 1981 event" (http://www.climate.washington.edu/stormking/November1981.html) which sounds very similar to the current threat of dual windstorms in only a few days. Here, from Wolf Read's page above, is the track of the first (stronger) storm in that November 1981 event:
You'll note that it made landfall over central Vancouver Island, similar to our current most-likely forecast swatch. However, this Nov 1981 storm had a much more south-north oriented track, which elongated the area of the coast that was affected by this storm and prolonged its effects. The current forecasted path for the Saturday storm is a little more west-east, which should decrease the amount of time exposed to high winds. The Nov 1981 storms did cause 12 deaths and "tens of millions" in damage, according to Wolf Read's summary. Also of note, the 520 floating bridge experienced some $300,000 of damage (in 1981) after taking waves driven by 75 mph winds on Lake Washington. This goes to show that even if lows don't exactly take the "classic" path for severe windstorms in central Puget Sound, Seattle can still see damaging events.
As the storms approach, our model solutions should converge on more likely forecast tracks with better estimates of the potential for wind damage. People from Vancouver Island down through Portland need to be on the lookout and make preparations for this storm. Be sure to frequently check your local National Weather Service office in Seattle (http://www.wrh.noaa.gov/sew/), Portland (http://www.wrh.noaa.gov/pqr/) or Environment Canada (https://weather.gc.ca/warnings/index_e.html?prov=bc) for the latest warnings and advisories. Stay safe!