As you undoubtedly noticed, we had the remains of a weak frontal system move over the area today and it will continue to move inland overnight. As it does, it will continue to weaken and behind the front, a weak ridge of high pressure will start to build along the coastal waters producing an onshore flow. Since the frontal system is weak and the high building along the coast is weak, the timing of these events is not well-defined.
The satellite picture and the Sea/Tac Doppler Radar give us a nice view of the current situation.
April 18 500MB 
April 18 Surface Analysis 
April 19 500MB 
April 19 Surface Forecast 
April 20 500MB 
April 20 Surface Forecast 
April 21 Surface Forecast
The surface charts for today, Friday, Saturday, and Sunday show a consistent progression of these systems. The general rule is that the first day the ridge builds will be the best day for breeze and that matches what we are seeing with the pressure gradient being tighter tomorrow and into tomorrow night and then starting to ease Saturday.
The good news for West Vancouver YC’s Southern Straits Race is that most of the racing will take place on the west side of the Straits where the northwesterly will fill first, be the strongest, and last the longest. The lighter air will be in the starting area while the onshore flow continues to build in the Straits. You should probably start logging the barometric pressure, wind speed, and wind direction at Sentry Shoal, Sisters(34 knots of SE at 1400hrs), Halibut Bank and Sand Heads to get a feel for how fast things are changing.
The starting area should see 5-10 knots of westerly. This will hold until about midday when you should expect the wind to build to 10-15 knots of west-northwesterly as you beat across the Straits. Think about hoisting your first headsail in the port groove so you can do a quicker tack change as the wind builds.
It’s mid to late afternoon where the models diverge as far as wind velocity goes. One model has the wind building to 15-25 knots while the GFS model has it holding at 10-15 knots until midnight when it will build to 15-20 with higher gusts closer to Vancouver Island. This should hold until about midnight when the gradient should ease and the wind begins to drop. As the gradient does ease, expect lighter air the closer you get to the finish as the onshore flow will begin to conflict with the downslope drainage northeasterly coming down the mountains.
For the Long Course, it looks like the TP 52’s will make it around in about 19 hours finishing around 0500 hours. On the Medium Course, the Beneteau 40.7’s should be finishing about the same time. My apologies to the Short Course boats, I just don’t have any of your polars.
Good luck, be safe, and have a great race.
Bruce has raced and cruised the Pacific Northwest his entire life. He earned a Bachelor’s of Science from the University of Washington in Biological Oceanography and learned meteorology “to keep from getting kicked around on the race course.” Bruce spent nearly two decades as Associate Publisher for Northwest Yachting Magazine, retiring in mid-2015, and was the chairman of the board of trustees for the Northwest Marine Trade Association in 2014. (photo of Bruce driving Playstation is a bit dated, but cool)