It was toasty this Memorial Day weekend! Portland hit 82 on Saturday, 84 on Sunday, and 88 today, According to chief KPTV meteorologist Mark Nelsen, this was the warmest Memorial Day weekend by average high temperature since 1992, which had an average high of 86 degrees over the three days and a peak of 93 degrees on 5/24. Salem Airport hit 90 degrees today, and further south, most of California’s Central Valley hit the low 100s today.
The combination of low 100s inland, sea-surface-temperatures in the low 50s, and a robust seabreeze this afternoon/evening resulted in a nearly 40 degree range in temperatures across the San Francisco Bay Area, and a 50 degree range if you head further east to the Central Valley.
Wow, look at the range in temps over the Bay Area this evening. Low 100s over the Central Valley due to a strong ridge overhead, but mid 50s over San Francisco due to a strong #seabreeze and cooler-than-average waters offshore. #cawx #heatwave pic.twitter.com/PEEKy6HtsL
— Charlie Phillips (@GeoduckChuck) June 1, 2021
At 6:00 pm, a weather station near the southern portion of the Golden Gate Bridge was at 56, while a National Weather Service weather station in northern San Rafael approximately 20 miles north of the GG Bridge was at 95 due to downslope warming from this westerly flow sinking down the lee slopes of the North Bay Mountains. The next time somebody tells you that California meteorologists have it easy and just get paid to forecast sunshine and smog, tell them about the many microclimates of the San Francisco Bay Area! I can’t imagine a Bay Area weatherman would remain employed for very long if he used the same temperature forecast for San Francisco and San Rafael.
Forecast
Tomorrow should be the hottest day of our entire heatwave. The ridge of high pressure will amplify and create a thermally-induced trough along the Coast Range. The ridge amplification will result in warmer temperatures aloft and at the surface (which will thus induce the thermal trough, since warm air is less dense than cold air and is associated with lower pressure), and the thermal trough will create offshore winds and downslope flow along the western Cascade slopes, resulting in further warming there.
Credit: University of Washington
Because the thermal trough will be over the Coast Range and not offshore, the immediate coastline should see onshore flow tomorrow, and many places along the immediate coastline (Newport, Lincoln City, Yachats, etc.) should stay in the 60s. Areas along the northern Oregon Coast could reach the low 70s, and areas just a little further inland (Tillamook, Astoria, etc.) should reach the upper 70s or lower 80s due to weaker onshore flow there.
The graphic below shows the expected temperatures at 4pm tomorrow (which is near the time of maximum high temps for most spots) and does a great job of showing the sharp temperature gradient along the coast. It may not be as intense as that in the Bay Area, but it is impressive nonetheless. And don’t forget about the widespread low-mid 90s across the Willamette Valley! Portland’s record on 6/1 is 94 and was set in 1970, so we will come very close to tying or exceeding that mark.
Forecast model: 6/1/2021 00Z HRRR
Credit: WeatherBell
On Wednesday, the thermal trough will move into Eastern Washington, resulting in widespread triple-digit heat there. Onshore flow should help decrease temperatures by a couple of degrees over the Willamette Valley, but they should still be up the upper 80s or lower 90s for most locations on Wednesday.
Image Credit: WeatherBell
A stronger cooling trend will occur Thursday and Friday as onshore flow increases, and by the weekend, highs should fall all the way to the upper 60s/low 70s on Saturday and the low 60s on Sunday with a chance of rain. As the below “spaghetti” plot shows (where each line represents one GFS ensemble member), there is pretty good ensemble agreement on this cooldown. The current forecast from 6/7-8 is for 850 hPa temps (850 hPa is approximately 5000 feet) to bottom out near freezing… a sharp contrast from the +22C (72F) temps expected Tuesday afternoon/evening!
Next weekend, the only memory of the heat wave will be high streamflows on some of the rivers flowing off the North/Central Cascades and Northern Rockies due to all the heat. My cousins have a house on the Wenatchee River near Peshastin, WA and are avid river rafters. Anna, Alec, Matt – if you are reading this, this Friday and the weekend will have super high flows down the Wenatchee! I don’t know if this is good or “too high” for river rafting, but regardless, be careful out there!
Enjoy the warmth this week before things turn rainy and chilly again this weekend!
