Relief On The Way, But More Heat Next Week

The past three days have been exceptionally hot throughout the Pacific Northwest. On Thursday, the coast sizzled in 90 degree heat, with Quillayute hitting 96, Port Angeles and Sequim hitting 94, and Forks hitting 93. While Quillayute and Forks average a day or two at 90 per year, 90s are practically unheard of along the Strait of Juan de Fuca, with Port Angeles only hitting 90 once every five years.

On Friday, 90s spread over the entire area. The coast was still sizzling, and with highs of 96 and 95 at Quillayute and Hoqiuam, respectively, Friday was only the second time on record that both of these locations hit at least 95 degrees (last time was September 2, 1988). Further inland, Seattle jumped up 8 degrees from Thursday’s high of 87 to hit 95, and Olympia hit 96. Warmed by easterly downslope flow, some communities near the Cascade foothills, like Duvall, jumped into the triple digits. Isolated from the relatively cool waters of the Puget Sound and the Pacific Ocean, many communities in the Willamette Valley also soared into the triple digits, with Salem and Eugene both hitting 104 and the Portland Airport hitting the century mark. Needless to say, plenty of temperature records were broken west of the Cascade crest.

Today, the coast cooled off significantly as the flow switched to onshore. While Hoqiuam was at 95 at 2 pm Friday afternoon, they were only at 61 degrees on Saturday afternoon. However, the wind shift hadn’t yet made it to the I-5 corridor, as Seattle still topped out at 91 degrees, making today only the 6th time since the 1890s that back-to-back 90+ degree days had been measured at Sea-Tac after August 15th. But this wind shift is marching inland as we speak, and by morning, many of you will wake up to cloudy skies and cool temperatures.

One of the best ways to track cooling, onshore wind shifts or “marine pushes” during these heat waves is to look at the pressure gradients between coastal stations and stations further inland. Below, I’ve listed pressure gradients for the last 24 hours from various stations across the Pacific Northwest and highlighted the HQM-SEA (Hoquiam-Seattle) pressure gradient, as this gradient shows the pressure difference from the Pacific Coast to the the Puget Sound Lowlands. A positive number means that Hoqiuam has a higher pressure, and since air flows from higher pressure to lower pressure, it implies that there is cool, onshore flow coming off the Pacific and streaming into the Puget Sound lowlands. The stronger the gradient, the stronger the onshore flow. Generally speaking, a light marine push occurs when the gradient is between +1 and +2 hPa, a moderate push occurs from +2 to +3.5 hPa, and strong pushes occur when the pressure gradient is greater than +3.5 hPa.

With a gradient currently at +2.3 hPa, it would seem we would expect just a “moderate” push. However, because we’ve experienced record high temperatures over the past couple days and the gradient was previously offshore, the push will seem much stronger. For example, Shelton, which was 97 degrees at 5 pm, plummeted to 69 degrees by 9 pm.

NASA’s MODIS instrument on their TERRA satellite shot a spectacular image of the marine push making its way inland along the coast early this afternoon. Also note the smoke from the Godkin and Hayes fires in the Olympics and the sizable algae bloom in the Strait of Georgia. Sunday morning, expect the majority of the Pacific Northwest west of the Cascade Crest to be covered in low clouds.

Taking a look at current observations around the Pacific Northwest, you can clearly see all that coastal areas have transitioned to a cooler, moister onshore flow, but not all inland areas have. KHQM (Hoquiam) has a current temperature of 57 with fog and 10 knot winds from the SW, but much of the Puget Sound area is still in the 70s with northerly winds. Interestingly enough, KDLS (The Dalles) has westerly flow, but with a temperature of 90 and dewpoint of 43, it doesn’t appear as though this flow has marine origins. However, they should get a marine push before the night is over as well.

Looking ahead, a weak upper-level trough should drop by Sunday evening into Monday, cooling us off even more and making Monday morning’s marine push even stronger than Sunday morning’s. However, in its wake, high pressure once again develops over the Eastern Pacific.

This high pressure strengthens as the week goes on, and by the end of the week, a thermal trough should extend northward from California into our region, creating lower pressure offshore and thus giving us offshore flow. I would not be surprised to see Seattle once again hit the 90s next week, especially Thursday or Friday. Thankfully, models also show a dramatic marine push next weekend just like they do this weekend, so the 90s should be short-lived.

Hard to believe that Thanksgiving is just over three months away!

Thanks for reading!
Charlie