11:00 pm Monday
What a shift in weather we’ve seen over the last week! In my last blog on 5/31, I was talking about the potential for record heat in Portland on Tuesday, 6/1. Portland indeed set a record that day, hitting 95 and breaking the record of 94 set back in 1970. Perhaps even more impressive was that Portland only dropped to 65 degrees by Wednesday 6/2 morning, and we were still at 78 degrees at midnight Tuesday. We still hit 90 degrees Wednesday, but temperatures mercifully fell to 67 by midnight Wednesday as cooler, marine air began to breach the Coast Range and flood into the Willamette Valley.
Before I delve too much into last week’s weather, here some main highlights for the forecast this week and beyond:
- Expect cool weather through Friday as a series of troughs stall off the coast. Tuesday will likely feature a few afternoon/evening showers, Wednesday could feature more showers (but they will mainly be over the south/central Oregon Cascades and over Eastern Oregon), and Thursday could feature a few more showers, though they will probably be lighter than either Tuesday or Wednesday.
- An unseasonably strong cold front will bring a shot of rain Friday morning. Friday should be the coldest day of the week, with highs only reaching the mid-60s for most spots in the Willamette Valley.
- We’ll see a warming trend over the weekend as a massive ridge/heatwave centered over the Desert SW expands over much of the Western US and the upper-level trough responsible for Friday’s cold front moves to the south, giving us southwesterly flow on Saturday that will turn due southerly by Monday. However, this warming trend will be accompanied by a threat of thunderstorms, particularly Sunday through Tuesday, with the greatest threat over and east of the Cascades.
- The latter half of next week looks dry and warm, but current forecasts give us onshore flow, which should keep temperatures from rising to the levels experienced last week.
Temperatures continued to fall for the remainder of the week, and by the weekend, temperatures were below-average for NW Oregon and well below-average for Western Washington. In fact, Seattle only hit 55 degrees on Sunday 6/6, making it the coolest June day since 6/4/2012.
A cool, wet June day so far. High temp at Sea-Tac Airport so far is 55°. That is the coldest high in June in 9 years (since 55° on June 4th, 2012). However, the record coolest max temp for Sea-Tac Airport in June is 51° set in 6/2/1966 & 6/11/1952. #wawx #Junuary
— NWS Seattle (@NWSSeattle) June 7, 2021
These cooler temps were associated with a deep, mature low-pressure system that approached the Pacific NW on Saturday and dragged a front across Western Washington during most of Sunday. On Saturday afternoon and evening, the showery, unstable, onshore flow ahead of the front supported a strong, persistent Puget Sound Convergence Zone over much of Snohomish County that extended westward to eastern Jefferson County on the Olympic Peninsula. Right in the middle of the convergence zone was Hidden Valley Camp, a summer camp near Granite Falls that I went to as a camper and counselor. Saturday’s convergence zone alone dropped nearly an inch of rain on the camp. I remember the summer rainstorms there as a kid and how they soaked our tents and turned the horse corral into a mud pit. Thankfully, summer camp hasn’t yet begun (it is open this year after being closed for Covid last year!), so this rain will hopefully help encourage some of the grass on the playfield to grow a bit more before the campers (usually the boys) come and pick it away!
Pretty classic convergence zone today. Here is the rainfall for the last 12 hours ending at 10 pm. One characteristic of the convergence zone is the dry area south of the zone which shows up well on this analysis. #wawx pic.twitter.com/YDLDQ4UNut
— NWS Seattle (@NWSSeattle) June 6, 2021
The convergence had a classic look on satellite, with a well-defined east-west band of clouds over the north-central Puget Sound area and clear skies to the north and south due to the air sinking on either side of this convergence zone and inhibiting cloud formation.
Credit: College of Dupage
Snow fell all the way down to 4000 feet, with flakes observed at both Stevens and Crystal. In fact, Crystal opened Green Valley this weekend for skiing!
Sunrise at Stevens Pass bouncing through a little cloud somehow pic.twitter.com/OsnbBXOJ2Q
— NWS Seattle (@NWSSeattle) June 7, 2021
So with all the talk of awful snowpack throughout the West (and it is indeed awful, particularly for the Klamath Basin and points south), the Washington Cascades are a bit of a bright spot. The current snow water equivalent (SWE) chart illustrates this well.
In total, most of Western Washington saw 0.25 – 0.75″ of rain over the weekend, with locally higher amounts where the Puget Sound Convergence Zone set up Saturday afternoon through early Sunday morning. As usual, the coast and Olympics/Cascades saw the highest amounts. The Willamette Valley was much drier, with Portland only receiving 0.11″ through this storm cycle. However, some weather stations in the Coast Range directly to the east of Portland recorded nearly 2 inches of rain, showing how big of an impact terrain has on rainfall amounts, particularly when winds are perpendicular (in this case, westerly) to the terrain.
Credit: NWS
This Week’s Forecast
If you want some more cold, showery June weather, you are in luck! A deep trough (visible as the broad “dip” in the upper-level height lines (green) below) will slowly move over the Pacific NW, bringing us cool/showery weather with unseasonably low snow levels. This is the infamous “Juneuary” weather regime, although I bet most Pacific Northwesterners will trade some cool and cloudy days for a bit of drought/fire danger relief. I know I will!
Credit: University of Washington
This trough will sit offshore Tuesday, keeping us in a cool and marginally unstable environment. This will support a few afternoon and evening showers across Western Washington and Western Oregon. These are the type of “pop-up” showers that seem to come out of nowhere and can be quite heavy, so be sure to check the radar before heading out and about tomorrow afternoon/evening! You don’t want to get caught under one of those soakers…
On Wednesday, the trough will move inland to the south, resulting in more widespread showers over the south/central Oregon Cascades and much of Eastern Oregon. Some lightning is possible, but this shouldn’t be a prolific lightning producer and should help the fire threat down there by wetting some of the fuels. Additionally, note the next trough in the Gulf of Alaska, slated to impact the Pacific NW.
Credit: Pivotal Weather
Friday morning should be wet. The aforementioned Gulf of Alaska trough will deepen, move SE, and direct a robust cold front into the Pacific NW. Unusual weather just 10 days away from the summer solstice, but not unheard of… it’s not called “Juneuary” for nothing!
Credit: Pivotal Weather
Credit: University of Washington
It’s good news that we are seeing this rain this week, as I’m closely watching a potential pattern transition over the weekend that could give us thunderstorms over the weekend into early next week. A massive ridge of high pressure is expected to build across the Western US, bringing the Desert SW and much of the Intermountain West its warmest temperatures of the year. Meanwhile, Friday’s trough will weaken and drop to the south, directing southerly flow aloft into the Pacific NW.
Credit: Pivotal Weather
This is the type of pattern that gives relatively dry, lightning-heavy thunderstorms to the Cascades and Eastern Washington/Oregon, as it directs warm, moist air into the region at the surface and increases instability. Additionally, the area upstream (to the NE) of a trough and downstream (to the SW) of a ridge is a preferred area for storm formation, as there tends to be more upper-level divergence in this region, which supports surface convergence, dynamic lift, and associated weather systems/thunderstorms.
The below image shows the precipitable water (amount of liquid water in a column of the atmosphere if all the water vapor in it were condensed) at 5pm Monday, the same time as the upper-level height chart above. The precipitable water chart shows the stream of subtropical moisture flowing right into the Pacific Northwest, giving us warm and unusually “sticky” weather with dewpoints in the 60s and an unstable environment favorable for convection.
Right now, the moisture doesn’t look quite deep enough or the wind profile favorable enough for severe thunderstorms, but that could change (conversely, the pattern could become much less favorable for t-storms). Regardless of thunderstorms, widespread heat looks like a pretty good bet for a broad portion of the Western US, particularly the Desert SW, Intermountain West, and Central/Southern California, and even the Pacific NW should be warmer-than-average due to us being on the periphery of this ridge.
Have a great week and enjoy the showers Tuesday/Wednesday (and potentially Thursday) and the frontal system Friday morning before we transition to a warmer and potentially stormier pattern this weekend as flow turns more southwesterly.
Charlie
