FXUS66 KPQR 042204 AFDPQR

Area Forecast Discussion National Weather Service Portland OR 304 PM PDT Thu Jul 4 2024

SYNOPSIS
It’s going to get hot! A long duration, impactful, and historic heatwave is expected to develop by Friday, lingering through the weekend into early next week as an anomalous high pressure strengthens aloft. Across the inland valleys anticipate an almost 4-5 day stretch of near to above triple digit temperatures. Nights will struggle to cool off, leading to minimal overnight relief, especially in urban areas.
At least guidance has been trending towards the return of somewhat cooler temperatures around the middle of next week, albeit staying above normal. No precipitation is expected through the middle of next week.

DISCUSSION
Today through next Thursday...As we enter the “unofficial” start of summer across the Pacific Northwest (commonly joked about as July 4th/5th) Mother Nature has outdone herself as we embark into a prolonged stretch of dangerously hot conditions for inland portions of SW Washington and NW Oregon. Daily high temperature records will likely be broken for many inland sites going forward.

KEY POINTS:

1. A major heat wave will occur for SW Washington and NW Oregon except for the immediate coast. temperatures peak Friday through Monday, and now potentially even into Tuesday, with dangerously hot conditions.

2. The hottest daytime conditions will likely be Saturday, Sunday, and Monday.

3. This heat wave will likely be remembered more for its duration than its intensity. Exploring historical weather data at Portland, Salem, and Eugene reveals there have only been 3-5 comparable events in which there was a 4-5 day streak of consecutive 100 degree days. Four to five consecutive days of 100-106 degree heat with little relief at night is dangerous, especially to those sensitive to heat struggling to find a place to cool off. The longevity of this event will likely compound and exacerbate impacts.

4. The coast will be the place to go to cool off, though the cooling influence of the ocean will only extend a few miles inland.
Daytime sea breezes should keep coastal communities mostly in the 70s and 80s for highs.

5. Dry northerly winds will accompany the hot temperatures along the Coast Range and Willamette Valley, resulting in rapid drying of fine vegetation. Fire danger will increase as fuels dry out over the weekend into early next week.

A strong upper-level ridge of high pressure has begun to build into Western CONUS with guidance continuing to show better agreement regarding the finer details of the heatwave expected to last into early next week. One of biggest changes today has been the extension of near to above 100 degree temperatures into at least Monday of next week. Fortunately, it appears the most extreme model solutions suggesting 110+ temperatures have continued to wane. Latest NBM probabilistic guidance now suggests there is only a 10-20% chance anywhere in our forecast area will reach 110 deg F with this heat wave - with the best chance being in the Tualatin Valley extending towards the Portland metro Sunday afternoon. Still, this is just a minor consolation to the added duration of heatwave into at least Monday maintained by most deterministic and ensemble guidance.
Models begin to show the axis of the strong upper-level ridge slowly meandering eastward Tuesday into Tuesday night but have delayed its shift slightly. Should it stay on the latest project track it won’t be enough for any appreciable cool down across our inland areas with NBM probabilistic guidance now showing a 40-60% chance to meet or exceed 100 degrees Tuesday afternoon - the current forecast reflects this with high temperatures in the upper 90s to near 100 degrees.

As of the 13z NBM run, here are temperature probabilities for the Willamette Valley on Sunday, where/when the most intense heat is expected:

Greater than or equal to 100 degrees: 80-95% Greater than or equal to 105 degrees: 50-60% (highest north)
Greater than or equal to 110 degrees: 5-20% (highest north)
Greater than or equal to 115 degrees: 0-3%

As recent studies have shown, night time temperatures are often more important during a heat wave, because warm nights can limit the ability to cool off and reset for the next hot day.
This compounding effect of multiple hot days with warm nights will exacerbate the the threat of heat-related illness, especially for those who are sensitive to heat and lack the ability to cool off. In that sense, this several day-long stretch of 100-106 degree highs and lows struggling to fall below 70 degrees is potentially more dangerous than just a day or two of intense 110 degree heat followed by a quick cooldown.
NBM suggests a 30-50% chance of lows at KPDX failing to reach 70 degrees Saturday night, Sunday night, and Monday night.
Unfortunately this chance in reality is likely much higher than that for much of the Portland/Vancouver Metro due to microscale effects like the urban heat island effect which can cause temperatures to cool slower than they normally would at night.

Part of the reason models continue to back off from the more extreme hot solutions is the NW flow aloft and associated shortwave energy over Canada providing some resistance to the ridge strengthening and bulging northward across the Pac NW. How this NW flow and the northern periphery of the ridge evolve over the weekend into Monday and Tuesday of next week will be paramount as to whether or not this heat wave gets reinforced for a second week. We have seen multiple occasions this year where westerly flow in the NE Pacific "bridges the gap" across the northern periphery of a ridge, causing it to be weaker than it was depicted in early model runs. Either way, all WPC clusters suggest the upper level ridge axis shifts east of the Cascades by Wednesday, which should allow at least some onshore flow during the midweek. With 500 mb heights still well above normal and the rather slow progression of the upper-level pattern, the cooling influence of any offshore flow will be limited east of the coast range due to a shallow marine layer.
Although it should begin to cool down on Wednesday temperatures will very likely remain 10-15 degrees above above normal inland through most if not all of next week. It’s worth noting the “normal” high temperature for this time of year should be around 80-82 degrees from Portland to Eugene. -Schuldt/Weagle
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