DISCUSSION
Today through next Thursday...Well, it is often said across the Pacific Northwest that the Fourth of July is our unofficial start of peak summer weather, and Mother Nature appears to be right on time this year. A major heat wave will begin to develop today, then continue through the weekend for inland portions of SW Washington and NW Oregon.

KEY POINTS:

1. A major heat wave will occur for SW Washington and NW Oregon except for the immediate coast. Temperatures begin to heat up today, with dangerously hot conditions continuing through at least Sunday.

2. The hottest days will likely be Saturday and Sunday.

3. This heat wave will likely be remembered more for its duration than its intensity. Temperatures appear unlikely to reach the extremes encountered during the June 2021 Pac NW heat event. However, three to five consecutive days of 100-107 degree heat with little relief at night is still dangerous, especially to those sensitive to heat struggling to find a place to cool off.

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 rapidly as fuels dry out.

Forecast models finally appear fairly locked on to the weather scenario expected to play out over the next few days. We had always known it was going to get hot as a strong upper ridge built over the region - the question was how hot. Fortunately, it appears the most extreme hottest model solutions have fallen off the table with the 00z model/ensemble suite. NBM probabilistic guidance now suggests there is only a 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 Sunday afternoon. On the flip side, it is now a near-certainty (80-100% chance) that much of the Willamette Valley will reach 100 degrees by Sunday. This should happen for most of our inland valleys Friday afternoon, with intense heat persisting with our upper level ridge all the way through the weekend.

As of the 07z 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: 80-100% Greater than or equal to 105 degrees: 40-70% (highest north)
Greater than or equal to 110 degrees: 5-20% (highest north)
Greater than or equal to 115 degrees: Less than 5%

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 100-107 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 30-40% chance of lows at KPDX failing to reach 70 degrees Saturday night, but the chance is likely much higher than that for much of Portland due to microscale effects like the urban heat island effect which can make temperatures slower to cool off at night.

Part of the reason models have backed off from the more extreme hot solutions is the NW flow aloft and associated shortwave energy 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 will be very important 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 Tuesday, which should allow at least some onshore flow by midweek. With 500 mb heights still well above normal, the cooling influence of any offshore flow will be limited east of the coast range due to a shallow marine layer
So
although it should begin to cool down Tuesday night, temperatures will very likely remain above normal inland through most if not all of next week. Weagle

FIRE WEATHER
Increasingly hot weather will combine with dry and gusty afternoon northerly winds this afternoon/evening and again Friday afternoon/evening as thermal low pressure strengthens over SW Oregon. While fuels are not quite ready for our fire weather zones to be Red Flag Warning eligible, these conditions will quickly dry out vegetation over the coming days. Forecast guidance suggests winds will be lighter Saturday, but it will still be very hot, dry, and unstable with thermal low pressure overhead. Those working out on the line today or any day through this weekend should pay special attention to staying hydrated, as the expected conditions can easily cause heat-related illness or worse. Weagle

AVIATION
Little change in the pattern, with some coastal marine stratus. The only terminal impacted for the rest of the night will be KAST, which will see IFR cigs continuing through much of the night, lifting around 16-18z Thu. VFR conditions are expected afterwards for the rest of the TAF period. Central/southern coastal terminals (KONP) tonight will mostly just remain VFR, though there's around a 25% chance of intermittent MVFR cigs at times before 16z. Inland terminals will just remain VFR through the night, with NNW winds between 5-8 kts.

Daytime hours will just remain VFR for all terminals, and winds will gust right up to 15-20 kt during afternoon and evening hours.

PDX AND APPROACHES...VFR conditions throughout the TAF period.
Wednesday afternoon brings NW winds up to 10 kt starting at 20z Thu. Winds weaken going into Thu night. /JL

MARINE
High pressure over the northeast Pacific with a thermally induced low pressure over California/Oregon and the Great Basin. This pattern will persist through the remainder of the week and likely into the start of next week. Small Craft Conditions continue to develop across all the waters each afternoon and evening, with weakening winds in the second half of each night and early mornings going forward. Expect gusts up to 30 kt with similar conditions each afternoon and evening as winds will closely follow a diurnal cycle. Seas will be predominately wind- driven resulting in 5-8 ft at 8-10 seconds throughout the week.

PQR WATCHES/WARNINGS/ADVISORIES

OR...Excessive Heat Warning from noon today to 11 PM PDT Sunday for ORZ104>125.

Heat Advisory from noon today to 11 PM PDT Sunday for ORZ126>128.

WA...Excessive Heat Warning from noon today to 11 PM PDT Sunday for WAZ202>210.

Heat Advisory from noon today to 11 PM PDT Sunday for WAZ211.

PZ...Small Craft Advisory until 5 AM PDT Friday for PZZ272-273.