National Weather Service (2024)

Area Forecast Discussion
Issued by NWS Portland, OR

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261FXUS66 KPQR 041034AFDPQRArea Forecast DiscussionNational Weather Service Portland OR330 AM PDT Thu Jul 4 2024.SYNOPSIS...

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The hottest weather so far this year will develop byFriday, lingering through the weekend as high pressure strengthensaloft. The heat will be prolonged and dangerous with multiple daysexpected to reach 100 degrees for most inland valleys. Nights willstruggle to cool off, leading to minimal overnight relief, especiallyin urban areas. This heat wave will continue through at least Sunday,with some potential for it to last several days into next week. Norain is expected through the middle of next week.

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Today through Saturday...Well, it is often said acrossthe Pacific Northwest that the Fourth of July is our unofficial startof peak summer weather, and Mother Nature appears to be right on timethis year. A major heat wave will begin to develop today, thencontinue through the weekend for inland portions of SW Washington andNW Oregon.KEY POINTS:1. A major heat wave will occur for SW Washington and NW Oregonexcept 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 durationthan its intensity. Temperatures appear unlikely to reach theextremes encountered during the June 2021 Pac NW heat event. However,three to five consecutive days of 100-107 degree heat with littlerelief at night is still dangerous, especially to those sensitive toheat struggling to find a place to cool off.4. The coast will be the place to go to cool off, though the coolinginfluence of the ocean will only extend a few miles inland.Daytime sea breezes should keep coastal communities mostly in the 70sand 80s for highs.5. Dry northerly winds will accompany the hot temperatures along theCoast Range and Willamette Valley, resulting in rapid drying of finevegetation. Fire danger will increase rapidly as fuels dry out.Forecast models finally appear fairly locked on to the weatherscenario expected to play out over the next few days. We had alwaysknown it was going to get hot as a strong upper ridge built over theregion - the question was how hot. Fortunately, it appears the mostextreme hottest model solutions have fallen off the table with the00z model/ensemble suite. NBM probabilistic guidance now suggeststhere is only a 20% chance anywhere in our forecast area will reach110 deg F with this heat wave - with the best chance being in theTualatin Valley Sunday afternoon. On the flip side, it is now anear-certainty (80-100% chance) that much of the Willamette Valleywill reach 100 degrees by Sunday. This should happen for most of ourinland valleys Friday afternoon, with intense heat persisting withour upper level ridge all the way through the weekend.As of the 07z NBM run, here are temperature probabilities for theWillamette Valley on Sunday, where/when the most intense heat isexpected: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 moreimportant during a heat wave, because warm nights can limit theability to cool off and reset for the next hot day. This compoundingeffect of multiple hot days with warm nights will exacerbate the thethreat of heat-related illness, especially for those who aresensitive to heat and lack the ability to cool off. In that sense,this several day-long stretch of 100-107 degree highs and lowsstruggling to fall below 70 degrees is potentially more dangerousthan just a day or two of intense 110 degree heat followed by a quickcooldown. NBM suggests a 30-40% chance of lows at KPDX failing toreach 70 degrees Saturday night, but the chance is likely much higherthan that for much of Portland due to microscale effects like theurban heat island effect which can make temperatures slower to cooloff at night.Part of the reason models have backed off from the more extreme hotsolutions is the NW flow aloft and associated shortwave energyproviding some resistance to the ridge strengthening and bulgingnorthward across the Pac NW. How this NW flow and the northernperiphery of the ridge evolve over the weekend will be very importantto whether or not this heat wave gets reinforced for a second week.We have seen multiple occasions this year where westerly flow in theNE Pacific "bridges the gap" across the northern periphery of aridge, causing it to be weaker than it was depicted in early modelruns. Either way, all WPC clusters suggest the upper level ridge axisshifts east of the Cascades by Tuesday, which should allow at leastsome onshore flow by midweek. With 500 mb heights still well abovenormal, the cooling influence of any offshore flow will be limitedeast of the coast range due to a shallow marine layer. So... althoughit should begin to cool down Tuesday night, temperatures will verylikely remain above normal inland through most if not all of nextweek. Weagle.FIRE WEATHER...Increasingly hot weather will combine with dry andgusty afternoon northerly winds this afternoon/evening and againFriday afternoon/evening as thermal low pressure strengthens over SWOregon. While fuels are not quite ready for our fire weather zones tobe Red Flag Warning eligible, these conditions will quickly dry outvegetation over the coming days. Forecast guidance suggests windswill be lighter Saturday, but it will still be very hot, dry, andunstable with thermal low pressure overhead. Those working out on theline today or any day through this weekend should pay specialattention to staying hydrated, as the expected conditions can easilycause heat-related illness or worse. Weagle

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Little change in the pattern, with some coastal marinestratus. The only terminal impacted for the rest of the night willbe KAST, which will see IFR cigs continuing through much of thenight, lifting around 16-18z Thu. VFR conditions are expectedafterwards for the rest of the TAF period. Central/southerncoastal terminals (KONP) tonight will mostly just remain VFR,though there`s around a 25% chance of intermittent MVFR cigs attimes before 16z. Inland terminals will just remain VFR throughthe night, with NNW winds between 5-8 kts.Daytime hours will just remain VFR for all terminals, and windswill 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 20zThu. Winds weaken going into Thu night. /JL

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High pressure over the northeast Pacific with athermally induced low pressure over California/Oregon and theGreat Basin. This pattern will persist through the remainder ofthe week and likely into the start of next week. Small CraftConditions continue to develop across all the waters eachafternoon and evening, with weakening winds in the second half ofeach night and early mornings going forward. Expect gusts up to30 kt with similar conditions each afternoon and evening as windswill closely follow a diurnal cycle. Seas will be predominatelywind- driven resulting in 5-8 ft at 8-10 seconds throughout theweek.

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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.

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