National Weather Service (2024)

Area Forecast Discussion
Issued by NWS Portland, OR

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-- Discussion containing changed information from previous version are highlighted. --456FXUS66 KPQR 072155AFDPQRArea Forecast DiscussionNational Weather Service Portland OR255 PM PDT Wed Aug 7 2024.SYNOPSIS...
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Hot, dry, hazy and/or smoky conditions will continuethrough Friday, with the hottest and smokiest conditions expected onThursday. Onshore flow at the coast will keep conditions much cooler.Offshore flow develops Thursday and will push additional wildfiresmoke and/or haze westward into the Portland/Vancouver metro, CoastRange, and much of the Willamette Valley. Cooling off this weekendinto early next week as upper level troughing develops over the area,bringing morning cloud cover and a 10-20% chance of light drizzlealong/near the coast and in the south WA/north OR Cascades.
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Now through Friday night...Satellite and surfaceweather observations from Wednesday afternoon depicted increasingwildfire smoke across the Columbia River Gorge, Portland/Vancouvermetro, the south WA Cascades/foothills, portions of the north ORCascades and eastern Lane County. The smoke is coming from wildfiresburning in various locations across the region, especially theongoing fires in Lane County, the Whiskey Creek Fire burning in theGorge and the Williams Mine Fire near Mt. Adams. Although surfacewinds remain onshore, mid-level winds are out of the east, hence thereason for increasing haze across much of northwest OR and southwestWA this afternoon. Locations that are experiencing smoke at thesurface and/or increasing ozone pollution will be dealing withperiods of degraded air quality over the next 36-48 hours; a varietyof Air Quality Advisories are now in effect, including thePortland/Vancouver metro, Marion County, and eastern Lane County.This includes Salem and Oakridge.Aside from air quality concerns, smoke is also impacting thetemperature forecast. While the NBM typically performs quite well fortemperatures this time of year, this is not the case when significantsmoke and haze are a factor as the NBM does not directly account forthe impact of smoke on temperatures. As such, the NBM typically runstoo warm on days with wildfire smoke, assuming smoke layers aloftand/or near the surface are dense enough. This is already proving tobe true today in some locations; the deterministic NBM is calling forhigh temps between 91-93 degrees across the Willamette Valley andPortland/Vancouver metro, however observed temps as of 2pm were stuckin the upper 70s over the Portland metro where upper level smoke wasmost dense. Warming another 15 degrees over the next 2-3 hours seemslike quite a stretch, so it is safe to say the NBM is already runningat least 3-6 degrees too warm. This is also the case when comparingthe latest NBM 1-hourly guidance with observed hourly temps.Given the fact that wildfire smoke will be increasing both aloft andnear the surface tonight through Thursday as offshore winds developthrough the Columbia River Gorge and push additional smoke into thearea, suspect the deterministic NBM will remain around 5 degrees toowarm. As such, have decided to nudge towards the NBM 10th percentileby 50% for the high temperature forecast on Thursday (for locationsto the east of the Coast Range where smoke and/or haze will be mostprevalent). This lowered the high temperature forecast by 3-5degrees, which also pushed the HeatRisk category down from major tomoderate for the Portland/Vancouver metro. Given the ongoing smokeimpacts, high temps will likely peak between 90-95F for inlandvalleys on Thursday rather than 95-100F. Still hot, but not quite ashot as initially expected.The flow regime turns back onshore on Friday, bringing improving airquality for most of northwest OR and southwest WA, except in easternLane County and immediately downwind of fires burning in Linn County,the Columbia River Gorge and south WA/north OR Cascades. Temps willalso begin to gradually cool each day, falling from the upper80s/lower 90s on Friday to the mid/upper 80s on Saturday. Inaddition, patchy light drizzle is possible along the central ORcoast Friday into Friday night (15-25% chance) as a passing upperlevel shortwave helps deepen the marine layer.-TK
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Saturday through Tuesday night...WPC cluster analysesare in agreement for broad upper level troughing over the PacificNorthwest this weekend. We`ll likely remain warm and dry as themajority of ensemble members suggest 500 mb heights will be nearaverage, but excessive heat is unlikely. NBM probability of 95degrees or hotter this weekend in the Willamette Valley is onlyat 5-25%. The most likely outcome for high temps this weekend is midto upper 80s for inland valleys and upper 60s to lower 70s at thecoast. There is also a 10-25% chance of light drizzle near the coastSaturday night into Sunday morning as the marine layer will likely bedeep enough. The western slopes of the coastal mountains will alsohave chances for drizzle given the light upslope winds.Monday to Tuesday, the majority of ensemble members stillsuggest broad troughing over the Pacific Northwest with relativelystronger onshore flow. 20% of members now show 500 mb heights fallingslightly below average, while the remaining 80% show near normalheights. This would lead to a cool-down to average or slightly belowaverage temperatures. Therefore, the current forecast calls for hightemperatures in the upper 70s to low 80s for interior valleys earlynext week. Any meaningful precipitation with this trough still looksunlikely, but there is a 10-20% chance of 48 hour QPF amounts greaterthan 0.05 inch across northwest Oregon and southwest Washington(ending 5 AM Wednesday). At the very least, would suspect increasingchances for coastal drizzle with this type of pattern in place alongwith morning cloud cover for inland valleys. -TK/Alviz
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Thursday...Easterly winds will develop over theColumbia River Gorge and portions of the Mt. Hood and G.P. NationalForests (strongest along exposed ridges and east-west orientedvalleys), resulting in near critical fire weather conditions for afew hours Thursday afternoon. Expect sustained winds around 10-15 mphwith gusts to 20-25 mph, except gusts as high as 30-35 mph possibleon exposed ridges prone to easterly winds (20% chance). These windswill be occurring simultaneously with critical relative humidityvalues in the 15-20% range. While red flag warning criteria will mostlikely not be met across most of our Columbia River Gorge fire zone,criteria may be met in localized areas for a few hours (e.g. exposedridges prone to east winds). Although criteria is not being metacross a large enough area to pull the trigger on a red flag warning,conditions will still be favorable for an increase in fire activityand smoke production. Fortunately, east winds look to be quiteshort-lived, weakening in strength Thursday night into Friday morningbefore winds turn back onshore Friday afternoon. -TK
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VFR throughout the airspace. Around 06Z Thursday weakonshore flow will likely bring back IFR/LIFR conditions to thecoast (60%-70% probability) before pulling back from the coastaround 18Z Thursday. Generally north/northwesterly winds 5 to 10kt with gusts up to 15 kt. Could see some gusts up to 25 kt alongthe coast and up to 20 kt for KSLE and southward from 20ZWednesday through 02Z ThursdayWildfires burning across the WA, OR and CA as well as localwildfires located near Mount Adams (NE of KPDX), Central ORCascades (east of KEUG) as well as within the Columbia RiverGorge (east of KPDX and KTTD) could result in some slant rangevisibility issues.PDX AND APPROACHES...VFR conditions through the forecast period.North/northwesterly winds around 5-10 kt. Gusts up to 20 ktpossible from 20Z Wednesday through 02Z Thursday. /42
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Generally north/northwesterly winds through the weekend.Small Craft Advisory gusts up to 30 kt starting this afternoonand persisting through tonight. Zones PZZ272 and PZZ273 willmaintain these conditions the longest, with other zones fallingbelow criteria through this evening. Seas 3-7 ft withpredominately wind driven conditions through the weekend. /42
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OR...None.WA...None.PZ...Small Craft Advisory until 11 PM PDT this evening for PZZ251-271. Small Craft Advisory until 11 PM PDT this evening for PZZ252. Small Craft Advisory until 8 PM PDT this evening for PZZ253. Small Craft Advisory until 5 AM PDT Thursday for PZZ272-273.
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