058 FXUS66 KPQR 210450 AAA AFDPQR Area Forecast Discussion...UPDATED National Weather Service Portland OR 950 PM PDT Wed Aug 20 2025 Updated Aviation Discussion... .SYNOPSIS...High pressure will build over the region Thursday through the weekend, bringing a prolonged stretch of hot and dry weather with Moderate to Major HeatRisk for inland valleys Friday through Sunday with high temperatures of 95-101F. Remaining warmer than average early next week with a 80-90% chance for high temps of 90F or warmer. Dry and breezy conditions over the southern Willamette Valley Thursday afternoon and early evening will increase fire weather concerns. && .DISCUSSION...Wednesday night through Tuesday night...A significant pattern change begins tomorrow for NW Oregon and SW Washington as a hot and dry conditions begin to develop across the region. This pattern shift is in response to an upper level ridge of high pressure currently centered over the Four Corners region, which will be building northward into the Pacific Northwest. Meanwhile, a thermally induced surface trough in southwest Oregon will shift northward over the Willamette Valley Thursday night into Friday with high pressure east of the Cascades. This set-up will bring a transition from low-level onshore flow to offshore flow, causing an increase in north to northeast winds for the Willamette Valley, Cascades and Columbia River Gorge (except due east in the western Gorge and high Cascades). Max wind gusts are still forecast to reach 20-25 mph, except 30-35 mph in the western Gorge. This will lead to elevated fire weather concerns in some areas, which is discussed below in the fire weather discussion. In regards to forecast temperatures, there hasn't been much change with today's update. The deterministic NBM continues to suggest heat will peak Friday and Saturday with widespread highs ranging between 95-101F across inland valleys. Overall model spread is minimal on Friday, suggesting confidence is high to start the heat event. Spread increases a few degrees on Saturday, with the NBM 90th percentile for high temps approaching 105F. Spread increases more considerably Sunday through Tuesday, suggesting confidence is relatively low regarding the exact duration of this heat event (e.g. will it only last 2-3 days, or will it last 4-5 days)? The latest NBM run (21z Aug. 20th) shows NBM 10th-90th percentile values ranging from the upper 80s/lower 90s to the lower 100s on Sunday and Monday, and mid 80s to around 100 degrees on Tuesday. For now, the most likely outcome is for high temps somewhere in the 90s on all three days (80-90% chance). Given the ongoing uncertainty regarding exact temperatures early next week, have left the current timing of the Extreme Heat Watch as is, covering Friday afternoon through Sunday night for inland valleys away from the coast. However, have decided to expand the Extreme Heat Watch to include the south Washington Cascade foothills and the remainder of the Oregon Cascade foothills, as these locations are likely to see temps peak in the 95-100F range with a Moderate to Major HeatRisk. Note these areas will also have mild overnight low temps in the 60s, similar to urban areas in the Willamette Valley. Note the coast is not included in any heat headlines as temperatures will be much cooler there with highs in the 70s. The main reason Major HeatRisk is not more widespread is due to the overnight low temperatures most nights remaining in the low to mid 60s for many locations. However, an increase in those overnight temperatures even 1-3 degrees could result in Major HeatRisk expanding in coverage. Either way, daytime temperatures will be very warm and will impact those who don't have access to adequate cooling, including those spending time outdoors. If you have extended outdoor plans, especially ones that include strenuous activity, it is important to stay extra hydrated to avoid heat-related illness. Another thing that comes into play early next week is the potential for monsoonal moisture moving into the area from the south, bringing increasing chances for showers and thunderstorms in the Cascades beginning as early as Saturday afternoon or Sunday afternoon when an upper level impulse will push northward towards the Cascades. Note steering flow looks to be very weak, suggesting any storms that do develop would move very slowly. Given the high PWAT values in place (over 1"), heavy rain would be possible with any thunderstorm that develops. As of right now, confidence is very low regarding the exact location and coverage of storms. NBM PoPs are generally around 15% or less, but have the potential to increase with future forecast updates. Even if thunderstorms develop east of the Cascade crest, there is the potential for convective cloud debris to impact high temperatures depending on the coverage and timing. -TK/HEC && .FIRE WEATHER...Expect hot and dry conditions Friday through at least Sunday for areas away from the coast. Critical fire weather conditions are expected to occur over the southern Willamette Valley between 2 and 8 PM Thursday due to dry and windy conditions. During that time, expect northerly winds sustained around 15 mph with gusts up to 25 mph. Relative humidity values will fall as low as 15-25%. Main concern is for rapid fire spread with any grass fire that may ignite. A Red Flag Warning is in effect for fire weather zone 685 from 2 to 8 PM PDT Thursday to cover this threat. Modest easterly winds develop across the Cascade ridgetops and western Columbia River Gorge Thursday night into Friday. Wind gusts up to 35 mph will be possible in the western Gorge during that time. While winds of this magnitude would normally warrant a headline given the dry conditions in place, have decided not to issue any fire weather headlines for the Columbia River Gorge due to the round of wetting rain that fell August 16th. Would be more concerned if the breezy winds were set to occur near or after the end of the upcoming heat event, as fuels would have several more days of hot weather to dry out. Winds weaken beginning late Friday afternoon. Strong nighttime thermal belts will produce poor overnight relative humidity recoveries across the higher terrain Thursday night through at least Saturday night, especially for elevations above 3000 ft where maximum relative humidity values in the 20s and 30s are forecast. Note relative humidity values most likely won't follow a typical diurnal curve, as humidity values are likely to peak in the evening before dropping rapidly overnight. There is also a 10-15% chance of thunderstorms over the Willamette N.F. near the Cascade crest Saturday through early next week during the afternoon/evening hours each day. Note any thunderstorm that does develop will most likely be slow moving and wet, which would help decrease the potential for widespread new fire starts. -TK && .AVIATION...With high pressure overhead and mostly clear skies through the period, there is low confidence (guidance suggests a 10-25% chance) for LIFR/IFR stratus along the coast between 12-16Z Thursday. Otherwise, high confidence for widespread VFR conditions to continue through the TAF period. Coastal terminals will see mostly northwesterly winds, with wind speeds under 10 kt until 18Z Thursday, then becoming 13-18 kt with gusts up to 25 kt. Inland terminals will generally stay under 10 kt through most of the period, but will increase to around 7-10 kt by 20-23Z Thursday, with gusts up to 20 kt possible at KPDX, KSLE, and KEUG. PDX AND APPROACHES...VFR and mostly clear skies through the TAF period, with northwesterly winds expected around 7-10 kt with gusts up to 20 kt by 19-23Z Thursday. ~Hall && .MARINE...High pressure will remain offshore over the northeast Pacific through the remainder of the week and through the weekend. This will maintain northerly winds across the coastal waters, with strongest winds diurnally driven each afternoon and evening as pressure gradients tighten. Expect winds to gust up to 25 kt by late this afternoon south of Cape Falcon. Winds ease a bit by early Thursday morning, then increase again, with gusts up to 25-30 kt, with gusty northerly spreading northward as well. Expect similar conditions through the end of the week. So, have maintained the Small Craft Advisories through Friday afternoon. Seas generally remain wind driven, at around 4 to 6 ft with a dominate period of around 8-9 seconds. /Hartsock && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Extreme Heat Watch from Friday morning through late Sunday night for ORZ108>125. Red Flag Warning from 2 PM to 8 PM PDT Thursday for ORZ685. WA...Extreme Heat Watch from Friday morning through late Sunday night for WAZ204>210. PZ...Small Craft Advisory from 2 PM Thursday to 5 PM PDT Friday for PZZ251-271. Small Craft Advisory until 5 PM PDT Friday for PZZ252-253-272- 273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland