Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
844 FXUS66 KPQR 071106 AFDPQR Area Forecast Discussion National Weather Service Portland OR 405 AM PDT Sun Jul 7 2024 .SYNOPSIS...Northwest Oregon and southwest Washington remain within the grip of a prolonged heat wave that will persist through Tuesday. The strong ridge of high pressure aloft responsible for our heat wave will slowly migrate eastward over the coming days. As it does so, thermal low pressure will focus on areas east of the Cascades, allowing onshore flow to strengthen Tuesday night and Wednesday. This will bring an end to the record hot temperatures, but above normal temperatures are very likely to persist through the end of the week. No rain is expected through next weekend. && .DISCUSSION...Today through Saturday...The overall forecast picture has changed little over the last 48 hours, and remains on track for oppressive heat to persist Coast Range eastward through Tuesday. Confidence continues to increase that Tuesday night will be the definitive end to this heat event as onshore flow increases and begins to moderate temperatures. Coastal communities generally had high temps in the 80s Saturday; some of these towns may remain stuck in the 70s today as thermal low pressure strengthens over the Willamette Valley and draws sea breezes onshore. The marine layer has deepened slightly over the past 24 hours according to the Astoria PSL profiler, which adds confidence to at least slight cooling along the coast this morning. Marine stratus has also developed near the coast overnight, but mostly appears limited to areas with colder SSTs due to upwelling. That said, the perfect sunny beach days way be ending for the coast, as the daytime sea breezes may advect some of this stratus onshore. KEY POINTS: 1. Record or near-record heat will persist inland through Tuesday. 2. High temperatures will likely approach 105 degrees for the Willamette and Hood River Valleys Monday and Tuesday. For most of these valleys, latest probabilistic guidance suggests a 40-70% chance of reaching 105 degrees each day. 3. Nights will be slow to cool off in urban areas through Monday night. Strengthening onshore flow will bring some relief Tuesday night into Wednesday. 4. The prolonged and compounding nature of this heat wave will make it increasingly hard for people, pets, and plants to cope with the heat. This will especially be the case for those with medical conditions that make them sensitive to heat. 5. Coastal areas will remain cooler, with highs generally in the mid 70s to mid 80s today and Monday, then cooler for Tuesday. It is important to impress on everyone that aside from the coast, we have not yet seen the worst of this heatwave. The most brutal period of heat appears to be Monday through Tuesday for the Willamette Valley and Lower Cowlitz eastward. Today`s temps will be similar to the last two days, but models have been very insistent on temps climbing another 2-4 deg F for Mon-Tue. Additionally, Monday night appears to offer the least overnight relief from the heat, with NBM probabilities of lows 70 deg F or higher at around 60-90% for the Portland metro area and even 30-60% probabilities for much of the Willamette Valley. So... folks need to treat this heatwave like a marathon and not a sprint. Pace yourself, try to limit outdoor exposure to the cooler morning or night hours. Most importantly, stay hydrated. Heat-related illness can sneak up on you, and this is the type of heat that puts just about anyone at risk. Also, don`t forget about your pets: hot surfaces such as concrete and asphalt can easily burn paws. Best to take your pets out during the cooler morning and night hours, if possible. It appears much of the uncertainty we were dealing with a couple days ago regarding the duration of the heatwave has been cleared up, now that models show reasonable agreement on downstream features such as the recurving of Beryl in the Gulf of Mexico. This will make room for our upper level ridge to expand and migrate eastward, focusing the most intense heat east of the Cascades. As the Columbia Basin becomes hotter than the west side, thermal low pressure will strengthen, pulling increasing onshore flow across much of western WA/OR and through the Columbia Gorge Tuesday night and beyond. The onshore flow will bring some moderation in temperatures, pulling them down from record territory. That said, NBM probabilistic guidance still suggests a 30-60% chance of the Willamette and Hood River Valleys climbing to 95 deg Wednesday. Our forecast leaned toward NBM 25th percentile temperatures Wednesday onward, as an approaching weak upper trough should weaken subsidence and allow the marine layer to expand some. For the Willamette Valley, the record number of days in a row with highs temps of 100 degrees of warmer is 5 days, set in mid-July 1941 at Portland airport. Many areas in the Willamette Valley/Clark County have seen 4 days in row a few times, such as back in August 2023, or 2015 or in 1981. For this heatwave, it looks like 4 days of 100+ deg heat is a good bet at Salem and Eugene, but will see if the record 5 days is reached. Portland has just missed 100F by one degree the past two afternoons. Here are the odds of reaching 100+/105+ deg at select cities, based on the 07z iteration of the NBM on July 7th via the 1D Viewer: Sun Mon Tue Wed July 7 July 8July 9 July 10 100 105 100105 100 105 100 105 ======================================================== Kelso 50% <1% 70%15%80% 30% 0% 0% Portland* 90% 20% 95%50%99% 70% 10% 0% Salem 95% 40% 99%70%99% 80% 10% 0% Eugene 95% 25% 95%50%90% 40% 5% 0% Hood River 85% 15% 95%65%99% 80% 25% 0% The % of reaching 110 or higher is <10% for all areas, except on Tuesday when there is a 15-30% chance across the Willamette Valley (best chance in Salem, second best chance in Portland). *Portland is for Greater Portland/Vancouver metro area. One little wrinkle to the forecast Monday night into Tuesday is some weak vorticity lifting northward from California, which will enhance instability over the forecast area. While mid-level lapse rates steepen impressively, it looks like we will lack the moisture needed for thunderstorms. The steepening lapse rates in the presence of thermal low pressure may end up being more of a fire weather concern, as it can promote deeper columns in any fires which may start. Meanwhile, fuels are drying out impressively, and all our fire zones now appear dry enough to be eligible for Red Flag Warning consideration. We considered issuing a Fire Weather Watch for our inland areas Monday through Tuesday, but opted to allow the day shift to coordinate with our fire partners regarding the status of fuels. Weagle && .AVIATION...Upper level ridge shifts over the region with light westerly flow aloft. High pressure will maintain clear skies and VFR conditions for inland sites through the period with hot daytime temperatures. As a result, expect the potential for high density altitude effects on takeoffs during peaking heating in the afternoon hours. Air mass along the coast cools later tonight into Sunday morning, with pockets of IFR stratus and fog forming just onshore before clearing later in the morning. PDX AND APPROACHES...High pressure maintains VFR conditions through the period. Light winds becoming northwest 5-7 kt by the afternoon. /DH && .MARINE...Not much in way of change. Thermal low pressure just onshore extending southward along much of the coast. Meanwhile, high pressure remains anchored well offshore. This will maintain northerly winds on the waters into early next week. Gradients tight enough to pop gusts to near 20 kt in afternoons/evenings, but that will be generally south of Cascade Head Sunday. High pressure builds on Monday, tightening pressure gradients with wind gusts up to 25 kt possible by Monday afternoon. Seas mostly a mix of wave fetches: one at 1 to 2 ft from the southwest, with the other from the west- northwest at 2 to 4 ft. As such, overall combined seas stay in the 3 to 4 ft range into early next week. && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Excessive Heat Warning until 10 PM PDT Tuesday for ORZ104>125. Heat Advisory until 10 PM PDT Tuesday for ORZ126>128. WA...Excessive Heat Warning until 10 PM PDT Tuesday for WAZ202>210. Heat Advisory until 10 PM PDT Tuesday for WAZ211. PZ...None. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland