Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
880 FXUS66 KPQR 051739 AAB AFDPQR Area Forecast Discussion...UPDATED National Weather Service Portland OR 1014 AM PDT Fri Jul 5 2024 Updated Aviation Discussion .SYNOPSIS...Long duration and dangerous heat wave begins in earnest today, with inland high temperatures expected to warm several degrees over Friday`s already-very warm temperatures. Temperatures will be slow to cool each evening, especially in urban areas. The coast will be the place to go to escape the heat this weekend, as beaches will be cooled by afternoon sea breezes. Little change in this overall pattern is expected through Sunday, and latest forecast models are strongly suggesting the heat will persist through Monday if not Tuesday. Confidence increases in some degree of cooling as onshore flow strengthens midweek, but forecast models suggest strongly that above normal temps will persist all of next week. No rain is expected through at least the middle of next week. .DISCUSSION...Today through next Friday...Overall, not much has changed forecast-wise over the past 24 hours. Southwest Washington and northwest Oregon will experience a dangerously persistent heat wave, with at least the next four days expected to approach or exceed 100 degrees for the inland valleys. Adding to the concern is that temperatures will be slow to cool off each night, with urban and suburban areas only dipping below 70 degrees for a few hours (if that) before heating back up the next morning. Rural outlying areas will see a little more relief with overnight lows dipping closer to 60 degrees, but evenings will be uncomfortably warm even there. KEY POINTS: 1. A major heat wave begins in earnest today for SW Washington and NW Oregon except for the immediate coast. 2. This heat wave will likely be remembered more for its duration than its intensity. Inland valleys can expect maximum temperatures of 100 to 105 each afternoon Friday, Saturday, Sunday, and Monday. There is a possibility the heat will continue even longer than that. 3. Nights will be slow to cool off, especially for urban and suburban areas. Of most concern are highly urbanized areas where temperatures will be even slower to cool off due to the urban heat island effect. Once temperatures warm above 70 degrees this morning, some inner Portland neighborhoods may not see temperatures fall below 70 degrees again until Monday morning, or perhaps even longer. 4. The combination of multiple consecutive days with triple-digit heat and only modest relief at night will make this heat wave particularly dangerous. Those sensitive to heat will struggle each night to recover from the hot days if they do not have access to adequate cooling. The longevity of this event will likely compound and exacerbate impacts. 5. 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. As recent studies have shown, overnight temperatures are often more important than daytime temperatures 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 100-106 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 30-50% chance of lows at KPDX failing to reach 70 degrees Saturday night, Sunday night, and Monday night. Unfortunately this chance in reality is likely much higher than that for inner portions of the Portland/Vancouver Metro due to microscale effects like the urban heat island effect which can cause temperatures to cool slower than they normally would at night. Looking at the big picture, strong high pressure aloft anchored off the California coast is amplifying rapidly just off the US West Coast. Meanwhile there is weak shortwave energy moving into Montana from western Canada on NW flow aloft. This NW flow aloft has lingered long enough to provide some resistance for the amplification of the upper ridge, which is a big reason the hottest model solutions have mostly fallen off the board. This NW flow aloft is expected to diminish by Sunday. On one hand, this will allow slightly hotter temperatures aloft to build northward into the northwestern US, but on the other hand it will also allow the 500 mb ridge axis to shift slowly eastward. As the Columbia Basin heats up, thermal low pressure will eventually become stronger there than west of the Cascades, encouraging the onshore flow which should bring an end to this episode of excessive heat. That said, even with the onshore flow temperatures will be well above normal as we move into midweek. Substantial positive 500 mb height anomalies are likely to keep the marine layer too shallow to have a dramatic impact on inland temperatures. Latest NBM probabilistic guidance shows no more than a 30 percent chance of high temps lowering to early July normals (generally the lower 80s) for the Willamette Valley by next Friday. So while the intense heat may end by midweek, it will still be very warm for several more days after that.Weagle/Schuldt && .FIRE WEATHER...Increasingly hot weather will combine with dry and gusty afternoon northerly winds again this afternoon/evening as thermal low pressure strengthens over SW Oregon tightening surface pressure gradients, especially across the southern Willamette Valley. While fuels today are not quite ready for our fire weather zones to be eligible for a Red Flag Warning, the hot and dry weather will rapidly dry out vegetation over the coming days. Unfortunately overnight relative humidity recoveries appear poor over the higher terrain for several nights moving forward, further working to chip away at fuel moisture. Forecast guidance still suggests winds will be lighter Saturday, but it will still be very hot, dry, and unstable near the surface with thermal low pressure overhead. Those working out on the line any day through this weekend into early next week should pay special attention to staying hydrated, as the expected conditions can easily cause heat-related illness or worse. -Schuldt/Weagle && .AVIATION...Widespread VFR conditions across the airspace. Calm winds each night will give way to gusty northerly winds each afternoon (19Z-21Z) through late evening (05Z-07Z) Gusts up to 25 kt along the coast and gusts up to 20 kt for inland locations. Given the hot daytime temperatures, which are forecasted to be above 100F for an extended period of time, high density altitude issues could present a problem starting 20Z Friday. PDX AND APPROACHES...VFR with clear skies. Gusty northerly winds each afternoon (20Z) through late evening (06Z) with gusts up to 20 kt. /42 && .MARINE...The synoptic pattern has high pressure over the eastern Pacific paired with a thermally induced low pressure over California/Southern Oregon and the Great Basin. This pattern will persist through the weekend and likely into the start of next week. Therefore, expect Small Craft conditions across all waters with gusts up to 30 kt. Winds continue to follow a diurnal cycle, reaching Small Craft Advisory criteria each afternoon and evening, decreasing each night and morning. 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 until 8 PM PDT Monday for ORZ104>125. Heat Advisory until 8 PM PDT Monday for ORZ126>128. WA...Excessive Heat Warning until 8 PM PDT Monday for WAZ202>210. Heat Advisory until 8 PM PDT Monday for WAZ211. PZ...Small Craft Advisory from 2 PM this afternoon to 2 AM PDT Saturday for PZZ251>253-271. Small Craft Advisory until 8 AM PDT Saturday for PZZ272-273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland