FXUS66 KPQR 032238 AFDPQR Area Forecast Discussion National Weather Service Portland OR 338 PM PDT Tue Aug 3 2021 .SYNOPSIS... A pattern shift is expected Thursday, as the first in a series of upper level lows brings an increasing chance for showers and isolated thunderstorms. Cooler temperatures and more widespread precipitation follows Friday from a cold front, bringing the best, most widespread chance for rain in weeks. An overall troughing pattern may last into early next week to keep the cooler pattern going. && .SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/...Tonight, a vort max passing along the OR/WA border will result in some localized moisture and instability to be pushed northeastward through the overnight hours. While this will likely have little impact to our area, expect some increasing cloud cover along the northern areas of our CWA as this vort max moves eastward. Elsewhere tonight, areas will remain mostly clear except along the coast as the marine layer returns. Given that the overall upper level flow remains out of the south/southeast through Thursday, an Air Quality Advisory for smoke is in effect for areas of far eastern Lane County including the Oakridge area through Thursday. Overall, confidence at this time is moderate but, caution must be used as models both global and high resolution are vastly different for the upcoming forecast. As the models are showing differences in timing and location of the incoming lows over the next few days. Given these differences, have chosen to lean into the NBM for short term forecast. Over the next few days, both global and high resolution models continue to show a pattern shift taking place over the Pacific NW. In general, the models are show the upper level high pressure that is anchored over the Sonoran Desert, flatting out and giving way to a series of upper level lows. The first low, models are in good agreement that it will move northeast from off the California coast, coming onshore in northern California late Wednesday night/Thursday. While this low will not impact the weather much as today and Wednesday will be very similar. The exception to this will be Wednesday night as cloud cover will slowly increase through the overnight hours as this low moves inland. Thursday, will likely be the start of the pattern change from what the region has experienced over the past few weeks. This comes in the form of a cold front extending from the second strongest low, which will be hugging the western coast of Canada. As the first low pushes eastward through Thursday afternoon, showers along with a slight chance of thunderstorms enter the forecast. Then as the cold front associated with the second low enters the region late Thursday afternoon/Thursday evening, expect precipitation chances to increase from west to east as the front pushes inland through the overnight hours, with a transition from showers to stratiform precipitation by very late Thursday night. Current guidance has most areas within the CWA seeing at least 0.01" of precipitation by Thursday night. However, some areas could see some more, with areas along the coast possibly seeing around 0.05" to 0.10", with higher elevations within the Cascades possibly seeing up to 0.05". /42 && .LONG TERM /FRIDAY THROUGH TUESDAY/...Friday looks to be the day with a very noticeable change as the environment has become post frontal, which will mean cooler and likely wetter weather. Even through models continue to have the bulk of the second upper low staying close to Haida Gwaii, guidance still suggests that most of the CWA could have at least 0.10" of total precipitation by Saturday. However, locations along the Coast, in the Coast Range and in the Cascades could see high total precipitation values. Saturday and Sunday, look to be dry and cool with highs in the mid 70s to low 80s. As the second low exits the region, operational and ensemble models have the last in this series of lows dropping out of western Canada. However, there are differences in strength, location and timing of this third low. Monday and Tuesday are expected to be dry, with Monday slightly cooler that Tuesday. /42 && .AVIATION...VFR prevails across the vast majority of the forecast area this afternoon, as coastal stratus has cleared for the most part, even over the coastal waters. This is likely due to some enhanced mixing associated with a weak upper trough moving onshore into Washington. This system is also adding a bit of moisture and instability aloft, which may still trigger a thunderstorm or two later today north of Mt St Helens and Mt Adams. Inland terminals should remain VFR through tonight, though smoke may reduce visibility at times over eastern Lane County. The 12z HREF shows a high level of confidence that marine stratus will return as the boundary layer cools overnight. With the marine layer as shallow as it is, suspect this will manifest as an IFR/LIFR deck along the coast. Onshore MSLP gradients remain robust, so IFR/low MVFR stratus may push as far up the Columbia River as KKLS Wed morning. For detailed regional Pac NW aviation weather information, go online to: https://weather.gov/zse KPDX AND APPROACHES...VFR likely to prevail for the next 24 hours, with light to moderate W-NW sfc winds. Smoke aloft from fires to the south may impact slant-range vsbys at times, but impact on sfc vsby appears likely to remain minimal through Wednesday. There is roughly a 10-20% chance of a brief intrusion of MVFR marine stratus around sunrise, but confidence is too low to include this in our TAFs for the time being. Weagle && .MARINE...Winds across the coastal waters have grossly underperformed versus most model guidance today, and thus we have dropped today's Small Craft Advisories. We suspect winds remained lighter today in response to a weak upper trough moving onshore into Washington...it must have been strong enough to produce a weak surface reflection which weakened our offshore high pressure system. The upper trough is expected to move well east of the region by Wednesday, perhaps allowing a return to our more typical northerly wind regime with gusts up to 25 kt across our southern waters. Seas will remain primarily wind-driven through Wednesday. High pressure weakens again offshore Wed night, allowing a weak Gulf of Alaska cold front to move onshore later Thu, perhaps followed by another over the weekend. Westerly swells associated with the parent system should arrive Wed night/Thu, but should generally be 5 ft or less through the upcoming weekend. Weagle && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Air Quality Advisory for far east Lane County through Thursday. WA...None. PZ...Small Craft Advisory from noon to 10 PM PDT Wednesday for coastal waters from Cascade Head OR to Florence OR out 60 NM. && $$