Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
627 FXUS66 KPQR 171037 AFDPQR Area Forecast Discussion National Weather Service Portland OR 337 AM PDT Tue Sep 17 2024 .SYNOPSIS...An incoming frontal system will bring increasing clouds, cooler temps, and increasing chances for rain on Tuesday. Mainly dry Wednesday through Friday with onshore flow and slightly below average temps. Rain potential returns this weekend, but confidence is low in the overall pattern. && .SHORT TERM...Now through Wednesday Night...The high pressure ridge that brought sunny and warm weather to the region will shift rather abruptly on Tuesday as the next upper level trough digs south over the NE Pacific and sends a frontal boundary through the area, bringing the next round of rain. However, forecast rain amounts continue to decrease and the timing has shifted a bit with rain now expected to begin later in the day compared to previous forecast updates. Now expecting rain to arrive along the coast during the late morning to early afternoon hours before spreading into the Willamette Valley in the mid to late afternoon. In fact, the HREF suggests rain will most likely hold off until 3-5 pm for Clark and Cowlitz Counties as well as the Portland/Vancouver metro and Willamette Valley. The latest possible start time in these locations is 7-10pm, suggesting there is a possibility for nearly the entire day to stay dry (albeit <10% of HREF members suggest a start time this late and 3-5pm remains the most likely start time). As mentioned earlier, there has also been a notable downward trend in rain amounts with the latest model guidance, especially with hi-res model guidance and the HREF. This downward trend seems to make sense from a conceptual standpoint with the trough remaining offshore the Oregon Coast on Tuesday and evolving into another closed low over northern California by Wednesday morning. As such, rain amounts east of the Coast Range remain around 0.10-0.20 inches from Kelso to Portland to Salem. The probability for rain amounts over 0.25 inches have fallen from around 20 percent down to 10 percent in these areas according to the 00z iteration of the HREF, with the best chances (40-60%) found in the south valley around Eugene and also in orographically favored parts of the Cascades. Probabilities remain higher along the coast to the south of Astoria and along the west slopes of the Coast Range (generally above 70%). That said, forecast rain amounts have decreased even in these areas, down from 0.50-0.75 inches to 0.25-0.50 inches. A few showers may linger across the area Tuesday night into Wednesday morning as the low resides over northern California, but expect this activity to diminish fairly quickly as a positively tilted ridge noses into the region from the Pacific. This will yield seasonable conditions with temperatures in the low to mid 70s from Wednesday into the latter half of the week. Areas of fog are likely to develop early Wednesday morning for locations that experience some clearing given the wet ground conditions and calm winds. That said, it appears cloud cover will be too expansive for widespread fog to develop. Locations in the Willamette Valley to the west of I-5 and the Coast Range lowlands have the best chance for some clearing and subsequent fog development (25-50% chance for surface visibilities below 1/2 mile due to fog according to the HREF).-TK/CB .LONG TERM...Thursday through Monday...Thursday and Friday will feature benign, pleasant September weather as the ridge axis shifts slowly southward across the region, opening the door to increased onshore flow as we head into the end of the week. Expect inland high temperatures generally in the low to mid 70s each day. Forecast confidence remains low headed into the weekend as ensemble clusters remain fairly evenly split between ridging or troughing developing over the region Sunday into Monday. Meanwhile, most deterministic solutions depict a somewhat zonal flow pattern Saturday into Monday. This uncertainty is reflected by the continuing high spread between the 25th and 75th percentile NBM temperature guidance, which places highs for next Sunday and Monday anywhere between the mid 60s and lower 80s across the interior lowlands. As such, the deterministic forecast continues to stick closely to the NBM mean which depicts seasonable temperatures and broad brushed chance to slight chance PoPs across the area this weekend into early next week. -CB/TK && .AVIATION...Widespread VFR, except at the coast where marine stratus with low-end MVFR CIGs around 1000-1500 ft. Guidance suggests there is a 40-60% chance that conditions deteriorate to IFR this morning, then 60-80% chance this afternoon into the evening hours. Stratus likely pushing into the Willamette Valley 12-15Z Tuesday, then filling in for the remainder of inland areas 15-19Z Tuesday. Cigs most likely in the 2500-3500 ft range, but there is a 20% chance for cigs 1500-2000 ft. Light rain is likely to begin at the coast around 16z Tuesday morning, and gradually spreading inland by 20-22z Tuesday afternoon. PDX APPROACHES...VFR with variable high clouds expected through at least 12z Tuesday. Then, stratus begins building across the area with chances for MVFR CIGs increasing to 40-60% after 14z Tue morning. Chances for light rain increases through Tuesday afternoon. Northwest winds become light and variable after 08Z Tuesday. /mh /DH && .MARINE...Later today into Wednesday a weather system drops down from British Columbia. However by the time it reaches the waters, it will elongate and weaken. Will see winds back to the south today as a front approaches. Peak wind gusts, up to 20 kt, over the coastal waters looks to be this afternoon as the front nudges against the coastline, increasing southerly winds. Winds should then relax tonight as the system falls apart. There is a slight chance for thunderstorms tonight primarily over the outer zones. Northerly winds return Wednesday as high pressure rebuilds offshore. Winds may gust 20 to 25 kt at times, primarily over the outer zones. Seas around 4 to 6 ft today and tonight, increasing to around 8 ft later Wednesday into Thursday, then settling to around 5 ft late in the week. /mh && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...None. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland