Area Forecast Discussion
Issued by NWS Portland, OR

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769 FXUS66 KPQR 202350 AFDPQR Area Forecast Discussion National Weather Service Portland OR 449 PM PDT Tue Apr 20 2021 .SYNOPSIS...High pressure will remain over the region through Wednesday, with daytime temperatures continuing well above normal. Increasing onshore flow Thursday brings cooler conditions. The pattern looks to hold through Friday followed by a change to unsettled conditions. && .SHORT TERM...Tonight through Friday...Afternoon GOES-17 water vapor imagery revealed strong high pressure over the eastern Pacific and extending over the the Pacific Northwest. Meanwhile, a compact upper level low could be seen over northern California. This feature has produced several lightning strikes, but all of them south of our forecast area. In addition, dry air aloft was pushing south through western Oregon. At 23Z, many inland locations observed humidity values in mid teens to lower 20s. Finally, visible imagery indicated areas of stratus hugging portions of the coastline, especially in the vicinity of Capes. The high pressure centered offshore moves closer to the coastline tonight and Wednesday. This results in northeast flow aloft. A thermally-induced surface trough is expected to reside over the interior valleys overnight, but shift to the Cascades mid to late Wed afternoon. The NAM suggests slightly stronger onshore low-level flow developing late in the day, but this should only impact the coast and coastal valleys. Model 850 mb temps warm another couple of degrees Wed. Thus, interior valleys are likely to be in the mid to uppere 70s. Subtle changes begin to occur Wed evening as the upper flow pattern backs to the northwest. This will help to induce stronger onshore low-level flow and allow marine stratus to spread into coastal valleys. Models show a short-wave disturbance dropping south through central and eastern Washington and Oregon Thu afternoon as the 500 mb ridge re-centers itself along 132W. Expect increasing onshore flow, which will result in several degrees of cooling inland. The upper ridge migrates to the coastline Fri morning and then shifts east of the Cascades by early afternoon. The leading edge of an approaching frontal system is expected to be just offshore by 00Z Sat. Have indicated slight chance POPS for the south Washington and north Oregon coast mid to late Thu afternoon. The NBM 1-D viewer for KAST suggests a couple hundredths late in the day. Weishaar .LONG TERM...Friday night through Tuesday...By Saturday normal Pac NW mid-spring conditions return. The GFS and ECMWF deterministic runs both indicate a large 500 mb trough settling over the central and eastern Pacific by 00Z Sun. Some much-needed rainfall is expected Fri night and Sat. The ECMWF ensembles for KPDX show anywhere from 0.2 to 0.4 inches Sat, with the mean value close to one-third of an inch. The control run is just slightly lower. Ensembles for KEUG show a little less. The upper trough remains over the Pac NW Sat night through Mon, although the deterministic GFS wants to shift the upper trough east a little sooner. The overall WPC mean 500 mb cluster valid Sun holds the upper trough near the Oregon and Washington coast. However, nearly 80 percent of the GFS ensemble members suggest the upper trough will be located slightly more to the east. In any event, confidence is moderate to high that unsettled conditions will continue Sun. Snow levels fall to just below the Cascade passes late Sat night and Sun. The last vestiges of the upper trough move through the region Mon, followed by a transitory ridge and improving conditions Tue. Weishaar && .AVIATION...high pressure over the region will maintain VFR across the interior tonight through Wed. But, shallow marine layer remains along the coast. IFR stratus mostly along the coastline and extending offshore, as northerly winds keeping it from moving much onshore. But, as north winds weaken this evening, will see that stratus return and spread onshore, affecting nearly all coastal areas. Expect widespread IFR to low end MVFR will tonight through early Wed afternoon. In fact, hints are that the stratus may persist through Wed night, with only some lifting of the CIGs. For detailed regional Pac NW aviation weather information, go online to: https://weather.gov/zse KPDX AND APPROACHES...High pressure and dry northerly flow aloft will maintain VFR with clear skies tonight through Wed. /Rockey && .MARINE...Rather breezy along the coast this evening, thanks to high pressure offshore and thermal trough along the far south Oregon coast into nw Calif. Winds will gradually ease later tonight into Wed, but will maintain northerly component. Will hold current Small Craft Advisory through tonight. Overall, seas running 7 to 9 ft, with seas 9 to 10 ft with locally choppy and steep waves where more windy areas tonight. High pressure weakens Thursday and Friday, although, low pressure will develop over the offshore waters. This low possibly gets close enough to shore to bring a cold front across the coastal waters Saturday. Seas will remain less than 10 feet through remain rather choppy with a strong wind wave influence. /Bonkey && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory until 5 AM PDT Thursday for Coastal waters from Cascade Head to Florence OR out 10 NM-coastal waters from Cape Shoalwater WA to Florence OR from 10 to 60 NM. Small Craft Advisory from 11 AM to 11 PM PDT Wednesday for Coastal waters from Cape Shoalwater WA to Cascade Head OR out 10 NM. && $$ Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland

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