Area Forecast Discussion
Issued by NWS Portland, OR

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000 FXUS66 KPQR 251739 AFDPQR Area Forecast Discussion National Weather Service Portland OR 1039 AM PDT Mon May 25 2020 Updated Aviation section .SYNOPSIS...Occasional light rain will likely spread across far northwest Oregon and southwest Washington today. Otherwise, expect mainly dry weather with increasing temperatures through Thursday. Temperatures should then gradually trend downward over next weekend with rain chances increasing substantially late Saturday into Sunday. && .SHORT TERM...Today through Thursday...Water vapor satellite imagery early this morning reveals a broad longwave trough over the north Pacific and a low amplitude shortwave ridge over the Inland Northwest. Sandwiched between is a southwest to northeast oriented weak atmospheric river over the northeast Pacific. Low to mid level warm advection ahead of the low level front is already beginning to produce light rain along the Olympic Peninsula. As the shortwave ridge shifts eastward and flattens today, expect this area of light rain to sag southward into southwest Washington and far northwest Oregon. At least some light rain appears likely north of a line extending between Lincoln City-McMinnville-Mt Hood today. Rain chances appear to diminish rapidly between Salem and Eugene, but even then brief light rain is not out of the question later today across Lane County. Expect a few areas of light rain and drizzle to linger overnight across the higher terrain as the front stalls and falls apart across the CWA. In general, rain chances should really begin to diminish late this afternoon and evening across the CWA, though. Thereafter, a weak shortwave ridge will develop over the northeast Pacific and transition across the area midweek. This will result in the area drying out entirely late Tuesday through Thursday. Varying degrees of low level northwesterly flow will persist across the area. This will keep temperatures from spiking into the 90s, but given building 500mb heights and warming 850mb temperatures, expect inland valleys to warm well into the 80s Wednesday and particularly Thursday. /Neuman .LONG TERM...Thursday night through Monday...Models and their ensembles are coming into better agreement with regards to details during the early part of the extended forecast. First, the aforementioned shortwave ridge will shift east of the region Thursday night. This will be in response to a shortwave trough lifting northeastward off the Pacific Northwest coast. Models suggest some elevated convection may develop off the Washington coast before lifting northeastward into British Columbia Thursday night. There is no indication in the models it will impact our waters, though. The main impact from this system skirting the region will be a stronger marine push that should allow more marine air to filter into the lower Columbia and Willamette River valleys Friday and result in temperatures cooling at least a couple of degrees. Meanwhile, a cutoff low pressure currently found well off the Baja/California coast will meander for the next few days before lifting northward across the Pacific Northwest this weekend. Models and their ensembles have trended slower and now lift the main vort max northward across the area late Saturday into Sunday. Still cannot completely rule out a faster scenario so still have some slight chance PoPs over the Cascades Friday night, but rain chances do not appear to really ramp up across the CWA until late Saturday into Sunday. 12 hour NBM thunder guidance is much higher relative to 24 hours ago for late Saturday and does at least suggest a slight chance of thunder is warranted in the Willamette Valley Saturday evening. GFS soundings do not appear favorable, but the pattern still seems quite conducive so went ahead and used the 12 hour NBM thunder guidance for populating a mention of thunder in the grids. Otherwise, a weak shortwave trough over the Pacific Northwest and onshore flow then appears likely to persist into early next week. This should keep temperatures closer to average during this time. Cannot completely rule out a few showers, but rain chances appear considerably lower than late Saturday into Sunday. /Neuman && .AVIATION...
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A front will continue slowly moving over the area today, bringing an area of light stratiform rain mainly for areas north of KSLE. The latest CAM model guidance suggests there will be a break from the rain this evening and tonight as high pressure builds in and the front moves out of the area. IFR cigs have moved into KAST earlier this morning, and are expected to continue through today, before possibly lowering to LIFR cigs this evening. Low clouds will attempt lifting back to MVFR later Saturday morning for KAST. At the same time, low clouds should move southward into KONP tonight and linger through tomorrow morning. MVFR to VFR cigs are expected for inland locations. KPDX AND APPROACHES...Light and variable winds along with light stratiform rain expected through this afternoon, before rain becomes more spotty this evening and gradually begins to diminish. Otherwise, expect mainly VFR cigs with brief periods of MVFR cigs throughout the 18z TAF period. -TK
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&& .MARINE...The high pressure that once sat over northwest Oregon and southwest Washington is beginning to erode as a weak front advances towards the area early Tue morning. This front will bring ample rain to the forecast area, but will likely have little influence over the overall water conditions. Seas will remain 5 to 6 ft with a 12 second period. Conditions will change on Tue as the front exits, and high pressure develops over the Pacific. This high pressure will intensify the northerly flow over the waters. While the overall swell will remain the same, it will be mostly comprised of a wind driven wave. While winds are expected to be rather calm and from the southwest today, the flow will shift late Mon/early Tue to a northerly pattern. Expect winds 10 to 15 kt with gusts up to 20 kt through Wed. Isolated gusts to 25 kt is possible in some locations, but not expected to be widespread. -Muessle && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...None. && && $$ Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland This discussion is for Northwest oregon and Southwest Washington from the Cascade crest to 60 nm offshore. This area is commonly referred to as the forecast area.

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