Area Forecast Discussion
Issued by NWS Portland, OR

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912 FXUS66 KPQR 170941 AFDPQR Area Forecast Discussion National Weather Service Portland OR 238 AM PDT Tue Sep 17 2019 .SYNOPSIS...A cold front will produce moderate to heavy rain, breezy winds, and cooler temperatures today. A cool upper trough will support showers and possibly isolated thunderstorms tonight and Wednesday. Drier weather is expected for THursday and Friday, but cloudy skies and a lingering cool air mass will keep temperatures cooler than usual for this time of the year. && .SHORT TERM...Today through Friday...Showers have become isolated overnight, but satellite imagery shows the next rainmaker, a cold front, approaching the coast early this morning. Satellite derived tropical precipitable water (TPW) values are around 1.3 inches with the front, and models indicate that the TPW will remain around an inch as the front moves inland today. This means there is plenty of moisture for the front to tap into, and have increased confidence that moderate to heavy rain will accompany this front as it moves over land today. Rain rates up to 0.5 inch per hour are possible with the front passage today, with the coast and coast range the most likely places to see such rain rates. Heavy rain is possible for the interior valleys and the Cascade Foothills as well, but West 700-850mb winds suggests that the interior valleys may experience some rain shadow effects from the coastal mountains. With that said, generally expect 1 to 2 inches of rain for the coast, coast range, and the Cascade foothills through mid morning through this evening, with 0.5 to 0.75 inch for the interior valleys. Impacts from today`s heavy rain should be localized, and most noticeable in places where there is poor drainage, or where high rain rates persist for a couple of hours. The north to south pressure gradient tightens with the front today and expect south wind gusts up to 45 mph along the coast and gusts of 25 to 35 mph for the Willamette Valley. These winds are below the strength that we would normally expect to see wind impacts with, but will not be surprised if we see some downed branches or trees today due to the moist soil conditions, the full foliage on the trees, and that we have not had similar frontal winds in a while. I am not confident enough in these impacts to issue a impact-based advisory, but do think there is some potential for minor tree damage today. A cooler airmass and a cloudy day will result in temperatures barely making it to 60 degrees today with is much cooler than the normal temperatures for this time of the year. A cool upper trough behind the front will increase atmospheric instability this afternoon and evening and support showers and isolated thunderstorms through Wednesday evening. The threat for thunderstorms will mostly be on the coast and over the coastal waters tonight and Wednesday morning with a better chance for thunderstorms inland Wednesday afternoon. The environmental conditions on Wednesday will be very similar to what was observed on Monday, and yes, small waterspouts will be possible for the coastal waters. Showers will decrease Wednesday night and Thursday morning as an upper ridge builds over the Pacific NW. A passing upper shortwave trough will maintain a threat for light rain Thursday night and Friday. Models indicate that there is enough low-level moisture and onshore flow to support a decent chance for showers on Friday, particularly to the north coastal areas where orographics will aid lift. Cloudy skies will keep temperatures cooler than the seasonal normal, but the inland afternoon high temperatures should warm into the upper 60s. ~TJ .LONG TERM...Friday night through Monday... Model soundings show Ridging aloft should bring a dry day Saturday, then models agree a fairly strong shortwave crosses the Pacific Northwest Sunday bringing the next chances for rain. Operational models remain in decent agreement after that with another ridge likely returning dry weather for next Monday. && .AVIATION...Primarily VFR across the region as of 09Z, but a few areas of MVFR exist. Frontal system just offshore at 09Z expected to reach the south Washington and north Oregon coast between 15Z and 18Z. Coastal conditions will worsen and IFR/MVFR mix by 12Z and remain in that range through the afternoon. Inland TAF sites to remain VFR through about 14Z and then lower to MVFR. Conditions likely to improve to VFR mid to late afternoon. Post- frontal air mass will result in brief periods of MVFR after 00Z Wed. Higher terrain, such as the SW Washington Willapa Hills, Oregon Coast Range and Cascades will become obscured this morning. Coastal areas will see south to southwest wind gusts 35 to 40 kt with the front and inland valley areas may see gusts 25 to 30 kt. Wind speeds diminish this afternoon and evening. KPDX AND APPROACHES...VFR at the terminal as of 09Z. Cigs gradually lower through 13Z. Rain reaches the area between 11Z and 13Z, with periods of moderate rain 15Z through 21Z. MVFR cigs and/or vis 15Z through mid-afternoon. Improvement to VFR late afternoon, but brief periods of MVFR possible 00Z Wed through 12Z Wed in post-frontal unstable air mass. KTTD may see LLWS ahead of the front with E-SE surface wind around 10 kt and much stronger S-SW wind at FL020. Weishaar && .MARINE...Rather potent cold front for mid-September moves inside 125W by 12Z and is forecast to reach the coastline by 18Z. The 00Z models show a narrow ribbon of 30-35 kt boundary layer wind speeds moving through the waters this morning. Will maintain the current gale warning, although wind speeds may may below gale thresholds before the 21Z ending time of the gale warning. The surface low slides south to the south Washington waters this evening and to the north and central Oregon waters by Wed morning. Northwest to North wind 15 kt or less develops by Wed afternoon and continues through Sat morning. The next threat for small craft advisory level wind looks to be late Sat night and Sun morning. Combined seas build to around 12 feet today and will be largely dominated by a steep, wind-driven component. Seas hover in the 8 to 10 ft range tonight through Thu morning, then lower to around 5 to 6 ft as high pressure settles over the waters. Weishaar && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Gale Warning until 2 PM PDT this afternoon for Coastal Waters from Cape Shoalwater WA to Florence OR out 60 NM. && $$ 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 nautical miles offshore. This area is commonly referred to as the forecast area.

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