Area Forecast Discussion
Issued by NWS Portland, OR

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626 FXUS66 KPQR 220418 AAA AFDPQR Area Forecast Discussion...Updated National Weather Service Portland OR 918 PM PDT Fri Sep 21 2018 Updated aviation and marine sections. .SYNOPSIS...A weakening storm system will spread light rain showers across the area tonight and Saturday before much warmer and drier weather return next week. && .UPDATE...Made a quick update to the forecast this evening to adjust/refine the amount and timing of precipitation. Based on latest radar imagery and observations, it appears the front is just about to move onshore into Washington and should reach the north Oregon coast around 9-10 PM this evening. With the best upper level dynamics racing out ahead of the surface front, it appears likely that at least some of the Willamette Valley will miss out on measurable precip as the front moves through tonight, especially rain shadowed areas west of I-5. Post-frontal showers will persist into Saturday, tending to favor the higher terrain. These showers may end up being more productive in terms of QPF for the inland valleys. The rest of the forecast remains unchanged. Weagle .SHORT TERM...Today through Monday...Visible satellite imagery this afternoon reveals a surface low pressure spinning off the Pacific Northwest coast. A band of deeper clouds with embedded rain showers are apparent via infrared satellite imagery and KLGX radar imagery along a southwest to northeast oriented cold front. Models are in good agreement this band of rain showers will drop southeastward and gradually weaken as it moves into over the south Washington and north Oregon coast between 4-7pm, the northern Willamette Valley towards 8-11pm and the southern Willamette Valley after midnight. This initial band of precipitation should generally produce a tenth of an inch or less of rain across even the wettest west facing slopes of the Coast Range and Cascades with perhaps just a few sprinkles to a couple hundredths of an inch in the Willamette Valley. Low level instability behind the front will gradually deepen enough to allow for additional showers to spread onto the coast and into the higher terrain, which should produce a patchwork of heavier rain totals overnight. As solar heating helps to destabilize interior locations Saturday midday, expect shower coverage and intensity to pickup Saturday afternoon and evening, particularly for areas north of Salem. Storm total rainfall amounts look rather meager overall, but a few of normally wetter locations in the Coast Range and Cascades, particularly farther north, could see upwards of a 0.50-0.75" of rain. Expect shower coverage and intensity to decrease Saturday night as the loss of solar heating helps to stabilize the lower atmosphere. However, showers shouldn`t dissipate entirely as a secondary shortwave trough drops southeastward across the Pacific Northwest. By Sunday, drier north to northwesterly flow will overspread the region and result in decreasing clouds, but a rather pleasant fall-like day. Shortwave ridging will begin to build eastward over the Pacific Northwest Sunday night and Monday, which should result in interior locations warming into the 70s. Breezy northerly winds may develop along the coast and in the Willamette Valley as thermal low pressure begins to strengthen over southwestern Oregon. /Neuman .LONG TERM...Monday night through Friday...The long term part of the forecast can be characterized as a brief return to summer or first real taste of Indian Summer. The operational ECMWF, GFS and CMC in good agreement through the period. In addition, the WPC forecast is close to the GFS and its ensembles. A 590 dm 500 mb high pressure center will be centered out near 45N 130-135W Tue and Wed. North to Northeast 700 mb flow and a Northeast to East 850 mb wind regime will result in a dry air mass over the area. Model forecast 850 mb temps Tue and Wed are 15 to 18C. These values would typically produce mid to upper 80s for the interior valleys. However, with shorter days and stronger night and morning inversions, full mixing will likely not be realized. In any event, inland valleys should still reach the lower 80s. The offshore low-level flow will aid in warming, especially along the coast. A thermally-induced surface trough is expected to reside near the coast Monday night then drift to the interior valleys Tue. The offshore gradient Tue and Wed does not appear to be all that impressive, but would still expect locally breezy to windy conditions. By Thu the CMC and GFS show the forecast area within a col area. The 500 mb high center closes off out near 140W with a closed low directly to the south. Thus, would expect above-normal daytime temperatures to continue Thu, but with weaker offshore low-level flow. 12Z operational models then start to retrograde the strong north 500 mb flow forecast to be present over the Northern Rockies. This would bring cooler air through Eastern Washington and Northeast Oregon. Some of this would slip through the Central Columbia River Gorge and the Upper Hood River Valley. Bottom line: No precipitation expected through the period and above-normal daytime temperatures. Weishaar && .AVIATION...A weak cold front will move through the region tonight. Associated light rain and MVFR cigs have already developed along the coast and will persist through most of the night. There may be some local IFR as well. Breezy pre-frontal southerly winds will persist through much of the night. Precipitation will spread into the interior later this evening and overnight. Mid- level clouds will increase for the interior TAF sites over the next few hours, and then expect some MVFR cigs to develop between 09Z-12Z Sat. Do not expect the interior MVFR to be solid, more likely a mix of VFR and MVFR conditions. Conditions should begin improving by mid to late morning in post-frontal showery air mass. There may still be some occasional MVFR cigs around tomorrow afternoon under heavier showers. Pyle KPDX AND APPROACHES...VFR conditions through the evening. An approaching cold front is expected to bring a mix of VFR and MVFR cigs in overnight through mid-morning Sat, with occasional light rain as well. Expect showers to increase Sat afternoon. Conditions to be mainly VFR Sat afternoon and evening, but some occasional MVFR possible with showers. Pyle && .MARINE...A well developed low pressure continues to push towards Vancouver Island. This low will keep southerly winds gusting to around 25 kt over our northern waters through tonight. Winds will be less over the central Oregon waters, but expect occasional gusts over the outer central waters as well. Winds will subside Sat morning as a cold front moves onshore. Winds will gradually shift to W then NW through the day Sat. Then look for northerly winds to return Sunday and continue through at least the first half of next week. Gusts up to 30 kt will be possible for the central Oregon waters late Monday through Tuesday night. Seas will build to 7 to 9 ft tonight. Expect the largest seas north of Cascade Head and beyond 20 nm from shore. Seas will become steep amd choppy tonight across the waters off the north Oregon and south Washington coast. As a result there is a small craft advisory for hazardous seas in effect now for the northern waters. Think seas will remain just below small craft advisory thresholds for the central Oregon waters. Seas will gradually ease off during the day Sat, reaching 6 ft by Saturday night and holding fairly steady through the start of next week. Pyle/42 && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory for winds until 8 AM PDT Saturday for Coastal Waters from Cape Shoalwater WA to Cascade Head OR out 60 NM. Small Craft Advisory for hazardous seas until 10 PM PDT Saturday for Coastal Waters from Cape Shoalwater WA to Cascade Head OR out 60 NM. Small Craft Advisory for winds until 5 AM PDT Saturday for Waters from Cascade Head to Florence OR from 10 to 60 NM. Small Craft Advisory for Rough Columbia River Bar from 2 AM to 6 AM PDT Saturday. Small Craft Advisory for Rough Columbia River Bar from 2 PM to 6 PM PDT Saturday. && $$ 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 CWA or forecast area.

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