724 FXUS66 KPQR 200413 AFDPQR Area Forecast Discussion...Update National Weather Service Portland OR 911 PM PDT Mon Mar 19 2018 Updated short term, aviation, and marine discussions .SYNOPSIS...Rather quiet weather with morning clouds and fog in the valleys continues through Tue under weak high pres. Wed and Wed night low pres off the CA coast will spread moisture north over the region as a cold front from the nw pushes through bringing rain, and then a cool showery air mass for the end of the week. && .SHORT TERM...Tonight through Thursday...Dropped overnight low temperatures a couple degrees, so now temperatures in the north and central Willamette Valley will be flirting with freezing. No additional changes. Previous discussion follows. -McCoy The weak upper low off the Oregon coast near 43N 129W is expected to swing inland across central and southern Oregon late tonight into Tue. This returns a chance for some showers to the far southern part of the forecast area, mainly the Lane County Cascades tonight and Tue. To the north, the area will remain under a weak surface high pres, with light winds favorable for radiational fog to develop again in the valleys late tonight and continuing through Tue morning. Tue night and Wed models indicate some moisture creeps up again from the south as the closed low well off the CA coast begins to open up. This spread chances for rain northward, and by late Wed deeper moisture arrives bringing the prospects for more substantial rain. As the deeper moisture spreads up from the south, warmer air associated with it likely raises snow levels above the passes Wed night.The warmer air will be short-lived though, as a cold front ahead of a deep trough of low pres from the Gulf of Alaska pushes in Wed night, adding additional lift for a wet night. Showers will decrease behind the cold front Thu as low level onshore flow temporarily weakens as a surface ridge of high pres moves in. Cooler air arrives Thu as the upper trough moves closer. Thickness values indicate snow levels to drop well below Cascade pass levels by Thu afternoon. .LONG TERM...Thursday night through Sunday...Models in good agreement moving an upper low, with origins in the Gulf of Alaska, down the BC coast Thu night and Fri, then inland across the region Sat. This brings a showery and quite cool air mass across the region. Thickness values down into the range of 522 to 525 dm Fri and Sat morning suggest snow levels well down into the Coast Range and Cascade Foothills, with maybe some flakes seen in the higher hills of the low elevation valley zones. Sun the upper trough is expected to move east with weak ridging following. A weak short wave is hinted at Mon in both the GFS and EC, so will need to hang on to some low pops Sun and Mon, with temps warming a bit, but still likely cooler than seasonal normals. && .AVIATION...VFR conditions prevail this evening. Skies are clear aside from some high clouds KEUG southward. High pressure and clear skies will result in excellent radiational cooling conditions overnight, making IFR/LIFR fog likely for most terminals by sunrise Tuesday. The exception may be along the coast, where light offshore flow may keep fog from forming. Fog is expected to lift/clear between 16z-18z Tue morning, though light northerly flow may jam low clouds against the south end of the Willamette Valley until about 20z...similar to today. VFR is expected Tuesday after the fog burns off. KPDX AND APPROACHES...VFR conditions will likely give way to IFR or LIFR fog/stratus by 13z Tue morning. Fog is expected to slowly lift during the mid to late morning hours Tuesday, then scatter out to VFR conditions with onshore flow by 19z. Weagle && .MARINE...Expect winds to generally remain less than 20 kt through midweek, but could see some gusts 20-25 kt Tue afternoon and evening as high pressure to the NW and low pressure to the SW squeeze the local pressure gradient just a bit. Strongest winds will be in the southern waters. Models continue to show a more active weather pattern returning Thursday and continuing through the weekend. Small Craft Advisory level wind gusts of 25 to 30 kt and seas climbing above 10 ft appear most likely during this time. Bowen/Weagle && .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 nautical miles offshore. This area is commonly referred to as the CWA or forecast area.
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