Area Forecast Discussion
Issued by NWS Portland, OR

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000 FXUS66 KPQR 191104 AFDPQR Area Forecast Discussion National Weather Service Portland OR 246 AM PST Tue Nov 19 2019 .SYNOPSIS...After showery weather today, expect cool and dry weather to return tonight and persist through at least the first half of the weekend. A return to more active weather may occur next week. && .SHORT TERM...Today through Friday...Water vapor satellite imagery early this morning reveals a shortwave trough dropping southeastward across the Pacific Northwest. There are two main features accompanying this shortwave trough that will impact our weather today. The first feature is a surface low pressure spinning off the central Oregon coast. Most of the 00z model guidance suggested it would make landfall along the north Oregon coast, but based on current satellite imagery and surface pressure trends, it appears most likely to move ashore somewhere between Newport and North Bend. This should result in showers continuing to spread across Lane, Lincoln, Benton and Linn Counties through at least early this afternoon. The second feature of note is a vort max and attendant 500mb cold pool of -26C currently located over far northwest Oregon and western Washington. Infrared satellite imagery indicates cloud tops are continuing to cool just ahead of this feature and doppler radar supports this notion as widespread showers have developed over northwest Oregon and southwest Washington early this morning. Models are in good agreement this pocket of instabiltiy shift east of the Cascades this afternoon. Between temperatures warming aloft and drier air aloft advecting into the region under increasing northerly flow, we should see clouds and showers decrease from north to south across the area this afternoon and evening. Quieter weather will then be in store for the remainder of the work week across southwest Washington and northwest Oregon as a shortwave ridge builds over the Pacific Northwest during the second half of the week. In the meantime, expect a mix of valley fog and low clouds to develop tonight, but increasing winds aloft and possibly even at the surface in the Willamette Valley make it somewhat uncertain how much fog will develop. And even if fog does develop initially, it may lift into a low stratus deck as the night progresses so will leave out dense fog wording at this point in time. A secondary vort max currently located off Haida Gwaii will drop southward offshore today into tonight and help to reinforce northerly flow across the area tonight into Wednesday. This should result in any low clouds and fog dissipating rather quickly across the area except in the southern Willamette Valley where the low level northerly flow will upslope and likely result in a slower dissipation of clouds. This should also keep high temperatures cooler in places like Corvallis and Eugene than areas farther north in the Willamette and lower Columbia River valleys. Drier air working its way into the region on Wednesday should allow temperatures to drop near to below freezing for most locations Wednesday night. In addition, colder air spreading into the Columbia Basin from Canada will also aid in the development of increasing east winds near the mouth of the Columbia River Gorge Wednesday night into Thursday. Models suggest the east winds will be short lived and should decrease substantially towards Friday. Nighttime temperatures should moderate accordingly and morning valley fog coverage should begin to increase again. Otherwise, expect dry weather, near average temperatures and a decent amount of sunshine across most of the area Thursday and Friday. /Neuman .LONG TERM...Friday night through Tuesday...Models and their ensembles suggest the shortwave ridge over the region to begin the weekend will likely shift southward and allow increasing westerly flow aloft to spread into the region late in the weekend into early next week. This will likely open the door to shortwave troughs pushing east-southeastward into the region from the Gulf of Alaska. The first storm system may arrive as early as late Saturday or early Sunday, but some model scenarios either keep the area dry or only skirt our northern zones with light precipitation. Thereafter, additional storm systems certainly appear possible early next week so trended PoPs towards the latest NBM guidance, which brings PoPs near climatology towards early next week. This pattern would also be favorable for snow levels to lower below the Cascade passes and for snow to begin to actually accumulate in the Cascades. /Neuman && .AVIATION...The front on the leading edge of a low pressure system in the Pacific is the main weather maker through Tue. Cigs will vary between MVFR and IFR through Tue as the front continues to move east inland through the day on Tue. Showers will cease after 00Z Wed. After around 00Z Wed when the front has transitioned out of the forecast area, ceilings will rise to low level VFR (around 4000 ft AGL) where they will remain until the evening. With the atmosphere and ground moistened with rain from Mon and Tue morning, after sunset on Tue there is a strong possibility of fog forming throughout the Willamette Valley and along the coast. This is only enhanced by winds becoming light after 22Z Tue. One challenge to fog formation is potential for what would be fog to lift to a lower stratus deck around 1000 ft AGL. If fog does form after 03Z Wed, it will most likely be patchy and more concentrated in the central/southern Willamette Valley. High pressure will begin to develop over the forecast area after 06Z Wed. KPDX AND APPROACHES...Ceilings will start VFR then vary between VFR and MVFR through 21Z Tue as the front moves through the area. Chances for reduced visibility possible with heavier showers through 00Z Wed. Chance for patchy fog after 03Z Wed. -Muessle && .MARINE...Small craft winds and seas remain as low pressure just to the west of the waters moves south towards the Oregon/California border. Winds are expected at 15 to 20 kt with gusts to 25 kt. Isolated gusts to 30 kt are possible especially in the outer waters past 30 nm. Seas will be 10 to 12 ft building to near 14 ft Tue evening. As the low transitions south out of the area, winds will quickly ease to 10 to 15 kt with gusts to 20 kt Tue evening. Seas will follow suit subsiding to 8 ft. High pressure building over the area taking on a rex blocking pattern will keep conditions fairly neutral through Thu. The next system is expected Thu afternoon through Sat. At this time there is quite a bit of uncertainty with how a low and cold front will advance towards the area on Thu. If it moves closer to shore, winds could increase to gusts at 35 kt or more with seas building to 17 ft. However, at this time confidence remains quite low due to a lack of model consistency. -Muessle && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory until 4 AM PST Wednesday for coastal waters from Cape Shoalwater WA to Florence OR out 60 NM. Small Craft Advisory from 8 AM this morning to 1 PM PST this afternoon for Columbia River Bar. && $$ 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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