Area Forecast Discussion
Issued by NWS Portland, OR

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000 FXUS66 KPQR 290458 AAA AFDPQR Area Forecast Discussion...UPDATED National Weather Service Portland OR 858 PM PST Tue Jan 28 2020 UPDATE: for evening aviation discussion .SYNOPSIS...An active weather pattern will continue to produce above average temperatures, periods of rain and high mountain snow through the weekend. Cooler temperatures and lower snow levels appear likely early next week, though. && .SHORT TERM...Today through Friday...Water vapor satellite imagery shows a shortwave trough over the Pacific Northwest digging southeastward into the Great Basin. A trailing surface-850mb low pressure is located off the north Oregon coast this afternoon. Models are in good agreement it will continue to weaken as it pushes ashore late this afternoon. Strong low to mid level flow in a conditionally unstable environment will continue to wring out showers to the south and east of the low pressure track late this afternoon and early this evening. Snow levels are currently hovering between 3000-4000 feet this afternoon and will plan on letting the Winter Wx Advisory for the Cascades expiring at 4 PM. Models are in good agreement shortwave ridging will shift eastward towards the region this afternoon and evening, which will result in a subsidence inversion lowering to near 850mb by late this evening. This should result in showers decreasing in both coverage and intensity through that time. The break in the wet weather will be short lived however. The next storm system currently found in the Gulf of Alaska will drop southeastward towards the region later tonight and Wednesday. Models are in good agreement this will result in another round of rain spreading southeastward across the region during this time. This storm system appears progressive enough that hydrology impacts should be limited, but brief sharp rises can be expected on our smaller and faster responding rivers once again. Given the elevated levels currently found on the Grays River, it will likely approach minor flood stage once again tomorrow. More uncertainty creeps into the forecast Wednesday night and Thursday. Most model scenarios result in a warm front lifting back northeastward across the region. While rain appears likely across our northwestern zones regardless of model choice, particularly Thursday afternoon and evening, there appears to be a decent chance that our southern and inland zones could end up dry for most of the day. As a result, have begun to lower PoPs, but even then rain chances may be a bit overzealous. Models then suggest an atmospheric river will take aim at western Washington and Vancouver Island Friday. There is a lot of uncertainty in where the southern extent of the precipitation will end up. Current NBM guidance suggests areas north of Salem will end up at least a little wet while areas south of Salem may in fact stay dry the entire day. Anecdotally, it seems the southern periphery of precipitation is often modeled too far south in these scenarios so opted to keep temperatures on the warm side of model guidance. Our current temperature forecast assumes that most areas from Portland southward will dry out for at least a few hours Friday afternoon and high temperatures will top out around 60F. /Neuman .LONG TERM...Friday night through Tuesday...Guidance continues to show the bulk of the moisture streaming into western WA late Friday so will trend PoPs down for our southern zones Friday night into early Saturday. Models then show the cold front sagging south across the forecast area on Saturday which will result in another round of valley rain and mountain snow. The upper level trough looks to then swing inland late Saturday into Sunday which will keep unsettled weather over the region, with snow levels lowering to the Cascade Foothill and Coast Range elevations and temps falling from around 5 to 10 degrees above normal to near normal. A colder air mass looks to then settle in over the region late Sunday into early next week, with snow levels at night lowering to near the valley floor and temps cooling to near freezing. This could result in a brief period of lowland snow Sunday night, but due to model uncertainty will keep a mix of rain and snow in the forecast. Despite the low snow levels on Monday it also looks like rain chances will be waning so cannot rule out the development of patchy fog, freezing fog, and frost Sunday night. Transient shortwave ridging looks to then dry us out even further late Monday, but with saturated soils and near freezing temps expect areas of fog, freezing fog, and front across the forecast area Monday night. Guidance then shows rain returning on Tuesday as another disturbance moves across the Pac NW. /64 && .AVIATION...
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Stable air mass across the region this evening. Mix of VFR with quite a bit of MVFR along the coast. Better inland, with widespread low VFR with cigs generally 3500 to 5000 ft, and only few pockets of MVFR. Cconditions will gradually deteriorate later overnight, as a warm front lifts across the region. Expect increasing MVFR and IFR along the coast after 10Z, with areas of rain. But, main rain arrives after 15Z, with peristent upper IFR and lower MVFR along the coast. Farther inland, will see cigs gradually lower to MVFR after 15Z, with spotty light rain to north of Salem but staying VFR to the south. Rain increasing inland after 18Z, with MVFR becoming dominant. Expect widespread terrain obscurations in clouds. KPDX AND APPROACHES...Mostly VFR until 12Z, with cigs 3500 to 4500 ft. But, cigs lowering to MVFR 12Z to 15Z, with spotty light rain. Rain increasing by 18Z, with continued MVFR. /Rockey
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&& .MARINE...Timely 19Z ASCAT pass revealed a surface low pres center just north of Cape Disappointment. This can also be surmised from the 20Z wind speed observations. Buoy 46029 had minimal wind, generally around 5 kts, while 46050 reported gusts just above 25 kt. The 12Z GFS, ECMWF and even the 18Z NAM did not seem to have a good handle on location and depth of the surface low. The back edge of the bent back occlusion appears to have moved through the central and south Oregon waters as of 18Z. 12Z model runs show a gradually weakening surface gradient through the evening, with wind gusts south of Tillamook Bar falling below 20 kt by 06Z Wed. The next warm front reaches the north waters by 12Z Wed. The NAM and GFS suggest boundary layer wind speeds of 30-35 kt over PZZ270 around 12Z. Gale force wind gusts look likely for PZZ270 and PZZ250 beginning around 12Z and then spreading into the northern portions of PZZ275 by 15Z. The RAP, Hi-Res ARW, NAM and higher resolution HRRR suggest 30 kt boundary layer wind speeds spreading over the majority of the area by 15Z Wed. Will go ahead and issue gale warnings for the waters with valid times 12Z-21Z for PZZ250 and PZZ270 and 15Z-21Z for PZZ255 and PZZ275. Confidence is a little lower south Heceta Head, but feel the area from Cascade Head to Heceta Head has a high probability of gale force gusts. Yet another warm front moves through the waters Thu, but not anticipating gale force gusts with this system as models show it more north. Eventually, a sharp cold front swings through the waters Sat, which may produce near gale-force northwest wind gusts. Seas will hold steady in the 12-15 ft range through Wed and then may reach 14-17 ft Wed evening through Wed night. Seas settle back into the 11 to 13 ft range late Thu, but build into the mid-teens Sat. There is concern for sneaker waves Thu with wave heights 13-15 ft and relatively long periods of 15-17 seconds. The weekend also poses potential sneaker wave concern, although shorter periods of 12-13 seconds are expected. The predominant swell direction will be 240-270 degrees through Sat. Models then show a 13-15 ft northwest swell becoming the dominant train Sat night through Sun. Weishaar && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Winter Weather Advisory until 4 PM PST this afternoon for Cascades in Lane County-Northern Oregon Cascades. WA...Winter Weather Advisory until 4 PM PST this afternoon for South Washington Cascades. PZ...Small Craft Advisory until 4 AM PST Wednesday for coastal waters from Cape Shoalwater WA to Cascade Head OR out 60 NM. Gale Warning from 4 AM to 1 PM PST Wednesday for coastal waters from Cape Shoalwater WA to Cascade Head OR out 60 NM. Small Craft Advisory until 7 AM PST Wednesday for coastal waters from Cascade Head OR to Florence OR out 60 NM. Gale Warning from 7 AM to 1 PM PST Wednesday for coastal waters from Cascade Head OR to Florence OR out 60 NM. Small Craft Advisory until 4 PM PST Wednesday 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 nm offshore. This area is commonly referred to as the forecast area.

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