Scientific Forecaster Discussion

000 fxus66 kpqr 240445 aaa afdpqr Area forecast discussion Weather Service Portland or 944 PM PDT Thu may 23 2019 Aviation discussion updated. Synopsis...a weak ridge of high pressure will move over southwest Washington and northwest Oregon tonight with northerly flow aloft. A few showers or thunderstorms are possible mainly over the Cascades and Coast Range late this afternoon and evening. Friday, an upper low will move south down the British Columbia coast and over Washington and northwest Oregon bringing scattered showers to the area. Cooler weather will accompany this system with temperatures falling below normal for the weekend. By Saturday, colder air will bring snow levels down to the higher Cascade passes. No significant accumulations are expected but those planning recreation for the Holiday weekend will be impacted. Weak ridging late Sunday and Monday will reduce the shower threat. && Short term...today through Sunday... weak upper ridge and northeast flow over the area this afternoon with a few showers popping up over the Coast Range early this afternoon. Enough instability for showers and a slight threat of thunderstorms over Cascades this afternoon and evening but conditions over the valleys are more stable today as a result of all the low level marine air. Good onshore flow will continue tonight and expect deep marine layer with lots of clouds after midnight. On Friday a shortwave is dropping south out of British Columbia and into Washington and northern Oregon. This will increase the showers over the area. GFS/ec/NAM show quite a good area of precip...although light, but nbm pops are only in the chance category. We may need to up the pops if the low becomes more active as it develops but climatology for this time of year is against widespread stratiform precipitation. On Friday night models have the upper low over eastern Oregon with another shortwave dropping south into the longwave trough. That brings another significant threat of showers to southwest Washington and northwest Oregon. By this time cooler air aloft has reached the Cascades and snow levels drop to around 4500 feet. This should bring some snow showers to the passes, but surfaces will be warm enough to prevent much accumulation on roadways, unless there is a heavy shower. Still the cold air and moderate elevation snow will make for less than ideal camping and recreating conditions for early in the Holiday weekend. Sunday the main upper low moves south into California and northerly flow returns, much like yesterday and today. That will allow more breaks in the clouds and some sunshine in the valleys. The increased surface heating along the NE flow aloft could result in thunderstorms again over the mountains. Schneider Long term...Sunday night through Thursday...a broad, strong upper- level trough finally starts to kick this persistent upper-level ridge that has been nearly stationary off the West Coast off to the east, meaning a pattern change is in store next week. With the shift of high pressure inland early next week, we will see partly cloudy skies and little to no shower activity Tuesday through Thursday. We will also see temperatures inland get up into the mid-to-upper 70s. Persisting onshore flow should keep temperatures from getting too warm next week. && Aviation...expect the inland taf sites to see MVFR cigs return between 12z and 16z. An upper level trough and the associated remains of an offshore front will move onshore Friday morning and the associated lift may result in some marginal lifting in the cigs, but also possibly produce some drizzle or light showers as the moisture deepens over the region. Interior cigs should improve to VFR by Friday afternoon, but coastal sites are likely to remain MVFR or IFR through the day Friday. Kpdx and approaches...VFR conditions prevail through this evening. Expect MVFR stratus to return by 16z Friday. Some drizzle or light showers may develop in the mid- to late morning. Cigs improve to VFR by 20z Friday. Bowen && Marine...surface high pressure will remain over the NE Pacific through this weekend. With thermal low pressure over northern California, winds are northerly over the coastal waters today. They will be gusting in the 20 to 25 kt range over the outer waters later this afternoon and this evening. A Small Craft Advisory is now in effect. A couple of upper level disturbances will drop south along the Washington and Oregon coast on Friday and again on Saturday. Winds will increase over the Washington and far northern Oregon waters on Friday in response to this first system, with gusts likely reaching 30 kt. Isolated gusts to 35 kt aren't out of the question beyond 40 nm, but don't expect we will need a Gale Warning at this time. Winds will gradually subside Sat and should remain fairly benign through the rest of the weekend. No significant marine impacts are expected over the first half of next week. Seas will be a bit of nuisance over the next couple of days. Current observations show that they remain on the choppy side, at around 7 to 8 seconds with a dominant period of 9 to 10 seconds. Decided to issue a Small Craft Advisory for pzz270 starting late this evening, as model spectral guidance suggests that square seas will return. In the meantime, strong winds off of Vancouver Island will result in seas building Friday into the weekend. Expect seas to build into the low teens across our coastal waters Fri morning, and remain above 10 ft through at least Sat morning. Seas are expected to gradually subside Sat, reaching 5 ft or less by sun. Pyle && Pqr watches/warnings/advisories... or...none. Washington...none. Pz...Small Craft Advisory for winds until 11 am PDT Saturday for waters from Cape Shoalwater Washington to Florence or from 10 to 60 nm. Small Craft Advisory for hazardous seas from 11 PM this evening to 5 PM PDT Saturday for waters from Cape Shoalwater Washington to Cascade Head or from 10 to 60 nm. Small Craft Advisory for winds from 5 am to 11 PM PDT Friday for coastal waters from Cape Shoalwater Washington to Cascade Head or out 10 nm. Small Craft Advisory for hazardous seas from 8 am Friday to 5 PM PDT Saturday for coastal waters from Cape Shoalwater Washington to Cascade Head or out 10 nm-waters from Cascade Head to Florence or from 10 to 60 nm. Small Craft Advisory for rough Columbia River bar from 7 am to 11 am PDT Friday. && $$ Interact with US via social media: www.Facebook.Com/nwsportland

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