957 FXUS66 KPQR 170414 AFDPQR Area Forecast Discussion National Weather Service Portland OR 815 PM PST Mon Dec 16 2019 .SYNOPSIS...Fairly benign weather through Wednesday will likely give way to a multi-day stretch of notably wet and active weather by late this week. Heavy Cascade snow with pockets of freezing rain are possible for the Upper Hood River Valley. River flooding is possible late Thursday through Saturday. High winds will be possible along the coast Thursday. Conditions ease up by Sunday. && .DISCUSSION... This afternoon through Wednesday: Progressive upper ridging is traversing the area this afternoon, keeping the forecast area dry. Surface flow has turned lightly offshore as high pressure moves east of the Cascades today. Models suggest a peak TTD-DLS gradient of about -7 mb tonight into Tuesday, so the typically windy spots will be windy, but not significantly enough to expect much in the way of impacts. The next upper level trough is currently located around 135W and will shift east over the next 48 hours. As it does, its energy is expected to split with some energy going into Central California and some going north into northern Washington. This is most likely to result in a dry Tuesday locally with light rain beginning to push in Tuesday night or Wednesday morning. There remains enough disagreement between the models that light rain could begin as late as Wednesday afternoon, so have kept PoPs limited overnight Tuesday. Snow levels will remain low in the Cascades with the initial light precip, so expect a little bit of snow Wednesday down to 2000 or 2500 ft. Offshore flow is also likely to keep snow levels even lower in the Central Columbia River Gorge and Upper Hood River Valley, where precip may start off as snow with light accumulations through Wednesday afternoon. Bowen Wednesday night through Saturday: The primary period of concern continues to be Wednesday night through at least Saturday when an atmospheric river will set up somewhere along the Pacific Northwest Coast. At this time, the focus of heavy precipitation appears to be aligned longest over the North Oregon Coast Range and northeast to cover the South Washington Cascades. As always, am expecting subtle variations in the position of the core of the atmospheric river as these events are notoriously difficult to predict with confidence more than 24-36 hours in advance. Starting Wednesday night through Thursday evening, the leading edge of the atmospheric river core will fall into a lingering cool air mass, especially along and north of the Columbia River. Expect moderate to heavy snow will begin accumulating over the South Washington Cascades as the snow level starts lifting higher from 3500 feet. By later overnight/early Thursday morning, precip rates increase as snow levels lift toward 4000 feet. However, have significant concerns that cold air will remain trapped in the Upper Hood River Valley until later Thursday morning. Early indications are that somewhere 0.25"-0.35" of liquid will fall over the valley to produce a marginal ice storm event. At the same time, snow levels over the Oregon Cascades will have lifted to near 4500-5000 feet. But with lighter QPF amounts, that area is currently expected to be south of the primary atmospheric river core. Still, amounts are high enough to bring concern for warning level snow accumulations. 24 hour snow amounts look to reach 1-2 feet above 3500 feet in our Washington Cascades and Mt Hood region above 4500 feet. The passes will vary by elevation from around 5 inches at Government Camp/Blue Box Pass to 9-14" near Willamette Pass and Santiam pass respectively. This will, of course, fall as rain across at lower elevations. Rain rates will generally average 0.40-0.80 per 6 hours under the core during this stretch. Rates will increase to near 1.50" per 6 hours across the terrain by mid-day Thursday. Should start to see rises on area streams and rivers with perhaps larger concern for urban and small stream flooding becoming possible wherever these peak rates occur. From Thursday evening through Friday afternoon, the atmospheric river core has lifted far enough north to bring snow levels above 6000 feet to the South Washington Cascades and higher yet to areas farther south. This is when the primary thrust of precipitation will start hitting the area. QPF amounts really hit their stride with favored SW slopes now potentially receiving around 2" per 6 hours. It's this time where larger creek and river flooding really become the concern, although the higher Cascades of Washington may pick up their last slug of snow before the snow levels rise above the 6000 foot mark for the time being. Again, the details of where the precip core gets focused usually shift north and south with time. But by Friday afternoon, the dominant upper trough that is driving this scenario, digs offshore to bring more of SSW to NNE alignment to the flow. Not to be completely outdone on Friday, the coast has potential for a borderline high wind event. The various models are representing a surface low swinging north, offshore but coming inland somewhere over the Washington coast. At this time, peak gusts along the coast will hover around 60 mph, but could also see some notable gusts for the north Willamette Valley depending on how the low tracks. All told, the inland lowlands stand to receive 2"-4" of rain with the higher terrain seeing 4" to 8" of liquid equivalent as a mix of snow and rain.As such, have issued a Winter Storm Watch for the Cascades plus the Upper Hood River Valley to cover the aforementioned winter impacts. Have also issued a Hydrologic Outlook noting concerns for potential flooding impacts. Models begin to diverge on the big picture beginning Friday evening when the GFS starts splitting the upper trough, sending most of the energy into northern California. The ECMWF, however, keeps the trough more congealed with consolidated precip streamed west of the Cascades. In either case, snow levels will generally remain elevated, but start to come down, especially in Washington. But if the EC solution holds true, flooded rivers may hold steady well into Saturday until it also shows the atmospheric river core shifting south of our area. /JBonk Saturday night through Tuesday: Models agree that the main moisture axis will move south of the local area Saturday night, bringing an end to the heavy rain. Expect precip to turn more showery at this point. At the same time, snow levels are likely to lower back below the Cascade passes, but with limited moisture availability, don't expect significant travel impacts. As the upper low possibly closes off and shifts south into California, we'll likely see some ridging into Tuesday, at least of the shortwave variety. Models are in decent agreement, though, that the overall pattern will remain progressive. Bowen && .AVIATION...Dry with southwesterly flow aloft. System stays well offshore, with variable high clouds tonight into tomorrow. Will see patchy IFR fog forming again across the region, primarily in coastal rivers estuaries and bays, as well as inland. Overall, not much change tonight into early Tue am. But, on Tue, will see increasing mid level clouds as system offshore inches closer to the coast. KPDX AND APPROACHES...VFR conditions tonight and Tue, with light east winds. Likely to see patchy fog forming around PDX, mainly south to west. May see this in/around the field through 10Z, but am not expecting the fog to be deep enough to create problems. Fog should be gone by 12Z, with east wind increases a tad, generally staying in 10 to 12 kt range. /Rockey && .MARINE...Winds will remain above SCA conditions through this evening for the outer waters as a diminishing front passes through. Seas will stay above 10 ft building to 15 ft in the outer waters and 10 ft within the inner waters by tomorrow night. A Pacific low will track eastward toward our waters today and the effects of a strong offshore gradient will put a Gale watch into effect starting tomorrow evening and continue till Wednesday morning. The current operational GFS run shows that this system is expected to stay in our outer waters, track northeast and break up along northern Washington's coast. Hi-res models agree on the intensity of approaching winds behind the warm front through tomorrow morning but diverge quickly on where the low will track and break up. This will dictate whether the watch is bumped up to a warning. Two wave trains will become apparent Tuesday evening with a 10 ft, 10 sec southwesterly swell coupled with a 10 ft, 15 sec. westerly swell. As these wave trains pass each other square sea conditions are likely. Adding to these swells will be significant wind waves from strong winds producing overall wave heights between 13 to 15 ft through tomorrow night. An increase in sneaker wave threats will be present with the long period westerly swell. -BPhillips && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Winter Storm Watch from Wednesday evening through Thursday morning for Upper Hood River Valley. Winter Storm Watch from Wednesday afternoon through Thursday afternoon for Cascades in Lane County-Northern Oregon Cascades. WA...Winter Storm Watch from Wednesday afternoon through Thursday evening for South Washington Cascades. PZ...Small Craft Advisory from 10 AM Tuesday to 4 AM PST Wednesday for coastal waters from Cape Shoalwater WA to Florence OR out 10 NM. Gale Watch from Tuesday afternoon through late Tuesday night for coastal waters from Cape Shoalwater WA to Florence OR from 10 to 60 NM. Small Craft Advisory until 4 PM PST Tuesday for coastal waters from Cape Shoalwater WA to Florence OR from 10 to 60 NM. Small Craft Advisory until 10 PM PST this evening for Columbia River Bar. Small Craft Advisory from 6 AM to 11 AM PST Tuesday for Columbia River Bar. && $$