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SYNOPSIS

A deep, upper level trough over the northeast Pacific will continue to maintain cooler temperatures and scattered to widespread rain/snow showers across the forecast area through at least the latter part of the week. Colder air looks to maintain daytime temperatures around 5 to 10 degrees F cooler than normal, while also lowering snow levels towards 500-1000 ft each night/morning. Given the showery nature of the precipitation, there is a 40-60% chance for snow flurries or "conversational snow" to be observed down to the valley floor through at least late Thursday morning. There is the potential (45-60% chance) for 1 to 2 inches, or more of snowfall occurring below 500-1000 ft SOMEWHERE in NW Oregon/SW Washington, Wednesday evening into Thursday morning. Shower activity is expected to wane at the end of the week as high pressure returns briefly to the region. As high pressure builds, snow levels are expected to climb back towards over 3000 ft by Sunday as a warmer frontal system develops offshore.

DISCUSSION...Now through Monday

Current radar imagery this afternoon shows scattered showers across the forecast area. These showers are expected to continue to ease through Wednesday morning as a low pressure near the south Washington coast weakens. Any showers through tonight will have the potential to also produce some small hail/graupel. Any hail/graupel would be associated with a shower and quickly melt/dissipate after the associated shower leaves the area. Most inland locations are likely to remain dry tonight though a few light rain/snow showers will be possible through early Wednesday morning. With clearing and relatively drier conditions tonight, expect overnight lows to fall below freezing, while a few rural locations could see temperatures fall into the mid to upper 20s. This could lead to icy roads and slick surfaces as any lingering moisture would likely freeze by early Wednesday morning.

As Wednesday evening approaches we move into the more active weather for the week. While there is still a lot of uncertainty as to when and where snow will fall, we have moderate confidence (45-60% chance) that somewhere along the I-5 corridor will see at least 1 to 2 inches of snowfall. The GEFS, ECMWF and Canadian models and their ensembles are maintaining 850 mb temperatures of -5 to -7 degrees C. The fact that models continue to be consistent with the 850 mb temperatures has helped to bolster our overall confidence in the fact that snow will accumulate somewhere along the I-5 corridor. However, the timing and track of the low and associated frontal boundary still remains in flux. Now, there is some good news as the potential low tracks have narrowed slightly. Still, we are looking at three options when it comes to the higher probability tracks. Track One (20% chance), looks to move inland around Astoria, OR and push southeastward towards Kelso/Longview, WA and the Portland/Vancouver area. Track two (40% chance), looks to come in around Newport to Lincoln City, OR. The path for this solution, has a "swish and flick" look as the low looks to "swish" northeastward towards Portland/Vancouver before "flicking" eastward towards Mt. Hood, OR. Track three (20% chance), looks to come onshore around Florence, OR and head southeast towards Medford, OR. Now, for those of you counting up the probabilities of each track at home, track one is 20%, track 2 is 40% and track 3 at 20%. You might be asking about the remaining 20% chance and well, that can all be gathered into an option, that covers outlier runs of the GEFS, ECMWF and Canadian models. These outlier runs have the low coming onshore anywhere from the Olympic Peninsula all the way down to California. While these outlier runs are not likely on their own, there is still a chance, albeit a small one, that they could verify. Now, onto the other variable that has somewhat come into focus: Precipitation start time. Assuming a band of snow does develop (~50% chance), snow would most likely (50% chance) start between 7PM and 9PM on Wednesday. The earliest start time would be around 5PM Wednesday (10% chance) and the latest possible start time would be around 10PM Wednesday to 12AM Thursday (10% chance). It is not clear yet exactly when the snow would end, but it appears snow would most likely end by 12PM Thursday as the low pressure system shifts eastward towards the Cascade Crest. Note, this timing is subject to change with future updates.

While, we have a lot of things that we don't know, we are very confident (85-90% chance) that freezing rain and/or very gusty winds will NOT occur with this system, no matter the track the low takes.

Also, no matter the track, snow accumulations for the Cascades and Coast Ranges look to be on track as hazardous travel conditions are expected to continue through the work week. We are expecting additional snow accumulations of 2 to 7 inches through Wednesday night and another 1 to 3 inches of snow Thursday morning through Thursday night for the Foothills of the Northern and Central Oregon Cascades, North and Central Coast Range Mountains of Oregon, Willapa Hills, and South Washington Cascade Foothills. As for the South Washington Cascades and the Northern and Central Cascades of Oregon, expect additional snow accumulations of 4 to 9 inches through Wednesday night and another 3 to 6 inches of snow Thursday morning through Thursday night.

As we move into Thursday, temperatures are expected to warm back into the low to mid 40s, so if any snow does fall and accumulates, it would most likely melt. Temperatures Thursday night into Friday morning will likely cool again into the upper 20s to lower 30s. But shower activity is expected to wane, and any precipitation that does fall would be very light, resulting in little to no accumulation. Looking towards Friday and into the weekend, WPC 500 mb cluster analysis shows some agreement that a weak high pressure ridge will build over the region towards the latter part of the week and into the weekend. As this ridge builds over the area, 850 mb temps are expected to warm towards -3 to 0 degrees C by Saturday and towards 2 to 4 degrees C by Sunday. This will result in snow levels rising towards 2500-3500 ft by Saturday and to 3500-4500 ft on Sunday. Sunday and Monday, in addition to the warming pattern mentioned above, models and their ensembles also bring precipitation back into the forecast. Which means that rain is likely to return for areas below 5000 ft. However, there is still high uncertainty in regards to precipitation amounts and location of the heaviest rainfall. Models bring precipitation all along the western US coast Sunday into Monday, with the highest probabilities of enhanced rainfall currently pointed at portions of southern Oregon and California. /42

AVIATION

Post-frontal showers continue this afternoon which are producing gusty winds, moderate to heavy rain, and even small hail. Most guidance is suggesting a continuation of these showers over the next 24-hours. CIGs have generally lifted to VFR, though there continues to be periods of MVFR conditions with these showers. Overall, an overcast day with a mixed-bag of conditions. Overnight, temperatures will lower to or just below freezing which may promote areas of mixed rain/snow showers. While snow is not expected to accumulate significantly, most areas in northwest Oregon will see around a 30% chance of snow or a rain/snow mix.

Wednesday will be a day of transition day as a stronger cold frontal system nears the region. While not expected to make landfall before 00Z Thu, will likely see showers ahead of the main frontal band. For a bit more flight planning and a heads up, this system incoming does have the potential to bring even more snow to the area with increasing probabilities for accumulations of 1 inch or more in the interior lowlands. Confidence is low, but future forecasts will be able to narrow in on more details.

KPDX AND APPROACHES...VFR conditions with moderate to heavy showers and breezy winds. With the heaviest showers, graupel and small hail have been reported. Main concern over the next 24 hours will be precipitation type, and the inconsistency in CIG heights. Cannot rule out a rain/snow mix overnight after 10Z Wednesday and improving after 21Z. -27

MARINE

Surface low pressure will linger near the south Washington coast today, maintaining westerly breezes across the waters. Wind speeds expected to be 20 kt or less though gusty winds will be possible with any passing shower. Seas in general have been in the 10-14 ft at 13 second range but have slowly begun to ease within the inner waters. This will be the trend through Wednesday afternoon as they begin to subside. The overall trend is the combined seas remaining below 10 ft through Friday.

Looking into the future, a stronger system is on deck for the weekend with the potential for stronger southerly winds. Easily will see Small Craft Advisory level winds on Saturday, but there is around a 40-50% chance of gales in the outer waters of PZZ273. At that time, the background swell will be weaker and combined seas will be driven by the wind waves. -27

BEACH HAZARDS

There is an enhanced threat of sneaker waves at the coast through this evening. This is due to an incoming mid period westerly swell. Sneaker waves can run up significantly farther on a beach than normal, including over rocks and jetties. These waves can suddenly knock people off of their feet and quickly pull them into the frigid ocean which may lead to serious injury or drowning. Caution should be used when in or near the water, especially those who will be razor clamming. Keep children and pets away from the surf zone. Keep off of jetties, rocks and logs near the surf zone. If you see someone swept into the sea, do not swim in after them. Instead , call 911 and keep an eye on them until help arrives.

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory until 4 AM PST Friday for ORZ106-107- 123>128.

WA...Winter Weather Advisory until 4 AM PST Friday for WAZ203-208-211.

PZ...Small Craft Advisory until 4 AM PST Wednesday for PZZ210.

Small Craft Advisory until 10 AM PST Wednesday for PZZ251>253.

Small Craft Advisory until 4 PM PST Wednesday for PZZ271>273.