textproduct: Portland

SYNOPSIS

A deep, upper level trough dropping south out of Canada will bring cooler temperatures along with scattered to widespread showers and the potential for lowland snow. Colder air is expected to maintain daytime temperatures around 5-10 degrees cooler than normal, while lowering snow levels towards 1000 ft. Showery weather is expected to continue through the week and will result in a rain/snow mix for lower elevations and snow for higher elevations. Given the showery nature of the precipitation, there is a 60-85% chance for snow flurries or "conversational snow" to be observed down to the valley floor through the latter part of this week. At the same time, 72-96 hour snow totals for the Cascades are around 1 to 2 feet. While not the most likely solution, there is potential (10-30% chance) for 1 to 2 inches of snowfall occurring below 1000 ft somewhere in NW Oregon/SW Washington Wed night into Thursday morning. Towards the end of next week, precipitation remains in the forecast, but snow levels start to rise towards 3000 ft or higher.

DISCUSSION...Now through Sunday

Looking at radar and satellite observations a cold front can be seen pushing through the area. This will bring about a more unstable atmosphere as cooler air is introduced into the region. Current model guidance is showing 850 mb temperatures around the -5 to -7 C range and using past observations and trends, this is the typical 850 temperature threshold that snow down to the valley floor enters into the conversation. As this colder air moves into the region, snow levels will continue to lower towards 1000 ft by this evening and as the sun sets, the snow levels are expected to drop further towards 500 ft. Also, with the convective nature of the precipitation, the wet bulb effect could also result in snow flurries/conversational snow being observed down to the valley floor. Again, am not expecting impacts for lower elevations as accumulations will be light, if any. Still, don't be surprised if on Tuesday morning, there is some minor accumulation on the grass and elevated surfaces. Given all of that information, the current suite of Winter Weather Advisories for the Coast Range, Cascades and Cascade foothills remain on track, with at least a 70-80% chance of exceeding 6 inches at pass level through Tuesday morning, with similar probabilities of at least 2 inches of snowfall for the Coast Range and Cascade foothills.

As Tuesday approaches, a deep upper low looks to be positioned over the Pacific NW, which will keep 850 mb temperatures in the -5 to -7 C range and under onshore flow. The upper level low as well as the associated surface low, will slowly push southeastward towards the Central Oregon Coast. Surface low pressure will likely remain offshore near the Washington or north Oregon coast. Cooler 850 mb temperatures will support an unstable and showery pattern. Current model sounding are showing CAPE values around 200-300 J/kg, which is resulting in around a 10-25% chance for thunderstorms on Tuesday. So, while general snow accumulations will be unlikely for the lowlands. Locally heavier showers could briefly enhance snowfall rates, via the wet bulb effect and would likely melt on contact or shortly after the shower has moved out of the area. Any thunderstorms that do form would have the potential for small hail or graupel, along with gusty outflow winds.

As we look towards Tuesday and Wednesday, ensembles and hi-res models suggest a bit of a lull in shower activity Tuesday night into Wednesday morning. Overnight temperatures are expected to fall below freezing across most of the forecast area. Any showers that do occur early Wednesday would likely produce snow showers, but should be light and widely scattered. Another concern would be any wet roads from precipitation on Tuesday would have the chance to freeze and become icy by Wednesday morning.

Wednesday night into Thursday morning becomes a bit more interesting as a few models and ensemble members are hinting at a scenario that could produce accumulating snow for the lowlands, particularly during the morning commute. Models continue to maintain 850 mb temperatures in the -6 to -7 C range, which increases the chance for snow to be observed as well as accumulate for the valley floor. However, the biggest cause of uncertainty at this time is the exact track of the low and associated front. Diving into the ECWMF, GEFS and Canadian models and their ensembles, it looks like there are three general tracks that could occur.

Track one would take the low into the Olympic Peninsula, track two would bring the low inland around Astoria, OR and track three would result in the low coming onshore around Florence, OR. For track one details, the path would result in a higher chance for rain for elevations below 1000-1500 ft as relatively warm southerly flow will keep 850 mb temperatures towards 0 to -3 degrees C. While snow being observed cannot be ruled out for track 1, the overall probability would be around 10% for accumulating snow. Track two, would be the "snowiest" a.k.a. the highest impact solution out of the three tracks. 850 mb temperatures around -6 to -8 degrees C would be expected along with precipitation amounts around 0.05 to 0.15 inches of QPF. If track two occurs, the probability of snow accumulations of 1-2 inches or more across the lowlands would be around 30-40%, mainly over the Portland/Vancouver metro and Cowlitz Valley, including Kelso and Longview. Lastly, the third track would also result in snow accumulations for the lowlands, but with the bulk of the precipitation pointed towards the central or southern Willamette Valley. This track would favor snow from Salem to Eugene-Springfield, with little to no snow north of Salem. No matter the track, snow accumulations for the Cascades and Coast Ranges look to be on track where hazardous travel conditions are expected to continue through the work week. While overall snow totals will vary, depending on the track we are still anticipating advisory level snow accumulations somewhere in the Willamette Valley or Cowlitz Valley. Overall, the confidence for lowland snow for Wednesday night into Thursday morning scenario is low.

As we move into the latter part of the week and into the weekend, precipitation remains in the forecast, but 850 mb temperatures are expected to warm towards 0 to -2 degrees C. That would result in snow levels rising towards 1500-2000 ft by Saturday and Sunday. /42

AVIATION

Late this evening (as of ~05-06z) the I-5 corridor sits in a bit of a lull with mainly VFR conditions currently in place at inland terminals. However, satellite and radar show ample shower activity along the coast slowly starting to work inland with an arrival generally between 07-10z bringing a return to predominately MVFR cigs. The airmass is already cold enough for snow showers or a rain/snow mix down to the valley floor once showers arrive, in fact KAST has already reported snow in heavier showers. Expect off-and-on snow showers or a rain/snow mix to then continue through 15-18z Tuesday until activity moves northeastward and then decreases in frequency. Since temperatures will be marginal and precipitation will be showery, expect very little to no snow accumulation at the terminals, with a dusting of slushy snow on grass at best. Any snow that does accumulate should melt fairly quickly by the midday hours as temperatures warm into the 40s. Through the rest of the day, any heavier showers could still produce a rain/snow mix at times. CIGs trend higher to VFR in the afternoon hours Tuesday as well.

KPDX AND APPROACHES...Showers will increase in coverage again by 09-10z tonight and given snow levels are near the surface, this round of showers will likely fall in the form of a rain/snow mix or wet snow - 30-40% chance for IFR conditions due to reduced VIS within showers. However, little to no snow accumulation expected at the terminal as temperatures will be marginal and precipitation will be showery in nature. At best, a dusting of snow on grass may occur. Any snow that does accumulate should melt fairly quickly by late morning as temperatures warm into the 40s. -99/23

MARINE

A cold front moved inland early Monday afternoon, with westerly winds now in place over the coastal waters along with scattered showers and isolated thunderstorms. However, stronger showers or thunderstorms that develop will have the potential to produce gusty and erratic outflow wind gusts up to 30 kt.

Surface low pressure drops south along the Washington coast late Monday into Tuesday, bringing more south to southwesterly winds across the waters. The low pressure likely weakens as it continues to drift south through the Oregon coastal waters resulting in easing wind speeds.

A mid-period west-northwesterly swell will push into the coastal waters Monday night, building seas to around 12 to 14 ft on Tuesday, likely highest over the central and southern waters. The southern outer waters may see significant wave heights as high as 15-17 ft (10% chance). Have extended the Small Craft Advisory for all coastal waters through Wednesday morning.

Seas gradually subside Wednesday into Thursday with wind gusts most likely ranging between 15-25 kt and seas falling to around 10 ft or less. Winds and seas will likely increase next weekend as there is potential for a stronger system near the waters. There is currently a 60-80% chance for gale force wind gusts over 34 kt February 21-22 with a 50% chance for seas of 12 to 14 ft and a 5-10% chance for seas as high as 16-19 ft. -23/DH

BEACH HAZARDS

There is a high threat of sneaker waves at the coast on Tuesday, February 17th. This is due to an incoming long 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- 271>273.