textproduct: Portland

SYNOPSIS

High pressure will bring dry and calm weather with mild afternoon temperatures through Tuesday. Starting on Wednesday, a series of frontal systems stemming from a broad low in the Gulf of Alaska will move over the region. We are looking at a prolonged period of rain, wind and Cascade snow. A trend towards a moderate Atmospheric River (AR) on Friday will be accompanied by gusty south to southwesterly winds. Snow levels will begin to drop over the weekend with snow possible along the passes.

LONG TERM...Wednesday through Sunday night

Fall is in full swing as the first of multiple frontal systems approach the region. These systems are stemming from a large "parent" low in the Gulf of Alaska. This low is cold wrapped and producing several short and long wave troughs. The first trough is fairly organized as observed in the ensemble low centers. This system will move over northwest Oregon and southwest Washington early Wednesday morning but will be showery as we sit on the southern edge of the rain band. At this point, the atmosphere is still fairly warm so there is a low probability of thunderstorms, but accumulating precipitation chances are increasing. Any rain that does accumulate will be minimal with a 35% chance of 0.1 inch of rain in the northern Willamette Valley, around 65% along the coast near Astoria and lowering probabilities as you move southward. As this system quickly exits, zonal flow with advancing cooler air develops. Thursday will see minimal if any rain and mostly cloudy skies.

Friday is when things really begin to get spicy in the weather world. An moderate Atmospheric River (AR) will begin to push inland from the southwest. This AR will bring ample precipitation, strong south to southwesterly winds, and a burst of warmer air before trailing cooler air moves in. As of right now there is still a wide range of probabilities for both wind and rain. Impact wise, rain has a lower probability of issues as accumulation rates are not high enough to produce significant urban flooding at this time. Global ensembles are showing around 1 inch in 3 hours which is a little more than a quarter inch per hour. With these rain rates, cleared drains shouldn't have any issues ridding water. However, given the time of year and fallen leaves, those clogged drains could produce a problem causing localized ponding that could cause impacts to traffic.

Wind will be another concern but one that has significantly more variability. The coast will see the highest wind speeds, especially along the beaches and headlands. Along coastal cities, winds could be as low as 30 mph, or as high as 60 mph (the 10th to 90th percentile). Inland, gusts could range, based on the 10th to 90th, between 20 mph to nearly 50 mph on Friday afternoon into Saturday morning. This spread makes narrowing down on an exact forecast difficult. For rainfall totals too will be higher as the stronger band of precipitation runs over on Friday. Ultimately the rain combined with wind and full leafed trees could lead to downed branches and some heavier trees.

Also worth mentioning is lowering snow levels Saturday into Saturday night and Sunday as colder air aloft settles over the area. The deterministic NBM suggests snow levels will fall to 3500-4500 feet by Saturday night, while the 10th percentile suggests snow levels down to 3000-3500 feet (10% chance of occurring). In both cases, accumulating snow would occur at pass level, resulting in travel impacts.

Rain will continue into early next week. -27

AVIATION

Westerly flow aloft with weak ridge of high pressure over the region. Mostly clear skies with light winds tonight will bring areas of fog, particularly across the southern Willamette Valley. Likely LIFR/IFR conditions between 12-18z Tuesday morning where fog does develop. Otherwise, predominately VFR conditions expected.

KPDX AND APPROACHES...Predominately VFR conditions expected. Main exception will be for chances of fog developing between 12-18z Tuesday; 30-40% probability of MVFR or lower. /DH

MARINE

Steep seas, hazardous to small craft continue across the waters into the evening. Seas are currently around 10-12 feet at 14 seconds but are expected to continue to subside below Small Craft criteria around 8 pm. The break late this evening and into the overnight hours is expected to be short lived as another potent northwesterly swell then arrives early Tuesday. Seas will be pushed to 15 to 18 feet at 16 to 18 seconds coming down to 10 feet around 15 seconds by late Tuesday morning.

After a brief break Wednesday, an active weather pattern is then expected to continue Thursday through Saturday night with a period of gale force, possibly storm force southerly wind gusts Thursday night into Friday morning. Probabilities for gale force gusts are 40-70% while the probabilities for storm force have come down to around 20% for the inner waters only. Note that probabilities are highest over the inner waters due to the potential development of a coastal jet during that time. This system will likely push seas up to around 20 ft again, with a 10% chance for seas over 20 ft.

Seas are expected to remain elevated through the weekend as more storm systems approach the region, particularly Saturday night into Sunday. -19/23

BEACH HAZARDS

Razor clam diggers at the coast will need to exercise caution for sneaker waves this week, especially on Tuesday and Wednesday when a long period westerly swell will bring a high threat for sneaker waves. Sneaker waves can run up significantly farther on a beach than normal, including over rocks, logs, and jetties. Sneaker waves can suddenly knock people off of their feet and quickly pull them into the ocean which may lead to serious injury or drowning. Caution should be used when in or near the water. Always keep an eye on the waves, and be especially watchful of children and pets. Remember, never turn your back on the ocean. A Beach Hazards Statement is in effect across the central and northern Oregon coast and the south Washington coast Tuesday morning through Wednesday afternoon to help message this threat. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Hazardous Seas Warning from 8 AM Tuesday to 8 AM PDT Wednesday for PZZ210.

Hazardous Seas Warning from 5 AM Tuesday to 8 AM PDT Wednesday for PZZ251>253-271>273.