textproduct: Portland

SYNOPSIS

Seasonably cool and showery conditions continue through the workweek as repeated weather systems support persistent chances for rain. A drying trend beginning Friday may bring better trick-or-treating conditions by late Saturday.

SHORT TERM...Now through Tuesday

A vertically stacked surface and upper-level closed low, currently centered about 30 miles west of Astoria as of 2 PM PDT Monday, will continue to slowly push eastward and inland over the north Oregon coast this evening. Scattered to numerous rain showers ongoing this afternoon will diminish in coverage late this evening as the upper feature passes overhead, but an upper shortwave which will approach the coast will see another round of increased showers from tonight through midday Tuesday. Observed 36-hour rainfall totals through midday Monday have generally ranged from 0.75-1.5 inches, with slightly lower amounts near 0.5 inches on the east side of the Coast Range and the highest totals of 1.5-2.5 inches on the west slopes of the Coast Range and Cascades. Additional rainfall of 0.25-0.5 inches is most likely through Tuesday, with up to 1 inch in favored upslope enhancement areas on the west/southwest slopes of area terrain. Snow levels around 5500 ft as of midday Monday will fall closer to 5000 ft by early Tuesday morning, allowing for light accumulations at pass level. Chances for 1 inch of snow at Santiam and Willamette Passes through midday Tuesday remain 50-60%, with only 15% chances at passes near Mt. Hood. Seasonable temperatures continue across the region with afternoon highs in the 50s and early morning lows in the upper 30s to mid 40s. -36

LONG TERM...Tuesday Night through Monday

Another, broader closed upper low and its associated surface low will drop south from the Gulf of Alaska over Vancouver Island and into the Pacific Northwest from Tuesday night into Wednesday morning. This will bring another round of precipitation to the region through Friday. Falling mid-level temperatures will see snow levels decline to around 3500 ft by Thursday morning, with a 10% chance of falling all the way to 2500 ft. This will support 35-65% chances for 2 inches of snow at Cascade passes, highest at Santiam Pass, but 5% or lower chances of 6 inches. Those planning travel through the Cascades should prepare for winter driving conditions. At lower elevations, another 0.5-1.0 inches of rain is most likely, with lower amounts of 0.25-0.5 inches on the east slopes of the Coast Range and higher amounts of 1.0-2.0 inches on the western and southwestern slopes of area terrain.

From Friday through the weekend, upper-level ridging is favored to build into the region from the west, supporting a drying trend locally. While chances for rain therefore decrease through the day on Saturday, there continue to be about 20% chances for lingering rain showers for Halloween trick-or-treaters in the afternoon and evening. The drying and clearing trend will allow for colder overnight temperatures beginning Friday night, with upper 20s to mid 30s most likely for most inland valleys nightly through the weekend and into early next week. -36

AVIATION...Through 00Z Wednesday

Late this afternoon conditions continue to be a mix of VFR to MVFR as an upper- level low pressure system spins showers overhead. Headed into the evening and overnight hours most sites should see a lull in shower activity before another round arrives west to east ~07-12z before persisting through a good chunk of Tuesday. While CIGs/VIS likely remain a mixed bag during the lull overnight, the trend will be back towards MVFR as that next batch of showers arrives. It's worth noting a heavier shower either early this evening or on Tuesday could push conditions to IFR briefly. The NBM gives a 10-25% chance for a period of IFR conditions 13-16z Tuesday as the second wave of showers peaks. Winds generally remain south to southwesterly at 5-10 knots although expect gusts up to 15-25 knots at KONP and in the central/south Willamette valley into the evening hours.

KPDX AND APPROACHES...Frequent showers overhead likely decrease this evening followed by a lull in activity for a portion of the night - a mix of MVFR/VFR conditions persist. Another wave of showers arrives around 10-12z with decent confidence in MVFR conditions through 16-17z. -99

MARINE

This afternoon a surface low pressure system continues to slowly wander westward into the north Oregon coastline with buoys reporting wind gusts of 20-25+ knots south of Cape Foulweather while seas hold around 6-8 ft at 8-9 seconds. Given the current progression of this low feature driving the winds, Small Craft Conditions likely continue near Newport through 5-6pm after which point winds diminish. A Small Craft Advisory is also in effect for the Columbia River bar late this afternoon due to a very strong ebb mixed with seas near 7ft. Looking forward at Tuesday yet another weather system drops southward across the eastern Pacific from the Gulf of Alaska introducing a reinforcing northwesterly swell late Tuesday into Wednesday while also increasing wind gusts to 20-25 knots north of Cape Foulweather. There continues to be 10-35% chance for seas to meet or exceed 10 ft on Wednesday with the highest relative probabilities for the waters beyond 10 NM. Seas slowly ease Thursday into Friday ending up back in the 4-6ft range at 12-13 seconds by Saturday. -99

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.