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 06Z Wednesday

Moist, westerly flow aloft continues tonight into Tuesday morning as an upper level trough moves over the region. A mix of VFR to MVFR conditions persists through this evening across the area. After a brief lull, showers increase again overnight while flight conditions trend toward predominately MVFR. Brief periods of IFR will also be possible, though generally just a 10-20% chance at any given hour. Conditions are likely to improve after 19-21z Tuesday as showers decrease and CIGs lift toward VFR. Winds generally remain south to southwest at 5-10 kt.

KPDX AND APPROACHES...A brief lull in showers with low-end VFR trends toward more MVFR conditions after 06-08z Tue. Shower activity expected to increase after 08z with chances for MVFR CIGs and VIS bouncing between 40-70% through 20z Tue. Predominately VFR likely returns by 21-23z Tue as showers decrease again. Southwest winds expected around 3-7 kt through the period. /DH

MARINE

Surface low pressure continues to slowly drift eastward into the north Oregon coastline with winds and seas easing. On Tuesday, another weather system drops southward 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 subside Thursday into Friday ending up back in the 4 to 6 ft range at 12-13 seconds by Saturday. DH/99

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 5 PM Tuesday to 11 AM PDT Wednesday for PZZ210-251-252.

Small Craft Advisory from 8 AM Tuesday to 2 PM PDT Wednesday for PZZ271-272.