textproduct: Portland

SYNOPSIS

Showers and isolated thunderstorms this afternoon and evening will diminish tonight. Above 3500 feet including at the major Cascade passes, snowfall will continue to accumulate while showers persist, and Winter Weather Advisories remain in effect along the Cascades through this evening. A warming and drying trend is then expected from Sunday through the workweek.

DISCUSSION...Now through Friday night

An upper-level shortwave overhead will continue to support numerous rain and mountain snow showers this afternoon and evening. As mid-level temperatures within the trough reach their coldest values of -2C to -3C this evening, peak instability (surface-based CAPE values of 100-250 J/kg) will result in a 15-35% chance of isolated thunderstorms, but their overall intensity will be limited by the low buoyancy and their shallow nature as equilibrium levels struggle to reach 500 hPa. Thundershowers may nonetheless produce brief heavy rain, gusty winds, and small hail. Those recreating outdoors should seek shelter if thunder is heard.

Cold air arriving aloft has seen snow levels fall to around 4000 ft this afternoon, and they are expected to further fall to between 3500 ft by late this evening. Snowfall continues to accumulate at and above pass level, with a 50% chance of an additional 1-4 inches, and a 20-25% of 6 inches or more, by late this evening. There is a 10% chance of light accumulations as low as 3000 ft elevation. A Winter Weather Advisory therefore remains in effect along the Cascades through 11 PM PDT today as higher elevation road surface are cold enough to support accumulating snow resulting in slippery travel conditions.

The trough will exit to the south and east with shower coverage diminishing after midnight. An upper-level ridge building over the Northeastern Pacific will then yield a warming and drying trend through the week. Weak upper shortwaves may continue to track overhead as the ridge amplifies and shifts eastward, but ensembles favor only minor, 5% or less, chances of rain. Uncertainty in the eastward progression of the ridge axis supports increasing uncertainty in potential high temperatures later in the week. Chances to exceed 80 degrees rise from less near 0% on Sunday to 10-25% within inland valleys on Thursday and Friday, with a more eastward ridge axis favoring a warmer solution, while a more offshore ridge axis may maintain onshore flow and cooler temperatures. -36

AVIATION

Scattered rain showers, mostly VFR conditions, and northwesterly winds around 8 kt with occasional gusts up to 20 kt continue as of 04Z Sun. Expect rain showers to taper off from north to south, winds become light, and current 10-20% chances of MVFR conditions to lower towards 06-08Z Sun. Beyond 06-08Z Sun, expect dry conditions and VFR conditions through at least 06Z Mon. Around 18-21Z Sun, northwesterly winds will increase to 10-15 kt with gusts up to 20-25 kt at the coast and 7-10 kt with gusts up to 17-20 kt possible at inland terminals. Greatest chances for the upper end of these winds and wind gusts are located to the south, around KONP and KEUG.

KPDX AND APPROACHES...VFR through at least 06Z Mon. Scattered rain showers will taper off by 06-08Z Sun. Northwesterly winds around 8 kt become light by 06-08Z Sun, then increase to 7-9 kt by 18-21Z Sun. ~12

MARINE

Scattered showers continue to taper off as of early tonight. A mid- period northwest swell will support seas to around 10 to 12 ft, then gradually subsiding back below 10 ft by early Sunday. The Small Craft Advisory remains in place for all waters through 2 AM Sunday. The Small Craft for the Columbia River Bar has been extended to 8 AM for a very strong ebb current expected to peak around 530 AM Sunday morning. Strong ebb currents for the Columbia River Bar are expected during the morning hours into next week, potentially yielding steep and hazardous seas for small craft crossing area bars through much of the week. North to northwesterly winds return Sunday into early next week while seas ease to around 4 to 8 ft as high pressure builds offshore. Sunday afternoon, northerly winds increase to over 21 kt gusts over the inner waters from Cape Foulweather OR to Florence OR, therefore have issued a Small Craft Advisory from 3 PM to 9 PM Sunday. These gusty winds may reach beyond 10 NM from Cape Foulweather to Florence OR. Otherwise, winds will be strongest across the central Oregon coastal waters each afternoon and evening. ~12/19/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory until 5 AM PDT Sunday for ORZ126>128.

WA...Winter Weather Advisory until 5 AM PDT Sunday for WAZ211.

PZ...Small Craft Advisory until 8 AM PDT Sunday for PZZ210.

Small Craft Advisory until 2 AM PDT Sunday for PZZ251>253- 271>273.

Small Craft Advisory from 3 PM to 9 PM PDT Sunday for PZZ253.