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

Expecting mainly VFR conditions to prevail through the TAF period. Scattered showers expected into the evening and a 10-30% chance for thunderstorms through the afternoon, highest chances north to south. Any shower or storm that passes directly over a terminal may bring a brief period of sub-VFR conditions and could produce small hail and gusty, erratic winds. For coastal terminals, northwesterly winds around 10-15 kt with gusts to 20 kt. For inland terminals, expect westerly winds 9-12 kt and gradually shifting northwesterly towards 00Z Sun.

KPDX AND APPROACHES...Predominately VFR with scattered rain showers through the evening and a chance for thunderstorms through the afternoon. Any passing shower or storm may bring brief sub-VFR conditions. Westerly winds around 10-12 kt northwesterly by 00Z Sunday. -19

MARINE

Scattered showers continue through this evening, with erratic and gusty winds possible near stronger showers. 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. 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. Winds will be strongest across the central coastal waters each afternoon and evening. -19/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory until 11 PM PDT this evening for ORZ126>128.

WA...Winter Weather Advisory until 11 PM PDT this evening 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.