textproduct: Portland

SYNOPSIS

Persistent shower chances continue into the first half of the weekend beneath largely zonal flow aloft, before a more robust trough approaches the coast on Sunday, bringing cooler temperatures, widespread rain, and mountain snow potentially down to pass-level.

LONG TERM...Sunday through Thursday

As ridging builds inland over the Rockies, yet another elongated trough will approach the coast this weekend. The evolution of this next trough could look very similar to the current pattern, namely a developing cutoff low heading into California leaving a weak trough over the Pacific Northwest, however some global ensemble members instead favor a more coherent trough impacting the coast. Even modest troughing could usher in colder air aloft and allow snow levels to fall through midweek, however at this lead time, the majority of precipitation looks to fall while snow levels remain above Cascade passes through Monday night. Higher passes, namely Santiam and Willamette Passes, will most likely see 1-3 inches of accumulating snow through Tuesday, limiting travel impacts. Chances for more impactful amounts of 6 inches or more through Monday night are only 5-10% at pass-level given the lack of overlap of precipitation and sufficiently cold temperatures.

As repeated troughs bring cooler air overhead, many will see some of the coldest nights so far this season by the second half of the workweek, with most valley locations falling into the 30s overnight. With this cooler air mass in place, passing shortwaves may also see further accumulating mountain snow, yielding a 15-30% chance of 6 inches or more at Cascade passes from Wednesday through Thursday night. -36

AVIATION

Mixed flying conditions in place ahead of an approaching frontal system. Clearing earlier in the night has allowed areas of fog/mist or low stratus to develop across inland valleys early this morning, which are expected to linger through 15-18z Fri before mixing out. The highest chances for persistent IFR cigs/vis are in the southern Willamette and other sheltered valleys (KEUG, KCVO, KHIO) as well as along rivers (KSPB, KKLS). Elsewhere, intermittent restrictions are possible, with a 15-25% chance of occurrence, but too low to include in TAFs. Southeast winds at 5 kt or less this morning will build to 5-10 kt out of the south by midday.

Along the coast, rain shower coverage increases from south to north, with MVFR vis and increasing southerly winds expected to continue. Conditions should improve to VFR as diurnal overturning begins by 16-18z Fri, while southerly winds increase to 10-15 kt with gusts of 20-25 kt through the afternoon.

Rain will spread from northwest to southeast across the region later in the TAF period, beginning as early as 18-21z Fri on the coast, and spreading inland as early as 21-24z Fri. Most likely onset times of rain are not until 00z Sat at KAST, and 04-08z Sat at Portland-area, northern Willamette Valley, and other coastal terminals. Rain showers may fail to reach as far as KEUG at all.

KPDX AND APPROACHES...Largely VFR conditions expected through the remainder of the overnight period with pockets of restricted vis/cigs within fog/mist or beneath low stratus clouds. Increased mid-level cloud cover at 8-10 kft could help limit fog/mist extent after 15z Fri. Light southeast winds early turn out of the south at 5-10 kt by midday. Rain arrives late, as early as 21z Fri but more likely after 00z Sat, with MVFR cigs at 2-3 kft. -36

MARINE

Westerly swell is continuing to ease early this morning, however buoy observations maintain seas above 10 ft at 12 to 13 seconds. The Small Craft Advisory has therefore been extended through 1 PM PST Friday afternoon across the waters until seas fall below 10 ft. Meanwhile, southerly winds at 10-15 ft may see intermittent gusts above 20 kt this afternoon as a weak frontal boundary traverses the region and moves onshore. Chances for gusts up to 25 kt is the highest over the inner waters (within 10 NM) from Cape Shoalwater to Cape Foulweather including the mouth of the Columbia River and the Columbia River Bar through tonight. Small Craft Advisories in these areas may need to be further extended if the coverage of frequent gusts increases. Otherwise, seas will continue to subside to 6-8 ft through the weekend.

Another frontal system will traverse the waters on Sunday, bringing increased southwester winds before turning out of the northwest on Monday. Westerly swell rising to 10-13 ft behind the frontal passage will see seas of 12-14 ft by Monday morning, then falling below 10 ft again by early Tuesday. The active pattern continues as yet another weather system may bring hazardous winds and seas by mid to late next week. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 1 PM PST this afternoon for PZZ210- 251>253-271>273.