textproduct: Portland

UPDATED AVIATION DISCUSSION

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

Expect a mix of flying conditions through the TAF period as a frontal system currently hung up along the North Oregon and Southwest Washington coast slowly progresses it the rest of the region. Low stratus lingers in portion of the Willamette valley late this morning with a mix of LIFR/IFR CIGs showing slow improvement at KUAO through KSLE. Over the next several hours (by ~20z) these degraded conditions should improve back to MVFR/VFR but confidence is only moderate in this early afternoon timing.

Along the coast, light rain associated with the frontal band has already spread to KAST with a mix of MVFR/IFR conditions lasting through much of the TAF period due to reduced CIGs/VIS. Conversely VFR conditions at KONP well south of the frontal boundary likely persist through a good chunk of the day (until 22z Fri to 02z Sat). It's worth noting southerly winds remain breezy/gusty along the coast with gusts in the 15-30 knot range through the TAF period.

Rain will spread from northwest to southeast across the inland terminals mainly later in the TAF period. Most likely onset times of rain 05-08z Sat at Portland metro area and northern Willamette Valley. Rain showers may fail to reach as far south as KEUG however.

KPDX AND APPROACHES...Mid-level cloud cover (4-7kft) continues to increase ahead of an approaching frontal boundary but VFR CIGs likely (70-80%) hold in place into the evening hours. Light rain is the expected to slowly increase, most likely after 06-07z with MVFR cigs falling to 2-3 kft into Saturday morning. -99/36

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.