textproduct: Portland

SYNOPSIS

Drier weather returns on Friday as high pressure builds. A weak front will bring chances for light rain across southwest Washington and along the coast over the weekend into early next week. Drier and relatively warmer conditions return mid next week.

DISCUSSION...Now through Thursday

Westerly onshore flow persists in the post cold front environment. Widely scattered showers continue across the area, supporting light rain in the lowlands and light snow in the Cascades. Precipitation will continue to taper off this evening as high pressure builds in from the west. Scattered snow showers may continue over the higher terrain this evening but impacts should remain minimal at pass level.

Upper level ridging will build overnight and will support dry conditions across the area Friday and Saturday. Cloud cover is expected tonight but a few breaks may develop. Temperatures may drop close to freezing allowing for patchy frost to develop early Friday morning. Winds will turn offshore and will remain light. Temperatures will be closer to the upper 40s to low 50s along the coast and in the lowlands and upper 30s to low 40s for the higher terrain both Friday and Saturday. Light transport winds and low mixing heights will lead to a period of air stagnation throughout the Willamette Valley and around the Portland Vancouver Metro. An Air Stagnation Advisory has been issued and is in effect from 10 AM Friday through 10 AM Sunday. Conditions may cause deteriorating air quality which could cause issues for people with respiratory problems.

An upper level trough starting out south of the Aleutian Islands on Friday will track eastward into the Gulf of Alaska and toward the British Columbia coast. This will cause the upper level ridge to flatten and the axis to shift slightly inland toward the Great Basin. A cold front is expected to approach the southwest WA and northwest OR coasts but weakens substantially by the time it reaches the coast. Rainfall for most of the area is expected to be light or nonexistent Saturday night through Sunday. The bulk of the rain is expected north of the Portland CWA in the Olympic Peninsula of WA. However, some areas in the CWA will see rain, mainly Pacific and Wahkiakum counties in WA and Clatsop and Tillamook counties in OR. Totals Saturday night through Sunday could reach 0.5-1.0 inches. Along the central OR coast and southwest WA from Cowlitz to Skamania , totals could reach 0.1-0.25. The Portland/Vancouver metro, Willamette Valley, and OR Cascades should see at most a few hundredths if anything. Cluster analysis shows better agreement in the ridge amplifying on Tuesday and Wednesday, supporting dry and mild conditions across the area. Highs in the upper 50s to low 60s will be possible west of the Cascades. Ensemble agreement is still decent but there are signs of the ridge shifting or breaking down late next week. -19

AVIATION

Northwest flow aloft as high pressure continues to build over the region through Friday. Chances for MVFR ceilings remain around 40-60% through at least 08-10z Friday for most terminals, becoming VFR after. Showers are expected to decrease after 06z Friday. There's some uncertainty on whether skies will clear out across portions of Willamette Valley. If they do, this would allow for IFR to LIFR stratus or fog development near KUAO, KSLE, KEUG and KHIO after 10-12z Fri. These lowered conditions would return to VFR by 18-21z Fri. Southerly winds to around 10 kt become light and variable after 10-12z Fri.

KPDX AND APPROACHES...Conditions will fluctuate around high-end MVFR to low end VFR with chances for MVFR CIGs persisting around 40-50% through at least 10-12z Friday. High confidence in VFR conditions returning by 18-21z Fri with flight levels rising to above 10kft. Southwest winds around 8-10 kts becoming southeast below 6 kts after 10-12z Fri. -DH/03

MARINE

As of 9 PM PST Thursday, seas are slow to decrease with buoy observations indicating wave heights hovering around 15 to 17 ft at 14-16 seconds as the northwesterly swell continues to push through the waters. Expect seas to continue to slowly subside tonight through Friday, falling into the mid- teens overnight, and eventually below 10 ft by Friday evening. The Hazardous Seas Warning has been extended through 5 AM PST due to the lag in lowering seas.

Meanwhile, an area of strong high pressure off the northern California coast shifts east, causing westerly winds over the coastal waters to become more southerly by Friday. Strong high pressure inland along with an approaching front offshore will produce an offshore component to the wind as well. Southerly winds are expected to increase on Saturday as the front slowly approaches the coastal waters, increasing the pressure gradient across the waters. Expect widespread gusts up to 25-30 kt on Saturday and through much of the weekend. There is around a 20-40% chance of gale force wind gusts across the outer coastal waters beyond 20 NM, but that will depend on how far east the quasi-stationary front gets.

Seas will build again on Saturday as the southerly wind waves combine with a more southwest to westerly swell through the weekend. Seas around 12 to 13 ft are likely by Saturday evening, before building again Sunday evening into the mid-teens. Chances for exceeding 20 ft on Sunday are very low (less than 5%). High pressure likely returns early next week. -DH/03

BEACH HAZARDS

A high threat of sneaker waves remains for the coast into early next week due to a continued high-period swell. Sneaker waves can create potentially life-threatening conditions in the surf zone as waves may run farther up the beach than expected. These waves can easily catch people off guard and cause beachgoers to be knocked off their feet and pulled out into cold ocean waters. Never turn your back to the ocean. Caution should be used when in or near the water, and those with children should be especially watchful. Sneaker waves can lift or roll large, heavy logs or rocks which can lead to serious death or injury. Keep off rocks, logs, and jetties near the waterline on beaches. -03

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory from 10 AM Friday to 10 AM PST Sunday for ORZ109>111-114>118.

WA...Air Stagnation Advisory from 10 AM Friday to 10 AM PST Sunday for WAZ205-206.

PZ...Hazardous Seas Warning until 5 AM PST Friday for PZZ210-251>253- 271>273.

Small Craft Advisory from 5 AM to 4 PM PST Friday for PZZ210- 251>253-271>273.