textproduct: Portland

SYNOPSIS

Dry weather under higher pressure continues through late Saturday with another round of fog and frost development expected tonight into Saturday morning. A pattern shift early next week will favor a wet week for the upcoming holiday.

MORNING UPDATE

The Dense Fog Advisory across interior lowland valleys has been cancelled. Based on surface observations and webcams, visibility has risen above one quarter of a mile across most interior lowland valleys. Some locations may still experience patchy dense fog, however, conditions should improve by 10-11 AM. If driving in fog this morning, slow down, use your low beam headlights, and leave plenty of distance ahead of you. -10

DISCUSSION...Today through Friday

Fog should lift by around 10- 11 AM this morning. Low stratus across the Willamette Valley will likely remain throughout the day and struggle to dissipate. This is due to the lack of mixing and a relatively strong inversion aloft keeping the clouds low. High level clouds above this low stratus deck will also prevent daytime heating from breaking up the low stratus. A weak weather system slowly approaches the region today which will bring increasing precipitation chances along the north Oregon and SW Washington coast tonight. The cold frontal system will bring a widespread band of light rain tonight into Sunday, transitioning into showers Sunday afternoon/evening and continuing through Monday afternoon. Rain amounts with the Saturday through Monday system will be around 0.25-0.95 inches along the coast and Coast Range, 0.15-0.35 inches for inland locations, and 0.25-1.00 inches over the Cascades. Snow levels remain above Cascade pass level through Sunday, then they will begin falling Sunday night behind the cold front, falling to 2500-3000 ft by Monday morning. However, as conditions will be showery by this point, limited snow accumulation is expected at pass level, generally less than 2 inches. Winds are not expected to increase much with this system with gusts up to 30 mph along the coast Sunday during the day and over the Cascades Sunday night, remaining less than 20 mph for inland valleys. The start of next week, mornings again are expected to be on the cooler side even with widespread cloud cover. Monday night into early Tuesday is expected to be dry with another night of colder temperatures in the 30s across the interior lowlands. By midday Tuesday, widespread precipitation chances increase again as ensemble guidance members indicate the first in a series of back to back frontal systems move through the region. Current guidance has an active weather pattern continuing through the remainder of the week. Exact timing details remain uncertain this far out, but limited impacts are expected at this time.

Probability for 2 inches of rain in the 72 hour period from 4 AM Tuesday to 4 AM Friday is 5-10% for inland lowlands, 15-30% chance south of Highway 20 in Oregon, and 40-75% chance north of Highway 20. 3 inches of rain over the same 72 hour period are less than 10% for inland lowlands, 10-20% chance south of Highway 20 in Oregon, and 20-45% chance north of Highway 20. Given that these system will be warm, snow levels will rise above pass level. Looking beyond Thursday, ensembles indicate yet another weather system could bring additional rain on Friday into Saturday. Keep an eye on the forecast if you have holiday travel plans! -42/10

AVIATION

High pressure will remain in place through at least 12z Sunday, resulting in calm winds and an expansive/persistent low stratus deck with cigs in the 200-700 foot range across the entire Willamette Valley. With high clouds moving in from the west over the top of this stratus deck, combined with a low sun angle this time of year and little to no mixing, now expecting this stratus deck to remain in place through today and tonight with no significant clearing. Cigs in the mid to late afternoon will most likely range between 500-700 ft, before lowering back to 200-300 ft towards 03z Sunday. This is also when surface visibilities will likely lower to 1/4 to 3/4 SM at all inland terminals, aside from KTTD which may see relatively higher surface visibilities. While unlikely, KEUG and KTTD may attempt to scatter out very briefly towards sunset, however stratus/fog would fill right back in within a few hours even if this does occur. In summary, expect LIFR to IFR flight conditions for all inland terminals through at least 12z Sunday. After 12-15z Sunday, a front will begin moving inland, bringing light rain and increased mixing to the area. This will eventually help lift fog and cigs, with flight conditions improving to IFR or MVFR. That said, confidence is low regarding the exact timing cigs/visibilities will lift Sunday morning. Meanwhile, coastal terminals will most likely experience a mix of IFR/MVFR cigs throughout the TAF period with relatively higher surface visibilities.

KPDX AND APPROACHES...Expect low stratus to remain in place through at least 12-15z Sunday, with cigs hovering between 200-700 ft (highest from 21z Saturday-01z Sunday, lowest from 02-12z Sunday). By 03-06z Sunday, fog will likely redevelop with surface visibilities lowering to 1/2 SM or lower. There is a 20-30% chance surface visibilities will fall to 1/4 SM or lower. By 15z Sunday, an incoming front will bring light rain and increased mixing. This should help scour out fog and allow cigs to lift to IFR or low-end MVFR thresholds, however the exact timing this will occur is uncertain. -23

MARINE

High pressure over the area is maintaining fairly stable conditions with minimal change expected through Monday. Seas currently range from 10-14 ft at 14-16 seconds and are expected to slowly subside towards 9-12 ft by Monday. Seas are expected to fall below 10 ft around Monday afternoon and will persist through at least the middle of the upcoming week. On Sunday a frontal system will move over the waters causing winds to increase from the south with gusts up to 25 kt. Low pressure will move inland by late Sunday into Monday, allowing winds to shift to the northwest. -23/42

BEACH HAZARDS

An energetic westerly swell is expected along the northwestern Oregon and southwestern Washington coasts through Monday. Seas 10-14 ft at 14-16 seconds are forecast through the the weekend, resulting in a high sneaker wave threat. These energetic waves can run significantly farther up a beach than normal, including over rocks and jetties, creating potentially life- threatening conditions in the surf zone when beachgoers can be unexpectedly knocked off their feet and pulled out into cold ocean waters.

A Beach Hazards Statement has therefore been issued through 10 AM Monday. Caution should be used when in or near the water, and beachgoers with children should be especially watchful. Never turn your back on the ocean, and be sure to keep an eye on the waves. Those participating in razor clam digs should additionally exercise caution. /42

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 10 AM PST Monday for PZZ210-251>253- 271>273.