textproduct: Portland

SYNOPSIS

Mostly clear skies and calm winds may bring frost and/or fog development across much of NW OR and SW WA in the mornings through Monday. Mostly dry conditions through Monday. Low chances for scattered showers Monday night, then precipitation chances increase later next week with limited weather impacts expected.

DISCUSSION...Through Friday

Visible satellite imagery early Saturday afternoon shows areas of fog/stratus across some inland valleys with mostly clear skies elsewhere. Afternoon temperatures are expected to warm into the mid- 50s, before quickly falling into the 40s after sunset this evening. The upper trough that has been spinning just offshore all week became elongated offshore overnight and is slowly pushing inland today through tonight. This will bring a 20-40% chance of showers to mainly the Lane and Linn County Cascades and foothills this afternoon and evening, decreasing around midnight. Valley locations should remain dry with just some high clouds overhead.

Dry conditions continue Sunday into Monday as an upper level ridge of high pressure builds over the PacNW behind the trough passage. Skies are expected to clear after midnight tonight, which combined with calm winds is expected to lead to another round of widespread fog or low stratus across inland valleys Sunday morning. Radiational cooling is also expected under these conditions, which will allow minimum temperatures outside of urban areas to cool into the mid to upper 30s. Fog/low stratus should dissipate by midday with sunny skies and high temperatures in the 50s. Expect a rinse and repeat of this forecast on Monday.

Today's 12z ensemble guidance has come into much better agreement on the overall weather pattern through midweek next week, though some uncertainty on specifics still exist. Guidance indicates a low pressure system will develop in the eastern Pacific early next week as the high pressure moves inland over the PacNW. The majority of ensemble members indicate this low will move inland somewhere in northern CA or along the CA/OR border late Monday through Tuesday. Currently the forecast has a 10-25% chance of scattered showers Monday night into early Tuesday with very limited rain amounts, though depending on the strength and north to south extent of this low, these chances could decrease in subsequent forecasts. Tuesday through most of Wednesday look to dry out with temperatures warming to near 60 degrees for the interior lowlands. Then precipitation chances increase again late Wednesday into Friday as ensemble members indicate another front or two could bring a couple more rounds of moisture to the area. At this point, no significant precipitation impacts are expected with any of these weather systems, though there's a 25-50% chance of wind gusts increasing over 30 mph along the coast (5-20% chance inland) with each frontal passage late in the week. -03

AVIATION...Through 06Z Monday

Currently mostly VFR across the airspace with northward-moving mid to high level clouds gradually breaking and shifting east of the Cascades due to high pressure aloft. With skies gradually clearing through 9-13Z Sun, expect increasing chances for fog development. KAST and KUAO have already begun to experience deteriorated conditions, currently with MVFR to IFR visibility. After 9-13Z Sun, expect fog to develop at most inland terminals, with the least confidence for fog at KTTD. Probabilistic guidance currently suggests a 20-50% chance of LIFR conditions between 09-20Z Sun, with the highest chances, earliest occurrence, and longest duration at terminals within the southern Willamette Valley. Specifically, most terminals will improve back to VFR by 18-19Z Sun, while KSLE and KEUG may remain under deteriorated conditions until 20-21Z Sun. Winds will remain generally light and variable through the TAF period, further favoring fog development overnight. Light easterly wind at coastal terminals will likely limit conditions to VFR, with only a 15-25% chance of MVFR conditions between 10-18Z Sun.

KPDX AND APPROACHES...Currently VFR conditions with mid to high level clouds gradually clearing towards 13Z Sun. Clearing skies will bring favorable conditions for fog development, prompting probabilistic guidance to suggest a 20-30% chance of LIFR VIS anytime between 12-18Z Sun. VFR conditions will return by 18-19Z Sun. Generally light and variable winds through the TAF period. ~12

MARINE

Weak low pressure offshore gradually dissipates today as winds decrease below 10 kt. Seas around 4 to 5 ft continue to subside to 3 to 4 ft into Sunday morning. Offshore winds develop late Sunday night and persist into early next week. A long period west-northwest swell also arrives Sunday evening, building seas to around 7 to 9 ft at 16-19 seconds on Monday. Chances for seas over 10 ft on Monday remain at around 30-50% across the outer waters. This long period swell will result in a moderate threat for sneaker waves at the coast Sunday evening through Tuesday. Caution should be used at beaches when in or near the water.

A weak low pressure system approaches the waters mid next week, returning light southerly winds. By the end of the week, model guidance hints at a stronger system entering the waters, resulting in a 60-80% chance for maximum wind gusts exceeding 21 kt on Friday with a 30-50% chance for seas exceeding 15 ft. -10/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Dense Fog Advisory from 1 AM to 10 AM PDT Sunday for ORZ108-109- 111-112-114>118.

WA...Dense Fog Advisory from 1 AM to 10 AM PDT Sunday for WAZ204>207.

PZ...None.