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 Tuesday. Precipitation chances increase Wednesday through late next week with limited weather impacts expected.

DISCUSSION...Through Saturday

Surface observations and satellite imagery early Sunday morning reveal widespread low stratus and fog developing throughout the lowland valleys of SW Washington and NW Oregon. A Dense Fog Advisory is in effect through 10 AM Sunday across the area. Fog/low stratus should dissipate by midday with sunny skies and high temperatures in the 50s. Dry conditions are expected today into Monday as an upper level ridge of high pressure builds over the PacNW. Similar conditions are likely Monday morning as radiational cooling under clear skies and calm winds allows minimum temperatures outside of urban areas to cool into the 30s.

Latest model ensemble guidance is in good agreement that a low pressure system develops off the northern California coast late Monday into Tuesday and moves east, remaining south of the forecast area. In turn, precipitation chances have decreased to around 10% or less, while afternoon high temperatures are likely to warm into the upper 50s to lower 60s. High pressure returns late Tuesday into Wednesday as a shortwave upper level ridge moves over the region.

Behind this, a shortwave trough quickly pushes a frontal system toward the coast later Wednesday, increasing precipitation chances across the area through Wed night. Most ensembles indicate this system will be weakening with just light amounts of rainfall most likely. Cluster analysis of 500 mb heights is in relatively good agreement heading into the latter part of the week with zonal to southwest flow aloft allowing for a somewhat active pattern over the NE Pacific to approach the coast. There is still quite a bit of uncertainty in regards to rainfall coverage and amounts, but there is potential for multiple rounds of systems moving across the area into next weekend. At this point, no significant precipitation impacts are expected with any of these weather systems. /DH

AVIATION...Through 12Z Monday

Northwest winds aloft as an upper ridge of high pressure builds over the region today. Satellite imagery and surface observations show widespread low stratus and fog developing along the lower Columbia River and through the Willamette Valley. As of 12z, predominately LIFR conditions are present, with the exception of KTTD at the moment. But clouds and fog continue to fill in and are expected to persist through at least 18-20z Sunday as a subsidence inversion strengthens this morning. Most terminals should see improvement back to VFR by 19-21z this afternoon, likely persisting longest within the southern and central Willamette Valley, including KSLE and KEUG. There is relatively high confidence (40-50% chance) for similar IFR to LIFR conditions to redevelop later tonight after 06z Monday. Winds generally remain light and variable through the TAF period.

KPDX AND APPROACHES...Conditions have deteriorated to LIFR as stratus and fog continue to spread across the area. Guidance suggests conditions gradually improve between 18-20z with VFR likely after 20z this afternoon. There is around a 30-40% chance that IFR conditions redevelop by 12z Monday. Light northwest winds become more variable today. /DH

MARINE

Fairly benign conditions are expected across the coastal waters today as high pressure returns. Winds remain light below 10 kt through this evening. Offshore winds develop late tonight and persist into early next week. Seas have fallen to around 3 to 4 ft this morning, but an incoming long period west-northwest swell is expected to build seas, beginning later this afternoon, to around 8 to 10 ft at 15-18 seconds Monday morning. Have issued a Small Craft Advisory for the outer coastal waters for wave heights peaking at around 10 ft through Tuesday morning. 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 frontal system approaches the waters on Wednesday, returning increasing 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. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Dense Fog Advisory until 10 AM PDT this morning for ORZ108-109- 111-112-114>118.

WA...Dense Fog Advisory until 10 AM PDT this morning for WAZ204>207.

PZ...Small Craft Advisory from 7 AM Monday to 7 AM PDT Tuesday for PZZ271>273.