textproduct: Portland

UPDATE

Satellite and other surface observations show a rather prolific areal extent of low stratus and lingering fog late this morning. While this cloud layer will continue to lift into the afternoon hours helping visibility further improve outside of Cascade foothill/Coast Range areas where stratus interfaces with the terrain, it may be a challenge for locations along the I-5 corridor to completely clear out today. Often in these set- ups models mix the boundary layer too readily compared to reality. Thus, high temperatures were reduced 3-6 degrees across the Willamette Valley into the lower elevations of SW Washington, and cloud cover expanded as well, although potentially not enough given current trends. In any case, the ridge of high pressure and ample surface moisture won't be going away through Monday. The previous discussion remains below.

SYNOPSIS

Mostly clear skies and calm winds may bring frost and 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 18Z Monday

Satellite imagery and terminal observations as of late Sunday morning depicts widespread stratus across the Willamette Valley, resulting in LIFR/IFR CIGs across inland terminals. There is also some fog at KHIO and KEUG which is bringing VIS down to 1/2SM or less. Expect these trends to continue through at least 21-22z Sun as the lower sun angle at this time of year makes it slower to erode stratus. After 21-23z Sun, some stratus may clear out and return VFR conditions; however, there is high uncertainty for this. The only locations with high confidence for VFR conditions will be the coast, where mostly clear skies will prevail today.

Tonight will be another repeat of last night. High pressure will build overhead, resulting in a strong inversion overnight. Some stratus and VIS reductions to IFR/LIFR may begin as early as 06-08z Mon, but after 08-10z Mon stratus should fill in the Valley and return IFR/LIFR CIGs. Either radiational fog will develop if skies clear out today, or the stratus will be capped so close to the surface that it essentially becomes fog. Generally expect light and variable winds around 5 kt or less across all terminals through the TAF period.

KPDX AND APPROACHES...IFR/MVFR CIGs and VIS continue through 21-22z Sun. Afterwards, stratus should erode and return VFR conditions. Tonight, will see a return of LIFR/IFR CIGs and VIS due to either low stratus or fog. There remains uncertainty with fog development, but guidance suggests a 30-50% chance for VIS below 1/2SM due to fog at any given hour between 08-16z Mon. Variable winds under 5 kt. -10

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...None. WA...None. PZ...Small Craft Advisory from 7 AM Monday to 7 AM PST Tuesday for PZZ271>273.