textproduct: Portland

SYNOPSIS

Clear skies and calm winds may bring frost and/or fog development across much of NW OR and SW WA in the mornings Saturday through Monday. Mostly dry conditions return this weekend. More uncertainty for the return of precipitation early next week with limited weather impacts expected.

DISCUSSION...Now through Friday

Clearing skies and calm winds overnight has allowed for cooling temps and patchy fog to develop across the area this morning. A Frost Advisory is in effect through 9 AM this morning for much of the area. Will also be monitoring for the chance of dense fog this morning, while some rural valley locations may also experience freezing fog this morning. The low pressure system offshore will continue to weaken as the upper level low lifts northward into western Canada and the surface low dissipates offshore. This will lead to a mostly dry day today, except for increasing chances of showers over the Oregon Cascades and foothills this evening as the upper level trough moves over the region. Afternoon temperatures are expected to warm into the mid- 50s, before quickly falling into the 40s this evening after sunset. Dry conditions return on Sunday as an upper level ridge of high pressure builds over the region. Sunny skies expected after morning fog/clouds.

Ensemble guidance remains in relatively good agreement that upper level ridging and subsequent dry conditions remain over the region into early next week as another low pressure system develops in the eastern Pacific west of the OR/CA border and eventually moves inland over California midweek. However, there are a handful of ensemble members that bring the low pressure system inland over the PacNW sometime late Monday into Tuesday as a weak trough, allowing for a 5-20% chance of precipitation to return. If the drier solutions pan out, expect a slight warming trend with afternoon highs near 60 degrees for the interior lowlands through the middle of next week. Chances for rain increase later Wednesday through the end of next week as some ensemble members indicate a frontal system approaches the area, but there is high uncertainty in specifics. At this point, no significant impacts are expected with the upcoming weather pattern. -DH/03

AVIATION...Through 18Z Sunday

Satellite imagery and terminal observations as of late Saturday morning depicts a mix of LIFR/IFR CIGs across the Willamette Valley as moist conditions maintain low stratus. Central and southern Willamette Valley terminals also still remain under LIFR/IFR VIS due to fog, while VIS for northern Valley terminals are beginning to improve as fog clears out. Expect general LIFR/IFR conditions to persist through 19-21z Sat, with the slowest improvement in the south Valley including KEUG. Afterwards, VFR conditions return in the afternoon with BKN/OVC high clouds. Along the coast, offshore flow will maintain predominately VFR conditions through the TAF period.

Tonight, will see another around of LIFR conditions due to fog and low stratus across Valley terminals as high pressure builds aloft and brings increasing cloud breaks. Guidance suggests a 30-60% chance for VIS < 1/2SM across the Valley at any given hour between 12-17z Sun. However, typical foggy spots such as KHIO and KEUG could see fog as early as 06-07z Sun based on what we saw last night. Fog/stratus development is favored across the rest of the Valley by 11-13z Sun. Light north/northwest winds around 5 kt or less across the Valley today, becoming calm overnight. Light easterly winds continue at the coast.

KPDX AND APPROACHES...IFR CIGs through 19-20z Sat, improving to VFR afterwards with high clouds. Tomorrow morning, there is a 20-40% chance for LIFR VIS due to fog at any given hour between 12-17z Sun. Northwesterly winds around 5 kt or less today, becoming variable overnight. -10

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 and persist into early next week. A long period west- northwest swell also arrives Sunday evening, building seas to around 8 to 10 ft at 17-19 seconds on Monday. Chances for seas over 10 ft on Monday remain at around 30-60% 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. The next frontal system likely approaches the waters mid to late next week, returning southerly winds and building seas. -DH/10

PQR WATCHES/WARNINGS/ADVISORIES

OR...Dense Fog Advisory until noon PDT today for ORZ114>118.

WA...None.

PZ...None.