Area Forecast Discussion

SYNOPSIS

Dry and seasonably cool today and Tuesday, with increasing clouds overnight as a weak upper-level trough moves in. Drier and warmer conditions arrive Wednesday to Thursday with increasing easterly winds through the Columbia River Gorge and Cascades. Onshore flow returns Thursday afternoon. Dry and warm conditions continue Friday through the weekend, but there remains uncertainty with how warm we will get.

SHORT TERM...Today through Wednesday night

Satellite imagery and surface observations as of early Monday afternoon depicts scattered to broken cloud cover across northwest Oregon and southwest Washington as weak high pressure re-builds over the area following the low pressure this past weekend. Clouds were briefly breaking out late this morning, but daytime heating has resulted in some popcorn cumulus clouds across the area which may linger into the afternoon. As a result, expect seasonably cool temperatures today with afternoon highs peaking in the upper 60s to low 70s across interior valleys, and low to mid 60s along the coast.

Tonight, another upper-level shortwave trough will approach the Pacific Northwest, bringing increasing mid to high level clouds. Some of these clouds may be thin enough to keep skies clear enough to support patchy fog development across interior valleys. Clouds and any patchy fog will linger throughout the morning and then slowly break out Tuesday afternoon/evening. Expect high temperatures Tuesday afternoon to peak in the upper 60s to low 70s across interior valleys, similar to today due to the morning cloud cover. Despite the trough dropping down on Tuesday, limited moisture with this weak system will result in mostly dry conditions. Chances for measurable precipitation on Tuesday (>0.01") are only 20-30% across the north Oregon Coast Range, Willapa Hills, and the crest of the Cascades, and <5% everywhere else.

On Wednesday, the aforementioned upper trough will continue to drop into southern Oregon and northern California while an upper-level ridge amplifies offshore and its ridge axis extends into western Washington. At the surface, guidance is in agreement that thermal troughing will develop along the coast and Cascade foothills, resulting in light easterly winds through the Columbia River Gorge, and Coast Range/Cascade gaps. Ensemble guidance shows pressure gradients anywhere between -1 to -3 mb between KTTD and KDLS, which would translate to wind gusts peaking up to 20-25 mph through the Gorge and other east/west aligned terrain gaps. Across the Willamette Valley, winds will turn more northerly and be breeziest across the central and southern Valley. Chances for maximum wind gusts exceeding 30 mph on Wednesday are around 30-50% along the Gorge, and 15-20% for the central/southern Valley. Expect the strongest gusts over higher terrain and exposed ridgetops. Given that easterly winds are not looking too strong and afternoon humidities are well above 25%, minimal fire weather concerns are expected. The easterly offshore winds will also help with increasing temperatures, with highs rebounding into the upper 70s to low 80s across interior valleys Wednesday afternoon. -10

LONG TERM...Thursday through Sunday

Thursday is shaping up to be the warmest day of the week, with high confidence (70-90%) for high temperatures peaking in the low 80s across interior valleys as weak offshore flow continues through the morning. By Thursday afternoon, winds should begin to shift back into an onshore direction (west to east) as the thermal trough breaks down. The weak upper-level trough will also begin to gradually shift eastward into the Great Basin, and wrap-around moisture will support a 10-15% chance for thunderstorms along the crest of the Lane County Cascades. Everywhere else should remain dry and out of any thunderstorm threat.

Friday to the weekend, the majority of ensemble members continue to favor warm and dry conditions for the region, however, there is uncertainty with how warm we'll get. About 45% of the ensemble members continue to place weak upper-level troughing over the Great Basin, maintaining more mild temperatures. The other 55% of ensemble members show upper-level ridging building over the Pacific Northwest, which would favor warmer temperatures. As a result, there's a pretty wide range in high temperatures Friday to the weekend, with the coolest scenario being in the low 70s, and the warmest scenario being in the upper 80s and close to 90 degrees. -10

AVIATION...Through 06Z Wednesday

Northwest flow aloft becomes more westerly tonight as an upper level trough drops south out of western Canada. High clouds are beginning to move into the area from the north, with ceilings above 20 kft. By mid tomorrow morning, ceilings are expected to lower closer to 10 kft, maintaining VFR conditions. That said, there are a few potential exceptions. Probabilities for MVFR ceilings around 2500-3000 ft remain around 60-80% at KAST after 06-08Z Tuesday. Probabilities peak near 50% at KTTD and KONP. Confidence is high enough to include a mention of MVFR ceilings for KAST, but not high enough to include at KTTD and KONP. Lastly, fog is likely to develop at KEUG beginning at 12Z Tuesday given the light winds and mostly clear skies that will be in place. Fog at KEUG will most likely last 2-3 hours, similar to what was observed Monday morning. Confidence levels for fog development at KEUG are moderate.

KPDX AND APPROACHES...VFR conditions are expected through the TAF period, with clouds gradually scattering out this evening. High level clouds develop tonight into tomorrow morning with ceilings lowering to around 10 kft. Light northwest winds around 3-6 kt are expected throughout the TAF period. -23

MARINE

North to northwest winds have developed as expected this evening as high pressure builds offshore. Winds generally increase up to 10-15 kt tonight through Tuesday. A strengthening thermal trough along the coast is expected to increase pressure gradients across the coastal waters on Wednesday, increasing northerlies to 15-20 kt with at least an 80-90% chance for wind gusts exceeding 21 kt south of Cape Falcon. Winds are likely to ease later in the week as low pressure drifts offshore. A persistent northwest swell around 4 to 5 ft at 7 to 8 seconds is expected to continue through Tuesday. Seas then build to around 6 ft on Wednesday as wind chop increases with strengthening northerlies. -23/DH

PQR WATCHES / WARNINGS / ADVISORIES

OR...None. WA...None. PZ...None.