textproduct: Portland

SYNOPSIS

The region remains on the northern periphery of an upper level ridge through Friday, bringing much warmer than normal temperatures to most of northwest Oregon. Meanwhile, a weakening atmospheric river aimed at western Washington and Vancouver Island will occasionally wobble southward from time to time, bringing occasional light rain down to the north Oregon coast at times with more persistent rain over Pacific, Wahkiakum, and northern Cowlitz/Skamania counties. As the weekend approaches, cooler conditions with isolated light showers are expected through Monday. A pattern change is on the horizon as a broad, low pressure system comes down from the Gulf of Alaska, bringing widespread rain with breezy south to southwest winds by Tuesday. Precipitation in the Cascades on Tuesday will fall as either rain, snow, or a rain/snow mix depending on snow levels (which could range anywhere from 2000 to 9000 feet per model spread).

LONG TERM...Saturday through Wednesday

The long term forecast remains highlighted by a relatively cooler weather pattern with seasonable temperatures and mainly dry conditions, except Tuesday and Wednesday when wetter conditions arrive as a more robust system is expected to dive southward from the eastern Pacific. For the Saturday through Monday time frame, models and their ensembles are in good agreement for nearly zonal flow aloft with surface high pressure in place on the east side of the Cascades.

With the start of next week taken care of, lets focus on the middle of next week as a robust eastern Pacific system moves into the Pacific NW. This system looks to bring 850 mb temperatures from -8 to 5 C (yes that is the spread between ensembles) along with widespread rain. NBM probabilities for precipitation pick up on this well and peak between 65-90% by Tuesday evening/night. Although confidence is high precipitation will return, confidence is not as high regarding exact precipitation amounts as model spread for QPF is large. The NBM 10th percentile for 48-hour QPF amounts from 5 AM Tuesday to 5 AM Thursday ranges from around 0.10-0.50 inches, while the NBM 90th percentile ranges from 1.15-3.70 inches (highest amounts at the coast and across the western slopes of the Coast Range and Cascades). Regardless of the exact outcome, it appears flooding will not be a concern.

As mentioned previously, the almost 15 C degree spread within the ensembles for 850 mb temperatures, is resulting in a very high uncertainty in snow levels for Tuesday and Wednesday. The NBM 10-90th percentile for snow levels during that time ranges from 2000-9000 feet, respectively. The most likely outcome is around 2500-4500 feet, highest over the Lane and Linn County Cascades and lowest over the south Washington Cascades. As such, is not clear yet whether or not the Cascades will see rain, a rain/snow mix, or accumulating snow. It is also not clear yet if the Cascade passes will be impacted with accumulating snow. Current NBM probabilities for 6 inches of snow or more in a 48-hour period ending 5 PM Thursday are quite low, around 20% at pass level. This will be an area of focus as more data becomes available. Lastly, breezy south/southwest winds are also worth keeping an eye on, but current ensemble guidance has gusts likely staying below 35 mph with minimal impacts. /42

AVIATION

An upper level ridge to the south and a broad upper level trough over the Gulf of Alaska continues to direct an atmospheric river towards western Washington. The southern portion of this atmospheric river will bring light rain showers along the far north Oregon coast (KAST) throughout the TAF period. Can't rule out a few stray showers or sprinkles over the northern Willamette Valley. High confidence (60-80% chance) that MVFR CIGs persist at KAST throughout the day, while there is moderate confidence (30-50% chance) that LIFR stratus at KONP dissipates and returns VFR conditions by 19-21z Thu. High pressure aloft could easily keep the stratus below 500 feet at KONP as onshore flow maintains moist conditions.

Meanwhile, VFR conditions prevail for Willamette Valley terminals with high level clouds. CIGs across the Valley will gradually drop to around 5 kft or less Friday morning as the upper level ridge shifts eastward and troughing enters the area. Winds generally southerly to southwesterly across the area and under 10 kt, with occasional gusts up to 20 kt along the coast.

KPDX AND APPROACHES...VFR conditions with high clouds throughout the TAF period. CIGs begin to gradually fall to low-end VFR after 09-12z Fri as the upper level ridge shifts eastward. Southerly to southeasterly winds under 10 kt. -10

MARINE

Low pressure offshore will maintain breezy southerly winds through late tonight/early Friday morning, mainly for the waters north of Cape Falcon including the Columbia River Bar. A Small Craft Advisory has been issued for these areas through 5 AM Friday for southerly winds with gusts up to 25 kt. Winds remain weaker south of Cape Falcon. Seas around 7-9 ft at 10-11 sec through Friday. Also, for the Columbia River Bar, we are entering a strong to very strong ebb cycle through at least the weekend.

A broad, upper level low and associated weak surface cold front will result in winds becoming northerly by Saturday and remaining under 20 kt. Seas also gradually fall towards 5-7 ft at 10-12 sec. Benign marine conditions continue through Monday before another, more robust system returns Tuesday/Wednesday. This system will bring a return of stronger southerly winds across all waters. Current guidance suggests a 60-80% chance for widespread, small craft wind gusts of 25 kt or greater and a 40-60% chance for Gale gusts of 35 kt or greater on Tuesday. Seas on Tuesday and Wednesday are also expected to build in response to increasing wind waves and look to build towards 11-13 ft. /42

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 5 AM PDT Friday for PZZ210-251-271.