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. Trending cooler Saturday through Monday with mainly dry conditions aside from isolated light showers, mainly in the mountains. Widespread rain develops on Tuesday with breezy south to southwest winds. 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. 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 over southwest Washington and western Oregon. This will result in mainly dry conditions during that time, however there may be isolated post-frontal rain showers lingering on Saturday, mainly over the mountains early in the day (30-60% chance).

Attention then turns to the Tuesday to Wednesday time frame. This is when ensemble guidance depicts the arrival of a stronger Pacific cold front, bringing wetter conditions with widespread rain. NBM probabilities for precipitation pick up on this well and peak between 80-90% 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 5am Tuesday to 5am Thursday ranges from around 0.10-0.50 inches, while the NBM 90th percentile ranges from 1.00-3.50 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. Another aspect of the forecast on Tuesday and Wednesday that has high uncertainty is snow levels. The NBM 10-90th percentile for snow levels during that time ranges from 2000-9000 feet, respectively. The most likely outcome is around 5500-8000 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. Probabilities for 6 inches of snow or more in a 48-hour period ending at 5am Thursday are quite low as pass level (10-15%). This seems reasonable as only the coldest ensemble guidance suggests snow levels will fall low enough for impactful snow amounts at pass level. Also worth noting is breezy south to southwest winds with the frontal passage on Tuesday, albeit likely staying below 35 mph with minimal impacts. -23

AVIATION

An upper level ridge to the south and a broad upper level trough over the Gulf of Alaska continues to squeeze an atmospheric river, continuing light rain over the north Oregon and south Washington coast. Expect this light rain to become showery into tomorrow morning. For coastal terminals, expect a mix of MVFR/IFR conds through the TAF period, with an exception of possible LIFR conds at KONP before 18Z Thu, improving to VFR thereafter (20-40% chance of LIFR; 40-60% chance of IFR). As for inland terminals, expect VFR conds and high clouds through the TAF period.

Through the TAF period, winds will remain generally southerly and under 8 kt. One exception, terminals along the Columbia River will vary from southerly to southeasterly, while the rest of the Willamette Valley will see southerly to southwesterly winds. Could see gusts up to 20 kt at coastal terminals during the afternoon around 18-23Z Thu.

KPDX AND APPROACHES...VFR conditions and high clouds continue through the TAF period. Southerly to southeasterly winds under 10 kt. ~12

MARINE

Low pressure offshore will maintain breezy southerly winds through Thursday, 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 PM Thursday for southerly winds 15-20 kt with gusts up to 25 kt. Winds remain weaker south of Cape Falcon. Swells are forecast out of the southwest and northwest, with combined wave heights around 7-9 ft at 10-11 sec through Friday. High pressure builds this weekend, turning winds more northerly and remaining under 20 kt. Seas then gradually fall to 5-7 ft at 10-11 sec. Benign marine conditions continue through Monday before another system returns Tuesday- Wednesday. Guidance suggests a 50-60% chance for widespread, small craft wind gusts of 21 kt or greater on Tuesday. -10

PQR WATCHES/WARNINGS/ADVISORIES

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