textproduct: Portland

SYNOPSIS

Low pressure brings chances for showers Friday evening into Saturday across northwest Oregon and southwest Washington, with a 15-20% chance for thunderstorms over the Cascades and 5-10% chance elsewhere. Winds out of the west/southwest on Saturday. Seasonable weather returns Sunday through the middle of next week.

DISCUSSION...Today through Thursday

An upper level trough over the northeast Pacific will shift toward the PacNW today, supporting upper level cloud cover, a decrease in temps and a low chance for precipitation. Highs in the 80s are expected for interior lowland locations with the warmest temps in the upper 80s in the western Columbia River Gorge and Cascade foothill valleys. Along the coast, highs top out in the low to mid 60s.

Hi-res models seem to be struggling with the development and progression of precipitation across the area as the low shifts over the area. Most likely timing is this evening and into the overnight hours. As winds shift more southwesterly, PWAT values are expected to increase to between 1.0-1.30 inches across NW OR and SW WA. The biggest question will be CAPE over the region. SBCAPE on the order or 400-800 j/kg is possible over the Cascades but a strong capping inversion of 50-100 j/kg of CIN could stifle any development. MUCAPE values are a bit higher in the 700-1100 j/kg range over the Cascades which indicates some elevated instability in the mid levels of the atmosphere. Without a clear forcing mechanism, will maintain the slight chance PoPs and thunder (both 10-25%) over the area with the Cascades remaining most likely for any precipitation initially then shifting north of the OR/WA border as a weak cold front moves onshore late tonight into Saturday morning. Any showers or storms will be isolated with most locations likely remaining dry.

The upper level trough will continue to progress northeast through the weekend. The weak cold front moving through the area early Saturday will support temperatures cooler further into the upper 70s inland while also shifting winds more out of the west/southwest. Winds through west-east aligned mountain gaps could be gusty, especially through the western Columbia River Gorge. Gusts around 30 mph are expected with a 30-50% chance for isolated gusts around 35 mph Saturday afternoon.

Zonal flow aloft develops Sunday as the upper level trough continues to trek northeast away from the region. Seasonable highs in the upper 70s to low 80s are expected across the interior lowlands. Looking ahead into early next week, ensemble clusters favor ridging over the central CONUS and influencing the PacNW. Model spread for high temperatures on Monday increases with the 10th-90th percentiles ranging from the mid 70s to upper 80s for the interior lowlands. Confidence increases Tuesday and Wednesday with highs likely in the mid 80s. Clusters indicate another trough influencing the region by the middle of the week but exact timing and upper level pattern are uncertain but onshore flow and more seasonable temperatures would be favored. -19

AVIATION...Through 18Z Saturday

Marine stratus remains along the coast, maintaining LIFR/IFR conditions at coastal terminals. Expect these conditions to improve to IFR/MVFR by 20Z Fri, then once again back to LIFR/IFR conditions by 00-02Z Sat as marine stratus pushes back into the coast. Inland, expect VFR conditions and mid to high clouds above 15 kft through most of the TAF period, with the exception of a 30-40% chance of MVFR CIGs between 12-18Z Sat. Otherwise, expect mostly southwesterly winds between 5-10 kt through the TAF period, shifting more southerly tonight.

KPDX AND APPROACHES...Expect VFR conditions and mid to high clouds above 15 kft through most of the period, with the exception of a 30-40% chance of MVFR CIGs between 12-18Z Sat. Light and variable winds through most of the period, generally shifting northwesterly to southerly from this evening into tonight. ~12

MARINE

Winds shift to the southwest today as a low pressure system moves over the northeast Pacific. A cold front moves through Saturday,bringing a 40-70% chance for maximum wind gusts to exceed 21 kt for the waters north of Cape Foulweather, including the Columbia River Bar. For the rest of the waters, chances are 15-30%.

High pressure and northerly winds return by Sunday, with a 30-50% chance for northerly wind gusts to exceed 21 kts in the late afternoon and evening hours, with the highest chances south of Cape Falcon. Seas of 4-6 ft persist through the period. -19/12

PQR WATCHES/WARNINGS/ADVISORIES

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