textproduct: Portland

SYNOPSIS

Low pressure brings chances for showers tonight and into Saturday across northwest Oregon and southwest Washington, with a 15-20% chance for thunderstorms over the Cascades and 5-10% chance elsewhere. Southerly winds tonight will become more west/southwest on Saturday and return to a west/northwest pattern Sunday. Seasonable weather returns Sunday through the middle of next week.

DISCUSSION...Tonight through Thursday

An upper level trough over the northeast Pacific will continue to move eastward towards the PacNW through the overnight hours. This will support upper level cloud cover, a decrease in temps and a low chance for precipitation through Saturday.

Hi-res models seem to be struggling with the development and progression of precipitation across the area as the low shifts over the area. The most up to date CAMs continue to show the potential for light precipitation starting this evening and continuing 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. CAMs and high resolution models continue to show surface based CAPE (SBCAPE) values 400-800 j/kg is possible over the Cascades but a strong capping inversion of 75-175 J/kg of CIN could stifle any development. Most unstable CAPE (MUCAPE) values are a bit higher in the 700-1100 J/kg range over the Cascades which supports elevated instability in the mid levels of the atmosphere, this lends credence to the Cascades maintaining the highest probability of thunderstorms through the overnight hours. It should also be noted that CIN values over the Cascades are around 100-200 J/kg and that could be enough to stifle any development for the Cascades as well. Therefore, will continue to 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 daytime highs in the upper 70s to low 80s for inland valleys as well as highs in the 60s along the coast and 70s for the Cascades. Southerly winds which developed today will become more west/southwest tonight and into Saturday. 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 approaching 40 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 the PacNW. Model spread for high temperatures on Monday increases with the 10th-90th percentiles ranging from the upper 70s to low 90s for the interior lowlands. Confidence increases Tuesday and Wednesday with highs likely in the mid 70s to 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. -42/19

AVIATION...Through 06z Sunday

An upper-level low tracking to the north offshore will bring periods of vis/cig restrictions to area terminals throughout the period. Along the coast, widespread low stratus coverage will bring persistent IFR/low-end MVFR cigs at 500-1500 ft. Highest confidence in IFR cigs to the south at KONP (70-80% chance) while MVFR conditions are more likely at KAST. Periods of MVFR vis are likely overnight at coastal terminals. Cigs will tend to lift to 1.5-2.5 kft after 15-18z Sat while south-southwesterly winds increase to 12-16 kt with gusts of 20-25 kt through the afternoon. Cigs look to trend downward again late in the period as winds ease Saturday evening.

VFR conditions are favored to continue ahead of the inland penetration of low stratus for I-5 terminals. Chances for MVFR cigs are 40-60%, with the highest confidence in the southern Willamette Valley (KEUG) as early as 09z Sat, and less so in the central Willamette Valley and Portland area. If coverage of low clouds is high enough, restricted cigs are most likely from 13-17z Sat before mixing out and returning to VFR conditions. South to southwest winds will build to 6-12 kt by Saturday afternoon.

KPDX AND APPROACHES...VFR conditions favored through at least 12z Sat. Low cloud coverage will increase after 08z Sat, with the chances for MVFR cigs reaching 50-60% from 13-18z Sat. As low clouds mix out, VFR conditions are again highly favored after 18z Sat. South to southwest winds of 3-5 kt overnight will build to 8-10 kt by Saturday afternoon. -36

MARINE

South to southwest winds continue today as a low pressure system moves over the northeast Pacific. A cold front moves through Saturday, bringing a 50-80% chance for maximum wind gusts to exceed 21 kt for the inner waters, including the Columbia River Bar. As a result, a Small Craft Advisory has been issued for the inner waters from 5 AM to 5 PM Saturday and for the Columbia River Bar from 11 AM to 5 PM Saturday. For the outer waters, chances are 20-50% with the highest values of that range between 10 and 30 NM.

High pressure and northerly winds return by Sunday, with a 40-70% chance for northerly wind gusts to exceed 21 kts in the evening and continuing into Monday. Seas of 4-6 ft persist through the period.~12

PQR WATCHES/WARNINGS/ADVISORIES

OR...None.

WA...None.

PZ...Small Craft Advisory from 11 AM to 5 PM PDT Saturday for PZZ210.

Small Craft Advisory from 5 AM to 5 PM PDT Saturday for PZZ251>253.