textproduct: Portland

SYNOPSIS

A final warm day is expected on Friday before a return to more seasonable temperatures across the region beginning this weekend, when a passing front will being chances for light showers to the coast and Coast Range. Similar conditions persist into next week.

DISCUSSION...Now through Thursday

A pattern transition continues today as upper-level troughing develops over the Gulf of Alaska while the upper ridge axis over the Great Basin shifts eastward over the Rockies. For today, heights aloft remain high leading to one final day of heat as inland valley temperatures rise to afternoon highs in the mid to upper 80s, supporting widespread Minor HeatRisk. Continued onshore flow will see marine stratus penetrate inland, although the majority of low clouds will burn off by this afternoon except along the coast.

This weekend, a closed low over the Northeast Pacific will open into the developing upper-level trough before passing overhead as an upper shortwave. This will result in cooler temperatures aloft, reflected at the surface as slightly below-normal highs on Saturday, and near-normal on Sunday. Increased onshore flow and synoptic ascent maximized as the shortwave and associated weak surface front near the coast will result in modest chances, generally 15-25% from Florence to Tillamook and 25-45% from Tillamook north into southwest Washington, for light rain showers or drizzle Saturday into Sunday, most likely late Saturday night and early Sunday morning. Any rain accumulation will most likely be very light, 0.05 inches or less. Southerly to southwesterly winds will increase ahead of the frontal boundary Saturday afternoon, with peak gusts of 30-35 mph on the coast and 20-25 mph inland. Stronger west winds through the Columbia Gorge may also reach 30-35 mph.

By next week, the large-scale pattern of upper troughing over the Gulf of Alaska and ridging over the Rockies and Central Plains looks to remain fairly persistent. This pattern will favor continued onshore flow supporting seasonable temperatures with a fairly narrow distribution of ensemble solutions through much of the week given the lead time. Shortwaves transiting the region within the southwesterly flow aloft may bring further chances for light precipitation, particularly along the coast, but the uncertainty in the position and timing of these features is low at this time. -36

AVIATION...Through 12z Saturday

VFR conditions inland expected to prevail throughout the period, while latest satellite imagery depicts some low clouds reaching inland through gaps in coastal terrain as well as beginning to build along the Cascade foothills early this morning. Chances for MVFR cigs beneath low cloud cover is low (10-25%) at terminals along the I-5 corridor, but as high as 25-40% closer to the Cascades, most notably at KTTD. Any low clouds will mix out by midday Friday. Satellite imagery also shows widespread coverage of low clouds along the coast, yielding IFR conditions at coastal terminals along the central/northern OR and southern WA coasts. These conditions are likely to prevail through 17-19z Fri, when cloud bases lifting above 1-1.5 kft and clearing somewhat will instead favor VFR/MVFR conditions. Light and variable winds across the region early this morning will build out of the west to northwest to 6-12 kt by 18-21z Fri, then ease and turn out of the south to southwest late in the period.

KPDX AND APPROACHES...VFR conditions favored through the period with a 25% chance for MVFR cigs through 16-18z as low stratus develops and builds westward from the Cascade foothills through the remainder of the overnight period. Any low clouds are expected to erode by midday. Northwest winds at less than 5 kt early this morning will build to 5-10 kt this afternoon, before turning out of the south-southeast late in the period. -36

MARINE

Weak low pressure offshore continues to support light winds of 5-10 kt or less and seas of 4-6 ft early this morning. As a front approaches the waters on Saturday, southerly winds will increase to 15-25 kt with gusts up to 30 kt Saturday afternoon. Given high confidence (80-90%) in gusts to at least 25 kt across the waters, a Small Craft Advisory has been hoisted from 8 AM through 8 PM Saturday across the waters, and from 11 AM through 8 PM Saturday at the Columbia River Bar. Seas during this time will also build slightly to 5-8 ft with a dominant northwest swell at 8-9 seconds. Behind the frontal boundary, winds quickly ease down to around 10 kt out of the south and seas to 3-5 ft, persisting through at midweek. Seasonable northerly flow returns Wednesday, continuing through the end of the week. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...None.

WA...None.

PZ...Small Craft Advisory from 11 AM to 8 PM PDT Saturday for PZZ210.

Small Craft Advisory from 8 AM to 8 PM PDT Saturday for PZZ251>253-271>273.