textproduct: Portland

SYNOPSIS

Very isolated rain showers will continue to diminish in coverage as the region trends drier into the beginning of the weekend. Clear skies will see warm daytime highs and cool overnight lows, with widespread frost or freeze conditions expected tonight. Chances for additional rain returns Sunday into early next week, but long-term forecast confidence remains relatively low.

LONG TERM...Sunday through Thursday

The forecast through the long-term period hinges on the progression of an upper-level low tracking southward from the Gulf of Alaska over the Northeastern Pacific from Sunday into the middle of next week. There remains no firm consensus in long-range guidance, and resultant forecast confidence therefore also remains low. By Monday, about half of ensemble members position the low well to the southwest, off the northern California or southern Oregon coasts. Some of these solutions maintain stronger ridging overhead and therefore result in warmer and drier conditions. If the placement of the low is close enough to transport moisture from the south up toward the Cascades, these warmer temperatures could also support afternoon/evening convection Monday or Tuesday, however the chances of this remain too low to include in the forecast at this lead time. The other half of ensemble members place the low farther to the north and east, closer to the local area, yielding higher cloud cover, near to below-normal temperatures, and potential rain showers. In total, the forecast currently runs down the middle of these scenarios, but this could change substantially as guidance evolves toward a higher-confidence consensus in the coming days. Despite this period of high uncertainty, there is actually higher confidence toward the middle of the week as the low eventually tracks inland, and northwesterly onshore flow aloft tends to return temperatures to near-normal values with chances for rain on Wednesday, then warming and drying into Thursday as ridging builds anew. -36

AVIATION

At 21z Thursday, satellite and surface weather observations showed scattered to broken cumulus clouds across northwest OR and southwest WA with isolated light showers/sprinkles over the Cascade foothills, Coast Range, south WA/north OR coast, and western/northern portions of the Portland metro. A few light showers were drifting within the vicinity of KAST, KHIO, and KPDX. Cigs were generally VFR, aside from brief periods of high-end MVFR cigs. Expect this trend to continue through this afternoon before showers and clouds decrease in coverage this evening into tonight. Given the clearing skies and calm winds in place tonight, widespread frost is expected both inland and at the coast. One exception is at KAST and KPDX where temperatures will most likely stay above 36 degrees, thus limiting the potential for frost development. Note there is now a 45-50% chance for fog and/or low stratus below 500 feet at KHIO beginning around 14-15z Friday, and a 35-40% chance for cigs below 1000 ft at KPDX.

KPDX AND APPROACHES...Expect mainly VFR conditions today and tonight with light and variable winds around 5 kt or less. Light showers are in the vicinity of the KPDX terminal to the northwest as of 2030z Thursday. Expect some light showers to remain within or very near the vicinity through 00-01z Friday. There is a 35-40% chance for low stratus with cigs below 1000 ft between 13-17z Friday. Confidence in low stratus developing over the terminal is not high enough to reflect in the TAF. -23

MARINE

Weak northwesterly flow continues today with lingering showers decreasing this evening. Small Craft Advisory seas of 8-11 feet at 10-11 seconds persist this afternoon, mainly due to a northwesterly swell as wind speeds and wind waves are now minimal. Expect wave heights to slowly wane this evening into Saturday across both the inner and outer waters, falling below 10 ft. Note a very strong ebb current will bring steep seas of 7 to 8 ft within the main channel of the Columbia River Bar early to mid Friday morning. A Small Craft Advisory has been issued for the Columbia River Bar to cover the strong ebb current.

The ongoing northwesterly winds will become northeasterly Friday morning. Winds will then veer to the south this weekend ahead of a weak front. With the approaching front, guidance now suggests a 50-75% chance for max wind gusts over 21 kt across the outer waters Saturday evening/night. Chances decrease to 15-40% over the inner waters, with the lowest chances along the coast. Seas are expected to increase by 2-3 ft with the increase in winds, but will most likely stay under 10 ft. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...Frost Advisory from 1 AM to 9 AM PDT Friday for ORZ102>110- 112>115.

Freeze Warning from 1 AM to 9 AM PDT Friday for ORZ116>119-121- 123>125.

WA...Frost Advisory from 1 AM to 9 AM PDT Friday for WAZ203>205-207- 208.

PZ...Small Craft Advisory from 3 AM to 9 AM PDT Friday for PZZ210.

Small Craft Advisory until 5 PM PDT this afternoon for PZZ251>253-271>273.