Clackamas, OR Weather

Synopsis

Dry weather is expected through much of Saturday, before rain chances increase through the latter half of the weekend with persistent seasonable temperatures. By the start of next week, there is a 10-20% chance of very light precipitation across the forecast area through at least Wednesday. A relatively stronger system looks to drop down by mid-week, which looks to bring a slightly higher probability (20-30%) for showers.

Discussion

Today through Thursday...Dry weather looks to take hold over the area as a transient, upper level ridge moves across the region through Saturday. Expect daytime highs today in the low 50s to upper 50s on the coast, into the low to mid 60s for inland valleys and upper 40s to upper 50s for the Cascades. As the ridge axis shifts inland, light offshore flow will develop, peaking overnight in the Cascades and Coast Range. This will result in daytime high temperatures on Saturday warming into the upper 50s to low 60s on the coast, into the low to mid 60s for inland valleys and low 50s to low 60s for the Cascades. Overnight temperatures will again fall into the low to mid 30s for most locations across the CWA tonight, with Freeze Watches and Frost Advisories remaining in effect; Freeze Watches may be upgraded to Freeze Warnings if confidence increases in temperatures falling to near or below freezing.

There remains good consensus among long-range ensembles that another cutoff low will develop over the Gulf of Alaska on Saturday and track to the southeast offshore of Vancouver Island. By Sunday, there is less agreement on how the low will evolve as it nears the Pacific Northwest. For now, There are modest chances, near 50%, for rain on Sunday across the region, decreasing through the first half of next week as troughing may linger overhead. Cool air aloft will also likely continue to support seasonable temperatures for late October.

Looking beyond the weekend and into the start of next week, uncertainty in the forecast remains high. Ensemble clusters are split on the overall pattern with the ECWMF maintaining a ridge over the region as a broad, upper level low persists near 40N/140W which would bring a return of a warm and dry pattern. While the GFS ensembles lean into a broad trough spreading southward from far Western Canada thus maintaining a cooler and wetter pattern. As a result NBM is keeping a broad, 15-20% chance for precipitation, with little to no QPF across the forecast area, along with slightly warmer than seasonal daytime highs and overnight lows through the remainder of the forecast period. /42-36

Aviation

Through 18Z Saturday...Generally VFR conditions expected across the airspace through the majority of the TAF period. Stratus will continue to linger over the Cascades through approximately 19Z Saturday. Skies remain mostly clear west of the Cascades, aside from a brief period of scattered to broken ceilings around 3000-4000 ft between KTTD, KPDX, and KUAO and at KAST. These lower ceilings are not expected to last long and should clear out by 19Z.

There is a 10-20% chance of IFR to MVFR ceilings developing at KUAO, KSLE, KEUG, and KAST starting around 11Z Saturday and continuing through 18Z Saturday, however confidence is too low to justify putting a broken or overcast low stratus deck in the TAFs. Frost formation is more likely than fog and low stratus tonight as temperatures dip into the low to mid 30s at all TAF sites.

KPDX AND APPROACHES...VFR conditions will continue through 18Z Saturday with mostly clear skies and light and variable winds around 3-5 kt. Will note some scattered clouds around 3500-4000 ft are expected through 19Z Friday. Frost development is possible late tonight into early tomorrow morning as temperatures dip as low as 35-38 degrees. -23

Marine

Buoy observations as of 8 AM PDT Friday showed north/northwest winds around 10 to 15 kt with gusts up to 20-25 kt. Seas remained around 10 to 12 ft at 9 to 11 seconds, mainly driven by a northwest swell. Gusty north winds this morning will gradually decrease in strength Friday afternoon and evening while seas decrease and become less steep. Therefore, Hazardous Seas Warnings are no longer in effect. However, Small Craft Advisories are in effect for the inner and outer waters through 5 PM PDT Friday given conditions will still be hazardous to small craft as seas and winds will take some time to decrease.

A slow-moving low will approach the outer waters late Saturday into Sunday, bringing a transition to southerly winds with seas increasing again Saturday night into Sunday. Conditions will be hazardous for small craft during that time, before winds and seas decrease early next week. There is still uncertainty on the exact strength and track of this low, however probabilities for gale force wind gusts remain low at 5-15%. Chances for seas over 10 ft peak between 15-35% Sunday morning, with the highest probabilities beyond 20-30 NM offshore. -23