Clackamas, OR Weather

Synopsis

Dry weather is expected through Saturday with overnight and early morning frost/freeze conditions across much of the region. Rain chances increase early Sunday morning, then linger into the beginning of the workweek. The active pattern continues with additional rain chances midweek.

Discussion

Now through Friday...Transient upper-level ridging continues to build overhead, bringing clearing skies to the region. Cool temperatures aloft will support seasonable daytime highs in the 50s on the coast and in terrain and in the low to mid 60s within inland valleys. As the ridge axis shifts inland, light offshore flow will peak overnight in the Cascades and Coast Range. Clear and calmer valley conditions will again yield efficient nocturnal radiative cooling, with Saturday morning likely to be the coldest. Widespread morning lows in the 30s are expected, coldest in sheltered valleys including the southern Willamette and neighboring peripheral valleys, northern Coast Range valleys near Vernonia and Grand Ronde, and the Upper Hood River Valley, where Freeze Warnings are in effect from 11 PM PDT this evening through 10 AM PDT Saturday. Frost Advisories for temperatures in the low to mid 30s are in effect at the same times for most other areas outside of the south Washington coast and coastal lowlands. Those with exposed or uninsulated outdoor pipes, agricultural interests, or sensitive outdoor plant should take action to prevent frost/freeze damage tonight.

Seasonable conditions with milder overnight temperatures then persist through the remainder of the period as the weather pattern becomes more active by late this weekend. Mean upper troughing develops over the region from Sunday through much of next week as a series of upper lows drop from the Gulf of Alaska over the Pacific Northwest. There remains good agreement among mid-range ensembles that an initial system will move into the region on Sunday with rain beginning at the coast early Sunday morning and reaching inland through the afternoon, although there are modest, perhaps 10-20% chances, that the system tracks farther to the south with lower rain chances locally. As the low slowly exits eastward, At this point, the chances for 0.25 inches of rain or more from 5 AM Sunday through 5 AM Tuesday are 75-85% along the coast and Coast Range, 60-75% along the I-5 corridor, and 50-60% in the Cascades and foothills.

Forecast confidence decreases by midweek as this low departs eastward, although long-range guidance does continue to support additional chances for rain, with 30-40% of ensemble members bringing another weather system to the Northeast Pacific. Otherwise, seasonable temperatures are favored to continue into the final week of October. -36

Aviation

Through 06Z Sunday...Current widespread VFR conditions at coastal and inland terminals will likely continue through the TAF period. With skies expected to remain clear through at least 12Z Saturday, there is the potential for frost or fog at inland terminals, generally in the southern Willamette Valley. Guidance suggests a 15 to 25% chance of LIFR conditions between 08-16Z Saturday, with the highest chances at KHIO and KEUG. With these chances competing with frost potential tonight, confidence is too low to include broken or overnight low status in the TAFs, although isolated pockets of fog and low stratus in valleys cannot be ruled out. Expect winds to remain light and variable through most of the period at all terminals.

KPDX AND APPROACHES...VFR conditions will continue through the TAF period with light and variable winds around 3 to 5 kt. Frost development is possible tonight into early tomorrow morning as temperatures dip as low as 35-38 degrees. If temperatures do not drop low enough, there is a 15% chance of low stratus/fog between 12-17Z Sat. ~12

Marine

Seas continue to decrease tonight into Saturday, well below 10 ft. Winds remain weak and continue to decrease into Saturday. 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%, except 25-35% near buoy 029. Chances for seas over 10 ft peak between 15-35% Sunday morning, with the highest probabilities beyond 20-30 NM offshore. -23