Hillsboro, OR 97129 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 00Z Sunday...Widespread VFR conditions continue across northwest OR and southwest WA, including coastal and inland TAF sites. Skies are expected to remain clear to mostly clear now through at least 12Z Saturday, except at KTTD and KPDX where scattered to broken ceilings around 4000-5000 ft will continue this afternoon before dissipating sometime this evening. High clouds with ceilings above 15-20 kft will increase over the region between 12-18Z Saturday.

There is a 10-20% chance for ceilings around 1000 ft or lower developing at KUAO and KSLE between 11-18Z Saturday and a 30-40% chance at KAST and KEUG. Confidence is too low to justify putting a broken or overcast low stratus deck in the TAFs, especially because frost formation is more likely than fog and low stratus tonight as temperatures dip into the low to mid 30s at all TAF sites. However, some pockets of fog and low stratus cannot be completely ruled out, especially at KEUG.

KPDX AND APPROACHES...VFR conditions will continue through 00Z Sunday with light and variable winds around 3-5 kt. 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 2 PM PDT Friday showed northerly winds around 5-15 kt with seas around 7 to 10 ft at 9 to 11 seconds, mainly driven by a northwest swell. Although winds have weakened significantly Friday afternoon, seas remain steep enough to pose a hazard for small craft through the remainder of the afternoon. Therefore, Small Craft Advisories remain in effect for the inner and outer waters through 5 PM PDT Friday. Seas will decrease this evening into tonight with winds remaining weak, bringing improving conditions.

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