Hillsboro, OR Weather

Synopsis

Seasonable weather continues through much of the week with a first weather system bringing chances for rain today into Monday and a second from Wednesday into Thursday.

Short Term

Now through Tuesday...A vertically stacked surface and upper-level closed low centered about 75 miles west of the Olympic Peninsula early this afternoon will slowly track to the southeast, filling as it moves onshore on the north Oregon coast by midday Monday. An initial cold front continues to progress eastward across the region, extending from the Eugene area to the north-northeast over the western Columbia Gorge and south Washington Cascades as of early Sunday afternoon, and should move east of the Cascade crest by late this evening or very early Tuesday morning. Observed rainfall over the past 12 hours has already reached a half inch or more for many areas on the north and central Oregon coast and along the I-5 corridor north of Salem, with a quarter to a half inch to the south.

While short-range guidance favors a brief break from the rain this afternoon, rain shower coverage should increase again as the core of the upper low moves overhead beginning early this evening along the coast and by around midnight tonight inland. Shower coverage should persist through the day Tuesday as a shortwave aloft reinforces support for ascent, before trending drier later in the day. Up to an additional three-quarters of an inch may fall from this afternoon through Tuesday afternoon in valley areas and along the coast, with as much as 1.5-2 inches in the Cascades and foothills which have yet to see the main frontal rain band. Snow levels falling to near 5000 ft may allow for accumulations at or above pass level, with a 50-60% chance for 1" of snow at Santiam and Willamette Passes through tonight, but only a 10% chance at mountain passes near Mt. Hood. Temperatures through this period will remain near seasonal norms in the 50s to low 60s for afternoon highs and largely in the 40s for overnight lows. -36

Long Term

Tuesday Night through Sunday...Another, broader closed upper low and its associated surface low will drop south from the Gulf of Alaska over Vancouver Island and into the Pacific Northwest from Tuesday night into Wednesday morning, bringing another round of rain chances to the region from early Wednesday through much of Thursday. Even cooler air within the upper low will see seasonable temperatures continue, or even fall by a degree or two from earlier in the week. Snow levels will fall to 3500-4000 ft by Thursday morning, although much of the precipitation at pass level may occur before snow levels are low enough to see significant accumulations. Nonetheless, there are 30-60% chances for another inch or more at area Cascade Passes from Wednesday morning through Thursday night. At lower elevations, another 0.5-1.0 inches of rain may fall during this time. While the forecast trends drier as the upper low fills by this Saturday, it may still remain close enough to the region to support lingering rain showers for Halloween trick-or-treaters. At this point, there are 10-20% chances for light rain while temperatures fall through the 40s Saturday afternoon and evening, with higher confidence in dry weather by Sunday. -36

Aviation

Through 18z Monday...A cold frontal rain band is bisecting the airspace as of 17z Sun, extending from K6S2/Florence to the NNE across Portland-area terminals and into the south WA Cascades. This feature will progress eastward, passing east of the Willamette Valley by 00z Mon, and east of the Cascade crest by 06z Mon. While the front is overhead, MVFR cigs/vis are favored at area terminals, with persistent 15-25% chances for brief IFR cigs/vis if heavier rain elements pass overhead. Behind the boundary, a slight improvement of flying conditions is expected, as currently observed as KAST, although restricted vis/cigs may linger a few hours after precipitation eases. Post-frontal rain showers are also likely to persist through the remainder of the period, yielding brief periods of MVFR conditions if they pass over a terminal. More widespread MVFR cigs are again favored after 12z Mon within the nocturnal boundary layer.

South to southeast winds around 5-10 kt expected at inland terminals, veering out of the south behind the boundary along the Willamette Valley. Low pressure offshore will maintain enough of a pressure gradient to see gusty east winds continue near the western Columbia Gorge at KTTD. At the coast, south winds of 10-15 kt gusting to 20-25 kt will continue into this evening.

KPDX AND APPROACHES...MVFR cigs/vis with steady light rain favored as the frontal boundary progresses eastward through 00z Mon, with persistent 15-25% chances for period of IFR cigs/vis if heavier rain elements pass over the terminal. Post-frontal rain showers should accompany a slight improvement to low-end VFR after 00z Mon, with increasing chances for further MVFR cigs after 12z Mon. Southeast winds 5-10 kt persist through the period. -36