textproduct: Portland

SYNOPSIS

A relatively drier pattern with upper-level ridging atop a closed upper low over California will give way to a progressive and active pattern early next week. Repeated systems will bring cooler temperatures along with multiple chances for lowland rain and mountain snow to the region.

LONG TERM...Sunday through Friday

As upper-level ridging pushes farther to the east through the middle of next week, a progressive and active pattern takes hold in the Northwest. An initial deep but narrow trough will approach the coast Sunday into Monday, bringing a return of widespread rainfall chances to the region by Sunday afternoon. There remains some uncertainty around the evolution of this trough; the majority of global ensemble members depict its development similar to the current pattern, namely the flow may pinch the bottom of the trough into a cutoff low affecting California while the energy associated with the remaining weakened rump trough moves into the Pacific Northwest, or the minority of members suggest it could maintain a more coherent single elongated trough structure. Ultimately, the precipitation forecast may be the most sensitive to the details of the flow evolution, but both scenarios end with much cooler air aloft advecting into the region by Monday.

With cooler air in place, temperatures will drop toward more seasonable values during the workweek with daytime highs in the upper 40s to mid 50s and overnight lows in the mid 30s to low 40s. As showers continue while troughing exits eastward through Tuesday night, snow levels dropping to 3500-4500 ft could see light accumulations at pass-level in the Cascades where snowfall amounts remain very light through Tuesday. With less than a 10% chance of more impactful amounts near 6 inches at area passes, only minimal travel impacts are anticipated.

Following a brief dry stretch on Wednesday as narrow upper-level ridging transits the region, another cutoff low along the coast could see additional precipitation chances from Wednesday night through Thursday night. Snow levels remaining around 4000-4500 ft could again see pass-level accumulations, although the chances of 6 inches from 4 AM Wednesday through 4 AM Friday at area passes remain only 10-20%. -36

AVIATION

This morning the region continues to see a mix of flying conditions with VFR/MVFR to locally IFR (KTTD) at inland terminals, and prevailing IFR conditions along the coast. Light rain and mist/drizzle will gradually decrease south to north as the morning goes on with dry weather returning for the afternoon hours. Expect flight conditions to follow this trend as well at inland terminals as generally MVFR CIGs improve back to VFR 20-22z Sat, although there is still a 15-35% chance we hold onto widespread MVFR beyond this point Portland Metro. At least visible satellite looks promising with partial clearing beginning to take place across the southern Willamette Valley - a transition in-line with 12z high-res model guidance. However at the coast IFR conditions are favored through the TAF period outside of the Newport area where clearing likely yields a switch to MVFR CIGs this afternoon.

Come the evening and overnight hours the focus switches to fog formation which will be tied to the prevalence of clearing. Greater clearing of cloud cover would yield an environment more conducive to dense fog, especially in the Central/South Willamette Valley with widespread LIFR impacts, while more persistent low stratus at 2.5-3kft into the evening would trend CIGs more MVFR/IFR. Given the latest guidance and observational trends, confidence continues to grown for this fog scenario playing out at least at KEUG and KSLE with a 70-80% chance for <0.5 mile VIS by 10-12z Sun at these sites. Slightly lower confidence further north into the Portland Metro at this point with these probabilities closer to 40-50% - something to watch. Winds remain fairly light today, generally less than 10 knots.

KPDX AND APPROACHES...MVFR cigs prevail this morning with intermittent light rain and mist/drizzle through 19-21z after which point conditions likely begin to slowly improve to low-end VFR. East-southeast winds at 5-10 kt this morning will ease below 5 kt by this afternoon into the evening. Chances (40-50%) for further vis restrictions due to fog increasing 08-12z Sun, peaking in likelihood 14-18z Sunday morning. -99

MARINE

Seas are expected to persist at 6-8 ft through the weekend while southerly winds ease below 10 kt by this afternoon. Another frontal system traverses the waters Sunday into Monday, bringing increased southwesterly winds with gusts to 20 kt before turning out of the northwest. Behind the front, a building westerly swell will increase wave heights to 13-15 ft by Monday morning, before falling below 10 ft again on Tuesday. Should this forecast remain on track, an additional round of Small Craft Advisories will be needed across the waters for this event. Expect a fairly active weather pattern to continue through the rest of the week as yet another system may bring hazardous winds and seas later in the week, with increasing chances for seas at or above 15 ft Thursday into Friday. -36/99

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.