textproduct: Portland

SYNOPSIS

A weak system is set to bring light precipitation to the area Sunday morning into Sunday night. Conditions dry out Monday morning through Tuesday as weak ridging develops along with a thermal trough at the surface. A robust system is expected to drop southeast out of the Gulf of Alaska, sending a cold front toward the PacNW and supporting unsettled weather into early Friday. The system will bring widespread rain, Cascade snow, and gusty winds.

DISCUSSION...Now through Friday

Conditions remain pleasant today with partly cloudy conditions and highs warming into the low 60s inland and 50s along the coast. Saturday night lows are more mild, only dropping into the upper 30s to low 40s. Isolated frost may develop in the higher terrain and in more sheltered locations.

The pattern shifts tomorrow as a short wave trough over the Gulf of Alaska drops southeast toward the PacNW, returning precipitation chances to the area early Sunday morning at the coast and late morning/early afternoon inland. Precipitation totals from Sunday morning through Sunday night will be light with most locations seeing less than 0.10 inches, with the exception of the higher terrain of the Cascades which could see up to 0.25 inches. Snow levels Sunday morning over the Cascades start out around 4-6 kft but drop closer to 1-2 kft Sunday night into Monday morning. Snowfall for most of the Cascades will be minimal with only the crests and tops of the volcanoes receiving 1-3 inches. Cooler temperatures are expected Sunday night in the wake of the cold front. Northwest flow develops late Sunday with cloud cover thinning overnight. There is a 20-70% chance that Monday morning lows drop below freezing across the Willamette Valley and I-5 corridor in SW WA. The lowest chances are over the southern Willamette Valley where clouds will thin latest while the I-5 corridor in SW WA has the highest probability of reaching freezing. However, no snow is expected over the lowlands given moisture will be limited during this time.

Showery precipitation is expected to taper off by early Monday morning. Zonal flow develops aloft in the wake of the shortwave trough. High pressure at the surface off the OR/WA coast will try to build eastward late Monday into Tuesday. Tuesday conditions continue to look dry and mild as the zonal flow amplifies slightly over the PacNW. A weak thermal trough will also help support weak offshore flow on the western slopes of the Cascades and coast range. Highs creep back into the upper 50s to low 60s inland and low to mid 50s along the coast.

A more robust system may develop over the Gulf of Alaska early next week. Another trough is expected to dig southeast from the Gulf of Alaska, supporting the development of a surface low off the Canadian coast. A cold front associated with this low will approach the PacNW Wednesday, reaching the coast early Wednesday morning and bringing a decent shot of rain west of the Cascades and a good amount of snow to the Cascades. 48 hour QPF totals from Wednesday to Friday morning could eclipse 1 inch. Probabilities for the Willamette Valley are 30- 60% and 50-80% along the coast and coast range. Snowfall totals over the Cascades during the same timeframe have a 70-80% chance of reaching greater than 6 inches at pass level. Cascade peaks over central Oregon and the tops of the volcanoes have a 50-70% chance of seeing a foot of snow or more. There is also a chance for gusty winds associated with this system on Wednesday. There is a 30-60% chance for wind gusts to reach 40+ mph along the coast and a 15-30% 35+ mph gusts inland. Details are still uncertain at this time.

Shower activity behind the cold front is expected to decrease through the day on Friday as high pressure builds into the region. Cluster analysis shows good agreement in some degree of ridging developing over the PacNW by the end of next week. Conditions should trend drier and more mild. -19

AVIATION

Southwest flow aloft with broken mid to high level clouds streaming across the area. VFR flight conditions continue, except at KONP where MVFR cigs have developed. Generally northwest winds 10 kt or less expected to continue through late this evening, then becoming light and variable overnight. A weak front will approach the coast Sunday morning with light rain likely at KAST by 15z, before spreading down the coast to KNOP by approximately 18z, and then over the northern Willamette Valley and Portland metro area around 20z. KSLE to KEUG will likely stay dry, aside from a few potential sprinkles. High-end MVFR cigs will likely become more widespread along the north OR coast, northern Willamette Valley, and Portland metro once rain begins (50-60% chance at KAST, 30-50% chance at KHIO, KPDX, KTTD, 15-30% chance at KUAO, KSLE, and KEUG). Confidence was just high enough to lower cigs down to 3000 ft at KAST, KHIO, KPDX, and KTTD, but confidence is not high enough to bring cigs that low at KUAO, KSLE, and KEUG. For TAF sites that do wind up observing MVFR cigs, expect cigs to quickly lift back to VFR mid to late Sunday afternoon as rain begins to dissipate.

KPDX AND APPROACHES...Expect predominately VFR flight conditions to continue, except between 20z Sunday and 01z Monday when cigs may lower down to 3000-3500 feet (30-50% chance) with a period of light rain. Cigs should lift to 4000 ft or higher after 01z Monday as rain dissipates. -23

MARINE

High pressure continues to weaken over the waters through tonight. Northwest winds around 10-15 kt ease overnight while seas fall to around 5 ft. A weak front will move across the waters Sunday morning followed by breezy northwest winds. Northerly winds increase late Sunday into Monday, and remain elevated through Monday night, as high pressure rebuilds over the waters. This will likely lead to a period of Small Craft Advisory winds.

A more robust frontal system is expected to move across the waters sometime late Tuesday through Wednesday. Guidance suggests there is around a 80-90% chance of gale force wind gusts exceeding 34 kt with this system. Seas are expected to rapidly build on Wednesday becoming steep and hazardous into Thursday, likely peaking in the mid-teens. /DH

PQR WATCHES/WARNINGS/ADVISORIES

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