textproduct: Portland

SYNOPSIS

A warming trend continues into Saturday with mild spring conditions expected. Attention then turns to early next week, where moderate to high confidence exists in a developing upper- level-low offshore influencing temperatures and precipitation chances. A weak frontal system may bring a few light showers late Saturday, though impacts appear minimal.

LONG TERM...Monday through Thursday

The primary forecast concern early next week centers on the evolution of a developing upper-level low offshore, which will introduce increasing uncertainty in temperature trends and precipitation chances.

On Monday, a range of outcomes remains possible depending on the position of this low. A farther offshore track would support relatively mild and drier conditions, while a closer or more northerly solution would favor increased cloud cover and a higher likelihood of showers. At this time, low-end precipitation chances increase and become more widespread by late in the day, with temperatures generally above seasonal averages.

Tuesday, attention turns to a more organized Pacific frontal system approaching the region. Confidence increases that this system will bring a return to more widespread precipitation late Tuesday into Wednesday, though some timing differences remain among ensemble members. Specifically, current guidance suggests the start time of precipitation reaching inland will vary from any time between Tuesday evening and Wednesday afternoon. From 5 PM Monday to 5 PM Tuesday, precipitation totals in the lowlands will vary from 0 to 0.6 inches (highest along the coast) under best and worst scenarios, which continues to reflect the poor agreement between models at this time.

The most impactful period looks to be early Wednesday, with widespread rain expected across the region as the front moves through, bringing up to 0.6 to 0.9 inches to the lowlands from 5 PM Tuesday to 5 PM Wednesday in a worst-case scenario. While exact rainfall totals remain somewhat uncertain (a best case scenario could still result in 0 inches of rainfall totals), this system represents a more notable shift back to wetter conditions compared to earlier this week. Into Thursday, the pattern begins to transition once again as the upper-level system shifts inland and heights start to rise. This will support a trend toward decreasing precipitation and a gradual return to drier conditions. ~12

AVIATION

VFR conditions prevail across all terminals through the TAF period. Expect SCT/BKN high clouds across the region tonight. Terminals with the clearest skies tonight will experience more efficient cooling, and thus potential for frost development over metal surfaces. The highest chances (40-50%) for frost development will be across the southern Willamette Valley including KEUG. Threat for frost ends after 15z Sat once daytime heating progresses. High clouds will break out into mostly sunny skies by 18- 21z Sat as high pressure builds overhead. Variable winds around 5 kt or less for most terminals, except slight tightening of KTTD-KDLS pressure gradients will result in easterly winds around 5-10 kt in the far east Portland Metro (KTTD). Along the coast, light offshore winds tonight turn more southwesterly after 18z Sat. This offshore flow along the coast will help prevent low marine stratus from developing.

KPDX AND APPROACHES...VFR conditions through the TAF period with high SCT/BKN clouds tonight. High clouds gradually decrease on Saturday. Variable winds around 5 kt or less. Light easterly winds develops through the Columbia River Gorge after 12z Sat, but not expected to reach KPDX. -10

MARINE

Relatively benign marine conditions continue through this weekend. Northerly winds around 10 kt continue into tonight, then turn southerly ahead of a weakening cold front on Saturday. Gusts up to 20 kt may continue this afternoon south of Cape Falcon within 20 NM, before easing this evening. Seas will continue to fall to 3-5 ft by tonight, building only slightly to 4-6 ft as the weak front approaches.

There is higher forecast uncertainty for winds and seas early next week as an area of low pressure offshore approaches the coast. The placement of the low will determine the conditions across the waters, with the forecast at this lead time favoring increasing onshore flow through Wednesday, when WNW gusts may reach 20-25 kt beyond 30 NM. Higher winds and a freshly arriving NW swell may push seas to 10-12 ft Wednesday into Thursday.

Very strong ebb currents will continue across the Columbia River Bar each morning through Tuesday. With seas expected to build slightly by early next week, a steep ebb chop may necessitate Small Craft Advisories in the hours around peak flow each day. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...Frost Advisory from 1 AM to 8 AM PDT Saturday for ORZ116>118- 121.

WA...None.

PZ...None.