textproduct: Portland

SYNOPSIS

Dry and benign weather is expected across the region through much of the next week with high pressure remaining in place over the Northeast Pacific. Weak systems aloft may bring enhanced cloud cover on some days, but rain chances are minimal. Uncertainty in the forecast increases Sunday into Monday as chances for precipitation return.

DISCUSSION...Now through Monday night

Broad upper-level ridging continues to build over the Northeast Pacific with persistent northwesterly flow streaming onshore from the Gulf of Alaska. This pattern will remain largely the same through Wednesday, with weak upper shortwaves bringing marine air inland and allowing pockets of mist or drizzle along the coast through the overnight and early morning hours. Temperatures will remain seasonable, around 60 degrees on the coast and near 70 degrees along the I-5 corridor. As offshore ridging shifts east late in the week, temperatures will trend warmer, reaching the 60s to near 70 degrees on the coast and upper 70s to mid 80s inland. This will support widespread Minor HeatRisk excepting only the immediate coast and high Cascade crest. Chances for Moderate HeatRisk are 20-35% in Portland and the adjacent northern Willamette Valley, 5-20% in Salem and Albany/Corvallis, and 5% or less elsewhere. Concerns for hazardous weather otherwise remain low.

Friday through the holiday weekend, uncertainty in the evolution and possible break-down of the upper-level ridge yields growing uncertainty in forecast temperatures and precipitation. In general, as the ridge deamplifies, temperatures are more likely to trend cooler through the period, especially by Monday. The 80%-confidence envelope (the difference between the 10th and 90th percentiles) in temperature widens from 8-13 degrees on Friday and Saturday to 13-19 degrees on Sunday and Monday along the I-5 corridor, with warmer scenarios resulting from a more persistent ridge and cooler scenarios from a more rapid break down. There continues to be greater consensus of an upper-level trough digging over the Northeast Pacific on Monday, supporting much cooler temperatures and 30-60% chances for rain across the region, with the highest chances to the north of US-20 and in areas of higher terrain. -36

AVIATION

Northwest flow over continues over the forecast area supporting a mix of MVFR conditions along the coast and VFR inland. The summer-like pattern supporting marine stratus along the coast and MVFR CIGs is expected to persist through the overnight hours. VFR conditions inland are expected for a few more hours before chances for MVFR conditions increase. Guidance suggests some stratus backbuilding into the Willamette Valley from the Cascade Foothills. Chances for MVFR CIGs inland increase after 08z with all terminals seeing a 30-60% chance through 16-18z, highest in the southern Willamette Valley around KEUG and in the eastern Portland metro. Conditions expected to improve back to VFR by mid to late morning inland and early afternoon along the coast. Light winds overnight, becoming north/northwesterly Wednesday morning between 5-10 kt. Could see some gusts to around 20 kt at KONP during the afternoon.

KPDX AND APPROACHES...Mainly VFR conditions with a chance for a short period of MVFR CIGs earlier Wednesday morning. Guidance shows backbuilding clouds along the Cascades pushing eastward starting around 09Z Wed. 30-60% chance for MVFR conditions developing and impacting the eastern metro, mainly KTTD and KPDX with lower chances at KUAO and KHIO. If these conditions manifest, improvement towards VFR starting around 18Z Wed. Light winds overnight, becoming north/northwesterly 6-10 kt after 18z Wednesday. -19

MARINE

Northerlies are expected to persist through much of the week as the summer-like pattern settles in. Strongest winds are expected during the afternoon and evening periods as a surface thermal trough strengthens over the region. Therefore, have updated the current suite of Small Craft Advisories as gusts up to 25 kt are expected across all zones with the exception of PZZ251, this afternoon and Wednesday afternoon. Seas 6 to 9 feet are expected through the week as well. Chances increase on Thursday and Friday for widespread wind gusts for all marine zones.

Small Craft Advisories are in place for the Columbia River Bar Wednesday morning due to strong/very strong ebb currents which will produce conditions hazardous to small craft. Strong/very strong ebb currents are possible during the morning hours each day this week. -42/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None.

WA...None.

PZ...Small Craft Advisory from 6 AM to 10 AM PDT Wednesday for PZZ210.

Small Craft Advisory until 2 AM PDT Wednesday for PZZ252-253- 271-272.

Small Craft Advisory from 4 PM to 9 PM PDT Wednesday for PZZ252-271-272.

Small Craft Advisory from 1 PM to 9 PM PDT Wednesday for PZZ253.

Small Craft Advisory until 9 PM PDT Wednesday for PZZ273.