textproduct: Portland

SYNOPSIS

Onshore flow will support morning clouds and cooler temperatures today. Temperatures re-bound Sunday as high pressure re-builds offshore. The hottest days of the week are expected Monday and Tuesday, with widespread Moderate HeatRisk and localized Major HeatRisk for the Portland-Vancouver Metro Area. Warm low temperatures for interior valleys will provide limited overnight relief, especially in urban areas. There is some uncertainty on Wednesday with how quickly the heat diminishes, but confidence is increasing for cooler temperatures and precipitation chances by Thursday-Friday.

DISCUSSION...Today through Friday

The Pacific Northwest currently remains within the southern periphery of an upper- level trough dropping down into southern British Columbia and Alberta. While this system remains too far north to bring us any precipitation, we're still getting increasing onshore flow and moisture. As a result, satellite imagery and surface observations as of early Saturday morning depict widespread marine stratus along the coast with some stratus beginning to develop across the southern Willamette Valley and adjacent Cascade foothills. As the low level atmosphere moistens up this morning, stratus will gradually spread across parts of the Willamette Valley. Morning cloud cover and onshore flow will support cooler temperatures today with afternoon highs peaking in the upper 70s to near 80 across interior valleys and 60s along the coast. While stratus may linger along the coast today, inland stratus should break out by late morning or early afternoon as daytime heating progresses.

It's worth noting that the passing trough to the north will tighten the west to east surface pressure gradient across the Cascades on Saturday leading to breezy conditions through central Columbia River Gorge and Upper Hood River Valley, where westerly winds may gust up to 30-40 mph with a 25-35% chance for isolated and brief wind gusts exceeding 45 mph. These winds noticeable decrease Saturday night.

The second half of the weekend temperatures begin to rebound into the mid to upper 80s across interior valleys Sunday afternoon as ensemble guidance shows the upper-trough quickly shifting eastward and upper-level ridging rebuilding offshore over the northeast Pacific. The hottest days over the next week are still expected Monday and Tuesday during which the vast majority of ensemble members show the ridge sliding right over- top the Pacific Northwest. There is high confidence (>90% chance) for high temperatures exceeding 90 degrees F across the Willamette Valley and southwest Washington lowlands on Monday and Tuesday with a 20-40% chance for temperatures exceeding 100 degrees F from Salem to Portland-Vancouver on Monday, increasing to 40-60% on Tuesday. Overnight lows will also be warm, especially across the central Willamette Valley, Columbia River Gorge, and Portland-Vancouver Metro Area where there is a 30-50% chance for Tuesday morning lows to remain above 65 degrees (highest chances across Portland-Vancouver Metro due to urban heat islands effects). Chances for lows remaining above 65 degrees fall to 20-35% Wednesday morning as onshore flow increases. Overnight lows near or above 65 degrees would provide limited overnight relief from the heat. As a result, there will be widespread Moderate HeatRisk across much of northwest Oregon and southwest Washington during this period, with localized Major HeatRisk across the Portland-Vancvouer Metro Area through the Columbia River Gorge from the warm overnight temperatures. The Extreme Heat Watch for these two areas remain in effect beginning Monday morning and continuing through Tuesday evening. Make sure to practice heat safety, including staying hydrated, wearing light-colored and loose- fitted clothing, wearing sun protection, and taking frequent breaks from the heat. If possible, schedule outdoor plans during the early morning or late evening when temperatures will be less intense.

There still remains forecast uncertainty with exact progression of the upper-level ridge on Wednesday, however recent guidance has trended temperatures slightly warmer. Most of the global ensemble members (65%) show the ridge axis remaining over the Pacific Northwest and keeping temperatures into the 90s across interior valleys. Meanwhile, the remaining 35% of ensemble members have the ridge axis pushes further east into the Interior Northwest/Great Basin, which would usher in a return of weak onshore flow and "cooler" temperatures (80s to near 90). Still, within the latest NBM there is currently a 15-20 degree spread between the NBM 10th and 90th percentiles on Wednesday for the Willamette Valley and Portland Metro, with the coolest scenario being in the mid 80s while the warmest scenario pushes highs near 100 degrees F. Should the warming trend on Wednesday continue, heat related impacts may extend yet another day. At least there is more agreement among guidance for the ridge axis to nudging eastward Thursday, allowing temperatures to decrease by still above normal for June. By Friday, nearly 70% of ensemble members suggests troughing approaching the Pacific Northwest, returning chances for light precipitation and much cooler temperatures. Uncertainty remains with exactly how much precipitation we could see by the end of next week. -10/99

AVIATION

Satellite imagery as of early Saturday morning depicts widespread MVFR stratus along the coast and southern Willamette Valley (KEUG). Onshore flow will continue to support MVFR stratus backbuilding against the Cascade foothills and spreading across the Willamette Valley this morning between 12-18z Sat as the low level atmosphere moistens up. Guidance suggests that MVFR CIGs linger at along the north Oregon coast (KAST) throughout the TAF period, while the central Oregon coast (KONP) may get brief improvement to VFR conditions between 20z Sat-00z Sun. Willamette Valley terminals should improve to VFR after 18z Sat as daytime heating progresses and increases mixing. After 00z Sun, another surge of marine stratus will filter into the coast bringing a return of widespread IFR/MVFR CIGs.

KPDX AND APPROACHES...Guidance suggests a 35-55% chance for MVFR CIGs at any given hour between 12-17z Sat. After 17-18z Sat, conditions improve to VFR with high clouds. West-northwesterly winds under 10 kt. -10

MARINE

High pressure offshore continues to maintain northerly to northwesterly winds over the next seven days. A Small Craft Advisory remains in effect across the outer waters (beyond 10 NM offshore) through 11 PM Saturday. Northwesterly winds will gradually weaken below 20 kt this morning as pressure gradients ease. Steep and choppy seas will continue until late Saturday, around 8 to 11 ft at 9 to 10 seconds. Seas subside to 5 to 7 ft Saturday into Sunday. Seas remain around 6 to 8 ft or less into early next week, with only a 5% chance for seas exceeding 8 ft at any given hour Mon-Wed. -10/12

PQR WATCHES/WARNINGS/ADVISORIES

OR...Extreme Heat Watch from Monday morning through Tuesday evening for ORZ109-111-112-120-122.

WA...Extreme Heat Watch from Monday morning through Tuesday evening for WAZ206-207-209-210.

PZ...Small Craft Advisory until 11 PM PDT Saturday for PZZ271>273.