textproduct: Portland

SYNOPSIS

Hot and dry conditions continue today, bringing widespread Moderate HeatRisk across all interior valleys. Increasing onshore flow tonight into Wednesday will bring high temperatures down a few degrees Wednesday afternoon. Cooler and wetter conditions arrive Thursday through Saturday, with a slight chance for thunderstorms Saturday afternoon. Drier conditions return Sunday-Monday, but we'll still maintain slight chances for rain across the Coast Range and Cascades.

DISCUSSION...Today through Monday

Satellite imagery as of early Tuesday morning depicts widespread marine stratus along the coast and mostly clear skies inland with a few high clouds. As daytime heating progresses today, we'll see increased mixing and some stratus break-out by late morning. However, there is a 20-30% chance that stratus lingers throughout the day along the coast.

Hot and dry conditions continue today with afternoon high temperatures forecast in the upper 80s to mid 90s, warmest along the I-5 corridor from Salem to the Cowlitz Valley. This will result in widespread Moderate HeatRisk across all interior valleys. This level of heat will affect those who are sensitive to heat, especially those without adequate cooling or hydration. The Heat Advisory remains in effect through 11 PM tonight along the I-5 corridor from Salem to the Cowlitz Valley, the Columbia River Gorge, and Upper Hood River Valley. Make sure to stay hydrated, wear light-colored and loose-fitted clothing, wear sun protection, taking frequent breaks from the heat, and limiting strenuous outdoor activities during the hottest part of the day.

Onshore flow increases tonight into Wednesday as a weak trough passing through the region. The airmass is dry, so this trough is not expected to bring precipitation. Overnight lows in the low to mid 50s for most areas will provide overnight relief from the heat. An exception is the Portland/Vancouver Metro Area and Columbia River Gorge, where Wednesday morning between 60 to 65 degrees will provide limited relief from the heat. Because of urban heat island effects, the Portland/Vancouver Metro also has a 10-25% chance for Wednesday morning lows exceeding 65 degrees. Wednesday afternoon highs will still be very warm but a couple degrees cooler than today. Will also note that some high-resolution guidance is showing some very light precipitation over the Cascades this afternoon as the aforementioned trough moves through, however, there is low confidence it will actually reach the ground due to how dry the air is (aka it would fall as virga).

A large-scale pattern change to cooler and wetter conditions remains on track Thursday into the weekend as ensemble guidance is in agreement with an upper-level trough approaching the Pacific Northwest. Most ensemble members have rainfall reaching the coast by late Thursday afternoon/evening and then spreading into the I-5 corridor by late Thursday night into Friday morning. We'll remain in this cool and wet pattern through at least Saturday, but at least total rain amounts at this time appear beneficial and generally non-impactful. Chances for 48-hour rain amounts exceeding 0.50 inch from 5 AM Friday to 5 AM Sunday are around 60-80% for the coast and Coast Range, 50-70% along the I-5 corridor, and greater than 90% across the Cascades due to westerly flow bringing orographic enhancement. Will note that the westernmost parts of the Willamette Valley that are west of I-5 (including McMinnville and Corvallis) only have a 25-35% chance for exceeding 0.50 inch during this timeframe due to potential rainshadowing from the Coast Range.

On Saturday, the upper-level trough will transition into a closed upper-level low and move directly over the Pacific Northwest. Temperatures will cool down aloft, increasing atmospheric instability and bringing CAPE values of 100-200 J/kg across the region late Saturday morning into the afternoon. This instability plus sufficient lift from the low will support a slight chance (15-20%) for thunderstorms across northwest Oregon and southwest Washington. LREF guidance also suggests 0-500 mb wind shear values around 25-35 kt with limited direction shear, so it appears that these thunderstorms will at least trend non- severe. Any passing thunderstorms may produce lightning, brief heavy rain, erratic winds, and/or small hail. Also can't rule out a few cold air funnel clouds.

By Sunday-Monday, the majority of ensemble members are showing the upper-level low shifting eastward, returning relatively drier conditions and a warm-up of a few degrees. Will still maintain precipitation chances across the Coast Range and Cascades as westerly flow continues to support orographic enhancement of precipitation, however, any additional rain amounts appear light. Westerly winds throughout this cool and wet pattern will be breeziest along the central Columbia River Gorge and across Upper Hood River Valley, with chances for isolated wind gusts exceeding 45 mph around 10-30% from Friday to the end of the weekend. -10

AVIATION

Southwest flow aloft ahead of a weak upper level trough that will move across the region this afternoon. Continued dry conditions with scattered to broken high clouds will maintain predominately VFR conditions across inland terminals. The main exception is the shallow marine stratus along the coast. As of 17z, satellite imagery shows the stratus pulling offshore from KAST, but continues to linger from KTMK southward with predominately IFR CIGs. Visibilities should continue to improve through the late morning and early afternoon hours. There remains moderate chances for improvement to VFR near KONP, however there is a 30-50% chance for marine stratus to persist through the afternoon. Chances for IFR/LIFR stratus returning to the coast increase again after 02-03z Wed. Light winds under 5 kt become WNW, increasing to 7-10 kt in the afternoon across any given terminal.

In addition, high temperatures between 90 to 95 degrees are forecast across the Willamette Valley this afternoon. Be aware of high density altitude which may reduce aircraft performance.

KPDX AND APPROACHES...VFR through the TAF period with SCT/BKN high clouds through this afternoon. Light and variable winds expected to become northwesterly, increasing to 7-10 kt in the afternoon/evening. /DH

MARINE

High pressure will maintain north-northwesterly winds across the coastal waters through Wednesday. Winds generally expected to remain under 20 kt, but there is a 60-80% chance for isolated wind gusts up to 25 kt for the waters beyond 30 NM offshore. Given the low confidence for widespread and frequent wind gusts greater than 21 kt, decided against issuing Small Craft Advisories for the outer water zones beyond 10 NM offshore.

Weakening high pressure will give way to a frontal system approaching the area on Thursday. Winds are expected to back west to southwesterly before the cold front pushes across the coastal waters late Thursday into Friday. Southwest winds are expected to be strongest for the waters north of Cape Foulweather and up to 10 NM offshore, but there is only around a 10-20% chance of small craft wind gusts exceeding 21 kt at any given hour through Friday morning. West to northwest winds will persist behind the front later Friday and into the weekend as weak low pressure lingers near Vancouver Island. High pressure gradually builds across the waters late weekend. Seas remain around 5 to 7 ft through Wednesday, subsiding to 4 to 5 ft on Thursday. -10/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Heat Advisory until 11 PM PDT this evening for ORZ108>115- 119>123.

WA...Heat Advisory until 11 PM PDT this evening for WAZ204>210.

PZ...None.