textproduct: Portland

SYNOPSIS

Hot and dry conditions this afternoon as high pressure remains overhead. Moderate HeatRisk is expected along the I-5 Corridor north of Salem, including the Portland/Vancouver Metro. Cooler temperatures return Wednesday to Thursday as a weak shortwave trough swings through the region. Slight chances for showers along the coast, Coast Range, and Cascades on Wednesday. Cool trend continues at the end of the week as a deeper trough arrives from the Gulf of Alaska, returning widespread chances for rain Friday night into Saturday. Renewed chances for precipitation early next week.

SHORT TERM...Today through Thursday night

Satellite imagery as of early Tuesday afternoon depicts mostly sunny skies across northwest Oregon and southwest Washington has an upper-level ridge moves overhead. Temperatures are on track to peak in the upper 80s to low 90s across interior valleys, with the warmest temperatures across the Portland/Vancouver Metro Area due to urban heat effects. There is Moderate HeatRisk today across along the I-5 corridor from Salem to the Cowlitz Valley, as well as the Columbia River Gorge. This level of heat affects those who are sensitive to heat, especially those without cooling or hydration. Everywhere else will experience Minor HeatRisk.

Other than heat, surface observations as of early Tuesday afternoon are showing easterly winds with gusts up to 25 mph along the Columbia River Gorge and surrounding terrain. These easterly winds will gradually weaken this afternoon as KTTD-KDLS pressure gradients turn more positive (onshore) and westerly winds return.

Tonight, most places will experience overnight relief as low temperatures Wednesday morning fall below 60 degrees. An exception would be the Portland/Vancouver Metro Area, where there is moderate confidence (40-60% chance) that morning low temperatures remain above 60 degrees. Therefore, this Metro Area will be a little slower to cool off from today's heat compared to other areas.

The hot temperatures will be short lived as a negatively tilted trough along the leading edge of a low pressure system in the Gulf of Alaska drops down tomorrow (Wednesday). This trough will transition the flow into a zonal onshore flow pattern and thus usher in cooler and more moist air. There remains a chance for light showers, but the probabilities are quite low with the chances between 15-30% being along the coast, Coast Range and spots within the Cascades. The rest of the interior valleys remain predominately dry. The mass of dry air leading into this trough will decay most of the rain that would normally reach the ground in a more moist scenario. It is not uncommon for these troughs to be sacrificed to weaken the ridge allowing for more precipitation to follow. Thursday will generally be dry and sunnier with seasonable temperatures in the mid 70s inland and in the 60s along the coast. -10/27

LONG TERM...Friday through Monday

Very little change in the forecast on Friday as onshore flow persists and the low pressure system in the Gulf of Alaska remains nearly stationary. However, Friday night into Saturday is a different story as the low aloft drops down over the northern Washington coast, and is coupled with another surface. This is a very energetic system with strong vorticity advection and climatologically abnormal CAPE over the marine waters. With ample support, there are increasing chances for more widespread rain. However, the most robust portion of this system will be well to our north as we sit on the southern edge of the low. Therefore, could see less of a stratiform rain and trend towards a convective/showery pattern. Due to the combination of increased instability with cooler air aloft, there remains a 15-20% chance for thunderstorms over southwest Washington and far northwest Oregon Friday and Saturday afternoon/evening.

Accumulation of rainfall remains uncertain as there is a) still ample time for this system to evolve, b) the location of the low will come into play - if it shifts south we could see heavier rain, and c) the convective nature of the overall pattern. 24 hour precipitation as described by the NBM continues to show quite a large range of possibilities. The 19Z Run currently shows the 10th-90th percentile spread of 24 hour precipitation being between 0.10 inch to 0.80-1 inch across southwest Washington, north Oregon coast, and north Oregon Cascades. For all other areas, the spread is generally 0.00 inch to 0.60 inch from Salem northward, and 0.00 to 0.40 inch south of Salem. Overall, the rain is non-impactful.

Transitioning into Sunday and Monday there will be some lingering showers but overall weather appears to be more settled. One thing we will have to watch is a cut-off low that some of the ensembles are attempting to produce on Sunday night. This cut-off low creates almost like a double barrel low type situation along the coast. The impacts are uncertain at this time, but this would bring renewed chances for precipitation heading into early next week. -10/27

AVIATION

VFR conditions continue across the airspace as surface high pressure moves inland and weakens ahead of a decaying front which will approach the coast later in the period. Inland, coverage of high clouds continues to increase, with cigs lowering to 5-10 kft through tonight. Terrain-influenced winds out of the southwest to northwest continue at most terminals, while the weakening offshore pressure gradient should see gusty east winds through and west of the Columbia River Gorge to KTTD begin to ease this afternoon. Winds will ultimately turn southwesterly as the weak front nears the coast tonight into Wednesday morning. Along the coast, chances for restricted cigs increase above 50% by 06z Wed, reaching 70% chances of IFR at KONP and 60% chances of MVFR at KAST. There are 25-50% chances of light rain showers early Wednesday morning at coastal sites. West to southwest winds of 5-10 kt will persist through the period.

KPDX AND APPROACHES...VFR conditions expected throughout the period. Coverage of high clouds will continue to increase through this evening before cigs lower to around 10 kft tonight. West to northwest winds of 5-10 kt will shift out of the south-southwest tonight ahead of a weakening frontal boundary. -36

MARINE

Northwesterly flow of 5-10 kt will veer southwesterly this evening through much of Wednesday before a decaying front traverses the waters tomorrow afternoon. Hourly rain shower chances peak at 50-80% early Wednesday morning south of Cape Falcon, with 20-40% chances otherwise lingering through midday. Northerly to northwesterly flow returns across the waters by Wednesday evening behind the boundary. Seas of 4-6 ft at 10-12 seconds with a dominant northwest swell continue through Wednesday, before a more westerly swell builds behind the frontal passage. Seas then build to 6-8 ft at 10 seconds from Wednesday night into early Thursday morning.

Generally unsettled weather looks to persist over the Northeast Pacific through the remainder of the forecast period with repeated systems bringing chances for further rain showers and elevated winds, most notably from Friday night into Saturday. Despite the active pattern, winds and seas are favored to remain below hazardous thresholds aside from chances for wind gusts exceeding 20 kt when brief northerly flow redevelops Sunday afternoon and evening. -36

PQR WATCHES/WARNINGS/ADVISORIES

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