textproduct: Portland

SYNOPSIS

A cutoff low lingering offshore of northwestern Oregon and southwestern Washington will bring continued seasonable temperatures through the holiday weekend with morning incursions of marine clouds giving way to breaks of afternoon sun. Building high pressure will yield a warming trend early in the week, with the potential for high temperatures reaching the 90s and Moderate to Major HeatRisk Tuesday through at least Thursday across inland areas. Chances of scattered showers and isolated thunderstorms increase midweek.

LONG TERM...Tuesday through Saturday

Through the workweek, the aforementioned cutoff lows over the northeastern Pacific will begin to interact, with the nearby low retrograding westward while the more distant low begins to fill. In the wake of the retreating low, narrow upper-level ridging will build out of the Great Basin across the interior Pacific Northwest, while yet another weak but very important upper low will track northward and inland over northern California and ultimately western Oregon by Friday. This pattern will yield a warming trend across the region, with medium-range forecasts coming into better agreement on temperatures Tuesday and Wednesday, however there remains increased uncertainty in temperatures through the latter half of the week.

On Tuesday and Wednesday, slightly cooler temperatures have been maintained, with afternoon highs most likely in the low 90s through the Willamette Valley, 80s in the Lower Columbia and Cowlitz Valleys, Cascades, and Coast Range, and 60s to 70s near the coast. The Columbia Gorge and Hood River Valley are favored to see the hottest temperatures in the mid to upper 90s. Chances to reach 100 degrees on Wednesday have fallen to 15% or less across the region, with the exception of 30-60% heading east through the Columbia Gorge to the Hood River Valley. Overnight lows largely in the 50s to low 60s will help to minimize the HeatRisk to Minor or Moderate across most of the region, however warmer lows in the upper 60s through the Gorge and Hood River Valley maintain the coverage of Moderate to Major HeatRisk in those communities.

Some of this increased forecast temperature confidence is due to developing consensus on the position and intensity of the approaching upper low, but its arrival also raises the specter for chances of Cascade convection and attendant fire weather concerns. Anomalous southeasterly flow aloft ahead of the low, along with synoptic support for ascent and increasing elevated instability could favor convection along the Cascades, with storms and/or outflow clouds propagating downstream over the remainder of the forecast area. Model soundings also show abundant dry air beneath any elevated convection, which could allow for dry lightning through the afternoon and evening on Tuesday and Wednesday. Even with increasing moisture, column moistening would still result in periods of virga and lightning strikes could also reach well outside of any narrow rain cores. With the relative humidity falling to afternoon minimums below 25-30% along the Cascades and receptive fuels remaining in place, those with fire weather interests should continue to follow the forecast through the coming days.

Thursday and beyond, spread among long-range ensemble members increases with regard to the placement and intensity of the upper low as it passes overhead, with this uncertainty manifesting as decreased confidence in forecast temperatures. Afternoon highs will most likely remain in the low to mid 90s across interior valleys on Thursday, but chances to reach 100 degrees increase to 5-25% from the Portland/Vancouver Metro south through the Willamette Valley, and remain higher, 20-50%, through the Gorge and Hood River Valley. Temperatures begin to trend cooler into Friday and Saturday, yielding largely Minor HeatRisk across the region by the weekend. -Picard

AVIATION

Early this morning a marine layer is attempting to deepen along the coast while a weak upper-level low pressure system sits in the far northeastern Pacific. Based off current satellite trends and model guidance, degraded flight conditions due to the developing stratus will stay focused around and after ~12-13z for coastal sites. It still remains to be seen whether low clouds will be able to push inland with now only a 30-45% chance for low-end MVFR to IFR CIGs between ~13-18z across the Portland metro and north Willamette Valley terminals. KSLE and KEUG only have a 15-25% chance for MVFR CIGs/VIS or below, and most likely remain VFR. Overall confidence in the set-up is only low to moderate thanks to scattered high cloud cover over the region which can act as an inhibitor to low cloud formation, especially inland. VFR conditions likely prevail at inland terminals during the afternoon and evening hours Sunday.

PDX AND APPROACHES...Generally VFR conditions are expected to prevail through the forecast period. Still, we'll need to watch the 13-17z time period with a ~40-45% chance for MVFR to IFR CIGs. Any stratus that forms likely clears to VFR by 18-20z Sunday. Light winds of 5-10 kt or less are expected through the period. -Schuldt

MARINE

Overall fairly benign late summer-time weather conditions continue through next week. Expect a weak area of low pressure just off the coast to help maintain rather light winds across the coastal waters through Monday and Monday night. From there, confidence is high a ridge of surface high pressure builds over the waters Tuesday and Wednesday increasing northerly winds. The winds will be strongest in the afternoons and evenings, especially to the south where the surface pressure gradient will be the strongest. Peak wind gusts during this early to mid week period get close to small craft conditions (>21 knots gusts), mainly across the outer waters, although guidance keeps the probability to meet criteria only around 20-35%. Seas hold below 4 ft through Monday, with an increase in wind waves building seas to 5-6 ft Tuesday through Thursday. -Schuldt

PQR WATCHES/WARNINGS/ADVISORIES

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