textproduct: Portland

SYNOPSIS

Cooler than average temperatures continue today with chances with showers and isolated thunderstorms as a closed upper level low remains in place over the region. Trending drier Monday through Wednesday with temperatures gradually warming each day as low-level offshore flow develops. The forecast becomes more uncertain Thursday into Saturday, with the potential for weather conditions to range anywhere from cool and showery to warm and dry depending on the evolution of the large scale weather pattern.

DISCUSSION...Sunday through Saturday

Ensemble guidance is in good agreement that today will be the last day the persistent upper low pressure system churns over the PacNW before it finally moves east of the Cascades tonight into eastern Oregon, eastern Washington, and western Idaho. This low will bring continued chances for showers and isolated thunderstorms today across most of northwestern Oregon and southwestern Washington before chances decrease from west to east this evening as the low progresses eastward. Rain amounts will be noticeably higher for many inland valley locations when compared to Saturday. Although mainly dry conditions are favored at the coast, relatively wetter conditions are expected inland over the Willamette Valley, greater Portland/Vancouver metro area, Columbia River Gorge, Cascades/foothills, and Upper Hood River Valley. These areas have a 15-35% chance for 0.25 inches of rain or more through 11pm PDT Sunday, except 45-60% in the Cascade foothills and 55-85% in the Cascades. The additional rain will continue to dampen fire season and keep wildfire smoke out of the area. With the excess cloud cover and occasional rain on Sunday, temperatures will remain cooler than average for early September with highs mainly in the mid 60s to lower 70s, coolest at the coast and in the Cascade foothills.

Although the aforementioned low will be well to the east of the Cascades on Monday, a shortwave trough rotating counter-clockwise around the low is set to move over Washington and possibly far northern Oregon. Model ensemble guidance suggests this feature will most likely produce very little to no rain at all, and the NBM backs this up with probabilities less than 5% for 0.01 inches or more between 5am PDT Monday and 5am PDT Tuesday except for 10-15% for the Willapa Hills.

Temperatures will trend noticeably warmer Tuesday into Wednesday with high confidence for dry weather as low-level offshore flow begins to develop. On Tuesday, afternoon temperatures will warm into at least the mid to upper 70s, except lower 70s at the coast. Come Wednesday, widespread highs in the 80s are expected. Even the coast has a chance to reach 80 degrees or warmer on Wednesday if offshore winds last into the afternoon hours (5 to 15% chance, except a 20-50% chance for inland coastal communities such as Astoria and Tillamook).

Come Thursday into Saturday, uncertainty increases substantially in regards to the large scale weather pattern and whether or not we will remain warm and dry or transition back to a cool and showery weather pattern. On Thursday, a cluster analysis for 500 mb heights shows around 40% of the total ensemble space from the GEFS/ENS/GEPS produces weak upper level troughing over B.C., western WA and far northwest OR. The remaining 60% of the total ensemble space depicts some degree of ridging over the area. The current deterministic forecast aligns with the latter outcome, suggesting the continuation of dry weather with highs still in the 80s for inland valleys, albeit several degrees cooler than Wednesday. For Friday into Saturday, clusters come into better agreement for upper level troughing (around 90% of the total ensemble space shows some degree of troughing over or near the area). However, there are significant differences in the exact amplitude, location, and timing of this trough, which will impact precipitation chances and temperatures. As such, the NBM 10th to 90th percentile temperature spread is very large for each day, ranging from 70-90 degrees on Thursday and 65-85 degrees on Friday. Friday and Saturday currently have a higher chance for conditions to trend cooler and wetter compared to Thursday, which the latest deterministic NBM represents with a 10-20% chance of showers Friday night into Saturday. Anyone with outdoor plans late in the week should monitor the forecast for updates over the next several days. -23/03

AVIATION...Through 12Z Monday

A low pressure system remains overhead, but will gradually shift east of the Cascades Sunday night. Showers and isolated thunderstorms developed in central Oregon generally south of KTMK and KMMV around 10z Sun with just showers continuing over the Cascades and slowly pushing west into the lowlands north of this. Showers with a 15-25% chance of thunderstorms will continue through 00-06z Mon west of the Cascades, with showers continuing through 12z Mon over the Cascades. Thunderstorms could produce lightning, locally heavy rain, gusty and erratic winds, and small hail.

Confidence is low (10-20% chance) in whether MVFR or lower conditions will develop inland now that clouds have become more widespread. In areas that have patches of clearing, MVFR cigs could develop, becoming VFR by 18z Sun. Along the coast, conditions are expected to fluctuate between IFR and MVFR south of KTMK through 18- 20z Mon when mainly MVFR conditions become dominant with a chance for a few hours of VFR conditions late in the afternoon. There's high confidence widespread marine stratus brings MVFR/IFR cigs back to the entire coast after 00-04z Mon. Expect generally light and variable winds at all terminals, with the exception of winds up to 10 kt at KAST Sun afternoon.

KPDX AND APPROACHES...VFR conditions with Light and variable winds through the period. Scattered showers are possible through the day, with the potential for a stronger shower or two between 22z Sun to 04z Mon. -03

MARINE

Weak high pressure building offshore will produce light northerly winds of less than 15 kt through Monday. Rather benign conditions then continue through at least the middle of next week with surface high pressure strengthening as the stagnant upper low ejects eastward, favoring stronger north winds Tuesday into Wednesday. There's a 60-80% chance of north wind gusts exceeding 21 kts across the waters Tuesday afternoon into Wednesday morning. Seas continue at 3-6 ft with a dominant northwest swell at 8-10 seconds. -36/03

PQR WATCHES/WARNINGS/ADVISORIES

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