textproduct: Portland

SYNOPSIS

Cooler than normal conditions persist into early next week as drier weather returns albeit with areas of morning low clouds and fog. Our attention then turns to a ridge of high pressure building overhead mid to late week leading to the return of summer-like temperatures - highs in the 80s across the lowlands. A period of ENE flow centered around Wednesday and Thursday likely brings breezy east winds to the Columbia River Gorge, Cascades, and adjacent foothills as well. Lower forecast confidence Friday onward as the ridge of high pressure begins to break down.

DISCUSSION...Now through Saturday

A closed upper-level low pressure system remains overhead this afternoon as rain shower activity rotates into the area out of the N-NW. Headed into the evening and overnight hours confidence is high this low pressure continues to race eastward followed by building upper-level heights helping to decrease lingering showers, largely ending across the lower elevations this evening, and aiding in a bit of partial clearing. This latter variable will come into play by sunrise Monday morning combining with lighter winds and ample surface moisture to facilitate areas of fog and low cloud development. Given the latest model soundings for Monday morning, would hedge towards a more prevalent low stratus/cloud layer than prolific fog.

Overall quieter and dry conditions are anticipated early next week with weak ridging beginning to develop offshore Monday followed by an amplifying upper-level shortwave trough dropping southward into the Pacific northwest from British Columbia late Tuesday. Compared to Monday morning, the sunrise hours Tuesday present a better set-up (stronger low level inversion) for morning fog in our protected valley areas across the region. Fog will greatly reduce visibility where ever present. Both Monday and Tuesday interior high temperatures should reach into the upper 60s to low 70s, around 5 to 7 degrees below normal for mid-September. If you're a glutton for warmer conditions you won't have to wait long for the proverbial temperature pendulum to swing the other direction.

A noticeable warm-up begins Wednesday into Thursday as the the ridge of high pressure offshore begins to exert more influence over the Pacific Northwest while the previously mentioned shortwave from Tuesday transitions into a closed low and sets up over northern California - this'll come into play to aid easterly flow. Deterministic and ensemble guidance are still in good agreement showing maximum temperatures in the upper 70s to mid 80s across the interior valleys midweek, highest on Thursday. For Thursday afternoon specifically, the NBM only shows a 10-20% chance for high temperatures to meet or exceed 90 degrees in the northern Willamette Valley (lower probabilities elsewhere) which is a decrease from previous forecast cycles. So at least it appears temperatures won't get too warm midweek, but they'll still be running 6-12 degrees above climatology.

Beyond the increase in temperatures, the large scale pattern favors broad scale ENE winds throughout the depth of the atmosphere in addition to a near surface thermal trough surging northward. The strongest easterly winds currently appear most likely Wednesday into Thursday morning, with gusts of 20-30 mph and isolated gusts near 30-40 mph in wind-prone areas of the Columbia River Gorge, Cascades and adjacent foothills. The higher terrain of the Coast Range could become similarly breezy as well. This combination of warmer temperatures, somewhat lower humidity (30-35%) and winds may produce a period of elevated fire-weather concerns. We'll continue to closely monitor the evolution of the weather pattern midweek.

Come Friday into Saturday forecast uncertainty increases notably as ensembles struggle to handle the progression of the large scale upper-level pattern. Looking at Friday, around 60-70% of ensemble members have the ridge axis either overhead to close enough for a repeat of mid-80 degree high temperatures across the lowlands, while the other 30-40% point to a quicker ridge progression and thus temperatures cooling back into the 70s. A similar theme is seen on Saturday as well although the trend even among the warmest solutions is towards lower temperatures, it's just a matter of how low. In any case dry weather is favored (80-90%) to prevail through the late week and early weekend time period. -99

AVIATION...Through 18z Monday

An area of low pressure shifting inland will see persistent rain showers with low clouds through much of Sunday. Rain showers wrapping around the backside of the low are tending to move north-to-south, with higher coverage over Washington state and far northwestern Oregon late this morning. Flying conditions have varied as rain showers have passed over area terminals: largely VFR cigs, intermittent MVFR cigs, and occasional IFR cigs. Predominantly VFR conditions with persistent 20-40% chances of MVFR and 10% or less chances of IFR will likely continue as long as rain showers remain in the vicinity of terminals, generally through 21-24z Sun. Conditions will tend to improve after 00z Mon as skies partially clear and cloud bases lift to high-end MVFR along the coast and to around 6 kft inland. Given moist low levels, chances for further MVFR cigs beneath nocturnal low stratus increases after 10-14z Mon. South to southwest winds continue at 5-10 kt through this afternoon, with occasional gusts to 15-20 kt possible at KAST. Winds ease below 5 kt tonight across the airspace.

KPDX AND APPROACHES...VFR conditions expected to prevail as scattered rain showers continue through 22-23z Sun, although there remains persistent 30% chances of MVFR cigs if a heavier shower nears the terminal. IFR cigs are unlikely but cannot be ruled out, with less than a 10% chance of occurrence. Cigs lift and cloud coverage decreases somewhat after 00z Mon, before chances for further MVFR cigs increase after 12z Mon as low stratus develops. South to southwest winds of 5-10 kt will ease below 5 kt tonight. -36

MARINE

Rain showers end this afternoon as low pressure continues to move inland. In its wake, west to southwest winds at 5-10 kt south of Cape Falcon and 8-12 kt to the north will ease through tonight and veer out of the north by Monday evening as high pressure builds offshore. The ridge will continue to strengthen through midweek, which will see northerlies similarly intensify to 15-20 kt with gusts to 25 kt by Wednesday afternoon. There remains a greater than 50% chance sustained winds peak above 20 kt on Wednesday to the south of Cape Falcon, with lower chances, 10-40%, on both Tuesday and Thursday. Winds then ease into next weekend as high pressure weakens.

Seas around 5 ft early this week will similarly build to 6-7 ft as winds strengthen. A persistent northwesterly swell at 8-9 seconds otherwise continues through the week, with seas easing into next weekend as high pressure and winds weaken. -36

PQR WATCHES/WARNINGS/ADVISORIES

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