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 06z Tuesday

Dry, northwesterly flow aloft tonight into Monday as an upper level low departs the region. Predominately VFR conditions this evening with CIGs around 4-6 kft, except MVFR stratus at the coast near KONP. MVFR cigs at coastal terminals are very likely, 80% or greater chances by 06z Mon, as continued onshore flow sees low marine cloud coverage increase. MVFR cigs will be favored at inland terminals by 12-18z Mon, with chances reaching 50-70%, highest along the lower Columbia River, in the Portland area, and in the southern Willamette Valley. Chances for IFR cigs at any point overnight are generally 10-15% or less inland but 30-40% along the coast. High pressure beginning to build offshore will favor increased clearing and improvement to widespread VFR conditions by Monday afternoon.

South to southwest winds ease below 5 kt tonight. As winds rise to 5-10 kt through the day on Monday, flow will veer out of the north to northwest by the mid-afternoon.

KPDX AND APPROACHES...VFR conditions expected to prevail overnight through at least 12z Monday with cloud bases around 5 kft. Continued onshore flow will favor low marine clouds developing early Monday morning, with chances for MVFR CIGs increasing to around 50% by 13-15z Mon. Improved clearing by Monday afternoon support high confidence in a return to VFR conditions by 18-20z Mon. South winds around 5 kt this evening ease to 3 kt or less overnight, then build to around 5 kt again while veering out of the northwest by Monday afternoon. /DH

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.