textproduct: Portland

SYNOPSIS

Warm and dry conditions expected through the week with persistent onshore flow. A stronger onshore push will bring increasing clouds tomorrow and cooler temperatures. Mostly dry through the weekend, but there are indications of increasing chances for rain early next week (though chances remain low at this time).

DISCUSSION

Satellite imagery as of late Tuesday morning depicts mostly sunny skies across northwest Oregon and southwest Washington as morning stratus has dissipated. Will continue to see some marine clouds hugging the coast through this afternoon as we maintain onshore flow. This onshore flow will persist throughout the week and provide near-normal temperatures across the region for this time of year. Expect high temperatures in the mid 80s today across interior valley locations and 60s along the coast.

Slightly cooler conditions are forecast tomorrow (Wednesday) as the upper level pattern shifts from zonal flow to more troughing. Broad upper level troughing over British Columbia this evening becomes a bit more organized as it drops south into Washington and Oregon. This pattern typically corresponds with a stronger push of marine stratus inland by Wednesday morning. Like previous days, still expect the marine layer to generally remain below 2000 ft but could see the stratus push through into the I-5 corridor through the Coast Range gaps and Lower Columbia River Valley. Would not be surprised for the coast to see some drizzle Wed morning as well. With the marine layer lingering into the late morning and early afternoon, temperatures will be slower to warm resulting in highs reaching into the mid to upper 70s across the southwest Washington lowlands and Portland-Vancouver Metro Area, while temperatures in the central and southern Willamette Valley are more likely to warm into the low 80s. Most of the moisture associated with the aforementioned trough is expected to remain across portions of the northern half of Washington, keeping us dry.

There is agreement among the models and their ensembles for later in the week with the upper level pattern trending toward broad upper level troughing and a continuation of onshore flow. The potential range of afternoon high temperatures from the NBM for Thursday through Friday has generally consolidated into the lower to mid-80s, favoring near to slightly above average conditions for mid-August. More uncertainty into the weekend, but ensembles are suggesting weak upper level ridging may nudge back into the Pacific NW as low pressure deepens the upper trough offshore. This could result in a slight warming trend, though models suggest there is only a 15-20% chance of exceeding 90 degrees across the Willamette Valley on Sunday and Monday. Shortwave troughs within the more southwesterly flow aloft along with some increasing monsoonal moisture will also bring slight chances (10-20%) for thunderstorms along the Cascade crests of Oregon for Saturday afternoon, with the highest chances across the Lane County Cascades.

The question then turns to the possibility of rain early next week, but chances for measurable precipitation remains low (15-30%) across the area next Monday. Cluster analysis of 500 mb heights shows a wide range of the magnitudes and placements of the upper level trough offshore, while 25% of ensembles show the potential for any substantial rainfall (24 hour totals of 0.10-0.25") across the area. -10/DH

AVIATION...Through 18Z Wed

Predominately west to southwesterly flow aloft continues with dry weather. At 18z Tue, predominately VFR conditions observed across NW OR and SW WA with isolated pockets of stratus expected to dissipate over the next hour or two. A weather system approaching the region early Wednesday will bring a stronger marine push, allowing for the return of more widespread stratus along the coast and pushing inland. There's high confidence stratus with MVFR/IFR conditions will return to the coast between 00-04z Wed with a 40-60% chance and 60-85% chance of LIFR conditions at KAST and KONP, respectively. Stratus will push inland after 06-09z Wed with a 70-85% chance of MVFR cigs developing at terminals north of KMMV and a 40-60% chance for the rest of the inland terminals between 09-13z Wed. Lowered flight conditions are expected to improve by 18-21z Wed at all terminals. For winds, north to northwest winds continue across the area, increasing to around 8-12 kt in the afternoon and early evening, weakening by 04-06z Wed.

KPDX AND APPROACHES...Predominately VFR with clear skies through at least 06-08z Wed after which marine stratus is expected to form. There is a 75-85% chance for MVFR CIGs to develop by 10-13z Wed, improving to VFR again by 18-21z. Northwest winds increase to 8-10 kt after 18z Tue, weakening again by 06-08z Wed. -03

MARINE

High pressure offshore will continue to support northerly winds and fairly benign conditions across the coastal waters through the week and into the weekend. Winds from the north to northwest with gusts less than 21 kts will continue through at least the end of the week. Wind driven seas are expected to remain around 3 to 6 ft through the week. -03

PQR WATCHES/WARNINGS/ADVISORIES

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