textproduct: Portland

SYNOPSIS

The arrival of a weather system tonight facilitates a notable pattern change towards cooler/slight wetter conditions. This is kicked off by the potential (15-35%) for a period of high-based thunderstorms this evening into early Wednesday morning, mainly I-5 corridor and eastward. After rain decreases on Wednesday, showers linger at times through the end of the week, mainly over the Cascades and coast/coast range. Then another low from the Gulf of Alaska increases precipitation chances(30-60%) to start the weekend while maintaining near to slightly below normal temperatures.

DISCUSSION...Today through Monday

Broad southerly flow is in place this afternoon with mid to high cloud cover streaming across the region. Still, the added clouds haven't been robust enough to suppress temperatures too much, and as of 1500 this afternoon we're running about 3-8 degrees warmer across the Willamette Valley and Portland metro than we were at this time yesterday. Enjoy the conditions this afternoon if you're a fan of warmer weather, we likely won't see temperatures in the 70s and 80s for the remainder of the week.

The feature responsible for the added high cloud cover and above normal temperatures is an approaching negatively tilted shortwave trough which we'll have to watch closely this evening and overnight as it takes aim at the Pacific Northwest. While it's mainly beneficial from a precipitation/moisture standpoint once we get to Wednesday morning, this approaching feature will result in a period of increasing high-based instability this evening through a good chunk of tonight with a 15-35% chance of thunderstorms of the nocturnal variety. To add more of a challenge, any thunderstorms that do develop, will likely not be surface based and can be tough for models to properly resolve. The primary question of thunderstorm coverage and frequency remains, but there has been some added clarity with the latest CAMs (Convective Allowing Models) this morning. Out of the available CAMs, the 12/18z NAMNEST and 12z UW WRF are the most bullish, with storm development starting around 6-8pm in Lane County and rapidly spreading north along the Willamette Valley through SW WA and across Cascades by 9-11pm. This presents more of a "worse case" scenario in regards to convection but the NAMNEST at least should be taken with a grain of salt as it can sometimes run "hot" when it comes to these set-ups; the UW-WRF does add some credence to the scenario it's depicting however. Otherwise, the remainder of the high resolution ensemble space keeps development a bit delayed (10pm-midnight), more sporadic over the lower elevations, and further north/east largely placing activity in around the I-5 corridor in the north Willamette valley/SW Washington eastward into the Cascades. As the night goes on, the instability fueling convection continues to progress eastward eventually bringing and end to thunderstorm chances by the sunrise Wednesday. Just chances for plain old rain and showers follow.

Later Wednesday morning into the early afternoon, the axis of the shortwave trough will shift east of the area leading to a decreasing precipitation the remainder of the day. The added cloud cover, westerly flow, and cooler airmass likely pushes high temperatures down into upper 50s to mid 60s for much of the area, almost 10-20 degrees lower than the day prior across the inland valleys. Looking towards Thursday and Friday, a more zonal flow pattern emerges keeping temperatures near to below normal, and shower chances in place across the coast, coast range/Willapa Hills, and Cascades. Then late Friday into early Saturday most deterministic and ensemble guidance shows another shortwave trough dropping down from the Gulf of Alaska into the region. This likely increases chances for precipitation across the area and nudges high temperatures even lower into mainly the 50s to near 60 Saturday afternoon. For those dreaming of a return to daytime temperatures back in the 60s and 70s hope is not lost as there is moderate confidence in a ridge of high pressure beginning to build back overhead by early next week. -99/42

AVIATION

Mainly VFR conditions Tuesday across NW OR and SW WA except for along the coast. MVFR marine stratus continues along the coast south of KAST under southwesterly surface flow. There could be a few hours of lifting to VFR at KONP or fluctuations between MVFR and VFR ceilings between 22z Tue - 03z Wed. Across inland areas, westerly winds are increasing to around 8-10 kts. Winds will then ease and become more south to southwesterly after 06z Wed.

An upper level low pressure system approaches the region Tuesday night with a surface front pushing ahead. The front will move through the region between 06-15z Wed. Ahead of the front, there is a slight chance (15-25%) of scattered showers and thunderstorms 03-09Z Wed for all inland TAF locations. The chance for these showers or thunderstorms aren't high enough at any one location to add them to the TAFs at this time, but any strong shower or thunderstorm could produce heavy rain, small hail, lightning, and gusty and erratic winds. A more stratiform band of showers then pushes inland along and behind the front, beginning along the coast around 06-09z Wed then moving east and reaching inland terminals by 09-12z Wed. This band will be mainly east of the Cascades by 18z Wed with scattered showers continuing through the afternoon. There's a 60-80% chance of MVFR ceilings south of KSLE and a 30-60% chance at KSLE and north between 09-14z Wed. Winds increase along and behind the front around 09-12z along the coast and 12-15z inland. Expect winds around 8-12 kts with gusts up to 20-25 kts at times.

KPDX AND APPROACHES...VFR conditions with high clouds into Tuesday night. There is a slight chance (15-25%) of scattered showers and thunderstorms from 03Z-09Z Wed. Then a more stratiform band of rain will impact the terminal beginning around 10-14z Wed with a 30-45% chance of MVFR ceilings. Scattered showers continue behind this with occasional chances of MVFR conditions in showers. Winds becoming west to southwest up to 8-10 kt, then becoming lighter and more variable after 02z Wed. Winds become southwest again and increase to 8-12 kts with gusts up to 20 kts after 11-14z Wed. -03

MARINE

Relatively benign conditions expected through the week with predominately westerly winds, mainly under 10 kts. An upper level trough and associated front moves through the waters on Wednesday, which bring a 40-55% chance of occasional small craft gusts up to 25 kt across all waters. This will mainly be when the front is passing between 4 AM - 12 PM Wednesday. Not expecting gusts to last long enough over any specific area to issue a small craft advisory at this time, though we will continue to monitor conditions. Winds remain westerly but decrease once again Wednesday night. Seas generally below 8 ft around 10-12 seconds through the majority of week, but could see seas close to 10 ft by Friday/Saturday. -42/03

PQR WATCHES/WARNINGS/ADVISORIES

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