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

Moist, southerly flow aloft will continue to increase this evening as an upper level trough offshore approaches the region. MVFR marine stratus along the coast is expected to persist through much of the period. Elevated showers and thunderstorms have developed across the Willamette Valley and Cascade foothills and will continue to quickly move north through 07-09z Wed. Frequent lightning will be possible with these storms, especially near the Portland metro area.

A more stratiform band of showers then pushes inland along and behind the front, beginning at the coast around 07-09z Wed, lifting northeast across inland terminals by 09-12z Wed. This band will be mainly east of the Cascades by 18z Wed with scattered showers diminishing 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 09-18z Wed. Winds increase out of the southwest behind the front Wed morning to around 8-12 kts with gusts up to 20-25 kts at times.

KPDX AND APPROACHES...VFR conditions expected to continue with mid to high level clouds, though fast moving showers and elevated thunderstorms will continue to move north over or near the terminal through 09z this evening. A more stratiform band of rain will move across the area after 10z, with around a 30-40% chance of MVFR CIGs. Showers are expected to diminish with Northwest winds become light overnight before increasing again out of the southwest with gusts to around 20 kt after 14z Wednesday. /DH

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.