textproduct: Portland

SYNOPSIS

Calmer conditions are expected the next several days with lingering shower chances primarily pinned across the coast, coast range, and the terrain of southwest Washington. Rain chances then shift to central Oregon Saturday into Sunday as a weather system moving into northern California brushes the region, but confidence is only low to moderate on the exact northern extent of precipitatiion. Completely dry weather then returns early next week. Conditions likely remain within typical climatological normals through the forecast period with no significant weather impacts on the horizon.

DISCUSSION...Now through Tuesday

We've transitioned to a post-frontal airmass this morning as the atmospheric river which impacted the area has long departed the region. Partial clearing form the inland valleys to the coast in addition to light winds and ample surface moisture continues to support the development of patchy fog into the sunrise hours today, some of which may be locally dense (visibility <0.25 mile). Keep this in mind when you embark on your morning commute. Otherwise, conditions remain mostly dry today as high resolution guidance keeps any lingering shower activity pinned near the coast of northern oregon/Washington into the terrain features of the coast range and Cascades. All in all, much calmer and quieter weather compared to the past several days.

Both deterministic and ensemble guidance depict the flow aloft becoming zonal Thursday into Friday through the weekend. Low precipitation chances continue across the area through Friday morning, mainly limited to the north Oregon coast and Coast Range north of Lincoln City, far northern Oregon Cascades, and SW Washington. Limited accumulation is expected. Colder air is expected to be funneled into the PacNW as well. This cooler airmass along with another round of clearing skies across the majority of the interior lowlands leads to colder overnight temperatures tonight with a 60-90% chance of low temperatures Thursday morning falling below freezing in the central/southern Willamette Valley and coast range valleys, and a 40-60% chance for the SW Washington lowlands north of Kalama (70-90% chance to drop below freezing near Battle Ground/Yacolt). However, if skies don't clear as expected, locations with remaining cloud cover will not cool quite as much as in the forecast. It's worth noting there is a slight chance for a rain/snow mix shower above 700-1500ft tonight into Thursday morning as well, but impacts appear extremely limited.

Mostly dry conditions persist Friday through early Saturday with inland high temperatures in the 50s and minimum temperatures in the low to mid 30s. Uncertainty increases in the forecast Saturday afternoon into late Sunday, primarily in regards to precipitation chances/placement. Ensemble guidance is in agreement that a closed low, the remnant of the longwave trough which helped steer the atmospheric river overhead previous couple of days, progresses westward into California. However, uncertainty remains in how far north along the California coast this will occur. A track further north would yield better chances for precipitation. Latest forecasts indicate precipitation chances limited to roughly south of Highway 20 in Oregon. Additionally, probabilities for overnight temperatures falling below freezing in the interior lowlands remain low to moderate, mainly around 30-60%, due to the uncertainty of this closed low and mid to high cloud cover being ejected northward. A bit more of a northerly track than expected will likely keep low temperatures above freezing due to cloud cover, but a more southern track could keep the area from being impacted by thicker cloud cover associated with the upper- level low and allow for the jet stream to dip into Oregon and bring colder temperatures. Either way, the closed low is expected to move east by Monday as a ridge of high pressure builds overhead, leading to increasing confidence (80-90%) of dry weather Monday into Tuesday. However, the pattern appears rather progressive (i.e. the completely dry weather won't last forever) with nearly 50-60% of the ensemble space depicting a shortwave trough and frontal boundary introducing better chances for precipitation around Wednesday next week. -99/03

AVIATION

Surface observations continue to show wide mix of VFR to IFR and LIFR CIGs/VIS across the region approaching sunrise this morning thanks in-part to clearing and light winds facilitating fog and low stratus formation which is impacting several terminals. Expect fluctuations to continue through the morning as fog/low stratus starts to lift while "sloshing" around the inland valleys. Overall, high resolution guidance points to a trend back to MVFR by 18-20z followed by the return of VFR conditions this afternoon and evening - low to moderate confidence regarding the exact timing of this transition. Headed into Thursday morning, patchy fog will again be possible but confidence in placement and timing is to low to add it as a prevailing conditions at this time. Winds stay fairly light, generally less than 5-10 knots today and tonight.

KPDX AND APPROACHES...Low stratus and fog is developing within the immediate vicinity of the terminal this morning with the best chance (40-50%) for a period of IFR/LIFR conditions due to low CIGs/VIS between 13-16z, after which point we should quickly trend back to MVFR then VFR. It should be noted forecast confidence through 16-18z this morning is rather low given the patchy/sloshy nature of the fog shown on satellite observations - large hourly fluctuations are possible. -99

MARINE

With our previous weather disturbance now well off to the east both seas and winds have calmed with bouy observations early this morning showing seas around 5 to 7 ft at 9 seconds accompanied by wind gusts under 15-20 knots. Headed through Thursday and Thursday night, confidence is high seas remain below 8 ft with wind gusts holding between 10-20 knots (highest over the outer waters north of Cape Falcon). Local guidance shows a mixed bag of swells with a primary northwesterly swell and a southerly fresh swell that then transitions into a true secondary swell. While a complicated swell organization, we'll see widespread seas rise to ~10 ft on Friday into Saturday. The sea state diminishes again back into the 5-7ft range by early next week. -99/27

PQR WATCHES/WARNINGS/ADVISORIES

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