textproduct: Portland

SYNOPSIS

Expect a showery, unsettled airmass to remain overhead into the weekend with cooler than normal temperatures persisting. Conditions trend drier by the early to middle portion of next week as high temperatures gradually rise back closer to normal for early September. However, uncertainty in the large scale pattern grows noticeably around the middle to end of next week.

DISCUSSION...Saturday through Friday

The upper level low pressure system remains situated over the PacNW this weekend as the surface low has weakened over the coast. Very light winds early Saturday morning through Sunday morning with clearing upper level clouds will lead to the potential of low stratus or fog development in the morning hours each day along the coast and interior lowlands. Multiple weak shortwaves embedded within the larger upper low pressure circulation will bring additional shower chances over NW OR and SW WA, most likely during the daytime hours and over terrain features as diurnal heating likely plays a major role in shower generation. Also can't rule out stronger cells or a pulse-like thunderstorm or two forming in the afternoon and evening hours each day. Brief heavy rain, small hail, and gusty winds are possible with any strong shower or thunderstorm that forms. Confidence remains high that temperatures hold below normal for this time of year with upper 60s to mid 70s in the interior lowlands, 60s along the coast, and 40s to low 60s over the Cascades (coldest at higher elevations). If you're venturing up in the mountains to enjoy the weekend it may be worth packing a jacket.

Ensemble guidance is in high agreement that the upper low will finally move eastward Monday into Tuesday, into Idaho by late Monday and southern Canada by Tuesday. This will bring dry conditions by midday Monday and temperatures warming back to near normal by Tuesday as most ensemble members indicate zonal flow begins to take shape over the PacNW.

Wednesday into Thursday, ensembles begin to deviate from one another over the PacNW as they try to resolve the upper-level pattern, particularly in regards to a ridge of high pressure over the central US trying to amplify northwest over the Rockies into southeast Oregon while a trough amplifies over the eastern Pacific. About 80% of ensemble members indicate at least zonal flow if not slight ridging over the area on Wednesday, leading to the warmest temperatures of the week with a 75-85% chance of interior lowland temperatures exceeding 80 degrees and a 40-60% chance for exceeding 85 degrees. By Thursday, about 50% of the members hold on to this pattern with the other 50% indicating weak to moderate troughing developing over the area. By Friday, about 70-80% of the ensemble members suggest troughing impacting the region, though precipitation chances with this are extremely variable. This trough feature could be influenced by the eventual northerly progression of remnants from Hurricane Marie in the eastern Pacific with around 50-60% of ensemble members suggesting this in the Thursday/Friday timeframe. However, how much moisture lingers from Marie and how much this will impact the PacNW is very uncertain at this point. Will be keeping an eye on this as Marie progresses through the Pacific over the next week. -03/99

AVIATION...Through 12Z Sunday

A low pressure system remains in place over the region, maintaining some moist and variable flow aloft. Predominately VFR conditions, with the exception of KONP where conditions may bounce between IFR/MVFR/VFR through the night and become predominantly VFR by 18Z Sat. With wet ground and the potential for clearing skies tonight, and light and variable wind for areas south of a line from KTMK to KUAO, low stratus or fog development is possible. Guidance suggests a 40-60% chance of MVFR, 20-40% chance of IFR, and 5-20% chance of LIFR conditions between 12-18Z Sat for KONP, KSLE, and KEUG. Any lowered flight levels should improve to VFR by 16-19z Sat. There's a 50-60% chance of marine stratus redeveloping at KONP by 06-10z Sun with a 20-50% chance of low stratus or fog in the southern Willamette Valley after 09-12z Sun. The coast will see light southeasterly winds shift northwesterly in the afternoon and strengthen but remain under 8 kt.

A 15-25% chance of thunderstorms return between 18Z Sat through 03Z Sun, mostly focused over the Cascades and Coast Range. Any storms that do develop will have the potential to produce lightning, gusty/erratic winds, small hail, and heavy rain.

KPDX AND APPROACHES...VFR conditions could bounce between MVFR through 17Z Sat, with a 20-30% chance of more persistent low stratus or fog development producing MVFR/IFR conditions between 13-17Z Sat. Light and variable winds expected through the TAF period. ~12/03

MARINE

Surface low pressure will continue to weaken into early Saturday morning as it sits off the OR coast. Northerly winds that developed across the waters overnight are becoming light and variable this morning. Seas have fallen to around 4 to 5 ft at 10 seconds.

Northerly winds return later today into Sunday as weak high pressure rebuilds over the waters. Winds are expected to peak at around 15 kt or less. Seas remain at around 3 to 6 ft through the weekend. Conditions look to remain rather benign into early next week, though high pressure may strengthen toward the middle of next week. -DH/03

PQR WATCHES/WARNINGS/ADVISORIES

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