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 18z Sunday

A low pressure system continues to meander overhead, bring continued moist conditions with light winds and persistent low chances for rain showers. Nonetheless, VFR conditions are favored at all terminals after 19-21z Sat, with remaining low cigs expected to lift as diurnal mixing strengthens. Terminal impacts from any showers are unlikely, and would be very brief if they do occur. Showers are more likely in areas of terrain, particularly the Cascades, with 15-25% chances for an isolated thunderstorm which could produce locally heavy rain, gusty and erratic winds, and small hail through this afternoon. Otherwise, VFR cigs/vis continue with light winds of 5 kt or less across the area. Chances for low clouds and additional rain showers increase after 06-12z Sun.

KPDX AND APPROACHES...Low clouds this morning may result in brief MVFR/IFR conditions, but diurnal mixing will increasingly favor VFR cigs through 20-21z Sat. Light winds below 5 kt expected through the period, initially out of the northwest and becoming variable tonight after 06z Sun. Low chances, <10%, for rain showers persist through much of the day, but increase to 25-35% after 12z Sun. -36

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.