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...Today through Thursday

This afternoon a vertically stacked low pressure system continues to churn right along the central Oregon coastline with showers and isolated thunderstorms progressing largely south to north across western Oregon and southwest Washington. As daytime heating peaks, the latest HREF indicates MUCAPE values top out around 300-600 j/kg however with effective shear values near 8-13 knots only weaker pulse-like thunderstorms are anticipated. The strongest convection may produce infrequent lightning, heavy rain, small hail, and gusty outflow winds. Headed through the evening hours activity weakens and decreases thanks to the loss of daytime heating. Given the lighter winds, ample surface moisture, and partial clearing overnight, patchy fog is likely across portions of the region by sunrise Saturday.

This weekend, the majority of deterministic models and their ensemble counterparts continue to show the broad upper-level low pressure system holding over the Pacific Northwest with multiple weak shortwaves embedded within the larger circulation. Anticipated additional lingering 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 with similar impacts what we'll see today (Friday). 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.

Come Monday into early next week the majority of ensemble members finally show the upper low ejecting eastward favoring the return of dry conditions by the second half of Monday through Tuesday coinciding with a gradual warming trend. After Tuesday into Wednesday ensembles begin to deviate from one another when trying to resolve the upper-level pattern, particularly in regards to a ridge of high pressure trying to develop over the Rockies into southeast Oregon while a trough amplifies over the eastern Pacific. This latter trough feature is also going to be influenced by the eventual northerly progression of Hurricane Marie in the eastern Pacific. The majority of ensemble favor a warmer/drier solution on Wednesday with highs near to a touch above normal. It remains to be seen when or if the leftover remnant moisture from Marie swings into the Pacific northwest, but at least a decent subset of ensembles (30-40%) show this as a potential solution around Thursday/Friday of next week - something we'll have to watch. -99/03

AVIATION...Through 00Z Saturday

A low pressure system remains over the region late this afternoon with moist, southerly flow across the area. Most TAF sites remain VFR even with the showers and cloud cover rotating around the region but brief reductions to MVFR is possible at times with heavier showers. Chances for thunderstorms (15-30%) will persist for the next several hours this afternoon as well, ending by 02-04z. Any storms that do develop will have the potential to produce lightning, gusty/erratic winds, and small hail. Once showers decrease overnight we'll have to keep an eye on the amount of clearing approaching the sunrise hours as the recent rainfall and lighter winds will present the ingredients for patchy fog formation. Currently KSLE and KEUG have the best chance for patchy fog between 11-16z with a 15-20% chance for IFR or lower conditions. Higher confidence in widespread VFR CIGs/VIS 17-18z onward. Generally expect light and variable winds inland with southeasterly winds less than 10 kt along the coast.

KPDX AND APPROACHES...This afternoon as of 21z a band of showers and embedded T-storms are approaching the terminal from the south, likely passing to the north by 22-23z. Expect scattered showers into the evening with a additional chances for isolated thunderstorm activity until sunset. While VFR conditions are favored most hours, models do show a 15-20% chance for MVFR conditions 12-16z Saturday. Light winds expected through the TAF period. -99/DH

MARINE

Surface low pressure will continue to weaken as it sits off the central OR coast through tonight. Easterly winds will continue to ease today, becoming more northerly tonight across the northern waters, with light and variable winds expected by Saturday morning. Seas have fallen to around 6 ft at 10 seconds and will continue to subside to around 4 to 5 ft by Friday evening. Also, a 15% chance for thunderstorms exists across the inner waters south of Cape Foulweather Friday afternoon, which could produce lightning and gusty/erratic winds.

Northerly winds return later Saturday 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 5 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

PQR WATCHES/WARNINGS/ADVISORIES

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