textproduct: Portland

SYNOPSIS

Mostly dry weather today, though skies likely remain cloudy through at least this morning. Wetter and more active weather returns later Friday. Additional chances for rain late Saturday afternoon into Sunday, and Monday through the middle of next week. South winds become breezy later Saturday afternoon and evening.

DISCUSSION...Now through Wednesday

Satellite and radar imagery early Thursday morning shows scattered light rain showers pushing east and decreasing this morning while a mix of clearing skies and developing low stratus is occurring inland. Brief upper ridging today will see dry conditions continue through tonight, though skies likely remain somewhat cloudy through the afternoon.

A progressive pattern through the next week will bring repeated frontal systems into the Pacific Northwest, although light precipitation amounts are not expected to yield widespread flooding concerns. The next occluding cold front arrives early Friday morning. This weak system will result in less widespread rain, although the areas which do see more persistent rain are likely to end up with higher totals than the first system. Expected rainfall amounts are a half inch or more along the Clatsop and Pacific County coasts, between a tenth and a quarter inch elsewhere on the northern and central Oregon coast, near Portland, and across inland southwest Washington, less than a tenth of an inch elsewhere at low elevations inland, and minimal amounts in the Oregon Cascades.

Another brief period of upper-level ridging with southwest flow aloft will see temperatures warm into the 60s on Saturday with largely dry weather across the region through the morning. Yet another frontal system will bring widespread higher rainfall totals from late Saturday afternoon into Sunday. While remaining relatively progressive, vapor transport aloft is likely to peak in the "moderate" range for atmospheric rivers. While this may produce heavier rainfall rates, the short duration due to the north-south orientation will limit total accumulations from becoming too impactful. Rainfall amounts on the west slopes of area terrain may reach a half inch from Saturday evening through Sunday evening (70- 90% chance), those along the Willamette Valley are much less likely (15-30% chance). Given the warm nature of this system, snow levels are likely to remain well above pass level. Will also mention a chance for strong to high winds along the north Oregon and south Washington coast late Saturday afternoon when there is around a 10- 30% chance for southerly wind gusts exceeding 58 mph, with the more likely scenario showing gusts along the coast of around 40-50 mph.

Long-range guidance continues to support a fourth system affecting the region during the midweek period, but uncertainty in the evolution of this system, in terms of both location and intensity, remains high. There is potential for both higher vapor transport aloft as well as a longer duration of resulting precipitation over the region, which could combine to support more substantial rainfall accumulations. Given antecedent rainfall from earlier systems, some rise in background river levels is expected which could prime area rivers for potential flooding if rainfall amounts are high enough. A period of strong south-southwest winds may also occur at the frontal passage, with chances for peak wind gusts of 40 mph or higher reaching 30-40% along the coast and 5-15% along the I-5 corridor. 36/DH

AVIATION...Through 12Z Friday

Northwest flow aloft today as high pressure builds over the area. Satellite imagery and surface observations from 11Z Thu depict a mix of LIFR, MVFR and VFR conditions across the air space. Scattered light rain showers are diminishing, being replaced with low stratus developing through the Tualatin and northern Willamette Valley. Given increasing cloud breaks with light winds and wet ground conditions, areas of fog and low stratus are expected to continue develop, resulting in fluctuating ceilings and visibilities with a complicated mix of LIFR/IFR/MVFR flight conditions. The lowest ceilings and visibilities are expected to occur between 10-18Z Thursday before conditions improve to MVFR/VFR thresholds thereafter. VFR conditions are favored after 20z Thu.

KPDX AND APPROACHES...MVFR stratus has spread across the Portland Metro area and is expected to persist with CIGs gradually lifting through 17-18z Thu. If more clearing occurs, then potential for fog development and reduced visibility increases. Conditions will likely improve back to VFR after 18-20z Thu. Winds are expected to remain light and variable. /DH

MARINE

An active pattern continues through the weekend as multiple frontal systems move across the waters. Light westerly winds this morning turn southerly again ahead of the next approaching system, while a westerly swell builds seas to around 9 to 11 ft at 13 seconds. South winds increase late tonight through Friday as the front pushes closer. A Gale Warning is in effect Friday morning through afternoon for coastal water zones north of Cape Falcon with gusts up to 40 kt expected. Steep and hazardous seas will also be possible later Friday as significant wave heights likely build to 14 to 17 ft.

Winds ease briefly, but a stronger front is expected to quickly approach the area later Saturday, bringing more widespread chances for Gales (70-90%) across the coastal waters. There is also a 20-30% chance for brief Storm Force wind gusts north of Cape Falcon. Another westerly swell is expected to build seas into the mid to upper teens, with around a 25-45% chance of wave heights exceeding 20 ft. The active weather pattern likely continues into early next week. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 7 AM this morning to 7 AM PST Friday for PZZ210.

Gale Warning from 7 AM to 4 PM PST Friday for PZZ210-251.

Small Craft Advisory until 7 AM PST Friday for PZZ251.

Small Craft Advisory until 4 PM PST Friday for PZZ252-272.

Small Craft Advisory from 7 AM this morning to 4 PM PST Friday for PZZ253-273.

Small Craft Advisory until 4 AM PST Friday for PZZ271.

Gale Warning from 4 AM to 4 PM PST Friday for PZZ271.