textproduct: Portland

SYNOPSIS

A frontal system today and tomorrow will bring rain and breezy winds from the south. There remains a 10-20% chance for thunderstorms through this evening. High pressure re-builds Wednesday and Thursday, returning drier weather for most interior valley locations. Friday through the weekend, a stronger low pressure system may impact the region and bring another round of rain and potentially strong winds.

DISCUSSION...Now through Sunday

Radar imagery and surface observations as of early Monday afternoon depicts a band of widespread rain moving through northwest Oregon and southwest Washington as a frontal system enters the region. We're also experiencing some breezy southerly winds with gusts up to 30-35 mph along the coast and up to 20 mph along the I-5 corridor. Behind the front, satellite imagery reveals scattered/broken cloud cover. A relatively warmer surface with colder air aloft is resulting in increased atmospheric instability, with SPC mesoanalysis showing MUCAPE values of 100-250 J/kg across the offshore waters, central Oregon coast, and southern Willamette Valley. This instability will gradually spread northward over the next few hours as the front progresses northeastward. As a result, there is a 10-20% chance for thunderstorms across the region through 7 PM this evening. Any passing thunderstorms may produce lightning, heavy rain, erratic winds, and/or small hail.

On Tuesday, another upper-level shortwave trough will move into the Pacific Northwest, swinging another front into the area while upper-level ridging begins to amplify east of the Cascades. This ridging will act to limit rain to the coast, Coast Range, and inland areas north of US-20 Tuesday morning, then steadily displace rain chances further north into Washington through the afternoon. A rather sharp gradient in rainfall totals on Tuesday is expected from south to north, with little to no rain south of US-20, less than 0.2 inch for the interior lowlands south of Longview and the coast south of Tillamook, 0.25-0.5 inches for the SW WA Cascades and Tillamook to the state line, and 0.5-1 inch for Pacific County. Heavier rain associated with this atmospheric river is expected over the Olympic Peninsula. Expect another round of breezy southerly winds with gusts as high as 30-40 mph along the coast north of Tillamook and SW WA west of I-5 through Tuesday evening.

Upper-level ridging will reach its peak amplitude on Wednesday, supporting dry and unseasonably warm temperatures across interior valleys with high temperatures forecast in the low to mid 60s. Daily record high temperatures look to remain out of reach for most locations, though the record values of 64F at Portland (PDX) and Hillsboro (HIO) set in 1989 and 66F at Vancouver (VUO) set in 1904, 1907, and 1930 may be in jeopardy. Thursday is expected to be a transition day as upper ridging begins shifting east as upper troughing begins pushing into the northeastern Pacific. Daytime temperatures will lower a few degrees, though will still be unseasonably warm in the low 60s across interior valleys. Mostly dry weather is expected through the day except for a 20-40% chance of showers along the coast, mainly due to uncertainty on specific timing of the rain from the approaching trough.

There is high confidence that an upper-level low pressure system will become situated over the northeast Pacific west of Canada from late Thursday/Friday into the weekend, returning troughing to the Pacific Northwest. Multiple surface frontal systems are expected to move through the region during this time, bringing repeated chances for widespread rain and breezy winds. There is still uncertainty with specific rainfall amounts and peak wind gusts; however, we can say for certain that it will be wet. This is shaping up to be another atmospheric river event as long- range ensembles indicate a 60-70% chance for vapor transport to exceed 500 kg/(ms) at the leading edge of this trough on Friday somewhere along the Oregon coast with higher probabilities of vapor transport values remaining above 250 kg/(ms) through Saturday. Chances for rain amounts exceeding 2 inches from Friday morning through Sunday morning are 20-30% along the I-5 corridor and 60-80% for the coast and Coast Range. Additionally, the coast and Coast Range have a 20-40% chance of total rain amounts exceeding 3 inches of rain for the same time period. Increasingly saturated soils from multiple weather systems could allow for greater runoff into area rivers, meaning at least isolated flooding cannot be ruled out. People should also clear gutters and storm drains of any leaves or debris to prevent urban flooding. Across the Cascades, it appears that this system would be warm enough to keep snow levels between 5500-6500 feet, keeping the passes snow-free.

When it comes to winds, there is a 20-40% chance for maximum wind gusts exceeding 60 mph along the coast Friday and Saturday, with the highest chances on Friday. Wind gusts of this magnitude could result in scattered or widespread power outages or tree damage. Along the I-5 corridor, there is a 20-40% chance for maximum wind gusts exceeding 40 mph, which could result in isolated power outages or tree damage. To prepare, make sure to secure loose outdoor items. -10/03

AVIATION...Through 0Z Wednesday

Post-frontal showers will maintain a mix of VFR and MVFR conditions this evening. There is a 10-20% chance for thunderstorms along the coast this afternoon and inland this evening. There will be a brief break in rain late tonight. Cloudy skies should hinder radiation fog from developing. Winds will increase along the coast Tuesday morning with the next front. Rain with periodic MVFR CIGS for the coast and terminals north of Salem on Tuesday. There is a 10-20% chance for thunderstorms near the SW Washington coast Tuesday morning.

KPDX AND APPROACHES...Mostly VFR conditions tonight with rain showers decreasing between 06-09z. Rain will return Tuesday morning with a 20-30% chance for MVFR CIGS at any hour. Easterly winds will dominate and generally remain below 10 kt. -11

MARINE

Small craft conditions across the water tonight in the wake of a front. Winds will increase Tuesday and Wednesday as a front moves across the waters. There is a 50-90% chance for gales north of Cape Falcon where a Gale Warning has been issued. The winds south of Cape Falcon will either be low-end gales or high- end small craft and have issued a Gale Watch. Marginal gales continue Wednesday as a cold front moves into the waters. A building west swell will result in 14-18 ft seas at around 14 seconds.

A stronger frontal system will increase winds and seas on Friday. with a 10-20% chance for Storm force winds on Friday and a 60-80% chance of 20+ ft seas on Saturday. -11

BEACH HAZARDS

A moderate to high sneaker wave threat continues along the coast through this week as a westerly swell of 10-12 ft at 13-15 seconds increases to 14-16 ft midweek. This will result in waves running up significantly farther on a beach than normal, including over rocks and jetties. Sneaker waves can suddenly knock people off of their feet and quickly pull them into the frigid ocean which may lead to serious injury or drowning.

There is also the potential for tidal overflow flooding Wednesday and Thursday as the tide plus anomaly are forecasted between 10 and 11 ft (MLLW). This could lead to further water inundation and coastal erosion.

Additionally, confidence is increasing that a stronger westerly swell of 20-24 ft with a dominant period at 16-18 seconds will impact the coast on Saturday for dangerous surf conditions. Destructive waves may wash over beaches, jetties, and other structures unexpectedly.

Extreme caution should be used if you plan to visit beaches this week. The large wave runup may dislodge logs on area beaches and result in them rolling up and down area beaches, which have historically killed or seriously injured unsuspecting people, so there will be much more of a threat than just getting wet from a rogue wave. -11

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Gale Warning until 4 PM PST this afternoon for PZZ210-251>253- 271>273.

Gale Warning from 6 AM Tuesday to 4 PM PST Wednesday for PZZ210- 251-271.

Small Craft Advisory until 6 AM PST Tuesday for PZZ210-251>253- 271>273.

Gale Watch from Tuesday morning through Wednesday afternoon for PZZ252-253-272-273.