textproduct: Portland

SYNOPSIS

A weak front will maintain light scattered showers and drizzle across northwest OR and southwest WA this afternoon. Tonight, temperatures cool down behind this system, dropping snow levels to 2500-3500 ft, resulting in light snow showers through the Cascade passes. Shower activity decreases on Monday, but can't rule out some isolated lingering showers. The next system arrives Tuesday-Wednesday, returning widespread rain. Wet weather continues through the end of the week, with potential for much colder temperatures this weekend.

SHORT TERM...Today through Tuesday Night

Radar imagery as of early Sunday afternoon depicts light scattered showers/drizzle across northwest OR and southwest WA as a weak front progresses through the region. Rainfall amounts so far in the last 24 hours have generally been less than 0.10" for most places, however, higher amounts of 0.20-0.40" have been observed along the north OR and south WA coasts, and interior southwest WA. This weak front will continue to push southeastward, spreading light showers/drizzle to the southern Willamette Valley and Linn/Lane County Cascades this afternoon. Some drizzle may lead to slight visibility reductions, so make sure to use slow down and leave plenty of distance ahead of you if you're driving.

Shower activity gradually decreases this evening as the front exits the region. Southerly winds will turn more westerly behind the front, but generally remain light for most places. The breeziest areas will be the high Cascades and eastern Columbia River Gorge where westerly winds could gust up to 25-30 mph. Temperatures will also cool down behind this system, making for a chillier Monday morning with snow levels dropping below pass- level (2500-3500 ft). Remaining showers in the Cascades tonight into early Monday morning would fall as snow, with snow amounts forecast under 1" through the passes. Chances for snow amounts exceeding 1" tonight are around 30-40% for Highway 26 and Santiam Pass, and about 5% for Willamette Pass.

Will maintain a 20-40% chance for lingering showers on Monday, but drier conditions return by Monday night as a shortwave ridge builds over the Pacific Northwest ahead of the next system. Tuesday morning will be the chilliest night of the week, with low temperatures forecast to drop into the low to mid 30s across interior lowland valleys. With moist surface conditions, there will be potential for frost development early Tuesday morning, especially over grasses and elevated metal surfaces in outlying, rural areas. One factor that could inhibit frost development is cloud cover. Model soundings show that there will mainly be high clouds, so if these high clouds end up being too thick, then radiational cooling may not be as efficient for frost formation.

By Tuesday afternoon, ensemble guidance suggests that upper level ridging will build over the U.S. West Coast, however, the Pacific Northwest appears to be right at the top of the ridge. While 500 mb heights will climb and bring relatively warmer temperatures, we will be far from dry. The next system will ride the periphery of the ridge and enter the Pacific Northwest, returning widespread rain chances Tuesday afternoon through Wednesday. Since this would be a warmer system with 850 mb temperatures around 2-4 deg C, snow levels will climb above 6000 ft and result in rain over the Cascade passes. Conditions remain showery through Thursday as this system moves through the region. Chances for 48-hour liquid precipitation exceeding 1" from 4 AM Tue to 4 AM Thu are around 40-70% along the coast, Coast Range, Willapa Hills, and Cascades (highest chances further north, lowest toward Lane County), 15-30% across the Willamette Valley and Upper Hood River Valley, and 30-50% across the southwest WA lowlands. -10

LONG TERM...Wednesday through Saturday

The majority of ensemble members favors the potential for wet weather continuing through the end of the week. Ensembles show another system arriving Thursday, but there is uncertainty with the exact track and magnitude of the system, thus bringing uncertainty with precipitation amounts. Chances for 48-hour liquid precipitation exceeding 1" from 4 AM Thu to 4 AM Sat are around 30-50% along the the coast, Coast Range, Willapa Hills, and Cascades and 5-15% across interior lowland valleys.

By Friday, about 65% of ensemble members (mostly from the GEFS) show 500 mb heights falling across the region, suggesting that temperatures will begin to cool down as broad troughing drops down into the Pacific Northwest. There remains high uncertainty with the magnitude and track of this broad trough, but will likely maintain precipitation chances with this set-up. By Saturday, about 80%+ of members are showing below-average 500 mb heights across much of the Western U.S. This is hinting at a more significant cooldown heading into the weekend, but by how much is to be determined.

You may notice in late in the forecast period (Saturday night into Sunday morning) that there is a mention of a rain/snow mix in the lowlands, including the Willamette Valley and southwest WA lowlands. While this is possible, probabilities for this outcome to occur are very low (1-5%). In addition, overall model spread is very large for both temperatures and precipitation amounts. This means forecast confidence is very low, however, we'll be taking a closer look at this over the next few days. Even if a rain/snow mix does occur, accumulating snow and travel impacts in the lowlands would be highly unlikely. -10

AVIATION

Expect a mixed bag of LIFR/IFR/MVFR/VFR flight conditions today and tonight. IFR/MVFR conditions should be most prevalent through 03z Monday, then high-end MVFR/low-end VFR between 03z-09z Monday. Cigs will trend lower once again after 09z Monday. In addition, a slow-moving frontal boundary will continue moving over southwest WA and northwest OR today, bringing periods of light rain and/or drizzle to all terminals much of the time through 00-03z Monday. Precipitation should end from west to east after 03z Monday as the aforementioned front shifts over the Cascades. Surface visibilities are generally ranging between 2 and 3 SM where rain/drizzle is occurring.

KPDX AND APPROACHES...Expect periods of light rain/drizzle and IFR/MVFR cigs to continue through approximately 04z Monday, with surface visibilities fluctuating between 2 and 3 SM. After 04z Monday, there is a 70% chance cigs will temporarily lift to 3500-4000 ft before lowering back to MVFR thresholds or lower around 10z Monday when probabilities for cigs below 2000-3000 ft increase to 60%. Cannot rule out IFR cigs below 1000 ft at that time either (50% chance). -23

MARINE

Seas will remain around 10 to 14 ft through Sunday night before falling to 8 to 9 ft by Monday afternoon and then 6 to 8 ft on Tuesday. The next frontal passage arrives Tuesday night into Wednesday, bringing wind gusts up to at least 25-30 kt with a 30-50% chance for gale force wind gusts up to 35 kt. Confidence in gale force wind gusts materializing is not high enough to issue a Gale Watch at this time. Even if gale force wind gusts to 35 kt do occur, it appears the coverage and duration for wind gusts of this magnitude will be limited. Seas are forecast to peak around 10 ft on Wednesday, but could go higher if gale force wind gusts do materialize. As such, there is currently a 1-5% chance seas will peak as high as 13-15 ft. Seas are forecast to hold steady near 10-12 ft on Thursday as well. -23

BEACH HAZARDS

An energetic westerly swell is expected along the northwestern Oregon and southwestern Washington coasts through Monday. A westerly swell with wave heights around 10-14 ft and a predominant wave period between 14-16 seconds will result in a high sneaker wave threat. These energetic waves can run significantly farther up a beach than normal, including over rocks and jetties, creating potentially life-threatening conditions in the surf zone when beachgoers can be unexpectedly knocked off their feet and pulled out into cold ocean waters.

A Beach Hazards Statement remains in effect through 10 AM Monday to cover the high sneaker wave threat. Caution should be used when in or near the water, and beachgoers with children should be especially watchful. Never turn your back on the ocean, and be sure to keep an eye on the waves. Those participating in razor clam digs should also exercise caution. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 10 AM PST Monday for PZZ210-251>253- 271>273.