textproduct: Portland

SYNOPSIS

A prolonged wet pattern continues through early next week as multiple frontal systems push inland. The most notable period continues to focus on Monday through midweek, when a prolonged atmospheric river may bring significant rainfall and rising rivers across southwest Washington and northwest Oregon. Today and Sunday bring lighter rainfall, while Monday is expected to produce the highest totals across southwest Washington and northwest Oregon.

Discussion...Now through Friday

Weather will remain active through at least the middle of next week. Upper level zonal flow will keep conditions dreary through Sunday with another shortwave disturbance passing this evening and overnight. This will bring a decent round of rain to the area as IVT values increase to between 250-500 kg/ms. Rainfall with this system is expected to be around 0.5-0.75 inches across the interior lowlands, 0.75-1.5 inches along the coast, 1.5-3.0 inches along the coast range and Cascades with the exception of the Lane County Cascades which will be lower at 0.75-1.5 inches. Widespread impacts are not anticipated through Sunday; however, if rainfall rates sustain around 0.2 to 0.3 inches per hour for several hours, a few faster-reponding basins, such as the Grays at Rosburg and the Willapa River, could rise quickly. Breezy southerly to southwesterly winds are expected at times with gusts typically 30-40 mph, strongest near exposed coastal and elevated terrain. This system will be a precursor to an active few days where several inches of rain could fall across NW OR and SW WA, increasing the potential for river and urban flooding.

The synoptic set-up for the week starts with a ridge of high pressure centered over the Pacific between Hawaii and southern California. Clockwise circulation will advect sub-tropical moisture northward toward the PacNW in several round between Monday and Wednesday night. Mid level flow will predominantly be zonal as the top of the ridge extends northward along the West Coast. At the same time, several troughs will support surface lows and cold fronts that will provide lift over the region. The first slug of AR moisture enters the picture on Monday with IVT values along the coast peaking above 750 kg/ms and values inland peaking between 500-750 kg/ms. The heaviest rainfall with this first round is expected to fall between Monday and Tuesday morning. 45% of NCEP Ensemble members show rainfall totals between 4 pm Monday and 4 am Tuesday reaching or exceeding the 10 year Average Return Interval for the Portland area. This indicates that the expected rainfall totals during a 12 hour period has a 10% chance of occurring in any given year. This is notable given the expected widespread heavy precipitation and potential urban flooding impacts. This round of rain will be falling on already saturated soil which has prompted WPC to add a Day 3 Slight/Marginal ERO risk over the CWA. The slight risk covers all of SW WA, down the coast rang in OR and along the Cascades down to around Santiam pass. rainfall totals from 4 am Monday through 10 am Tuesday are expected to be 1.5-2.5 inches across the interior lowlands, 3.0-5.0 inches along the coast and coast range, and 3.0- 6.0 inches along the Cascades and Cascade Foothills. Amounts may be slightly lower farther south, mainly across Lane county. These rainfall amounts will lead to significant rises on area rivers, especially in the coast range.

The second round of high IVT values is expected to impact the region Tuesday night into Wednesday. IVT values are expected to be slightly lower than the first wave, peaking around 500 kg/ms both inland and along the coast. However, the duration of these higher values is expected to be slightly longer. Rainfall totals from late Tuesday through Wednesday night are expected to be slightly lower across much of the area except for the Cascades where totals are forecast to be slightly higher. As it sits now, rainfall totals from early Monday through Wednesday night will significant and impactful. While the chances for widespread impacts are rather low, it is almost certain that localized impacts will occur somewhere within the CWA.

Winds will be a concern Monday through Wednesday, with gusts up to at least 30 mph inland and up to at least 40 mph at the coast. There is a 10-20% chance for max wind gusts over 40 mph for inland areas, and a 20-40% chance for gusts over 50 mph along beaches and headlands. If wind gusts of this magnitude do materialize, expect scattered downed trees and power outages.

Showery conditions will linger into Thursday and Friday as the flow gradually weakens and dries. Snow levels are expected to remain high (above 6000 ft) during this warm weather event, keeping most precipitation as rain in the Cascades. Forecast confidence in exact timing and magnitudes is still moderate given ensemble spread. Overall, the Monday through Wednesday time period remains the primary focus for potential high-impact hydrologic (see hydrology section below for more details) and wind concerns. -19/12

AVIATION

A showery warm front will move over the area over the next 24 hours. Showers continue within the vicinity of the area with a mixture of flight categories. These showers will intensify after 10Z Sunday causing CIGs to become widespread MVFR, and winds to increase. The combination of wind and rain will cause visibility to decrease, though how limited is unknown - ultimately it will come down to rain rates. Given the south to southwesterly winds amplifying, CIGs will hover right around the MVFR/VFR threshold before decreasing to solid MVFR through the day. Wind will ease towards the end of the forecast period.

Do want to note for those flight planning, a strong atmospheric river system is going to move into the area on Monday. This system will be accompanied by strong wind and very heavy rain.

KPDX AND APPROACHES...Exact timing is difficult for PDX as there are a variety of factors that must come into play in order to get any one cat level. Overall will see a trend towards MVFR after 10Z Sun through at least 20Z. A diurnally driven weak easterly wind will aid in clearing conditions temporarily. If they do clear, the dewpoint depression will shrink and stratus may lower to IFR levels. There is around a 20-30% chance of that occurring at some point between 12-19Z Sun. With 30 kts of SSW wind at 2000 ft and 5-10 kt winds out of the SE to E at the surface, there will be a period of modest low level wind shear values as we approach 18z Sunday at KTTD and KPDX. Have included LLWS in the TAF at this point but if the easterly winds at the surface shift to the south then the risk will be far less. -27

MARINE

Seas will continue to slowly subside over the next 12-24 hours to less than 10 ft. A warm front will lift northeastward across the area late tonight into Sunday morning and result in southerly winds increasing across the waters. HREF and NBM guidance suggest there is at least an 80% chance that low end Gale Force wind gusts will occur across the waters south of Cape Falcon. Seas will temporarily rise and become more wind dominated due to these winds. Winds will then subside and result in decreasing seas late Sunday before the next warm front lifts northeastward across the waters. This suggests there is a 70% chance for stronger Gale Force winds to materialize across the waters Monday. This will push seas into the mid teens Monday afternoon with a 10-20% chance of seas climbing to at least 17-18 by 4pm Monday. Additional rounds of gusty southerly winds are in store across the waters next week, but there is around 40-50% chance for brief Gale Force wind gusts across the waters Tuesday and Wednesday. Southerly winds continue through the end of next week, but there is 10% chance or less of Gale Force winds materializing across the waters next Thursday, Friday and Saturday.

HYDROLOGY

A series of frontal systems will keep conditions very wet through the weekend and much of next week. The most notable period will be late Monday through late Wednesday, when a prolonged atmospheric river will bring significant rainfall and the potential for widespread river flooding across southwest Washington and northwest Oregon. Total rainfall amounts from 4am Monday to 4am Thursday (72 hour total) will most likely be around 4 to 6 inches at the coast and interior lowlands, and 5.5 to 9.0 inches in the Coast Range and Cascades. Lane County Cascades will be an exception, with 4-6.5 inches expected. There is a 10% chance for rain amounts up to one foot in the Cascades and Coast Range, up to 8-9 inches at the coast, and up to 7 inches in the Willamette Valley, Portland metro and Cowlitz Valley. While rain amounts this high are unlikely to occur, they represent a reasonable worst case scenario. This outcome would result in widespread major flooding.

Although rain will likely become lighter Thursday into Friday, the threat of flooding will linger along slow responding rivers. Rivers draining the Coast Range and Willapa Hills have a 30-60% chance of reaching moderate flood stage and a 20-40% chance of reaching major flood stage. Additionally, many Willamette River tributaries that drain from the Coast Range and Cascades have anywhere from a 40-70% chance of reaching minor flood stage and a 20-50% chance of reaching moderate flood stage. Flooding of the Willamette mainstem rivers is unlikely to occur (less than 5%). Probabilities for specific river points can be found at the National Water Prediction Service website.

There is also increasing confidence in the potential for flooding along small creeks and streams as well as urban flooding, including roadway flooding. Periods of heavy rain will also increase the risk for landslides in areas of steep terrain and debris flows over recently burned areas. People, structures, and roads located below steep slopes, in canyons, and near the mouths of canyons may be at serious risk from rapidly moving landslides.

Considering the probabilities of flooding for rivers and urban areas, a Flood Watch has been issued for all of southwest Washington and northwest Oregon from 4 AM Monday to 4 AM Friday.

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 4 PM PST Sunday for PZZ210-251-271. Gale Watch from Monday morning through Monday evening for PZZ210-251>253-271>273. Small Craft Advisory until 3 AM PST Sunday for PZZ252-253-272- 273. Gale Warning from 3 AM to noon PST Sunday for PZZ252-253-272- 273.