textproduct: Portland

SYNOPSIS

A continued active pattern will bring multiple weather hazards through the next week including heavy rain, urban, small stream, and river flooding, gusty winds, Cascade snow, and hazardous seas.

LONG TERM...Thursday through Monday

Another moderate to strong atmospheric river takes aim at the region Thursday into Friday, bringing additional rain on top of elevated rivers and saturated soils. Initially, heavy rain rates raise the risk for urban or small creek and stream flooding on Thursday, while the risk for river flooding increases late Thursday into Friday as the abundant runoff makes its way downstream. As is typical with ARs, the highest precipitation totals are likely to be over the coastal terrain and across the Cascades, and the warm dynamics will also favor increasing snow levels during the heaviest precipitation. Rainfall forecasts have tended to shift slightly between forecast updates, but guidance remains in good agreement that the AR will impinge on the northern OR coast before tracking southward toward the southern OR coast by Thursday night. Unlike recent ARs which have been directed moreso at the Washington coast, the highest rain totals are currently focused across the southern half of the region, generally south of US-26. Chances to exceed 2" of rainfall Thursday and Thursday night increase from 30% to 50% south along the I-5 corridor from Kelso/Longview to Eugene, and from 50% to 75% in the Coast Range and Cascades. Chances to exceed 4" of rainfall are 5-15% across inland valleys and 20-40% in areas of higher terrain.

This amount of precipitation over only 24 hours would support river flooding as saturated soils will allow most of the rain to make its way into area rivers as runoff. Chances for Minor to Moderate river flooding continue to increase, with the highest confidence in flooding for rivers draining the central OR Coast Range, both toward the coast and into the Willamette River. Those who live near areas prone to river flooding should keep a close eye on the forecast moving forward, and additional information can be found in the Hydrology discussion below.

The veritable parade of weather systems does not look to let up this weekend and into next week. While confidence in forecast details is low at this lead time, the active pattern and persistent chances for rain will most likely continue. -36

AVIATION

Late this evening lingering shower activity is primarily hugging higher terrain features across the region with a mix of VFR to MVFR conditions at inland terminals. Headed through tonight into Tuesday morning the arrival of a weak upper- level impulse will usher in a period of increased showers and higher chances for widespread MVFR conditions between ~09-17z before it departs and showers generally decrease into the afternoon. For the central Oregon coast (KONP), high confidence (60-80% chance or greater) IFR/LIFR conditions linger much of the TAF period. Our next weather disturbance begins to increase rainfall across the region in the last few hours of the TAF period (03-06z Wed) south to north with increasing chances for wholesale degraded conditions soon thereafter.

Winds across the area remain weaker overnight inland but will become breezy again after by Tuesday afternoon and evening with the next system - gusts to at least 20-30 kt across the region. It's worth highlighting much stronger winds developing aloft (50-55 knots at 2kft) between 03-06z Wed along the coast will present some low level wind shear concerns. Winds continue to ramp up headed through Tuesday night.

KPDX AND APPROACHES...Currently sitting with VFR conditions as of 06z with similar CIGs/VIS expected over the next few hours. Chances for MVFR CIGs rise to 50-60% around 10-12z as showers increase before a return to VFR late morning into the afternoon. Still will hold onto a 20-30% chance to MVFR conditions the rest of the day. -99

MARINE

Observations at buoy 46029 and 46050 as of early Monday afternoon show southerly winds with gusts to 30-35 kt and seas hovering around 14-16 ft at 9-10 sec. Should be seeing seas peak around 16-18 ft this afternoon with seas unlikely rising above 20 ft (only a 1-2% chance of exceeding 20 ft). High confidence that winds will gradually weaken this afternoon, decreasing to 10-15 kt with gusts up to 20 kt late this evening. Seas may remain steep and choppy during this brief lull; therefore, a Hazardous Seas Warning is in effect across all waters including the Columbia River Bar from 4 PM this afternoon to 4 AM Tuesday. Seas fall to 10-13 ft by late Tuesday morning.

The brief lull ends early Tuesday morning when another frontal system approaches the waters. This will return breezy south- southwesterly winds with widespread gusts ramping up to 25-30 kt between 4 AM-10 AM Tue, then 35-40 kt between 10 AM-4 PM Tue, and strong Gales with gusts up to 48 kt arriving after 4-7 PM Tue. For the inner waters out 10 NM including the Columbia River Bar, there is a 20-30% chance for isolated Storm force wind gusts up to 55 kt due to the potential for a coastal jet. It's a marginal set-up for a coastal jet since guidance isn't really showing much of an inversion in the low-level atmosphere to squeeze the strong winds toward the surface. Either way, it will still be very windy. Behind the front, winds turn westerly around 1-4 AM Wednesday. Seas will also re-build to 14-16 ft Tuesday evening, and peaking and holding around 18-20 ft at 10-11 sec between 1 AM-1 PM Wednesday. Another Gale Warning will likely be issued for this system this afternoon.

Going forward, an active weather pattern continues through the week bringing additional winds and wave hazards to the coastal waters. Seas are very likely to remain elevated above 10 ft (greater than 90% confidence) from Wednesday through Friday as additional weather systems move onshore. At least models are beginning to hint as some larger breaks between weather disturbances next weekend into the following week, but at this 7-8 day time-scale, the confidence in this slowdown materializing is only low to moderate at this time. -10/27

HYDROLOGY

Abundant rainfall is a relatively short period from Thursday through Thursday night atop saturated soils while area rivers remain high following well above normal rainfall thus far in December has resulted in an elevated risk for flooding late this week. As is typical in atmospheric rivers, the highest rainfall totals are likely to be over areas of terrain including the Coast Range and Cascades, while lesser totals can be expected across inland valleys. Nonetheless, rainfall is likely to be measured in inches across most of the region. Unlike recent other ARs, this system looks to favor a more southern placement along the central OR coast of these highest rainfall totals, which may allow some river basins to better handle these rainfall amounts without yielding flooding. Other areas to the north within the Coast Range in Lincoln, Tillamook, Benton, Polk, Yamhill, and Washington Counties may see more overlap between recently observed heavy rainfall and additional heavy rainfall with this upcoming system. Similarly, river basins covering the central OR Cascades in Lane and Linn Counties have a slightly more optimistic outlook than north into Marion and Clackamas Counties, but that does not mean flooding cannot happen in all of the aforementioned areas. Luckily, where rainfall has so far been the heaviest across southwestern Washington and far northwestern Oregon may see the lowest totals from this system, although even 1-2 inches of rain over 24 hours may be enough to cause additional flooding in these very saturated areas.

At this point, the rivers with the highest confidence of reaching flood stage are those which drain the central Oregon Coast Range, including the Wilson, Trask, Siletz, and Siuslaw draining toward the coast, and the Marys, Luckiamute, Yamhill, and Tualatin draining toward the Willamette. The chances for these rivers to reach at least Minor flood stage are 25-55% at this lead time. Rivers draining the Cascades, including the Mohawk, Santiam, Pudding, Molalla, Clackamas, and Sandy, have slightly lower but still significant chances of 15-45%. These rivers will begin to rise later on Thursday as heavy rain continues, not cresting until Friday in most cases. The Willamette River main stem itself also has a 5-10% chance of reaching flood stage above Willamette Falls, and slightly higher chances in tidally-influenced portions downstream of Oregon City, however runoff will take longer to reach the Willamette and flooding would be delayed later than in its tributaries. A Flood Watch has been issues across all of northwestern Oregon and southwestern Washington from 4 AM Thursday through 4 AM Saturday to highlight these risks.

These river forecasts remain very sensitive to rainfall forecasts, and interested parties should expect changes as the event nears. The latest river forecasts across the region can be found at www.water.noaa.gov/wfo/pqr. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...High Wind Warning from 4 PM Tuesday to 6 AM PST Wednesday for ORZ101>103. Wind Advisory from 4 PM Tuesday to 6 AM PST Wednesday for ORZ104>107. WA...High Wind Warning from 4 PM Tuesday to 6 AM PST Wednesday for WAZ201. Wind Advisory from 4 PM Tuesday to 6 AM PST Wednesday for WAZ202-203. Winter Weather Advisory from 10 PM Tuesday to 10 PM PST Wednesday for WAZ211. PZ...Hazardous Seas Warning until 4 AM PST Tuesday for PZZ210- 251>253-271>273. Small Craft Advisory from 4 AM to 4 PM PST Tuesday for PZZ210- 251>253-271>273. Gale Warning from 4 PM Tuesday to 10 AM PST Wednesday for PZZ210-251>253-271>273.