textproduct: Portland

SYNOPSIS

A strong frontal system brought gusty wind and heavy rain to the region this morning, with rain expected to persist through the remainder of the day. The next significant weather system is a strong atmospheric river that will arrive on Thursday. High probability of flooding, Cascade snow, and potentially another round of gusty winds. Rain persists through early next week.

LONG TERM...Saturday through Tuesday

The weekend will feel like the calm after the storm but it is anything but that. We will still see continued rainfall but it will be more climatologically normal. South to southwesterly flow will also bring breezy conditions to interior valleys and even the Columbia River Gorge. The component about Saturday to keep an eye on is that we will see continued flooding from the rising that occurred on Thursday and Friday. Many rivers are forecast to be in a flood stage on that day.

As moist zonal flow then continues through the weekend, persistent lowland rain and high elevations snow chances also continue. While the detail remain relative low confidence, current long-range guidance does not favor another high-end rainfall event through the middle of next week. But don't let that catch you off of your feet. 500 mb heights and other synoptic features are suggesting a broad trough taking over much of the northeast Pacific. This trough is associated with a cold wrapped low that will produce weak impulses. These disturbances will then push over the region. While not well realized at this time, rain will persist through Tuesday. -27/36

AVIATION

Post frontal showers this morning will be heavy at times with gusty winds aloft. At around 1000 ft MSL gusts as high as 30 kt have been observed from the west. Due to the unidirectional nature of the wind and the decaying system have omitted LLWS but cannot rule out periods during the heaviest showers. Generally VFR through the day with lingering rain showers. Conditions begin to degrade after 10Z Thursday as a strong atmospheric river approaches. Most of the strong winds and heavy rain will occur after the TAF period, but may experience some periods of heavier rain and gusty winds along the leading edge. There is a 45% chance of MVFR CIGs after 11Z Thu.

Due to a very strong front overnight, several ASOS and AWOS have been left without power and observations may be unavailable.

KPDX AND APPROACHES...VFR through most of the next 24-hrs. Isolated periods of LLWS with post frontal showers behind the strong front overnight. Showery conditions persist through the day with west to southwesterly winds. May see times without rain, but given the overall pattern will be short lived. -27

MARINE

Conditions overnight have been less intense than previously though aside from the wind forecast that remains on track. Overall have seen seas around 14 ft at 10 seconds aside from around buoy 46050 where seas of 17 ft were observed with the passing strong frontal system. One reason could be that the background swell was weaker than forecast and thus the seas were predominately wind wave driven. At this point, not expecting seas above 18 ft and if they do reach those heights, it would be individual and isolated waves. Winds will remain west to northwest with gusts greater than 35 kt through the morning before falling to 15 to 20 kt with gust sup to 30 kt. Seas will continue to be hazardous through the day.

With the incoming strong system gales are once again expected with similar speeds as experienced this morning. A weak low level jet will form Thursday morning and persist through the evening. Gusts will be highest within PZZ252-253. Seas will build to around 14-16 ft. There is around a 5-10% chance of seas of 18-20 ft but confidence is quite low at this time. -27

HYDROLOGY

Potential for river flooding is highest on Thursday and Friday due to the abundant rainfall from an atmospheric river. Heaviest rainfall occurs for a relatively short period of time from Thursday through Thursday night. The already saturated soils and high rivers combined with the incoming well above normal rainfall 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. One difference between the last series of ARs and the one arriving on Thursday is the placement of highest rainfall totals. This next system favors a more southern maximum, and thus areas that previously missed the heaviest rain will see the higher accumulations this time around.

Due to the lack of observed rain, particularly in Lane County, some basins will be more capable of handling heavy rainfall without experiencing 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 high totals with this upcoming system. Similarly, river basins covering the central OR Cascades in Lane and Linn Counties may 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 chances 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 30-70% at this time. Those draining toward the coast, particularly in Lincoln and Lane Counties where rainfall is likely to continue later into Friday may see minor tidal overflow as rivers crests coincide with high tide on Friday, though this occurrence remains lower confidence. 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 remains in effect 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 until 6 AM PST early this morning for ORZ101>103. Wind Advisory until 6 AM PST early this morning for ORZ104>107- 123>125. Winter Weather Advisory until 4 PM PST this afternoon for ORZ126>128. WA...High Wind Warning until 6 AM PST early this morning for WAZ201. Wind Advisory until 6 AM PST early this morning for WAZ202-203- 208. Winter Weather Advisory until 10 PM PST this evening for WAZ211. PZ...Gale Warning until 10 AM PST this morning for PZZ210-251>253- 271>273. Hazardous Seas Warning from 10 AM this morning to 10 PM PST this evening for PZZ210-251>253-271>273.