textproduct: Portland

SYNOPSIS

Wet conditions continue through Thanksgiving as another frontal system impacts the region, bringing additional widespread rain and breezy winds. Conditions gradually trend drier Friday into Saturday with showers largely pinned to higher terrain features. Snow levels drop to the passes by Saturday but limited accumulation is anticipated. A cooling trend impacts the region Sunday into early next week with additional chances for precipitation arriving on Tuesday.

DISCUSSION...Thursday morning through Wednesday

The forecast remains on track from the previous forecast package with very little change. Satellite imagery early Thursday morning depicts a compact low pressure system located around 400 miles west of the WA/OR border with a cold front approaching the WA and OR coast. Radar imagery shows the band of rain associated with the cold front moving inland along the coast. The cold front will move inland through the morning hours, reaching the Cascades by midday, as the closed low approaches the coast. Ensemble guidance is in very good agreement that the low will approach the coast right around or just north of the WA/OR border late this afternoon, then slowly weakening and dissipating over the coastline tonight. Widespread rain will push east of the Cascades by midday with conditions turning showery for the afternoon and evening, continuing into Friday morning as the upper trough associated with the frontal system exits the region. Rain amounts with this system through Friday morning will be similar to previous weather systems. Expect 0.15-0.45 inch for the central and southern Willamette Valley, 0.4-0.75 for the northern Willamette Valley into the SW Washington lowlands, 0.5-1.5 inches along the coast and Coast Range as well as the Oregon Cascades south of Clackamas County, and 0.8-2 inches for the north Oregon and SW Washington Cascades. Snow levels will remain just above pass level with this second system, as well.

Winds are expected to be the main impact with this system. As the front moves inland, winds will begin increasing along the coast this morning and inland this afternoon. Strongest winds will be late this afternoon through this evening as the low center reaches the coast and pressure gradients tighten east of the low center. Some discrepancies remain between the various sets of guidance as to just how breezy winds will get. The NBM remains lower than high resolution ensembles like the HREF and REFS. Given that winds aloft are expected to be elevated (~40-50 knots at 850 mb) with neutral to slightly unstable conditions present in the boundary layer to facilitate vertical mixing Thursday afternoon, nudged wind/wind gust values towards the high resolution ensemble means. With this in mind, expect peak gusts of 35-45 mph along the coast and Cascades crests, and up to 25-35 mph for inland valleys (highest Salem to Portland). However, it's worth highlighting there is a 10-25% chance for isolated peak gusts to exceed 45 mph in the north Willamette Valley into the Portland/Vancouver late afternoon and evening which would cause additional impacts. We'll continue to watch the progression of this low pressure closely. There is high confidence that winds decrease late tonight into Friday.

Ensemble guidance remains in good agreement that a strong upper ridge will build in the eastern Pacific Friday afternoon through the weekend, though a shortwave will move along the flow on Saturday, producing a slight chance of showers, mainly over the terrain. Then cold air is likely to funnel into the PacNW behind this shortwave.

However, uncertainty in the exact location of the shortwave continues amongst the ensembles, whether it will move over western WA and OR or more to the east in eastern WA, OR, and into Idaho. This will affect exactly how much cooler air will funnel into NW Oregon and SW Washington. If the wave is more to the west, more colder air will move into NW Oregon and SW Washington. If the wave is more to the east, the coldest air will remain east of the Cascades. Latest NBM guidance indicates less cooling on Sunday morning then previous forecasts with mainly a 20-40% chance of low temperatures falling below 32 degrees except for up to 50% chance in the Willamette Valley between Corvallis and Eugene. Monday morning still looks to have the coolest temperatures with a 40-80% chance of low temperatures falling below 32 degrees, with highest probabilities in typical colder spots outside of urban areas, especially in the Willamette Valley between Corvallis and Eugene. One thing to note is probabilities of sub- freezing temperatures around the Portland metro area are only around 5-15% for each day. Probability of temperatures falling to 25 degrees (Cold Weather Advisory criteria) have also decreased with most lowland locations below 5%, and the cold pocked in the central/south Willamette Valley mentioned earlier around a 10-25% chance.

By Tuesday, ensembles remain in agreement that a slightly stronger shortwave and associated frontal system will slide south from western Canada into the PacNW, breaking down the high pressure over the region and returning precipitation chances over the area through Wednesday. Quite a bit of uncertainty exists in the strength of this system and the impacts associated with it, though cold weather could stick around in the wake of it. -99/03

AVIATION

At 10z Thu, observations indicate that the fog and stratus that formed in the Willamette Valley during the break between fronts continues with conditions fluctuating between MVFR/IFR/LIFR cigs and vis. Additionally, radar imagery shows a widespread band of rain moving inland along the coast. As this rain continues moving east, reaching inland terminals by 11-13z Thu, the fog and stratus should mix out. However, conditions trending predominately MVFR through the day Thursday, though periods of IFR cigs are possible as the band of rain moves over terminals between 11-15z Thu. Southerly winds are also expected to increase today as a low pressure system over the eastern Pacific moves closer to the south Washington coast, with wind gusts up to 35 kt along the coast after 15z Thu and up to 25-30 kt inland after 20z Thu.

KPDX AND APPROACHES...Predominately MVFR conditions continue through the period with the return of rain by around 11-12z Thu. Brief periods of IFR cigs are possible in heavier rain through 15-16z Thu. Southeast winds around 10-14 kt expected to increase through Thursday, with gusts up to 25-30 kt after 20z Thu. -03

MARINE

A closed low pressure system currently located about 450 miles west of the mouth of the Columbia River will move toward the south Washington coast through Thursday evening, causing winds to increase during the day. An initial cold front is expected to move across the coastal waters Thursday morning, with isolated wind gusts up to 35 kt. More widespread gales are now expected behind the front as the closed low pressure system moves through the waters. South to southwesterly winds will continue increasing through Thursday afternoon, with wind gusts up to 40 kt likely. Seas will also increase through the day, becoming steep and hazardous Thursday afternoon through evening, peaking around 15 to 18 ft at 11-13 seconds. There remains around a 10% chance that significant wave heights reach 20 ft. Gale warnings are in place for all waters later Thu morning into Thu afternoon, and transition into hazardous seas to cover the large fresh swell that will move into the waters later Thursday. Conditions ease late Thursday night into Friday as the low weakens, with seas expected to fall below 10 ft by Friday afternoon. Hazardous conditions are not expected through the weekend as weak high pressure builds over the waters. /DH/03

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 11 AM PST this morning for PZZ210- 251>253.

Gale Warning from 11 AM this morning to 6 PM PST this evening for PZZ210-251>253.

Hazardous Seas Warning from 6 PM this evening to 2 AM PST Friday for PZZ210-251>253-271>273.

Small Craft Advisory until 7 AM PST this morning for PZZ271>273.

Gale Warning from 7 AM this morning to 6 PM PST this evening for PZZ271>273.