textproduct: Portland

SYNOPSIS

Showers continue across the region today with a brief break for most areas late this afternoon into this evening. The next frontal system impacts the region overnight tonight through Thursday night, bringing another round of widespread rain and breezy winds. Conditions gradually trend drier Friday into Saturday with lingering showers largely pinned to higher terrain features. Snow levels drop to the passes by late Friday and Saturday but limited accumulation expected due to the showery nature. A cooling trend impacts the region Sunday into early next week.

DISCUSSION...Wednesday Morning through Tuesday

As expected, the band of rain showers slowly moved south and stalled overnight along a weak frontal boundary over northern Oregon, with current radar imagery show it located generally from Newport through the Cascades north of Lane County between the Portland area and Eugene, though drizzle and occasional showers continue to occur outside of the main band. The moisture associated with the showers has been on the lower side, with less than 0.15 inch observed in the lowlands and anywhere from 0.3-0.75 inches along the coast, Coast Range, and Cascades since 8 PM Tuesday night. This band is expected to remain fairly stationary or maybe moving south a little bit more through around 7-10 AM before another low pressure system developing over the eastern Pacific begins moving towards the coast and pushing the rain back to the north through the afternoon and early evening hours. Rain intensity could increase again this morning as another pulse of warm Pacific moisture is fed into the front from the incoming low pressure system. Between 4 AM and 10 PM today, an additional 0.1-0.5 inch of rain is forecast in the lowlands and 0.25-0.75 inch along the coast, Coast Range, and Cascades, except for only up to 0.15 inch in the Lane County Cascades.

Confidence is increasing in a bit of a break in the rain after the current band of rain moves north into Washington and the next front ushers in another round of widespread rain. Ensemble and deterministic guidance continues to indicate that the low pressure system currently developing over the eastern Pacific will move east through the day on Wednesday and making landfall somewhere along the Washington or northern Oregon coast late Thursday. Uncertainty still remains in the exact placement of where the low will move inland, but the timing has come into much better agreement. The cold front associated with this low will approach the coast early Thursday morning and move inland through the day, bringing another round of widespread rain with scattered showers continuing into Friday morning. Rain amounts with this system are expected to be similar to the previous front with 0.1-0.40 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, and Oregon Cascades south of Clackamas County, and 0.75-2 inches for the north Oregon and SW Washington Cascades. Snow levels will remain above pass level with this warm system.

Winds will also increase as the front and low pressure center move inland. Expect winds along the coast to begin increasing late Thursday morning or early afternoon, then transitioning inland late Thursday afternoon into Thursday night as the low finally moves inland. Ensemble guidance indicates peak gusts up to 30-35 mph along the coast and Cascades and up to 20-25 mph for inland valleys. However, there's a 40-60% chance of peak gusts of at least 30 mph for inland valleys and 40 mph along the coast and Cascades. Probabilities of peak gusts of at least 50 mph are around 10-30% along the beaches and headlands of coast, highest between Seaside and Lincoln City. These probabilities are near zero for inland valleys.

Weather on Friday into Saturday looks pretty benign other than slight shower chances continuing. Ensemble guidance is in good agreement of a strong upper ridge beginning to build in the eastern Pacific through the weekend. A good number of ensemble members continue to suggest a weak shortwave will move along the flow over the PacNW Saturday, which is what is causing the slight rain chances. Then this wave is expected to deepen as it moves south as the ridge continues amplifying over the far eastern Pacific, which will allow colder arctic air to funnel into the region. However, there still remains decent uncertainty in the exact location of the shortwave, 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 cold 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 cooling Sunday morning with Monday morning being the coldest. Sunday morning has a 20-50% chance of low temperatures falling below 32 degrees with probabilities increasing to 40-80% for Monday morning. The highest probabilities are 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 10-25% for each day. This is due to an increase in onshore flow that will allow for warmer easterly winds through the Columbia River Gorge combined with the urban heat island effect to keep temperatures above freezing, though still likely to fall into the 30s. When looking at the probability of temperatures falling to 25 degrees (Cold Weather Advisory criteria), most lowland locations are below 10%, though the cold pocked in the Willamette Valley mentioned earlier has around a 15-30% chance.

By Tuesday, ensembles are in agreement that a 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. 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. -03

AVIATION

Moist, westerly flow aloft continues through the TAF period. Widespread MVFR cigs and/or vis with occasional IFR conditions are being observed across the region. These conditions are expected through most of the day. A band of rain showers located between KPDX and KEUG as of 12z Wed will slowly lift north through the day, moving into central Washington by 02-05z Thu. Heavier showers later today could produce periods of IFR conditions. As rain clears from south to north, conditions are expected to improve to VFR, beginning around 20-23z Wed in the southern Willamette Valley (KEUG), around 00-04z Thu in the central valley (KSLE, KUAO), and around 05-08z in the northern Willamette Valley and SW Washington (KPDX and surrounding terminals). Along the coast, expect lowered flight levels to continue through the TAF period. Winds will be from the south, generally less than 10 kts.

Near 12z Thu, another frontal system approaches the coast, bringing another round of widespread precipitation. Winds along the coast could increase as early as 10z Thu, but more likely after 12z Thu, with south gusts up to 20-25 kts.

KPDX AND APPROACHES...Predominately MVFR conditions expected with a 30-40% chance of CIGs falling to IFR after 12-15z Wed. Once CIGs fall to IFR or MVFR thresholds, expect very little to no improvement through 06-89z Thu as light rain continues. East to southeast winds less than 10 kts. -03

MARINE

Winds and seas have decreased this morning in a brief break between frontal systems. Generally north winds less than 15 kts with seas around 7-8 ft at 8-10 seconds are being observed. Winds will shift easterly to southerly as a warm front moves through the waters today, though gusts should remain below 15 kts. South winds begin increasing tonight into Thursday morning as the cold front and surface low pressure system approach the waters. Expecting strongest winds with widespread gusts up to 30 kts between 2 AM to 5 PM Thursday. During this time, there's a 70-85% chance for maximum wind gusts to peak around 35 kt for all marine zones, though hourly probabilities drop to 10-20%, indicating widespread gale force wind gusts are unlikely to occur. Seas will rise Thursday, as well, peaking around 15-18 ft at 12-13 seconds late Thursday afternoon through Thursday evening, though there is a 5-10% chance seas peak as high as 20-22 ft. The uncertainty in exact peak wind gusts and seas is due to some lingering uncertainty regarding the exact strength and track of the closed surface low and the frontal system accompanying the low. Regardless of the outcome for this system, winds and seas will lower significantly on Friday. Winds and seas over the upcoming weekend are currently forecast to stay below small craft advisory thresholds with a period of offshore flow. -03/23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 1 AM to 4 PM PST Thursday for PZZ210- 251>253.

Hazardous Seas Warning from 4 PM Thursday to 2 AM PST Friday for PZZ210-251>253.

Small Craft Advisory from 9 PM this evening to 10 AM PST Thursday for PZZ271>273.

Hazardous Seas Warning from 10 AM Thursday to 2 AM PST Friday for PZZ271>273.