textproduct: Portland

SYNOPSIS

A shortwave trough exiting the region will maintain isolated showers across northwest Oregon and southwest Washington this afternoon. A deeper trough shifts southward into the region Friday to Saturday, increasing onshore flow and returning more widespread chances for precipitation. Slight chances for thunderstorms Friday and Saturday afternoon. Drier weather returns Sunday before another trough returns chances for precipitation early next week.

SHORT TERM...Today through Friday night

Radar imagery as of early Wednesday afternoon depicts scattered showers across northwest Oregon and southwest Washington as an upper-level shortwave trough swings through the region. This trough will quickly exit eastward this afternoon, leading to decreasing showers through this evening. Following this quick-moving system, dry weather and sunny skies return on Thursday as dry westerly flow sets up over the region. Westerly winds at the surface will generally be light tomorrow, except for the central Columbia River Gorge and Upper Hood River Valley where westerly winds will be breeziest and gust up to 30 mph.

On Friday, a cooler and wetter pattern returns as a deeper trough drops approaches the Pacific Northwest from the Gulf of Alaska. As this upper trough moves in, it will push a weak front into northwest Oregon and southwest Washington. This will return more widespread chances for precipitation, with the highest chances over southwest Washington and the north Oregon coast, Coast Range, and Cascades. Given the showery nature of this system, rainfall amounts will vary across the area and the highest amounts will ultimately depend on where showers set-up. Overall, showers will be non-impactful. Chances for 24 hour precipitation exceeding 0.25 inch from 5 AM Friday to 5 AM Saturday are around 60-80% from Tillamook to the south WA coast, 50-70% across the south WA and north OR Cascades, 20-30% across the Cowlitz Valley, and 5% or less for the Willamette Valley and Upper Hood River Valley. Given the northern track of this trough, chances for precipitation are highest as you go further north. In addition, there is a 10-15% chance for thunderstorms across these areas as instability increases due to cold air aloft along with sufficient lift from the trough. Any passing thunderstorms could produce lightning, brief heavy rain, erratic winds, and/or small hail.

Onshore flow will also increase as this upper trough moves in. West-southwesterly winds won't be too impactful across most areas with gusts up to 20 mph along the I-5 corridor, however, typical breezy spots during onshore flow like the central Gorge and Upper Hood River Valley could gust up to 35-40 mph. There is also a 5-15% chance for isolated wind gusts exceeding 45 mph through these areas as well, with higher chances (25-35%) for higher terrain and exposed ridgetops. -10

LONG TERM...Saturday through Tuesday

The vast majority of ensemble members are in agreement that the aforementioned upper- level trough will shift over the Pacific Northwest on Saturday, maintaining rain showers across northwest Oregon and southwest Washington through Saturday afternoon. There still remains some uncertainty on exact rainfall amounts due to the showery nature of the precipitation. This trough still maintains it's more northerly track with us sitting on the southern edge where there is less moisture. At the same time, we are sitting in the right entrance region of the jet streak which is often associated with more energetic weather. Combine this with the divergent flow, rain, and orographic effects, cannot rule out another threat for thunderstorms in the afternoon. Of the two days (Friday and Saturday), Saturday is the most potent day with CAPE exceeding 150 J/kg in some locations. Therefore, there is a 15-30% chance for thunderstorms Saturday afternoon from Salem and Lincoln city northward, with the highest chances across southwest Washington and the Portland-Vancouver Metro Area. This would again bring the threat for lightning, brief heavy rain, erratic winds, and/or small hail.

Drier weather briefly returns on Sunday as a shortwave ridge passes over the Pacific Northwest. By Monday, about 75% of ensemble members are showing some sort of broad troughing and below-average 500 mb heights returning to the area. While this would return widespread chances for precipitation and cooler temperatures, there still remains uncertainty on the exact magnitude and track of this trough. The other 25% of ensemble members suggest zonal flow with dry weather on Monday. Greater uncertainty follows on Tuesday with exactly how this trough exits the area (if it does), but we'll still maintain chances for light, non-impact precipitation. -10/27

AVIATION

Radar imagery at 21Z Wednesday showed isolated light rain showers beginning to dissipate over the area. Expect predominately VFR ceilings and visibilities to continue through this evening, aside from IFR to MVFR conditions at KONP. Chances for IFR to MVFR ceilings increase tonight into early tomorrow morning both inland and at the coast. By 12Z Thursday, chances for ceilings below 3000 ft increase to 25-50% for inland TAF sites, except 60-75% at KSLE and KEUG. Chances for ceilings below 3000 ft range between 75-95% at KAST and KONP between 21Z Wednesday and 12Z Thursday.

KPDX AND APPROACHES...Aside from a few sprinkles now through 00Z Thursday, expect mainly dry conditions to continue with VFR ceilings/visibilities and southwest winds around 5-10 kt. There is a 30-40% chance for ceilings below 3000 ft between 08-17Z Thursday as lower clouds forming along the Cascade foothills attempt backbuilding westward towards the KPDX terminal. -23

MARINE

Buoy observations from early to mid Wednesday afternoon showed seas around 5 to 6 ft with southerly winds around 10 to 20 kt. Winds were gusting as high as 23 kt at times at buoy 029, however wind gusts of this magnitude are not expected to last long and are mainly associated with an ongoing frontal passage.

As this front exits Wednesday evening/night, winds will veer to the northwest while weakening in strength. Seas are expected to remain around 5 to 6 ft as a westerly swell persists. Seas then build to 6-8 ft at 11 to 12 seconds late Thursday night into Friday morning as a weak frontal boundary approaches the water. This front will also bring increasing westerly winds with gusts up to 20 kt.

A secondary front arrives on Saturday, bringing a shift to northwest winds. However, wind speeds are not expected to increase much with this frontal passage. The time period with highest wind speeds will likely occur on Monday as stronger system moves over the waters, bringing south winds up to 20-25 kt and seas approaching 9-10 ft. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.