textproduct: Portland

SYNOPSIS

Dry and warm weather this afternoon. A deeper trough shifts southward into the region Friday to Saturday increasing onshore flow and returning chances for precipitation. Slight chances for thunderstorms Friday and Saturday afternoon. Drier weather returns Sunday before another trough returns chances for precipitation early to mid next week.

SHORT TERM...Today through Saturday night

Satellite imagery as of early Thursday afternoon depicts decreasing clouds with mostly sunny skies as dry westerly flow sets up over the region. Onshore flow is also increasing - while winds are generally light throughout most areas, increasing KTTD-KDLS pressure gradients of +2.5 to +3.5 will lead to breezy westerly winds with gusts up to 30 mph through the central Columbia River Gorge and Upper Hood River Valley this afternoon and evening.

The overall forecast on Friday and Saturday has remained unchanged and models are in good agreement in the synoptic pattern. 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 chances for precipitation over southwest Washington and the north Oregon coast, Coast Range, and Cascades, while most interior valleys remain dry. 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. In addition, there is a 10-15% chance for isolated thunderstorms across southwest Washington Friday afternoon due to increasing instability from cold air aloft along with sufficient lift from the trough.

On Saturday, the aforementioned upper trough will move south and inland over the Pacific Northwest, bringing more widespread chances for rain across northwest Oregon and southwest Washington. At the same time, cooling temperatures aloft will further increase instability with CAPE values as high as 150-200 J/kg. This plus higher 500 mb vorticity will lead to a 15-25% chance for thunderstorms Saturday, mainly from Pacific City and Salem northward with the highest chances (20-25%) across southwest Washington. The highest threat for thunderstorms appears to be in the late morning to afternoon (11 AM-5 PM Saturday) when instability is the greatest. Any passing thunderstorms Friday and Saturday afternoon may produce lightning, brief heavy rain, erratic winds, and/or small hail.

Onshore flow will also increase Friday to Saturday 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 mph. There is also a 20-30% chance for isolated wind gusts of 40-45 mph through these areas as well, with higher chances (30-50%) for higher terrain and exposed ridgetops. If the low shifts any further north, these winds would end up weaker. -10

LONG TERM...Sunday through Wednesday

For those with Sunday plans, drier weather briefly returns on Sunday as a shortwave ridge passes over the Pacific Northwest. By Monday, the vast majority of ensemble members are showing 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.

Uncertainty follows on Tuesday to Wednesday with exactly how this trough progresses through the area, but most ensemble members (65%) keeps some sort of troughing moving through the Pacific Northwest, maintaining a cooler and wetter pattern. However, the other 35% of ensemble members show the trough either swinging southward into California or the trough exiting eastward with high pressure re-building. If this were to occur, then we'd end up drier. By Wednesday, there is greater uncertainty as ensemble guidance is half and half on whether or not troughing lingering overhead or exits the area. It appears models are most uncertain with the exact timing/duration of this trough. Either way, any precipitation that falls will likely be light and non-impactful. Chances for exceeding 1 inch of liquid precipitation in 48 hours between 5 AM Tuesday to 5 AM Thursday are only 15-20% west of the Cascades, and around 35-45% across the Cascades. -10

AVIATION

Light north to northwest winds around 5-10 kt will continue through 06Z Friday as onshore flow continues. Satellite and surface weather observations at 21Z Thursday showed mostly clear skies from KONP to KSLE to KEUG, with scattered clouds north of KSLE with ceilings above 4000 ft.

A broken or overcast VFR cloud deck will likely redevelop around and after 06Z Friday along the Cascade foothills and Willamette Valley with ceilings most likely staying above 4000 ft. Periods of MVFR ceilings are most likely to occur along the coast between 06-18Z Friday. This is also when a weak front will begin moving inland from the coastal waters, which will bring scattered light rain showers to the coast by approximately 12Z Friday. This front will also bring a shift to southwesterly surface winds by 18Z Friday.

KPDX AND APPROACHES...VFR ceilings are expected to continue through 00Z Saturday along with dry conditions. Northwest winds will continue around 5-10 kt through this evening, with gusts as high as 15-18 kt towards 01-02Z Friday. Winds become light and variable again tonight. -23

MARINE

Looking at fairly benign conditions over the next several days despite a series of frontal systems moving through. Westerly flow will continue through Saturday. 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 tonight into Friday morning as a weak frontal boundary approaches the waters. This front will also bring increasing southwesterly to westerly winds with gusts up to 15-20 kt. There is less than a 5-10% chance of wind gusts of 25 kt or greater.

A secondary front arrives on Saturday, bringing a shift to northwest winds in the afternoon. Wind speeds are not expected to increase much with this frontal passage, as there is less than a 15-30% chance of Small Craft Advisory wind gusts. The time period with highest wind speeds will likely occur on Monday as a stronger system moves over the waters, bringing south winds up to 20-25 kt and seas approaching 8-9 ft. -23

PQR WATCHES/WARNINGS/ADVISORIES

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