textproduct: Portland

SYNOPSIS

Mainly dry and seasonable weather expected through at least Wednesday as onshore flow persists. Temperatures gradually trend trend cooler through the week with areas of morning fog and low clouds giving way to afternoon sun. Upper level troughing over the Gulf of Alaska deepens on Thursday, approaching the region through Friday, bringing increasing chances for widespread rain. Conditions expected to trend drier on Saturday with chances for rain returning late weekend into early next week.

LONG TERM...Thursday through Monday

Rain is appearing more likely later this week as more amplified upper level troughing interacts with a plume of moisture over the Pacific. By Thursday, a strong jet streak will cause the upper trough over the Gulf of Alaska to dig southward, becoming more amplified off the coast of the Pacific NW. This will interact with an area of weak low pressure associated with a plume of moisture, potentially strengthening the surface low as it moves toward the coast. But, there still remains quite a bit of uncertainty regarding the positioning and strength of this low pressure system.

First, will discuss the precipitation potential, as the probabilities for rain continue to increase across the area for later Thursday into Friday. Latest NBM guidance suggests there is at least an 80% chance for measurable rainfall across NW Oregon and SW Washington, but rainfall amounts remain quite uncertain, though have been trending higher in the latest model runs. There is still a 10- 20% chance that conditions remain dry. There is also a 10% chance that 24-hour rainfall amounts exceed 1 inch anywhere across the forecast area. Those with outdoor plans late Thursday or Friday should monitor the forecast over the coming days to see if rain chances trend up or down.

While the probability for impactful wind with this system is small, it is still worth noting. A handful of ensemble members, including the latest 12z ECMWF operational run, develops a slightly stronger low that lifts NE toward the northern OR coast. There is around a 10% chance for wind gusts exceeding 40 mph along the coast and within the central Willamette Valley.

Upper-level heights look to rebound quickly in the wake of the shortwave trough, with high confidence for dry weather and slightly warmer temperatures on Saturday given chances for rain fall below 10%. However, high pressure building in behind this trough Friday night into the weekend will bring clearing skies and calm winds during the overnight hours. This will bring an ideal set-up for radiational cooling and chillier overnight low temperatures. Aside from the high Cascades, overnight lows will be coldest in the Upper Hood River Valley where frost development is possible Friday night, Saturday night, and Sunday night. The NBM is showing a 30-50% chance for morning lows below 36-37 degrees Friday night and Saturday night near Parkdale and a 5-10% chance near Odell. Those with outdoor vegetation that is susceptible to frost damage should monitor the forecast over the coming days.

Chances for precipitation return Sunday into Monday next week, but there exists quite a bit of uncertainty between the clusters with some indicating weak ridging building in but others maintains more zonal to weak troughing persisting. DH/23

AVIATION...Through 00Z Wednesday

High clouds continue to stream across the region late this afternoon with mainly VFR conditions in place. Headed through the evening and overnight hours, expect a similar progression to past days with MVFR to IFR stratus developing along the coast between 00-06z before spreading through the coast range gaps and along the Columbia river into the metro and Willamette Valley between 08-12z Tuesday. However, compared to the past few nights/mornings, there's slightly better confidence that CIGs at inland terminals remain MVFR (stratus deck as opposed to fog) with only a 15-25% chance for IFR CIGs between roughly 12-17z. This cloud layer eventually clears midday into the early afternoon (19-21z) followed by VFR conditions returning at inland terminals. Winds stay fairly light, generally less than 5-10 knots.

KPDX AND APPROACHES...Expect VFR conditions to persist through the evening and the majority of the overnight hours. Once again, stratus likely develops near sunrise Tuesday morning with period MVFR CIGs likely (~70%). VFR conditions return for the afternoon however confidence is only moderate regarding the exact timing of low stratus dissipation (19-20z). -99

MARINE

High pressure will maintain predominately northerly winds across the waters through Thursday. An exception would be Tuesday afternoon to Wednesday morning, when winds may turn more westerly for the waters north of Cape Falcon with the passage of a weak front. Winds become more northwesterly and increase slightly in strength over all of the coastal waters Thursday night into Friday with the passage of another front. This front will be relatively stronger than the frontal passage earlier in the week, with widespread rain likely. Wind gusts should generally stay below 20 kt through the week. The highest probabilities come late Thursday into Friday with a 15-40% chance, highest in the northern outer waters.

Seas are expected to increase to 6 to 8 ft this evening into the overnight with the arrival of a long period west- northwesterly swell. Seas then remain near that range through Friday, but are not expected to be particularly steep at any point in time due to the dominant wave periods of 10 to 13 seconds. -19/23

PQR WATCHES/WARNINGS/ADVISORIES

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