textproduct: Portland

SYNOPSIS

Temperatures trend cooler through the workweek as broad upper- level troughing develops over the Gulf of Alaska. Dry weather with areas of morning fog and low clouds giving way to afternoon will continue through Wednesday. The broad upper- level trough over the Gulf of Alaska will deepen and progress towards western Washington and Oregon late Thursday into Friday, bringing increasing chances for widespread rain. Trending dry over the weekend in the wake of this trough.

LONG TERM...Thursday through Sunday night

Troughing over the Gulf of Alaska will approach the region in earnest late Thursday into Friday as the trough becomes more amplified and progresses eastward towards Washington and Oregon. As such, the 24-hour probability for precipitation remains fairly high during that time, ranging between 55-65%. The main reason probabilities are not higher than that is due to a subset of ensemble guidance that remains dry, mainly with the GEPS as this ensemble system shows a higher likelihood of the trough deamplifying after moving inland with the bulk of the precipitation falling farther to the north over northwest Washington. While this is not the most likely outcome, it is worth mentioning as it is still within the realm of possibilities. 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. If rain does fall, do not expect any flooding impacts as rain amounts would most likely range somewhere between 0.1-0.6 inches, highest in the mountains and lowest over the Willamette Valley. The wettest model solutions produce 0.5-1.2 inches of rain, however chances for rain amounts this high are only at 10% as suggested by the NBM 90th percentile. Even these rain amounts would not be enough to produce flooding given the extended duration.

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. -23

AVIATION...Through 18Z Tuesday

As of 18Z Monday, low marine stratus had dissipated along the coast but remains in place through the interior lowlands with ceilings ranging from 600-1500 feet, highest around the Portland metro and lowest in the southern Willamette Valley. Stratus will continue to erode as daytime heating increases and northerly winds help mix out the cloud cover between 19-21Z for most locations, exact timing remains uncertain. VFR conditions return to all terminals by this afternoon with northerly to northwesterly winds 5-10 kt. Conditions this evening are expected to be similar to the past few evenings with stratus developing between 00-06z Tuesday and spreading through the coast range gaps and along the Columbia river into the metro and Willamette Valley between 08-12z Tuesday.

KPDX AND APPROACHES...At 18Z Monday, low stratus persists between 1000-1500 feet with VFR conditions returning by 20Z as skies clear back out. Northerly winds around 5 kt expected. Stratus develops and pushes eastward again late tonight into Tuesday morning. -19

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 under 20 kt through Friday, even with the two frontal passages.

Seas are expected to increase to 6 to 8 ft Monday evening into Monday night with the arrival of a long period west-northwestwerly 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. -23

PQR WATCHES/WARNINGS/ADVISORIES

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