textproduct: Portland

SYNOPSIS

Warm and dry conditions expected through next week with persistent onshore flow. Slight chances (10-20%) for thunderstorms or showers over the Cascades this afternoon and Saturday afternoon, with the highest chances along the crest of the Cascades.

DISCUSSION...Today through Thursday

Satellite imagery and surface observations as of early Friday morning depicts onshore flow supporting a solid marine layer along the coast with stratus clouds pushing through the gaps of the Coast Range and through the Lower Columbia River into the Cowlitz Valley. By 5-7 AM Friday, expect stratus clouds to make it's way into the rest of the I-5 corridor including the Willamette Valley. After 9-11 AM Friday, clouds across interior valleys should begin clearing out as daytime heating progresses and increases mixing. Meanwhile, there remains uncertainty with how much clearing will occur at the coast. There is about a 50-60% chance at any given hour between 2-7 PM for marine stratus breaking out along the north Oregon coast, but lower confidence for clearing along the central Oregon coast.

Expect dry and warm conditions today and through the weekend for the majority of locations as weak ridging builds over the Pacific Northwest and an upper level trough elongates well- offshore over the northeast Pacific Ocean. An exception would be the Cascades, where there is a 10-25% chance for isolated thunderstorms this afternoon and Saturday afternoon. As shortwaves from the broad parent trough offshore brings more southerly flow aloft over the area, will see increasing instability and monsoonal moisture supporting these thunderstorm chances. Any thunderstorms that develop may bring lightning, brief heavy rain, and erratic winds. Otherwise, the general airmass remains too dry to support any widespread precipitation west of the Cascades. Guidance suggests high confidence for near to slightly above average temperatures for mid-August through the weekend, with afternoon highs warming into the mid to upper 80s across interior valleys.

Early next week, cluster analysis of 500 mb heights continue to suggest dry and seasonably warm conditions as broad upper level troughing remains well-offshore and an upper level ridge axis over the Rockies tilts into the Pacific Northwest. By mid-week, ensembles diverge into various upper level patterns. A little over half of ensemble members are favoring a trough entering the Pacific Northwest and returning cooler temperatures, while the other half favors warmer conditions due to zonal flow or amplified ridging. Either way, no precipitation is forecast as we maintain a dry summertime airmass. -10

AVIATION...Through 06Z Sat

West to southwesterly flow aloft as an upper level trough remains over the airspace. MVFR/IFR conditions have developed along the coast in marine stratus. These conditions will continue through the majority of the TAF period with a 50-70% chance of LIFR cigs at any given hour through 18z Fri. There's a 20-40% chance that stratus could briefly improve to VFR along the coast at any point through 18z Fri through 02z Sat, though this improvement would not last long. Northwest to west winds less than 8 kts continue along the coast through the TAF period.

Predominately VFR conditions expected for inland terminals through the TAF period. There's the potential for marine stratus to push inland in the early morning hours with a 20-40% chance of MVFR cigs for most inland terminals except for a 50-70% chance at KEUG at any hour between 10-18z Fri. Additionally, smoke aloft continues to impact NW OR and SW WA and could mix down to the surface at times. Smoke could impact slant range visibilities for inland locations through Friday afternoon. North to northwest winds become light and variable overnight through Friday morning, becoming westerly after 18z Fri. KPDX AND APPROACHES...Generally VFR conditions through the TAF period. There's a 30-50% chance of MVFR cigs developing at any hour between 12-18z Fri. Smoke in the mid levels could impact slant range visibilities through Friday afternoon and mix to the surface at times. Northwest winds less than 6 kts become variable overnight through Friday afternoon, becoming northwest again after 22z Fri. -03

MARINE

No significant changes in the overall pattern as high pressure offshore will continue to support northerly winds along with benign conditions across all waters through next week. Winds from the north to northwest with gusts less than 21 kts will continue through at least the weekend, with wind driven seas expected to remain around 3-5 ft. -10/42

PQR WATCHES/WARNINGS/ADVISORIES

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