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.

UPDATE

Radar and satellite imagery around 2:30-3 AM early this morning showed thunderstorms popping up in the western Portland Metro Area. Mesoanalysis at the time indicated MUCAPE values around 100-500 J/kg. Given that we had a low-level inversion around 2000- 4000 feet this morning which is maintaining low stratus clouds, a lot of the instability and thunderstorms originated in the mid to upper levels of the atmosphere. This was evident by the METARs shown from KHIO and KSPB which reported thunderstorms with cloud bases around 9000-10000 ft. As of 5 AM Friday, these thunderstorms have moved into the northern Clark County and Cowlitz Valley areas. These seem to be very isolated thunderstorms, so a slight chance (15%) for thunderstorms were added to the forecast across parts of the Portland/Vancouver Metro, Cowlitz County, and Skamania County through at least 6 AM this morning. -10

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 12Z Sat

Satellite imagery and terminal observations as of early Friday morning depicts widespread marine stratus along the coast, with low-end MVFR CIGs (1-2 kft) along the north Oregon coast and LIFR CIGs along the central Oregon coast. Some of the stratus as made its way into the I-5 corridor also with low-end MVFR CIGs across the eastern Portland/Vancouver Metro area and the southern Willamette Valley. Still some uncertainty if these MVFR CIGs will make it into the central Willamette Valley. For the interior valleys with MVFR stratus already, there is high confidence these trends continue through the morning. After 17-19z Fri, daytime heating will increase mixing and help clear out stratus across interior valleys, returning VFR conditions through the rest of the TAF period. Along the coast, there is a 50-60% chance at any given hour between 21z Fri-01z Sat for brief improvement to VFR at KAST, but lower chances (20-30%) for KONP. Westerly to northwesterly winds generally around 5-10 kt today, strongest in the afternoon.

Will note there were some thunderstorms observed this morning around the Portland Metro Area. These thunderstorms have moved northward into the Cowlitz Valley and are not expected the rest of the day (<10% chance for thunderstorms).

KPDX AND APPROACHES...MVFR CIGs through 18z Fri, scattering out and improving to VFR after 18z Fri. Expect VFR conditions the rest of the TAF period with few high clouds. Northwesterly winds around 3-7 kt, strongest in the afternoon. -10

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.