textproduct: Portland

SYNOPSIS

Warm and dry conditions expected through the weekend with persistent onshore flow. Slight chance (10-15%) for thunderstorms or showers over the Cascades Friday to Saturday afternoon, with the highest chances along the crest. There are low chances for more widespread rain returning early next week, but significant rain does not appear likely at this time.

DISCUSSION...Today through Tuesday

Satellite imagery and surface observations as of early Wednesday afternoon depicts stratus clouds dissipating across interior valleys. So far, the central and southern Willamette Valley have cleared out, and we are beginning to see the northern Valley and southwest Washington lowlands clearing out as daytime heating progresses. We're also seeing some cumulus clouds popping up across the Cascades and Coast Range due to increasing instability and moisture from an upper level trough; however, expect to remain dry today. The better moisture for precipitation is further north across the northern half of Washington. Given the widespread cloud cover this morning, temperatures are forecast to peak in the upper 70s across interior valleys and 60s along the coast.

Tomorrow (Thursday), models and their ensembles continue to agree on upper level troughing broadening over the Pacific Northwest and maintaining onshore flow as a ridge over the northeast Pacific flattens out and weakens. One nuance is in the mid-levels of the atmosphere (5,000-10,000 ft above ground) tomorrow, where forecast soundings are showing light easterly winds returning, including near the Grasshopper Fire. This could cause some wildfire smoke to drift across portions of northwest Oregon and southwest Washington by Thursday afternoon, mainly causing hazy skies. Can't rule out some of this smoke mixing down to the surface as well. However, this smoke is expected to be short-lived as winds from the west return Thursday night into Friday morning and push smoke eastward and clear the air once again. Still not expecting precipitation, but some lingering moisture to the north could support a 10-15% chance for thunderstorms across the far northern portions of the south Washington Cascades Thursday afternoon.

By Friday through the weekend the aforementioned upper level trough broadens even further off the U.S. West Coast. As more southerly flow aloft returns over the area, will see increasing instability and monsoonal moisture that will support a 10-15% chance for thunderstorms across the Cascades each afternoon from Friday-Saturday, with the highest chances along the crest of the Cascades. Otherwise, the general airmass remains too dry to support any widespread precipitation from this trough. Guidance suggests high confidence for near to slightly above average temperatures for mid-August tomorrow through the weekend, with afternoon highs rebounding into the mid to upper 80s across interior valleys.

The question remains whether or not we get widespread rain early next week, but chances for measurable precipitation (>0.01") remains low (15-25%) across the area next Monday and Tuesday. Cluster analysis of 500 mb heights shows a wide range of the magnitudes and placements of the upper level trough offshore, while only 5-10% of ensembles show the potential for any substantial 24-hour rainfall totals above 0.10" across the area. -10/DH

AVIATION...Through 18Z Thu

Westerly flow aloft continues as an upper level trough moves over the region. Marine stratus has cleared at most inland terminals. Any lingering stratus should clear by 22z Wed. MVFR conditions are being observed along the coast. Lowered flight conditions are expected to persist through the TAF period along the coast with around a 50% chance of brief improvement to VFR/high MVFR in the afternoon hours through 00z Thu. Then there's a 60-80% chance that IFR/LIFR conditions return along the coast between 02-06z Thu. The marine push will be less strong tonight into Thursday morning, so chances of marine stratus pushing inland during this time is lower with a 25-40% chance of MVFR ceilings at any hour between 10-16z Thu at inland terminals. Any lowered ceilings that do form should improve by 18-21z Thu. North to northwest winds are expected to increase to around 8-10 kt this afternoon, decreasing by 04-08z Thu.

KPDX AND APPROACHES...Expect MVFR CIGs to persist through 19-21z Wed, improving to VFR after. Lingering afternoon clouds are expected to gradually lift and scatter out, with predominately VFR conditions likely through the rest of the TAF period. However, there is a 25-40% chance of MVFR CIGS developing again between 10-14z Thu. If these formed, expect improve to VFR conditions again by 18-20z Thu. Northwest winds around 5 kt are expected to increase to 8-10 kt after 23z Wed, decreasing after 06-08z Thu. -03

MARINE

No significant changes as high pressure offshore will continue to support northerly winds and fairly benign conditions across the coastal waters through the week and into the weekend. Winds from the north to northwest with gusts less than 21 kts will continue through at least the end of the week. Wind driven seas are expected to remain around 3 to 6 ft through the week. -DH/03

PQR WATCHES/WARNINGS/ADVISORIES

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

PQR WATCHES/WARNINGS/ADVISORIES

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