textproduct: Portland

SYNOPSIS

Warm and dry conditions expected through next week with persistent onshore flow. There is a 10-15% chance for isolated, high-based thunderstorms across interior valleys and the Cascades through this morning. Chances for thunderstorms decrease across interior valleys in the afternoon, while the Cascades maintain a 15-25% chance for thunderstorms. The highest chances are expected along the crest. Any passing thunderstorms may result in lightning, heavy rain, and erratic winds. Morning clouds expected to dissipate this afternoon.

DISCUSSION...Today through Friday

Radar imagery as of early Saturday morning depicts isolated showers across the Cascades due to an unstable layer in the mid-levels of the atmosphere. This instability remains above an inversion present around 3000-5000 ft. Mesoanalysis suggests MUCAPE values are currently 100-250 J/kg across interior valleys and the Cascades, which is supporting convection. Vorticity from an upper level trough as well as monsoonal moisture from the south is tapping into this instability. As a result, there is a 10-15% chance for isolated, high-based thunderstorms through this morning across interior valleys and the Cascades. Later this afternoon, the thunderstorm threat diminishes for interior valleys as conditions dry up. However, there still remains a 15-25% chance for isolated thunderstorms across the Cascades through the evening with the highest chances along the crest. Any passing thunderstorms may result in lightning, brief heavy rain, and erratic winds.

Below the aforementioned inversion around 3000-5000 ft, a moist low level atmosphere is returning a solid marine layer along the coast with stratus spreading into the I-5 corridor. Similar to the previous days, expect these clouds to last through the morning and break out by the early afternoon as daytime heating progresses and mixes the atmosphere. Keep in mind that even though there is an inversion, high-based thunderstorms can still develop above the inversion and impact locations, which is what we saw yesterday morning with some pop-up thunderstorms that progressed from the Hillsboro, OR area to Cowlitz/Skamania Counties.

Otherwise, expect mostly dry and seasonable temperatures today and through next week for the majority of locations. Guidance suggests high confidence for near normal temperatures for mid- August through at least early next week, with afternoon highs in the low to mid 80s across interior valleys. Cluster analysis of 500 mb heights continues to show broad upper level troughing remaining well-offshore and an upper level ridge axis over the Rockies tilting into the Pacific Northwest. Ensembles are beginning to agree that this general pattern continues through mid to late next week, however, there is some uncertainty with the exact magnitude of ridging over the Rockies and placement of the offshore trough. Therefore, there is still a wider spread of potential high temperatures late next week across interior valleys, ranging from the mid 70s to upper 80s. In any scenario, no precipitation is forecast as we maintain a dry summertime airmass. -10

AVIATION...Through 18z Sunday

Widespread marine stratus west of the Cascades continues to support low-end MVFR cigs at 1.5-2 kft across the airspace late this morning. Modest clearing will see a trend to VFR cigs inland by 19-21z Sat and high-end MVFR cigs with a 40-50% chance of VFR cigs at coastal terminals by 21-23z Sat. Isolated rain showers have largely dissipated aside from along the Cascades, with the chances for rain or thunder now at 10% or less west of the mountains. Otherwise, hazy skies may continue to restrict slantwise vis while terminal observations maintain 7-10 SM at the surface. Light winds this morning will build to 5-10 kt out of the northwest this afternoon, then ease below 5 kt tonight.

KPDX AND APPROACHES...Low-end MVFR cigs below 2 kft are expected to trend to VFR as skies clear and cloud bases lift, with a 50% chance of VFR conditions by 20z Sat. West to northwest winds expected throughout the period, increasing to around 5 kt this afternoon, then easing tonight. -36

MARINE

No significant changes in the overall pattern as high pressure offshore will continue to support north/northwesterly winds with gusts less than 21 kts. Seas also expected to remain around 3-6 ft. These benign conditions are expected across all waters through at least the start of next next week. -10/42

PQR WATCHES/WARNINGS/ADVISORIES

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