textproduct: Portland

SYNOPSIS

Warm and dry conditions expected through next week with persistent onshore flow. Slight chances (15-30%) for isolated thunderstorms or showers over the Cascades this afternoon and Saturday afternoon, with increasing chances for isolated elevated thunderstorms over the Willamette Valley, Coast Range, and Cascades after 2 AM Saturday.

DISCUSSION...Friday afternoon through Thursday

Upper level pattern continues to show longwave troughing over the Pacific Ocean with ridging over the central US. Shortwaves along the trough are slowly moving north from N CA into OR today and Saturday, bringing southerly flow and monsoonal moisture to the area. This pattern is producing a 15-30% chance of afternoon and evening thunderstorms to the Cascade crest today and Saturday. Storms that form will slowly move northeast off of the crest. These thunderstorms could produce heavy rain, gusty winds, small hail, and frequent lightning.

Additionally, latest CAMs are indicating increasing potential (10-15% chance) for elevated convection in the early morning hours of Saturday spreading across the entirety of NW OR and SW WA. A subtle shortwave along the flow could produce enough vorticity to tap into just enough existing elevated instability combined with the monsoonal moisture for isolated thunderstorms to form, mainly over the Willamette Valley and Cost Range, though can't rule any out over the coast or Cascades. If any stronger cells or thunderstorms do form, they would move generally north to northeast and could form anytime after 2 AM with chances lingering through the morning hours. Since bases will be higher with these, the main impacts would be frequent lightning with locally moderate to heavy rain. Gusty winds and small hail are not as likely. A similar pattern occurred early this morning, leading to a few isolated thunderstorms to form west of Portland around 2-3 AM and slowly move northeast into Skamania County, lingering there until late morning. This provides higher confidence in the forecast Saturday morning, though the area that could receive thunderstorms is wider than this morning.

Otherwise, expect mostly dry and seasonable temperatures today and through the weekend for the majority of locations. Marine stratus continues along the coast through the weekend due to the general onshore flow with stratus pushing inland at times. Guidance suggests high confidence for near normal temperatures for mid-August through the weekend, with afternoon highs warming into the low to mid 80s across interior valleys.

Early next week, cluster analysis of 500 mb heights continue to suggest dry and seasonable 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. -03/10

AVIATION...Through 00Z Sunday

Satellite and terminal observations show VFR conditions with mostly clear skies inland and persistent marine stratus along the coast. Also, observations are showing smoke/haze across the airspace, but above the surface and slowly clearing. Smoke/haze across the airspace could impact slant range and vertical visibilities anytime through 06Z Saturday for inland locations.

Coastal locations are expected to maintain MVFR conditions through at least 06Z Saturday. Afterwards, conditions along the coast are expected to fall back towards IFR/LIFR through at least 16Z Saturday, becoming MVFR once again through the majority of the rest of the TAF period. As for inland locations, MVFR conditions via marine stratus are expected to return around 10Z-12Z Saturday, improving to VFR by 18Z-21Z. West to northwest winds generally around 5-10 kt with the strongest in the afternoon. Around 03Z-05Z Saturday, winds are expected to become light and variable across the airspace.

Starting around 09Z Saturday and through at least 15Z Saturday, models are showing around a 10-15% chance for elevated thunderstorms developing anywhere within the Willamette Valley and Coast Range. Any thunderstorms that develop could result in frequent lightning, heavy rain, and gusty and erratic winds.

KPDX AND APPROACHES...VFR conditions with mostly clear skies, though elevated smoke could mix down at times and impact slant range and vertical visibilities through 06Z Saturday. MVFR conditions expected to return around 12Z-15Z Saturday, improving to VFR by 18Z-21Z. Northwesterly winds around 3-7 kt, strongest in the late afternoon and early evening. /42

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 3-5 ft. These benign conditions are expected across all waters through at least the start of next next week. /42

FIRE WEATHER

There's a 15-30% chance of isolated thunderstorms along the Cascade crest during the afternoon and early evening hours today and Saturday. Additionally, there's a 10-15% chance of isolated elevated thunderstorms forming in the Willamette Valley, Coast Range, and Cascades after 2 AM Saturday with chances continuing through the morning hours. Any thunderstorm that forms could have frequent lightning that could strike outside of areas receiving rain. However, due to persistent onshore flow (winds generally from the west to northwest), minimum relative humidities are well above critical levels (40s in the interior lowlands, 30s to 40s along the Cascade crest), and overnight recoveries will be exceptional (70s to 90s). Available water potential in the atmosphere (PWAT) values are around 0.75-0.8 inches over the Cascades and 1.0-1.25 inches over the interior lowlands, which means any thunderstorm that forms will likely produce moderate to heavy rain. Although storms over the Cascades could have brief periods of gusty and erratic winds, it will be difficult for these winds to reach where lightning might strike outside of rain cores, limiting concern for potential fire spread. Also, gusty winds are less of a concern for the Saturday morning thunderstorms as their bases will be much higher. Because of the combination of all of these factors, there is limited concern for rapid spread of fire if lightning does ignite new fires. Therefore, a Red Flag Warning has not been issued since conditions are not reaching critical, though there is still an elevated concern as lightning can still strike dry fuels outside of areas that have received rain. -03

PQR WATCHES/WARNINGS/ADVISORIES

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