textproduct: Portland

SYNOPSIS

Numerous showers and isolated thunderstorms Saturday afternoon will quickly dissipate from west to east Saturday evening. Weak high pressure will bring mainly dry conditions Saturday night through at least Sunday afternoon. Scattered light showers return late Sunday evening/night before widespread steady rain arrives on Monday (85-95% chance). Steady rain will then transition to off-and-on showers Monday evening and continue through Wednesday, maintaining cooler than average temperatures. Trending much warmer and drier June 11-15 with increasing heat concerns next weekend, especially for inland valleys.

DISCUSSION...Saturday afternoon through Friday night

Radar, satellite, and surface weather observations from Saturday afternoon showed numerous heavy showers and isolated thunderstorms over southwest WA and northwest OR. The strongest showers and storms are producing brief heavy downpours, small hail up to the size of peas, and gusty outflow winds up to around 30-35 mph. Cloud to ground lightning will also pose a safety hazard for any thunderstorm that develops. Areas that have observed repeated heavier showers or storms have picked up another from 0.2-0.9 inches of rain over the past 6-12 hours. Despite the heavy rain rates with the strongest cells, these showers and storms are moving too fast to warrant a threat of flooding as heavier bursts of rain are typically only lasting anywhere from 5-15 minutes. However, ponding of water on roads and reduced visibilities during heavier showers will be hazardous to motorists. The latest suite of CAM guidance remains in agreement for an abrupt end to the ongoing convection after 6 PM west of the Cascade foothills and after 10 PM for the Cascades and Cascade foothills. This timing seems reasonable given high pressure will be building into the area this evening while peak daytime heating wanes.

Weak high pressure will then remain in place tonight through at least Sunday afternoon, bringing a brief break from precipitation. However, cloud cover will be on the increase ahead of the next weather system, resulting in the continuation of below average high temperatures in the 60s. Model ensemble guidance has come into better agreement on the arrival time of rain with the aforementioned weather system. It now appears a steady stratiform rain will arrive Monday morning into the early afternoon (85-95% chance) before transitioning to off-and-on post-frontal showers Monday evening. We will then remain in a cool and showery onshore flow regime through Wednesday. There is also a 10-20% chance of short-lived thunderstorms Tuesday afternoon, however these would be sub-severe and isolated due to limited instability. The latest NBM 48-hr probabilities for rain amounts over 0.50" now peaks between 5 AM PDT Monday and 5 AM PDT Wednesday, showing anywhere from a 60-80% chance for most locations in southwest WA and northwest OR, except an 80-95% chance in the Cascades, Cascade foothills, and Coast Range.

Conditions should dry out with temperatures trending towards seasonal normals on Thursday as high pressure builds into the area. Our focus then shifts to a significant warming and drying trend late next week through next weekend. Cluster analysis for June 11-15 for 500 mb heights generally suggest upper level ridging will gradually develop over the region. This will bring a prolonged stretch of dry weather, with temperatures likely warming each day. However, there is still considerable uncertainty in regards to exactly how warm temperatures will get, which could wind up anywhere in the 80s, 90s, or even lower 100s for inland valleys. That said, confidence has increased for high temperatures of at least 90 degrees or warmer by June 13-15. By June 15, probabilities for highs above 90 degrees peak near 55-75% for inland valleys per the latest NBM guidance, and less than 10% at the coast. With the increase in temperatures, there is a 25-45% chance for a moderate HeatRisk or higher over inland valleys on June 12, a 50-75% chance on June 13, a 65-85% chance on June 14, and a 70-90% chance on June 15. There is a 5-20% chance for a major HeatRisk or higher on June 13, a 15-35% chance on June 14, and a 20-40% chance on June 15. Those who plan on swimming in lakes or rivers to cool down should be mindful of the dangers that come with cold water temperatures and/or swift currents. Anyone who is sensitive to heat and/or has outdoor plans June 12-15 should pay attention to the forecast over the coming days. -23

AVIATION

Predominately VFR conditions through Sunday evening, although a wet ground surface along with clearing skies and light winds will result in a 25-45% chance of MVFR clouds across the interior lowlands or fog development in sheltered locales at some point between 10-16z Sun. Any lowered flight conditions should improve to VFR by 18-21z Sun. West winds decreasing less than 6 kts with most locations becoming southerly or variable overnight as high pressure builds over the region, remaining so through Sunday.

KPDX AND APPROACHES...VFR conditions most likely to prevail through the period, though there's a 25-35% chance of MVFR ceilings developing sometime between 10-18z Sun due to the wet ground, clearing skies, and light winds. Any lowered flight conditions should improve to VFR by 18-21z Sun. Westerly winds less than 5 kts becoming variable overnight. -03

MARINE

An active weather pattern continues through midweek with repeated frontal systems bringing periods of rain and increased winds. Isolated rain showers this afternoon will rapidly decrease in coverage in the hours before sunset while west-northwest winds of 10-15 kt ease to around 5 kt and turn out of the south tonight. A brief dry break from Sunday into Monday as a shortwave ridge passes overhead will see southerly winds build to 15-20 kt with gusts up to 25 kt, peaking early Monday morning. Small Craft Advisories may be needed if confidence increases in the duration of wind gusts greater than 21 kt. Seas of 4-8 ft at 8-9 seconds continues with a dominant westerly swell. This next, more robust frontal system will arrive Monday night into Tuesday with additional rain and winds again turning west-northwesterly behind the boundary.

Beyond midweek, there is high confidence that high pressure will build offshore, turning winds northerly over the coastal waters. Diurnal winds will peak in the afternoon and evening, reaching 15-25 kt with wind gusts near 30 kt. Seas of 6-8 ft at 10 seconds with a dominant westerly swell look to continue. -36

PQR WATCHES/WARNINGS/ADVISORIES

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