textproduct: Portland

SYNOPSIS

Persistent onshore flow and varying degrees of morning cloud cover will bring seasonable temperatures July 8-12 along with continued dry weather, aside from a 10-15% chance of light rain or drizzle in Pacific and Clatsop counties on Friday. Temperatures trend warmer July 13-14 as upper level flow becomes southwesterly upstream of an amplifying upper level ridge over the central U.S.

DISCUSSION...Tuesday night through Monday night

The forecast is highlighted by persistent onshore flow, seasonable temperatures, and continued dry weather aside from a 10-15% chance of light rain or drizzle in Pacific and Clatsop counties on Friday. Model ensemble guidance is in good agreement for an extended period of near zonal flow aloft from Tuesday night through at least Thursday night. Upper level flow becomes more southwesterly late Friday into the weekend. At the surface, onshore flow is expected to persist each day. This typical summer-time weather pattern will bring temperatures that are close to average for this time of year, along with varying degrees of morning cloud cover and plenty of afternoon sun. Total model spread for temperatures Wednesday through Friday is very narrow, suggesting confidence is high in regards to the temperature forecast. Expect highs ranging between 77-83 degrees Thursday and Friday for inland valleys, coolest over the Cowlitz Valley and lower Columbia and warmest over the Willamette Valley, Portland metro, Columbia River Gorge, and Upper Hood River Valley. Expect highs in the 60s for the coast and high Cascades. Morning lows in the upper 40s to mid 50s are expected both inland and at the coast. As such, HeatRisk will stay in the Minor category. It is also worth mentioning a decaying cool front will bring a 10-15% chance of light rain or drizzle over Pacific and Clatsop counties on Friday, especially in the morning when forecast soundings depict saturated conditions in the lowest 0.75-1.0 km of the atmosphere. No impacts would occur with this precipitation if it does occur, with total rain amounts of only a trace to a few hundredths of an inch.

Model spread for temperatures begins to increase slightly over the weekend for inland valleys, with highs ranging anywhere from the mid 70s to mid 80s. The outcome will be highly dependent on how widespread morning cloud cover is and how early in the day cloud cover is able to scatter out. The current forecast favors more widespread and persistent cloud cover on Saturday when compared to Sunday, and the deterministic NBM reflects this by keeping highs in the mid 70s to near 80 degrees on Saturday versus low to mid 80s on Sunday.

Beyond the weekend, confidence is high temperatures will begin to warm early next week with less cloud cover. This warming trend is in response to increasing 500 mb heights and 850 mb temperatures upstream of an amplifying upper level ridge over the central United States. Since the ridge axis will be located east of the Rocky Mountains and far removed from the Pacific Northwest, extreme heat is not expected. Instead, a more modest warm-up is likely with high temperatures most likely peaking somewhere in the mid to upper 80s for inland valleys. Meanwhile, the coast will remain cooler with highs staying in the 60s. There is a 15-35% chance for highs of 90 degrees or warmer across the Willamette Valley, Portland/Vancouver metro, Columbia River Gorge, and Upper Hood River Valley on Monday. Chances for highs above 95 degrees are under 10%, which further backs up the expectation that extreme heat is not expected. Nevertheless, the slight warm-up may be just enough to result in an increase from a Minor HeatRisk to a Moderate HeatRisk; probabilistic HeatRisk guidance suggests there is a 20-40% chance for a Moderate HeatRisk in the aforementioned areas. -23

AVIATION

Southwest flow aloft with onshore flow in the lower levels as broad upper level troughing persists over the region today. As of 21z, marine stratus has retreated offshore. Coastal terminals may see VFR conditions continue through 00-01z, but this break will be brief as marine stratus is expected to build back in this evening with around a 30-50% chance of IFR CIGs after 03z Wed. Guidance shows higher chances for stratus developing along the lower Columbia and into the Portland metro area early Wed morning, with around a 40-60% chance of MVFR CIGs after 12z Wed. Lower chances (10-20%) for MVFR stratus exist for the southern Willamette Valley. Diurnally driven northwest winds increase this afternoon and evening with gusts up to 20 kt possible.

KPDX AND APPROACHES...VFR conditions expected through this evening under mostly clear skies. Stratus is likely to develop by 10-12z Wed morning with chances for MVFR CIGs increasing to around 50% after 12z Wed. Northwest winds increase to around 9-11 kt this afternoon. /DH

MARINE

High pressure will maintain north to northwesterly winds across the coastal waters through the week. A weak cold front near Vancouver Island will dissipate later this evening as winds generally remain 15 kt or less through Wednesday morning. Pressure gradients tighten across the waters later Wed afternoon as northerlies increase with gusts to at least 20 kt south of Cape Falcon through Wed night. There is around a 40-50% chance for wind gusts up to 25 kt south of Cape Foulweather, so will continue to monitor forecast trends to determine if a Small Craft Advisory is needed for the coastal waters off the central Oregon coast. Latest guidance suggests similar conditions continue into Thursday with winds easing on Friday as another weak front approaches the waters. Seas are expected to remain around 4 to 6 ft through the remainder of the week and into the weekend. /DH

PQR WATCHES/WARNINGS/ADVISORIES

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