textproduct: Portland

SYNOPSIS

High pressure will maintain very warm and dry conditions today. Moderate HeatRisk is expected along the I-5 Corridor north of Salem, including the Portland/Vancouver Metro. Cooler temperatures return Wednesday to Thursday as a weak shortwave trough swings through the region. Slight chances for showers along the coast, Coast Range, and Cascades on Wednesday. Cool trend continues at the end of the week as a deeper trough arrives from the Gulf of Alaska, returning widespread chances for rain Friday into the weekend.

LONG TERM...Friday through Monday

Very little change in the forecast on Friday as onshore flow persists and the low pressure system in the Gulf of Alaska remains nearly stationary. However, overnight into Saturday is a different story as the low aloft drops down over the northern Washington coast, and is coupled with a low pressure system at the surface. This is a very energetic system with strong vorticity advection and climatologically abnormal CAPE over the marine waters. With ample support, there are increasing chances for more widespread rain. However, the most robust portion of this system will be well to our north as we sit on the southern edge of the low. Therefore, could see less of a stratiform rain and trend towards a convective/showery pattern. Due to the combination of increased instability with cooler air aloft, there is around a 15-20% chance for thunderstorms over southwest Washington and far northwest Oregon on Saturday afternoon.

Accumulation of rainfall remains uncertain as there is a) still ample time for this system to evolve, b) the location of the low will come into play - if it shifts south we could see heavier rain, and c) the convective nature of the overall pattern. 24 hour precipitation as described by the NBM continues to show quite a large range of possibilities though the "high end" amounts are less than in previous runs. The 07Z Run currently shows the 10th-90th percentile spread around 0.00 inch to 0.60 inch along the coast and around the Portland area. The 90th percentile (high end amount) is around 1.0 inch in Battle Ground and 1.25 inch along the south Washington Cascades. Storm total 48 hour precipitation ending 5 AM Sunday has a high end precipitation total of around 1.0 inch along the coast, 0.25 inch in the southern Willamette Valley, around 0.75-0.9 inch in southwest Washington and northern Willamette Valley and around 1.25 inch in the Gifford Pinchot and Mt Hood Natl Forests. Overall, the rain is non-impactful.

Transitioning into Sunday and Monday there will be some lingering showers but overall weather appears to be more settled. One thing we will have to watch is a cut-off low that some of the ensembles are attempting to produce on Sunday night. This cut-off low creates almost like a double barrel low type situation along the coast. The impacts are unrealized at this time but again, something to watch moving forward. -27

AVIATION

Late this evening VFR conditions prevail across the region as high pressure begins to move onshore through Tuesday. Northwest winds have largely decreased tonight remaining a bit more variable and terrain driven overnight, although more noticeable east winds are expected to develop in the Cascades and through east-west oriented terrain gaps, including along and west of the Columbia River Gorge as far as KTTD, and possibly to KPDX by late Tuesday morning. At least for the latter site, these likely flip back northerly during the afternoon into the evening - low to moderate confidence regarding exact timing. We'll also have to watch marine clouds attempting to build up the coast Tuesday morning which has the potential to degrade conditions at KONP with a 30-50% chance for sub-MVFR conditions after 12-13z through 17z then again after 00z Wed.

KPDX AND APPROACHES...Expect VFR conditions to continue throughout the period aside from occasional high cirrus at or above 25 kft. There is moderate confidence in a period of easterly winds around 5-10 knots between roughly 16-22z Tuesday before shifting more N-NW later in the afternoon into the evening as onshore flow returns. -99

MARINE

Settled conditions as surface high pressure remains over the waters. The high will shift inland today which will allow for the north/south pressure gradient to decrease and thus winds continue to ease. Looking at winds north to northwesterly winds of 5-10 kt with gusts up to 20 kt through the early afternoon then they shift to the west. Seas will remain around 5-7 ft at 10 seconds. Winds turn southwesterly ahead of a decaying front tonight into Wednesday. Rain shower chances increase to 20-45% beyond 20-30 NM Wednesday morning, before winds back out of the northwest by Wednesday afternoon behind the boundary. Seas of 4-7 ft at 8-10 seconds with a dominant WNW swell continue through midweek.

Generally unsettled weather looks to remain over the Northeast Pacific through the remainder of the week. The dominant swell will shift out of the west, with seas rising to 6-9 ft at 10-12 seconds into the weekend. Northwest winds persist through Friday morning, when another front may bring further rain chances into the weekend.-27/36

PQR WATCHES/WARNINGS/ADVISORIES

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