textproduct: Portland

SYNOPSIS

A warming trend persists into Tuesday as high pressure re-builds overhead, albeit temporarily. Precipitation chances then return Tuesday evening as a trough approaches the West Coast. Can't rule out a period of high-based thunderstorms Tuesday evening into Wednesday morning (15-25% chance) as well. After lingering showers decrease Thursday, another low from the Gulf of Alaska maintains precipitation chances and a trend towards cooler temperatures Friday into early next weekend.

DISCUSSION...Today through Sunday

The region remains under a rather progressive upper-level weather pattern through the week into the weekend, but we begin the forecast period under fairly benign weather overall. This almost certainly will change Tuesday evening/night. Until then, mostly sunny/clear conditions are favored thanks to a ridge of high pressure building overhead although the axis of the feature begins to shift east of the area on Tuesday morning. For most locations high temperatures on Tuesday will likely be the warmest of the upcoming week, reaching into the 60s along the coast, 70s across the Cascades, and upper 70s to mid 80s for the inland valleys/lower Cascade valleys. Compared to the prior forecast cycle, these temperatures have decreased several degrees owing to a slightly faster progression of the trough of low pressure offshore (we'll dive into that feature more in a second), thus decreasing the moderate Heat Risk threat across the Portland/Metro and I-5 corridor. The latest NBM has lowered the probabilities for temperatures to exceed 90 degrees, giving the Portland metro only a ~10% change to do so while the rest of the inland valleys are <5%. At least heat related impacts should remain a bit more muted than previously anticipated as a result.

Our focus later Tuesday quickly shifts to a negatively tilted upper-level trough approaching the coastline which is slated to pass into western Oregon/Washington Wednesday morning. As the trough advances towards the region over the eastern Pacific, this feature begins to increase diffluence aloft while ejecting mid to upper-level moisture northward as well. At this point it's just a matter of forcing, and in this case time, whether convection in the form of showers and potentially thunderstorms (15-25% chance) are able initiate. It's worth quickly pointing out this activity is not anticipated to be surface based in the traditional sense. Yes, you could get enough localized lift during peak heating channeling over a higher terrain feature like the Cascade crests, or convergence/lift along a strong marine push boundary to spark convection but in general the initiation will come from subtle mid to upper-level forcing mechanisms completely decoupled from the surface/boundary layer. This is the case for almost all of our nocturnal thunderstorm set-ups and why they can be challenging to forecast. The first period to watch will actually be during the morning hours on Tuesday as an initial swath of mid to upper-level moisture (~700mb) and weak elevated instability works through the region. Given the rather dry airmass above and below this moisture layer, and a sub-par set-up overall, it likely results in only some isolated to scattered ACCAS before moving off to our north. The 18z NAMNEST is one of the only Convective Allowing Models (CAMs) to depict isolated high based showers/T-storms developing near the coast range during this first wave of moisture, and it typically runs "Hot" when it comes to convection, so given the majority of other CAMs aren't biting, for now thinking the morning remains fairly calm in this regard - still a period of watch.

Things change headed into the evening hours though as the shortwave trough gets closer to the coast and moisture/instability increases further. CAM Ensembles like the 12 HREF depict activity beginning near Lane County, particularly around the Cascades, progressing northward and potentially fanning out in coverage as the evening and overnight hours go on due to increasing lift provided by the aforementioned shortwave. Deterministic model soundings suggest elevated CAPE will protrude through the -10 to 20c layer needed for electrification. Since this high based activity is likely getting initiated around 700-600mb (~9,000-10,000ft) where winds aloft are higher, an individual storm's residence time over a single location should be fairly low. Taking high resolution guidance and the general synoptics into account, the best chances (15-25%) for thunderstorms during the Tuesday evening/overnight period generally falls from the I-5 corridor eastward. However, as with most nocturnal set-ups like this, there remains significant uncertainty in storm coverage and frequency that can be modulated by weak perturbations rotating around the incoming trough, and adjustments to the placement of the trough itself. Definitely keep an eye on the forecast over the next 25-36 hours.

Finally on Wednesday morning the axis of the trough swings into the area likely bringing an end to any high-based thunderstorm chances and ushering in a period of more stratiform rainfall. The added cloud cover and cooler airmass Wednesday afternoon pushes temperatures back into the 60s for much of the area too. From there, guidance indicates a bit more uncertainty in the evolution of the upper-level pattern Thursday through Saturday, however, beyond the continuation of westerly flow, most ensemble members continue to show another trough dipping down from the Gulf of Alaska by the end of the week around Friday. This will maintain chances for precipitation across the area and bring lower 500 mb heights, resulting in temperatures cooling down to or even below seasonal normals. -99

AVIATION

Southwesterly flow aloft with north/northwest flow at the surface continues Mon afternoon and evening. Expect predominately VFR conditions for the majority of the airspace. The only exception is a 40-60% chance for the redevelopment of MVFR/IFR ceilings around and south of KONP after 10-12z Tue. Elevated north/northwest surface winds around 10 kts. Gusts along the coast up to 25 kt from 19Z Mon through 03Z Tue. Winds become light and variable after 04-06z Tue, then become south to southwesterly by 16-19z Tue.

KPDX AND APPROACHES... VFR conditions expected through the period. North to northwesterly winds around 8-10 kts, decreasing below 6 kts by 04-06z Tue. Winds become south to southeast by 18-20z Tue. -03

MARINE

Upper level high pressure over the waters along with tightening pressure gradients will result in elevated north winds this afternoon and evening with gusts up to 20-25 kts. A Small Craft Advisory remains in effect for zones south of Cape Falcon through 2 AM PDT Tuesday and for zones north of Cape Falcon from 5 PM to 11 PM PDT today. Winds decrease on Tuesday morning and become variable as an upper level trough approaches the region from the west and a weak surface low forms over the waters off of northern Oregon and moves north through the morning. Winds become westerly by Tuesday afternoon, remaining mainly under 10 kts. As the upper trough and associated front moves through the waters Wednesday, west winds increase. Current forecast indicates maximum wind gusts up to 20 kts, though there is a 50-65% chance of occasional gusts up to 25 kts. Winds remain westerly but decrease once again Wednesday night. Seas will remain around 5-7 ft at 10-12 seconds through the majority of the week. -03

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 5 PM this afternoon to 11 PM PDT this evening for PZZ251-271. Small Craft Advisory until 2 AM PDT Tuesday for PZZ252-253-272- 273.