textproduct: Portland

SYNOPSIS

Warm conditions continue today, but attention turns to increasing shower and thunderstorm chances this afternoon and evening. Stronger storms are possible over the Cascades and the Upper Hood River Valley, with a lower (but non-zero) chance storms drift into the interior valleys including the Portland/Vancouver metro this evening. Cooler, showery weather follows Friday, then a drier weekend and another warm-up early next week.

DISCUSSION...Today through Wednesday

Early this Thursday morning, the region sits in a "between-systems" pattern: weak ridging nosing in from the eastern Pacific while a closed low over California and another low south of Alaska shape the larger flow. This weak ridging will keep temperatures elevated again today, with interior lowlands warming into the high 70s to mid 80s, while also setting the stage for increasing convective potential as moisture wraps northward into Oregon and Washington.

The focus for organized convection ramps up this afternoon and evening as stronger lift and convergence develop along and east of the Cascades. Model guidance continues to suggest ample instability for late May, with CAPE increasing into the 1200-1600 J/kg range across much of the north Oregon/south Washington Cascades and into Hood River and Skamania Counties, along with 0-6 km bulk shear around 30-40 kt. Instability should be comparatively higher earlier in the day across the Lane County Cascades, supporting earlier initiation there, but that southern instability is expected to diminish later as the axis of better moisture/heating and stronger CAPE shifts north into the north Oregon/south Washington Cascades during the late afternoon and evening. For the interior valleys, guidance suggests moderate instability as well, with CAPE around 500-900 J/kg (highest generally east of the I-5 corridor).

SPC highlights a Slight Risk of severe thunderstorms over portions of the north Oregon/south Washington Cascades and parts of Hood River County, with surrounding areas in a Marginal Risk. SPC also now clips the Willamette Valley (including the Portland/Vancouver metro) with the edge of a Marginal Risk, mainly to account for the chance that stronger Cascade storms drift west into the foothills and possibly into the interior lowlands (likely weakening as they encounter a more capped and dry environment). The primary severe hazards with the strongest storms would be quarter-size hail and damaging wind gusts near 60 mph. Thunderstorm development may begin as early as the afternoon in the Lane County Cascades, while the earliest window for any stronger storms reaching the valleys is closer to around 6 PM, with the main period of concern this evening (roughly 6-10 PM). While confidence remains low, low-level shear near the surface trough may increase late afternoon into evening, and if any embedded rotating segments can persist within a line, a very isolated tornado potential cannot be ruled out.

Heavy rainfall rates are an increasing concern, particularly if storms move off the terrain into the lowlands where impacts are more likely. Guidance and soundings indicate very high moisture for late May, with PWAT values around 1.2-1.3 inches over the interior lowlandsnear or above local late-May climatological maxima. In the Cascades, PWATs are lower but still unusually high (roughly 0.75-1.0 inch). Ensemble guidance suggests rain rates over the Cascades and western foothills could commonly reach 0.25-0.50 inch per hour, with a few higher-end solutions suggesting locally higher rates. For the interior lowlands (especially the central/northern Willamette Valley into the Portland/Vancouver metro), rates closer to 0.25-0.30 inch per hour are possible if storms can hold together as they drift west, and lower-probability solutions suggest localized bursts could briefly approach 0.50 inch per hour. These rates could cause localized urban/poor-drainage flooding and ponding, and would also raise concerns for landslides in more susceptible terrain if heavier rates occur.

Friday brings a notable change as the California low finally shifts east and broader troughing presses overhead. Shower chances persist into Friday morning, then trend more terrain- focused by afternoon, with temperatures dropping back into the mid 60s for many interior lowland locations Friday into Saturday.

Looking beyond the cool-down, guidance favors a drier weekend followed by a stronger warm-up Sunday through Tuesday. Temperatures rise back into the low 70s Sunday, low to mid 80s Monday, and upper 80s to low 90s by Tuesday. Current probabilistic guidance supports increasing odds of 90+ early next week, highest around and north of Salem Monday, and expanding farther south by Tuesday. This brings a Moderate HeatRisk to the Portland/Vancouver Metro on Monday and then expanded southward into Salem on Tuesday. Additionally, there is a 15-25% chance of Major Heat Risk on Tuesday from Kelso WA to Salem OR, with the highest chances in the Portland/Vancouver metro. ~12

AVIATION

Onshore surface flow continues along the coast, supporting MVFR/IFR stratus which is expected to persist through the TAF period. Could see a brief break late this afternoon as the stratus recedes just offshore but this would be short lived. VFR conditions inland through the evening expected with high level clouds streaming in from the east. Chances for MVFR CIGs at inland terminals increase to 30-60% after 06-08Z Fri as wrap-around moisture brings rain showers to the airspace. Mostly westerly to northwesterly winds increasing after 21-23Z to 10-15 kt and persist through at least 06Z Fri, bringing gusts up to 20 kt at most terminals, possibly up to 25 kt at coastal terminals.

Otherwise, the focus shifts to thunderstorm development along the Cascade crests during the mid afternoon and evening hours with moderate confidence this activity attempts to shift east to west towards the Willamette Valley approaching 03-07z Fri. Model guidance suggests thunderstorms likely weaken as they move off the Cascades, in which case they may end up as just an organized band of showers by the time they are within the vicinity of the inland terminals. A period of breezy/erratic winds are still possible in this scenario however. Given the lack of confidence regarding convective mode and timing late in the TAF period, did not add a mention of thunder to any sites in the 06z TAF package.

KPDX AND APPROACHES...VFR conditions through at least 06Z Fri. Beginning around 06-09Z Fri, there is another 10-20% chance of MVFR CIGs. After around 03Z Fri, will have to be on the lookout for weakening thunderstorms along the Cascades attempting to work westward towards the terminal. For now it appears the main thunderstorm impact may be moreso for any the east approach into the airport, something to keep an eye on. ~19/12

MARINE

High pressure offshore will maintain breezy north to northwesterly winds across the coastal waters through the remainder of the week. Expect frequent gusts to around 20 kt. Seas around 11 to 12 ft will persist today as a mid to long period northwest swell pushes through the waters. Wave heights then gradually subside back down to around 9 to 10 ft by Friday morning. A Small Craft Advisory for all coastal waters including the Columbia River Bar will remain in effect until at least 5 AM Friday. High pressure is expected to strengthen this weekend across the coastal waters leading to a return of breezy northerly winds. ~12/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None.

WA...None.

PZ...Small Craft Advisory until 5 AM PDT Friday for PZZ210-251>253- 271>273.