textproduct: Portland

SYNOPSIS

A departing upper low will allow warmer and drier weather to build in today as high pressure rebuilds. Wrap-around moisture then returns mid to late week, bringing increasing shower and thunderstorm chances Thursday afternoon and evening, focused mainly over the Cascades and the Upper Hood River Valley. Cooler temperatures return Friday, followed by a drier weekend. Early next week remains uncertain with a wide spread in potential afternoon temperatures.

DISCUSSION... Today through Tuesday

Early this Wednesday morning, the broad upper low that produced yesterdays scattered showers is sliding farther south into the Great Basin. In its wake, 500mb heights rise over the Pacific Northwest and the lowlands should trend warmer and drier through the day.

By this afternoon and evening, attention shifts back to the Cascades as wrap-around moisture curls northward out of the southeast. The best signal for convection today remains over the Lane County Cascades, where a 10-20% chance of thunderstorms persists along with a 30-60% chance of showers. Elsewhere west of the Cascades, most locations remain dry and warm, with mid to high clouds overhead at times.

Thursday has trended more active and is the primary day to watch for convective impacts. Strengthening easterly flow aloft pulls in substantial moisture (model soundings suggest PWAT values up to around 1.25 inches), which mainly increases rain efficiency and the potential for heavier downpours in any showers/storms that develop. Current forecast favors a 15-35% chance of thunderstorms across the Cascades and Upper Hood River Valley, with the highest odds over the Lane County Cascades. Guidance also suggests some shower potential expanding west into the I-5 corridor, especially later in the day.

The ingredients for a few stronger storms, some potentially severe, appear to overlap Thursday between roughly 11 AM and 10 PM. Joint probabilities for CAPE exceeding 1000 J/kg and 0-6 km bulk shear over 30 kt (a combination supportive of organized convection) run on the order of 40-80% across portions of the Cascades, with the strongest overlap favored in the south Washington and north Oregon Cascades. Model soundings broadly support around 1000-1500 J/kg of CAPE with 30-40 kt of bulk shear. SPC also highlights a Marginal Risk in the Cascades and Upper Hood River Valley. If thunderstorms do initiate over the Cascades, a few cells may be capable of drifting west into the foothills and the Willamette Valley, including the Portland/Vancouver metro later in the afternoon/evening, though confidence in westward storm extent remains lower than over the higher terrain.

The main hazards with any stronger storm that develops will be brief heavy rainfall could lead to localized ponding in poor-drainage/urban areas, along with hail ranging anywhere in size from peas to the size of quarters, and gusty outflow winds that could approach 40-60 mph in isolated instances. The REFS ensemble mean supports about 0.25 inch per hour rainfall rates through Thursday afternoon/evening across the valleys, with some members suggesting localized bursts near 0.50 inch per hour. A small subset (roughly 5-10%) hints at very intense 1-2 inch per hour rates, though this appears to be an unrealistic outlier signal at this time. Storm motion is currently projected near 20 kt, fast enough to limit duration over any one spot, but not fast enough to remove the threat of brief heavier downpours if storms train or repeatedly develop over the same area.

Warmth continues today into Thursday for the valleys as the ridge reasserts itself, though exact highs will depend on cloud and shower coverage, especially Thursday if convection becomes more widespread. Current guidance still supports highs running 5-10 degrees above normal, with the Portland/Vancouver metro having a 60-80% chance of exceeding 80 today and 80-90% Thursday. Other interior valleys carry a 30-60% chance of exceeding 80 both days.

Friday brings a turn back toward cooler temperatures as another trough approaches from the northeast Pacific. Shower chances linger Friday (especially near the Cascades) before a drier weekend develops. Looking into early next week, ensemble spread remains large, with Monday highs in the Portland area still ranging from the low 70s to the low 90s on Monday and upper 70s to upper 90s on Tuesday depending on ridge placement. Current guidance supports a 15-30% chance of 90+ in the greater Portland/Vancouver metro Monday and a 35-50% on Tuesday, with about 5-15% elsewhere across the Willamette Valley, southwest Washington lowlands, and the Upper Hood River Valley on Monday and 20-35% on Tuesday.~12

AVIATION

Currently a mixed bag of MVFR and VFR conditions throughout the airspace as of 11Z Wed. Expect MVFR conditions along the coast to continue until at least 20Z Wed, including a 20-40% chance of IFR CIGs before 20Z Wed. Thereafter, there is around a 20-40% chance for MVFR CIGs until 02-03Z Thu, then increasing to a 60-80% chance of MVFR and 20-40% chance of IFR CIGs through at least 12Z Thu. As for inland terminals, expect any current MVFR CIGs at the terminals to improve to VFR by 16-18Z Thu and continue as such through at least 12Z Thu.

Expect north to northwesterly winds to increase between 18Z Wed and 06Z Thu, to around 10-15 kt with gusts up to 20-25 kt (higher end of the range at coastal terminals, lower end at inland terminals).

KPDX AND APPROACHES...VFR through the next 24 hours with a 10-20% chance of MVFR CIG before 18Z Wed. Northwesterly winds increase to 10-12 kt with gusts up to 20 kt between 21Z Wed and 05Z Thu. ~12

MARINE

Westerly swell continues to move through the waters, keeping seas around 9 to 12 feet. Northerly winds with gusts up to 20-25 kt continues mainly over the outer waters. Small Craft Advisories remain in effect for all zones due to steep seas and frequent wind gusts over 20 kt. The Small Craft Advisories have been extended until 5 AM Thursday as buoys are still showing conditions hovering right on the edge of 10 ft and wind gusts above 20 kt and not expecting such conditions to weaken for any extended period of time within at least the next 24 hours. Another northwesterly swell is expected to move into the water on Thursday, steepening seas across all marine zones again. ~12/27

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 5 AM PDT Thursday for PZZ210- 251>253-271>273.