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 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

Northeast flow aloft today as an upper level low spins over central California. Expect predominately VFR conditions inland as residual stratus continues to scatter out this morning. Low-end MVFR marine stratus along the coast is likely to persist through at least 21z this afternoon, with around a 50-70% chance of improving to VFR afterwards at coastal terminals as stratus moves offshore. Chances for MVFR to IFR stratus returning to the coast increases by 05-07z Thursday. North to northwesterly surface winds increase this afternoon with gusts up to 20-25 kt.

KPDX AND APPROACHES...VFR conditions with few high clouds expected through tonight. Chances for MVFR stratus return to around 20-30% between 14-18z Thu. Northwest winds expected to increase to around 10-12 kt with gusts up to 20 kt later this afternoon. /DH

MARINE

Westerly swell continues to move through the waters but the peak wave heights subsided earlier this morning. Seas are generally below 15 ft and will continue to drop closer to 10 ft overnight. Winds from the northwest at around 10 kt. Small Craft Advisories remain in effect for all zones due to steep seas. Have extended the Small Craft until 1 AM as area buoys are still showing conditions hovering right on the edge of 10 ft. Winds though are beginning to ease which will aid in lowering seas. The Small Craft for the inner waters has been extended to 1 AM as and the outer waters extended through 5 PM Wednesday for steep seas and wind gusts up to 25 kt. Another northwesterly swell is expected to move into the water on Thursday, steepening seas across all marine zones again. Conditions are borderline for Hazardous Seas Warning or a Small Craft Advisory at this time. -27/19

PQR WATCHES/WARNINGS/ADVISORIES

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