textproduct: Portland

SYNOPSIS

A showery, cool pattern continues today as yesterdays front gives way to an upper-level low over the Oregon coastal waters. Scattered thunderstorms remain possible this afternoon (10-20%), with the best window late morning through the afternoon. Drier and warmer weather returns Wednesday into Thursday as high pressure rebuilds, but wrap- around moisture may keep shower chances going over the Cascades late week. Another cool-down is possible heading into Friday and through the weekend. Ridging and much warmer temperatures may return at the start of next week.

DISCUSSION...Today through Monday

Early this Tuesday morning, the region sits in the wake of Mondays cold frontal passage with an upper-level trough becoming a closed low over the Oregon coastal waters. As a result, expect a classic post- frontal day: variable clouds, scattered showers, and cool temperatures for late May. Any shower activity will be intermittent rather than organized, but brief heavier bursts are possible under stronger cells.

The potential for thunderstorms persists today, although instability remains modest. Latest guidance suggests CAPE generally around 100-200 J/kg by midday/afternoon, supporting a 10-20% chance of thunderstorms, most favored from late morning into the afternoon (roughly 11 AM to 5 PM). Compared with Monday, the better thunder potential today shifts more into the interior (including portions of the I-5 corridor), generally from Eugene to the Portland/Vancouver metro. Any thunderstorms that develop could produce lightning, brief downpours, small hail, and locally gusty/erratic winds.

By Wednesday and Thursday, confidence increases in a transition back toward a warmer and drier regime as the upper low drops into the Great Basin and 500 mb heights rise over the Pacific Northwest. High temperatures should rebound to near or slightly above normal, with the warmest outcomes favored across the Portland/Vancouver metro, Salem, and southwest Washington lowlands. Current probabilities for 80 degrees have increased: about 50-70% in the Portland/Vancouver metro Wednesday, increasing to 60-80% Thursday. Elsewhere across interior valleys, chances for 80 degrees are around 30-50% on Wednesday and Thursday (highest chances on Thursday).

One possible scenario late week is wrap-around moisture rotating back north on the east side of the departing low. This keeps a 50-80% chance for showers over the Cascades on Thursday, along with a 15-30% chance of thunderstorms (highest chances in the Marion, Linn, and Lane County Cascades). West of the Cascades, most locations stay dry, though a stray shower cant be ruled out if moisture wraps farther west than currently expected. Additionally, more extensive cloud cover would also limit highs (more lower 70s versus upper 70s/low 80s).

Friday into the weekend, ensembles suggest another trough approaching from the northeast Pacific, bringing cooler temperatures again. Lingering wrap-around moisture could maintain some shower chances into Friday (with current forecasted chances around 40-60% in the Oregon Cascades), while the weekend currently trends toward drier weather overall given limited moisture with the broader troughing. Saturday into Sunday, the trough digs south rather than shifting east towards the region, allowing for high pressure to build into the beginning of next week. Uncertainty in temperatures drastically increase for early next week, with ensemble guidance suggesting the possibility of an afternoon high temperature anywhere between 73 and 93 degrees on Monday. We will continue to monitor this range of values as ensemble guidance hones in on the potential outcomes of this late weekend into early next week ridge. ~12

AVIATION

Currently a mix bag of MVFR and VFR CIGs as of 11Z Tue. Showery activity continues to dwindle throughout the airspace, with rain expected to end by 13-15Z Tue and any deteriorated conditions improved back to VFR by 15-18Z Tue. In the meantime, some inland terminals may lower to MVFR CIGs (if not currently MVFR) as partial clearing skies improve radiational cooling early this morning. Conditions will likely stay VFR between 18Z Tue and 06Z Wed (70-90% chance of VFR conditions at the coast; greater than 90% inland), then potentially lower back to MVFR thereafter (40-60% chance of MVFR CIGs at the coast; 20-40% inland).

Winds will shift to the north around 17-19Z Tue as the low pressure system moves southward along the coast. All terminals could see some increased winds with gusts up to 20 kt possible between 21Z Tue and 03Z Wed.

There remains a 15% chance of thunderstorms today between 17Z Tue and 00Z Wed, with much less confidence for thunderstorm development due to the dry air present within the vicinity.

KPDX AND APPROACHES...Currently VFR with showers bringing occasional MVFR VIS. Rain showers continue to dwindle early this morning, expected to completely clear by 15Z Tue. Expect any deteriorated conditions to end and improve back to VFR by 18Z Tue. There is a 10-20% chance for MVFR CIGs to return after 06Z Wed. Winds shift northerly in the afternoon 18-21Z Tue which may produce gusts as high as 20 kt until 03Z Wed. Speeds are not nearly enough for any kind of LLWS, though those landing on E-W runways may experience localized weak LLWS of +/- 5 kt. ~12

MARINE

A westerly swell continues to push seas into the mid teens at around 14 to 16 seconds. Winds have weakened over the waters as of 4 AM this morning but expect another increase to around 20 to 25 kt gusts in the waters by this afternoon. A Hazardous Seas Warning continues for all the waters until 11 AM today. A Small Craft Advisory will follow, coming into effect from 11 AM today to 11 PM tonight. Seas subside to 10 to 12 ft by this evening and gradually subside into Wednesday. Seas will remain around 8 to 11 feet through Thursday, then dropping well below 10 ft Friday into the weekend. Winds will remain gusty around 20 to 25 kt mostly in the outer waters through Thursday, although gusts up to 30 kt is possible Wednesday afternoon for the outer waters south of Cape Falcon. ~12

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Hazardous Seas Warning until 11 AM PDT this morning for PZZ210- 251>253-271>273. Small Craft Advisory from 11 AM this morning to 11 PM PDT this evening for PZZ210-251>253-271>273.