textproduct: Portland

..Updated aviation discussion

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 80-100 J/kg by midday/afternoon, supporting a 10-15% 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 low pressure continues to drop southward. Moist northerly flow across the area is supporting lower cloud cover but should lift to low- end VFR this afternoon. Light showers remain possible through the first half of the TAF period, mainly over the higher terrain and in the southern Willamette Valley. Chances around terminals are relatively low so have kept out of the forecast at this time. North/northwest winds at 8-14 kt with gusts to around 20 kt possible between 22z Tuesday and 04z Wednesday.

There remains a 10-15% chance of thunderstorms today through 00Z Wednesday, mainly in the southern and central Willamette Valley around KSLE and KEUG.

KPDX AND APPROACHES...Mainly VFR conditions expected through the TAF period. Low pressure to the southwest is supporting northerly flow and SCT to BKN cloud cover around 1500-2500 ft. This is bringing brief periods of MVFR CIGs to the terminals but should lift to low-end VFR this afternoon. Northerly winds 8-12 kt with a few gusts to around 20 kt possible this afternoon. Could see MVFR CIGs redevelop late tonight after 12-14z Wednesday. There is a 20-50% chance over the eastern metro. -19

MARINE

A cold front continues to move east of the waters, leading to decreasing southerly winds through the day. Small Craft Advisories continue across all waters for steep and hazardous seas hazardous to small craft through 5 PM. A Westerly swell moves into the waters later this afternoon and evening, pushing seas in the mid teens around 14-16 seconds. Have upgraded the Hazardous Seas Watch to a Warning for all waters except the Columbia River Bar where conditions are more marginal. The Warning continues through 11 AM Tuesday and at the same time, the Small Craft Advisory for the Columbia River Bar has been extended for the same time frame. There is a 10-20% chance for thunderstorms across the waters this evening. Any passing thunderstorms may produce lightning, brief heavy downpours that reduce visibility, erratic winds, and/or small hail.Seas subside to 10-12 ft by Tuesday evening and gradually subside into Wednesday. -19

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 until 11 PM PDT this evening for PZZ210- 251>253-271>273.