Marine Weather and Tide Forecast for Portland, OR

SHORT TERM
Now through Saturday night...As of mid- afternoon, a transient shortwave ridge is lifting north across western Oregon and southwest Washington, supporting well above normal temperatures for mid April. Many inland locations are on track to reach the low to mid 70s today, with coastal areas remaining in the 60s under a modest onshore influence. High clouds are increasing from the south, though coverage remains scattered with ample breaks supporting efficient daytime heating.

Attention then turns to a vertically stacked low centered off the northern California coast. This system will gradually drift northward through today and into Friday, placing the region beneath strengthening southerly to southeasterly flow aloft.
Increasing mid-level moisture and modest instability are beginning to overspread the region this afternoon, marking a transition toward a more convective pattern. Over the past few hours, around 11 AM to 2 PM, thunderstorms have begun to develop around the OR/CA border and southwest Oregon.

The initial focus for showers and thunderstorms will be across Lane County and the central Oregon Cascades this afternoon and evening. Current guidance supports precipitation probabilities of 20-40% across much of Lane County and extending northwestward into portions of Linn, Marion, Benton, Lincoln, Polk, and southern Clackamas counties this afternoon and evening. The highest PoPs, 60-80%, remain over the Lane County Cascades and foothills. Thunderstorm probabilities generally range from 10-20% in the Lane County Cascade foothills, increasing to 20-30% in the Lane County Cascades where orographic lift and greater instability overlap. The Storm Prediction Center has outlined a Marginal Risk for portions of the Lane County Cascades and foothills today, consistent with the potential for isolated thunderstorms capable of brief heavy rain, small hail, and gusty winds.

Coverage expands Friday as deeper moisture rotates northward around the offshore low. Shower probabilities increase to 75-95% over the Cascades and foothills Friday afternoon and evening, with 30-60% probabilities across the lowlands of northwest Oregon and southwest Washington. Thunderstorm chances also increase compared to Thursday, reaching 15-35% across the Oregon Cascades, Willamette Valley, and potentially the Portland- Vancouver metro (highest along the Lane, Linn, and Marion County Cascades). Southeasterly flow aloft may allow a few storms to drift west of the foothills.

By Saturday, ensemble guidance indicates broad cyclonic flow over the Pacific Northwest. Shower chances peak area-wide, reaching 70-100% inland and 60-80% along the south Washington and north Oregon coast. Thunderstorm probabilities range from 15-30%, highest over the central Oregon Cascades and foothills.
Across all three days, the strongest cells may produce brief moderate to heavy rain, small hail, and gusty outflow winds.
Forecast soundings continue to show modest CAPE and limited shear, suggesting organized severe thunderstorms are unlikely.
While an isolated stronger storm cannot be entirely ruled out, particularly Friday afternoon, the probability of large hail or damaging winds above 58 mph remains low. Storms will move south to north; building cumulus to the south may signal approaching wind shifts and precipitation.

Temperatures trend slightly cooler Friday, with highs in the upper 60s to low 70s inland, then cool further Saturday as cloud cover and precipitation increase.


LONG TERM
Sunday through Wednesday night...A cool, showery pattern becomes established Sunday as the closed low settles over northern California and far southern Oregon. Wrap-around moisture on the northern periphery of the circulation will maintain scattered showers, especially during the afternoon.
Precipitation probabilities range from 60-90%, highest over the Oregon Cascades and foothills. Thunder chances diminish to below 10% as instability weakens. Sunday is expected to be much cooler compared to Saturday, with lowland highs in the upper 50s to low 60s.

On Monday, ensemble cluster analysis indicates the low shifting southeastward, with northwesterly flow developing aloft. A weak embedded shortwave may move through during the day, maintaining a 60-85% chance of light showers across the Cascades and a 30-60% westward. Temperatures remain near to slightly below seasonal normals. Shower coverage decreases Monday night into Tuesday as transient ridging and westerly flow briefly build overhead.
Attention then turns to the next Pacific frontal system expected sometime between late Tuesday and Wednesday. While timing differences persist among ensemble members (9+ hour arrival time difference), confidence is increasing that widespread rain will return during that window. Current NBM probabilities suggest a 40-60% chance of 0.25+ inches of rainfall over 24 hours between Tuesday evening and Wednesday evening across the lowlands, increasing to 50-75% in the Coast Range and Cascades.
Probabilities of 0.5+ inches range from 20-35% in the lowlands and 35-60% in the mountains. Chances of 1+ inches remain around 5-10% across the interior valleys and 20-35% in the higher terrain. ~12

AVIATION
Current observations show VFR conditions mid to high level clouds slowly moving from south to north as southerly upper level flow persists across the airspace. Showers persist along the coast through around 09Z Friday. Otherwise, the airspace will remain relatively dry through around 18Z-22Z Friday. Afterwards, showers return and will take a south to north track over the airspace. Thunderstorms also return to the majority of the airspace with a 15-30% chance of thunderstorms also spreading south to north from 18Z Friday through 06Z Saturday. Have included PROB30 groups for KEUG, KSLE and KUAO as those sites currently have the highest probability. Other inland locations do have the chance for thunderstorms, but at this time, those chances are less than 30%.
There is moderate confidence in that the probabilities of thunderstorms will increase with the 12Z TAF package.

It should be noted that locations that do see stronger showers or thunderstorms will have the potential for brief moderate to heavy rain, small hail, and gusty and erratic outflow wind gusts up to 35 kt. These showers could also result in periods of MVFR/IFR conditions, which will likely coincide with showers and thunderstorms.

KPDX AND APPROACHES...Expect predominately VFR flight conditions through the TAF period. Showers expected around 22Z Friday, with a 15-20% chance for thunderstorms starting around 00Z Saturday. Stronger showers or thunderstorms will have the potential for brief moderate to heavy rain, small hail, and gusty and erratic outflow wind gusts up to 35 kt. Showers/thunderstorms could also result in periods of MVFR/IFR conditions. /42

MARINE
Tranquil conditions are expected to persist over the coastal waters through early next week. North to west winds will continue over the coastal waters, before becoming southerly on Friday then becoming northerly again by Saturday, then more westerly by the start of the week. Winds expected to be below 20 kt through the weekend and into the middle of next week. Seas around 5 to 7 ft through the middle of next week as well.

There is a 5-10% chance of thunderstorms Thursday through Saturday afternoons across all waters. Any thunderstorms that do develop could produce hail, gusty and erratic winds as well as a less than 2% chance for a funnel cloud or waterspout.

It is unlikely Small Craft Advisory conditions will occur over waters at any point in time through Monday, aside from one potential minor exception. Between 4-9 PM PDT Thursday, marginal small craft advisory level wind gusts as high as 22 kt may occur over portions of the northern and central outer waters. Although probabilities for max wind gusts to peak around this range are fairly high at 30-50% over PZZ272 and it appears wind gusts of this magnitude would only occur for a 1-3 hour period at any given location. Given seas are only expected to range between 5 and 6 ft at that time, have decided not to issue a Small Craft Advisory as conditions appear too marginal and short-lived to justify a headline. /42-23

PQR WATCHES/WARNINGS/ADVISORIES
OR...None.
WA...None.
PZ...None.