Marine Weather and Tide Forecast for Portland, OR

SYNOPSIS
Temperatures will trend cooler this week as a series of fronts cross the region and bring beneficial valley rain and high mountain snow.

DISCUSSION
Satellite imagery this afternoon shows fog and low clouds hugging the coastline. High resolution models are in good agreement marine clouds will consolidate and push back into the coastal river valleys this evening bringing at least some patchy fog. Elsewhere, clear skies today will allow temperatures to rise into the 70s across the inland valleys.

Ensemble guidance continues to show a series of fronts dropping southeastward across the region this week as a longwave trough over the eastern Pacific and surface low pressure system originating in the Gulf of Alaska slide eastward. The first front will be the sacrificial lamb and primarily serve to moisten the lower and mid levels of the atmosphere as it approaches the coast tonight into Monday morning. Nonetheless, there is a high probability (>90% chance) that some light rain reaches the far north Oregon and south Washington coast towards midday Monday. As the front progresses southeastward it will weaken considerably so that measurable rain chances drop below mentionable thresholds (<15% chance) farther southeast towards eastern Lane County.

The aforementioned front will most likely hang up across the area late Monday into Tuesday with precipitation likely reforming along the frontal boundary as upper level support increases. As an embedded shortwave trough drops southeastward towards the region, another front will slide southeastward towards the region and produce another round of precipitation late Tuesday into Wednesday. Rainfall with all of these fronts appear most likely to be beneficial. There is a 80-90% chance that the entire area receives at least 1/2" of rain by the end of the work week. Even probabilities for 1" of rain by the end of the week are 70% or higher although the lowest probabilities are in the typically rain shadowed areas of the Willamette Valley near and west of Interstate 5 and in the central Columbia River Gorge near Hood River.

There is good agreement among the ensemble guidance that another shortwave trough will drop southeastward into the region and bring an additional round of precipitation to the region late Wednesday into Thursday. This third storm system appears most likely to lower snow levels down into the 4000-5000 ft range. As the global ensembles merge on a solution, confidence in accumulating snow in the Cascades is increasing. For example, there is a >80% chance that the first 1" of snow will fall at Santiam and Willamette Passes late Wednesday/Thursday.
Nonetheless, there are some details that remain quite uncertain that impact QPF, and thus snow amounts. As a result, NBM probabilities suggest only a 30-40% chance that snowfall amounts of 6" or more will fall at 4500-5000'. It should be noted that low percentage of ensemble solutions produce a much more impactful snow event for the 4500-5000' elevation band in the Cascades where the NBM suggests there is a 10% chance for 12" or more of snow falling Wednesday night into Thursday. This would certainly be enough snow to temporarily strand most vehicles on unmaintained roads.

How the large scale pattern will evolve Friday into next weekend remains rather uncertain. The main uncertainty revolves around what happens with the Wednesday night/Thursday shortwave trough once it departs the region. About 60% of the global ensemble guidance suggests the aforementioned shortwave trough will slide eastward into the eastern US. This produces a lower amplitude shortwave ridge over the Pacific Northwest and a broad longwave trough over the eastern Pacific. This would place the Pacific Northwest in a region more susceptible to an atmospheric river.
As an aside >90% of the EPS membership is on board with this general weather pattern coming to fruition.

Meanwhile, about 40% of the global ensemble guidance suggests that the Wednesday night/Thursday shortwave trough will dive southeastward into the Great Basin and form a closed low. This produces a stronger ridge of high pressure over the Pacific Northwest and allows the area to dry out for at least a day or two next weekend.

AVIATION
High pressure over the region, with dry southwest flow aloft will maintain VFR conditions inland until around 12 to 14Z. Will see areas of IFR stratus and fog reform tonight, mainly valleys along the coast into the Coast Range/Willapa Hills, as well as through out the interior from Cowlitz Valley southward through the Willamette Valley. This stratus/fog will persist into Monday morning before thinning.

PDX APPROACHES...VFR through this evening, with variable thin high clouds. Will see areas of IFR to lower end MVFR stratus/fog developing around 12Z to 14Z, and persisting well into Mon am.

MARINE
High pressure over the waters will shift inland tonight and Monday as the first in a series of fronts arrive.
This front will bring gusty south winds to 25 kt later Mon into Mon evening. Winds ease behind the front later Mon night, but a stronger front arrives later Tue night into Wed, with another round of gusty southerly winds. Another front will arrive late in the week.

Seas running 4 to 6 ft through tonight. But a modest fetch of west swell will arrive late tonight into Monday, with seas building to 9 to 12 ft at that time. Seas will gradually subside back to 6 to 9 ft for Tue and Wed. Seas build back to 10 to 12 ft by Thu/Fri.

PQR WATCHES/WARNINGS/ADVISORIES
OR...None.
WA...None.
PZ...Small Craft Advisory from 5 AM Monday to 8 AM PDT Tuesday for PZZ210-251>253-271>273.