Bowcast

Rainbow forecast for Portland

Portland, Oregon, United States: today's strongest rainbow forecast, the best interval to look up, and the week ahead.

45.52°N 122.68°W · about 40 ensemble members · 12 km radius

Today's peak · estimated chance

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Checking about 40 forecast ensemble members for sunlit rain near Portland…

Open the live map for Portland

Rainbow weather in Portland

Portland gets its rainbows from Pacific storm-light: frontal systems that break into showers as they cross the coast, with sharp clearings behind them. The hour after a front passes, when the western sky opens while rain still hangs to the east, is the classic setup here.

At 45.5° north, Portland's useful viewing window shifts with the seasons. Local winter brings longer periods of low sun, while summer usually concentrates the geometry near sunrise and sunset.

Wherever you are, the geometry is the same: a primary rainbow forms opposite the sun while the sun is less than about 42 degrees above the horizon. Bowcast's research-informed heuristics give extra weight to clearing showers. The gauge above applies them to ensemble forecasts to predict the rainbow chance for the 12 km around Portland.

The rainbow window through the year

in the window sun above 42° below the horizon
At 45.52 degrees north the 42 degree ceiling is never reached from Oct through Mar, so the whole of those days is usable. May closes for 7 hours around solar noon.
Share of daylight inside the window
Spring
69%
Summer
56%
Autumn
91%
Winter
100%

Rainbow questions, answered

When do rainbows appear in Portland?
Rainbows need sunlit rain with the sun lower than about 42 degrees, so in Portland useful windows often occur after sunrise or before sunset on showery days. A study in ZhaoSu, China found that about nine in ten observed rainbows occurred during the hour after rainfall. Bowcast uses that regional finding as research guidance, not a universal rate.
Where should I look for a rainbow in Portland?
Always directly opposite the sun: put the sun at your back and look toward the retreating rain. In the evening that means looking east; in the morning, west.
What is today's rainbow forecast in Portland?
Bowcast checks sunlit-rain ingredients around Portland, hour by hour, for the next 7 days. When ensemble guidance is available, it shows an Estimated chance from about 40 model members, not a probability calibrated against observed rainbow frequencies. When ensemble guidance is unavailable, it shows a Conditions score /100 instead. This page shows the strongest actionable interval; the Now board shows the interval containing the current time.
Can you predict a rainbow in Portland?
Not with certainty, but you can forecast the odds. Bowcast works as a rainbow predictor for Portland: it reads about 40 ensemble weather members for sunlit rain, checks that the sun sits lower than 42 degrees, and works out which way to face. Read it as an informed estimate of the chance, not a promise that a bow will appear.

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