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Ocean-effect snow on Cape Cod and the coast

December to February

Ocean-effect snow is the coast's version of lake-effect snow. When very cold air blows over ocean water that is warmer than the air, it can build narrow bands of snow showers that land mostly on Cape Cod, Nantucket, and the Plymouth County shore, and sometimes on Cape Ann, usually with very minor accumulations.

Why this time of year

Winter is when arctic air reaches the coast while the ocean is still warmer than that air. National Weather Service forecasters in Boston have written about ocean-effect snow in December, January, and February, and in some years in November and March.

How to notice it

  1. On a bitter, windy winter day, open a radar app and look off Cape Cod and the South Shore. Thin lines of snow trailing off the water may show.
  2. Read the National Weather Service Boston forecast discussion for the words 'ocean effect'. Forecasters there have written that wind direction 'will be everything': it decides whether the bands come ashore and where.
  3. If you are on the coast, watch for flurries falling from a band of low cloud coming in off the water.
  4. If you are inland, compare your sky with a coastal webcam or a friend on the Cape. On some of these days most of the region has full sun while flurries fall on the Outer Cape and Nantucket.

Try this with kids: Find the skinny snow stripes coming off the ocean on the radar map. Are they over the land or still out at sea?

Phrase to know: “Ocean-effect snow”: snow made when cold air crossing warmer ocean water turns unstable and rises, wringing moisture out of the air.

Safety

  • These cold outbreaks can bring dangerous wind chills. Dress for the wind before going out to watch.

Sources

  • bluehill.orgMechanism, and how it compares with lake effect (Blue Hill Observatory, 2025-12-13): 'The physics behind ocean effect snow is analogous to lake effect snow.' 'Cold air moving over significantly warmer water (be it lake or ocean water) causes low level instability.' 'Typically (although not always) the ocean effect snowfall is considerably less robust than its lake effect counterpart.' Also: snowfall 'locally a little heavier along the Plymouth County coast and Cape Cod.'
  • bluehill.orgDecember observation (Blue Hill, 2025-12-15, about 2025-12-14): Cape 'snowfall was enhanced by ocean effect'; 'Snowfall across the Cape appeared to average between 4 and 6 inches'; 'isolated 4+ inches fell at the tip of Cape Ann (e.g. Rockport)'; 'From looking at radar imagery, this also appeared to be the result of ocean effect enhancement'; 'generally 1 to 2 inches along and north of the Mass Pike.' This was ocean effect adding to a passing storm, not ocean effect alone.
  • bluehill.orgSouth Shore (Blue Hill, 2026-01-29): 'possibly some enhancement from ocean effect snow along the coast from Plymouth to Hull and Weymouth.'
  • mesonet.agron.iastate.eduNWS Boston forecast discussion, 2025-12-04 (archived copy of the original NWS text at the Iowa Environmental Mesonet): 'Despite full sun for most' ... 'bands of ocean effect clouds and perhaps some scattered flurries at times underneath them across the Outer Cape and Nantucket.' 'NWly winds then shut off' ... 'the ocean-effect generation.' This supports the flurries-under-a-mostly-sunny-forecast way of noticing it, and December.
  • mesonet.agron.iastate.eduNWS Boston discussion, 2021-01-27: 'NE flow combined with favorable low level temperature profile will support' ... 'bands of ocean effect snow on Cape Cod and Plymouth County.' Northeast wind; January.
  • mesonet.agron.iastate.eduNWS Boston discussion, 2021-01-27 08:48Z: 'The strong winds moving this cold air over ocean water temps in the upper 30s and lower 40s' 'will cause ocean-effect snow showers.' Water temperature figure (deg F) for that event.
  • mesonet.agron.iastate.eduNWS Boston discussion, 2026-02-01: 'ocean effect snow bands forming from the prolonged northeasterly flow' ... 'Thinking these bands will set up over Cape Ann and into Plymouth County.' Cape Ann; February.
  • mesonet.agron.iastate.eduNWS Boston discussion, 2026-02-03, on wind direction: 'Wind direction will be everything here' ... 'NW winds generally favor the coastal waters east of the Cape and islands.'
  • mesonet.agron.iastate.eduNWS Boston discussion, 2026-02-06: 'Behind the arctic front as winds turn northerly expect ocean effect snow showers to develop over Cape Cod and Nantucket.' 'the shallow boundary layer and marginal ocean induced CAPES will probably limit the intensity.'
  • mesonet.agron.iastate.eduNWS Boston discussion, 2026-02-07: 'ocean effect snow showers will linger on Cape Cod and Nantucket through the evening with additional accum bringing total accums to 2-4 inches here.' Also 'Dangerous cold for Saturday night into Sunday.' Supports the safety note.
  • cimss.ssec.wisc.eduNWS Boston discussion, 2022-12-24 (copy hosted by UW-Madison CIMSS): 'Most of the time our ocean effect snow events are dealing with northerly flow with a shallow inversion layer' ... 'often a limited fetch. This results in very minor snow accumulations in most cases.' 'Large temp differentials from the top of the mixed layer to the SST were on the order of 22C/23C.'
  • cimss.ssec.wisc.eduUniversity satellite blog (UW-Madison CIMSS, 2022-12-26) on the rare 2022-12-24/25 event with west-southwest winds, which hit Martha's Vineyard, Nantucket, and Cape Cod. Quotes NWS Boston: 'An extremely rare and significant snow band has been impacting Marthas Vineyard.'
  • ams.confex.comUMass Lowell paper at an AMS conference (Colby), on the 1999-01-13/15 ocean-effect storm: 'more than 25 cm of snow fell along the New England coastline'; 'Snow also fell inland, although in decreasing amounts as the distance from the coast increased.' This was an unusually large event.
  • mesonet.agron.iastate.eduNWS Boston discussion, 2019-11-18 (opened by the refute pass 2026-11-10): 'A band of ocean enhanced light snow showers/flurries was impacting central and parts of northeast MA'. November, and inland reach.
  • mesonet.agron.iastate.eduNWS Boston discussion, 2021-03-15 (opened by the refute pass 2026-11-10): 'Last of ocean effect snow showers was pushing off outer Cape'. March.

Checked against these sources on November 10, 2026.

In the guide for December, January, February

Also worth noticing