Significant Weather Blog: Saturday, December 13, 2025 6 PM

Snow for Southern New England Late Tonight and Sunday

A rather fast-moving storm will pass south of New England Sunday and bring at least some snow to southern New England late tonight and Sunday.  Light to moderate amounts are expected over the Cape, rest of southeast MA, and along the RI and CT south coast.  There is little change from the weather pattern as discussed yesterday except most models have trended a little further north.  Thus, have nudged snowfall amounts up a little.  It is now looking like the Cape and Islands as well as the south coast of New England have a higher probability of plowable snowfall.  To the north, it still looks like a light snowfall.

Discussion

The upper-level trough that will spawn a surface low pressure center off the mid-Atlantic coast tonight has a moderately significant amplitude.  It reaches the east coast as a positively tilted trough (i.e., its axis runs southwest to northeast) but transitions into a neutral north-south orientation as it passes offshore and eventually a more negative tilt orientation (southeast to northwest axis) much farther offshore and over the Maritime Canada.  The positive tilt orientation usually supports weaker and faster surface systems which in turn limits the amount and duration of precipitation.  As the upper trough becomes more neutrally oriented offshore, the surface low center is expected to intensify and gain more latitude.  Thus, a major or long duration snowfall is very unlikely for southern New England.  This upper trough, however, has enough amplitude to bring mid to upper moisture northward and at least some uplift to enable most areas to experience a period or two of light snow with the higher amounts closer to the south coast where the moisture will be more abundant and the atmospheric uplift (the process that causes water vapor to condense and fall out as precipitation) stronger.  Looking at jet dynamics, the southern part of southern New England should lie under the right entrance region of an upper-level jet, a favored area for upward motion.

Another factor is that there could be some ocean effect enhancement over the Cape as well as possibly the Plymouth County coast south of Boston.  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 and subsequently intense upward motion (albeit over a shallow layer of the atmosphere) which wrings moisture out of the air and bands of snow showers or squalls.  Typically (although not always) the ocean effect snowfall is considerably less robust than its lake effect counterpart.  Anyway, snowfall on Sunday could be locally a little heavier along the Plymouth County coast and Cape Cod as well as cause snow showers to linger longer into Sunday night, especially over the Cape unless this system is just suppressed too far south with dry and subsiding arctic air dominating.

The computer model output has come in better agreement over the past 24 hours with respect to track and intensity of the surface low center.  The GFS model had been tracking the low center quite far south but has come into line with other models over the past 24 hours.

The cold temperatures typically support higher snow to liquid ratios.  However, upward motion in the best dendrite growth zone is relatively weak and result in small snow to liquid ratios and less efficient snow accumulation.  A shift toward the north in the track could increase dendrite production.  Very latest model output does put the Cape and Islands as well as much of the south coast in a somewhat more favorable location for dendrite growth and higher snow to liquid ratios.

Given the most recent model trends with a slightly further north track, the snowfall forecast has been nudged upward a little.

Forecast Timing

Snow will spread rapidly west to east across southern New England around or shortly after midnight.  The snow will be light to moderate intensity along and southeast of the I-95 corridor and likely just light intensity to the northwest of the I-95 corridor.  Snow will end west to east across most of southern New England Sunday afternoon.  However, the snow may persist a while longer along the Plymouth County coast south of Boston until early evening and possibly well into Sunday night over the mid and outer Cape as well as Nantucket due to lingering ocean effect snow showers.

Projected Snowfall Totals

Cape and Islands:  3 to 5 inches (possibly even 4 to 6 inches on Nantucket)

Northwest of the Cape to along the I-95 corridor (southeast MA, southern RI, coastal CT):  2 to 4 inches

North or northwest of I-95 to south of the Mass Pike (I-90):  1 to 3 inches

Along the Mass Pike (I-90) corridor including Boston: 1 or 2 inches

North of the Mass Pike (I-90):  an inch or les

Next message will be the issuance of an outlook on Monday for any potential significant weather for the week ahead on Monday.

About this Blog and Disclaimer

The National Weather Service monitors the weather 24/7 and updates forecasts, watches, and warnings as necessary.  For most of southern New England, refer to your local Weather Forecast Office for the latest forecast/warning information at weather.gov/box.  For information from other Weather Forecast Offices, go to weather.gov and click on the location of interest on the map.

A blog is issued when there is a potential risk of hazardous weather to southern New England and serves to provide background information regarding the significant weather with somewhat of an educational slant.  In the case of hurricanes, the author will attempt to issue a message when any portion of the United States coastline may be seriously impacted.  It’s intended to complement and not serve as an alternative to official forecasts issued by the National Weather Service.  Readers are urged to check National Weather Service forecasts and warnings for the most timely and updated weather information.  This weather message is solely produced by the listed author and does not constitute an official product of the Blue Hill Observatory and Science Center (BHOSC).

The frequency of this blog will depend upon the nature of the weather threat and availability of the author.  Thus, for the latest information on weather forecasts and warnings, refer to the National Weather Service, which monitors and forecasts the weather 24/7.

Blue Hill Observatory and Science Center (BHOSC)

The BHOSC maintains a continuous daily observation record that dates from February 1, 1885 without interruption and provides educational information on weather and climate to area students and the community at large.  Your support of BHOSC is much appreciated and enables this nonprofit organization to maintain the long-term climate record at the Blue Hill Observatory (longest continuous record in North America) and provide valuable educational resources on weather and climate via outreach to schools, site visits/tours, webinars, and conferences.  For more information on the Blue Hill Observatory and Science Center, explore bluehill.org.

Message issued December 13, 2025 by:
Bob Thompson
Retired National Weather Service Meteorologist
Blue Hill Observatory and Science Center Board member