Update on Sunday PM/Monday AM Winter Event

Snow will begin to fall between 9-10 PM tonight as a weak weather system moves towards the Northeast.  Snow will begin to change to sleet, freezing rain and eventually plain rain in the morning.  Although this will be weak the storm will have a high impact on the AM commute.  The National Weather Service Boston office has issued a Winter Weather Advisory from 7 PM tonight until  1 PM Monday.
NWS Boston Headlines Sunday December 11


General Overview
Here is the surface fronts over the water vapor satellite.  High pressure has been in control for much of the week as temperatures were well below average.  Low pressure is tracking along a frontal boundary that is draped from Iowa to Maryland.
GOES Composite Water Vapor Satellite (Image NCEP/NOAA/NWS)
This boundary is advancing to the north and east into all of this cold air.  Here is a quick science lesson that many of you already know.  As the warm air meets the boundary it rises above the more dense cold air.  As the warm air rises it also condenses and falls as snow when the center of the storm is hundreds of miles away.    Here is the radar loop as of 330 PM.
NWS Northeast Regional Radar Loop
Snow will begin around 9-10 PM tonight across the whole region, including the south coast as temperatures initially are below freezing.  The Cape and Islands likely remain all rain.  As of 2 AM the mix/rain line is follows I-95 to Providence and then follows the coast to the Canal and up to Plymouth.
20z HRRR simulated radar 2 AM Monday (image Weatherbell)
By 6 AM the rain snow line is to the Southern MA border and pretty much to 495.  Snow flakes will be large and wet as the change to mix/rain.
20z HRRR Simulated radar Monday 6 AM (image Weatherbell)
Schools inside I-95 in Eastern MA and most of CT/RI should be fine for school with only minimal delays.    By 9 AM the rain/snow line will push all the way to Northern MA border.
20z HRRR simulated radar Monday 9 AM (image Weatherbell)
Any snow that falls in SE MA, RI, and CT will wash away quickly.  Snow will pile up the further north and west one goes in Southern New England.  The NW corner of the state will see 4-6 inches.
Elsewhere just a general 1-3, sometimes 4" event.  I think these NWS numbers are high.
NWS snowfall forecast Monday AM (image Weatherbell)

  I like the latest short range model best.
20z HRRR snowfall total through 9 AM (image Weatherbell)
Max temperatures surge into the 40's south of the Pike and near 50 in SE MA.
NWS max temperatures Monday (image Weatherbell)
Temperatures will steadily drop this week as another round of Arctic air arrives late week.  I'll post an update in the morning.  Brip, pack your lunch.

-Zack Green

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