By James M. Arnold, Weather Specialist
The National Weather Service has issued a “Winter Weather Advisory” for a period of freezing rain from late tonight into early tomorrow morning.? Their 10:30 A.M. updated forecast discussion is below, with a few editorial changes for ease of reading:
“Expect precipitation to develop across the western zones between 7 and 10 pm tonight and expand northeastward quickly.? This will also bring a significant warm layer into the region from the south at higher levels of the atmosphere. Temperatures look to be warm enough at the onset for precipitation to fall as rain.? However…surface temperatures will drop through the night…resulting in a change to a variety of precipitation types for much of the area. Expect a period of snow or rain mixed with snow for southern New Hampshire before a change to freezing rain as the warm layer reaches that far north.?? An hour or two of sleet mixed in with rain/snow is possible north of Route 2 but south of the MA/NH border.?? Elsewhere a change to freezing rain is expected.?? Am most confident in this change roughly west of 495 and north of the Mass Pike.?? However…freezing rain is also possible across northern CT parts of northern RI…and areas of MA back to the 95 corridor. This is all dependent on how quickly? temperatures fall at the surface.?? If temperatures remain above freezing at the surface… then precipitation will remain as rain in these areas.?? If they don/t then freezing rain will occur.?? Have issued winter weather advisories for southern New Hampshire and all of Massachusetts roughly west of 495.?? Advisories may be necessary outside of this area…but with the uncertainty in the surface temperatures…will allow the day shift to take another look.”
Meteorology 101 and experience says that freezing rain is the hardest form of precipitation to predict.? What is needed is a layer of warm air at the mid to high levels of the atmosphere and freezing or below conditions near the ground surface.? Precipitation begins to fall as snow at high levels, encounters the layer of warm air and melts to become rain.? Near the surface of the ground the rain encounters a thin layer of temperatures at freezing or below, causing the rain to become super cooled and freeze on contact with surfaces.? Freezing rain can also occur with surface temperatures a bit above freezing if the recent temperatures have been cold enough to chill pavements etc. to levels below freezing, which also causes the rain to freeze on contact.? I think temperatures will fall back to near or below freezing tonight, raising the potential for a glazing of ice in our area from late tonight into early tomorrow morning.
Friday will see warming temperatures and rain, some of which will be heavy with even the chance of thunder during the day.? Urban and street flooding is likely due in part to drainage systems being clogged with snow and being unable to handle the volume of water from the heavy rain and rapidly melting snow.? Mild temperatures will be with us throughout the weekend.? Colder weather and more storminess likely in the form of snow is on tap for next week.? More on that as it draws nearer.
James M. Arnold is a Weather Specialist working with Shrewsbury Emergency Management Agency; town of Princeton; Worcester Emergency Communications and Emergency Management Agency; Southborough Emergency Management Agency; town of Grafton and Wachusett Mountain Ski Area