An Arctic high pressure system building over the Great Lakes today will lead to a very cold day today with highs ranging from the low teens across the north to low 20s in southern New England. The pressure gradient between the high to the west and the departing storm to the east will keep the breeze around, leading to wind chills remaining in the single digits all day.
Sunday night will be the coldest of this stretch due to optimal radiational cooling conditions under the high pressure bringing light winds and clear skies. Lows will be in the single digits, on either side of zero, across the entire region. Lows in the -10s will be possible across the north.
The high pressure slips overhead on Monday, bringing continued cold temperatures. It will begin to slide away Monday evening, making way for a very festive clipper system. These clippers have been the dominant winter storm type for New England so far in this young winter season. Timing of this system has continued to trend earlier. Guidance is beginning to agree that the system will pass through New England Monday night into Tuesday. Initially, it was slated to move through Tuesday evening into Wednesday morning.
This does appear to be a standard clipper system, staying on the weaker side and moving quickly through the region. Standard clippers typically see better forcing for precipitation on the northeast side of the low pressure center. This system is currently expected to cut straight through New England, leaving northern New England on the juicier side while southern New England may get into a dry slot.
Snow showers are currently expected region-wide, but they are currently looking to persist for a longer period of time across the north. Snow showers will also likely become steady for a time just north of the low's track. With that said, the low's track will be important and determine exactly where the most snow will set up. No matter where the low ends up tracking, this will be a minor event all around.
As of now, central and northern Vermont are in the zone for several inches of snow. Totals may also get a boost along Maine's eastern coast as the system slowly strengthens, drawing in more moisture into the area from the ocean as it does so. Elsewhere in New England is slated to get a couple to a few inches, with lesser amounts across southern New England.
Below: Current probabilities of at least 2 inches of snow by Wednesday morning:
Temperatures will be moderating back to seasonable levels after the very cold start to the week. With that said, there will be plenty of residual cold air leftover for an all snow event for a majority of New England. Only southeast Massachusetts may see some mixing and rainfall as a coastal front sets up, leaving them on the warmer side. Snow ratios will be high, likely higher than 10-15:1, so it will be a fluffy snow.
We often talk about snow ratios during storms and this is what that means. Snow ratios, or snow to liquid ratio (SLR) is the amount of moisture contained within the snow. The SLR represents the amount of water that is contained within the snow. This is shown as a ratio. A 10:1 snow ratio means that for every 10 inches of snow received, it would equal 1 inch of water when melted down. The higher the snow ratio, the lighter and fluffier the snow. The lower the ratio, the heavier and wetter the snow.
Below: Average SLRs across the United States. New England's average snowstorm involves SLRs of 10-12:1, however, the region can range from as low as 6:1 to as high as 25:1 in a given winter:
There are many factors that can go into determining snow ratios, but one of the most important is the air temperature. When the temperature is right around freezing, or just above freezing, the air can hold more moisture, allowing the snow to be wetter and heavy with large flakes. Very cold air holds little moisture, so the snow will be light and fluffy with small flakes. When ratios are above 10:1, it is the fluffier kind; lower than 10:1 is the heavier kind.
With that aside, this clipper will likely be pulling away by Tuesday evening, leaving Christmas Day/start of Hanukkah dry, quiet and seasonable under a new high pressure building to the north of New England. This will keep the weather mostly quiet for the rest of the week, aside from quick and very weak disturbances that may trickle through. A highly amplified ridge will build over the eastern United States with warm temperatures aloft, but surface high pressure to the north of New England will likely prevent a major warm-up.
Below: Current weather map for Christmas morning. That high to the north of New England will remain in place for much of the week. Despite warming temperatures aloft, this high will likely keep colder air filtering into the surface for New England, preventing a notable warm-up:
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