For the last several days, New England has seen a milder air mass advect into the region aloft. This has not translated to the surface as a surface high pressure parked just to the north of New England has allowed colder air to remain trapped at the low-levels. With high pressure in charge, this has only led to chillier temperatures, but very calm and dry conditions. With the high pressure slowly pushing to the east, and a frontal system approaching, this setup will be conducive to periods of freezing rain.
For tonight into Saturday morning, a warm front will approach southern New England. This will allow for continued warm air advection into the region as the cold air from this past week gradually erodes away. At the surface, a cold air dam will keep temperatures sub-freezing longer. This will allow for some light freezing rain or drizzle to fall across southern and central New England early Saturday morning, if the timing is right.
Below: CMC showing potential weather in the pre-dawn hours Saturday:
The timing will need to be right for freezing rain to fall as the surface will be warming up, just slower than the temperatures aloft. Overnight lows tonight will likely bottom out early in the night before slowly rising for the rest of the night. Should temperatures manage to rise above freezing at the surface before precipitation arrives, it will be plain rain. The areas with the best chance to see freezing rain will be lower elevation areas of southern New England (Connecticut River Valley, Merrimack Valley).
A winter weather advisory is in place for a light glaze of ice across southern New England. This initial round of precipitation will be very light, resulting in no more than a couple hundredths of an inch of ice accretion. Recent trends have been for a faster warm-up of the surface with less freezing rain and more plain rain. An issue with this is that recent very cold conditions have left roadways very cold; cold enough that temperatures in the mid-30s may still allow for ice accretion on roadways even if it is plain rain showers Saturday morning.
The reason why its valleys that have the better chance to see some ice over the higher elevations is due to the inversion of warmer air aloft with colder air at the surface. This creates a cold air dam, where the warmer air rises over the colder surface air trapped in the valleys due to the terrain. This trapped colder air takes longer to scour out in these valleys while the inversion (when a layer of warmer air aloft sits over a layer of colder air at the surface) allows faster warming in the higher terrain quicker.
This initial round of precipitation on Saturday morning likely won't get much farther north than southern Vermont/New Hampshire. The warm front leading this precipitation will likely wash out across central New England. Further north, weaker forcing and dry air in place will likely keep things dry for Saturday. Any showers activity in southern and central New England Saturday midday and afternoon should be plain showers. These showers will be light and scattered, and much of the afternoon could end up dry in southern areas as well.
Below: GFS showing potential weather early Saturday afternoon:
Saturday night through Sunday, a frontal system will slowly approach New England from the west. This will allow for continued rounds of scattered showers to move through New England during this time frame. All of New England will be under the gun for precipitation beginning Saturday night. The day Sunday doesn't appear to be washed out, just unsettled with plenty of clouds and scattered showers around. These showers will generally move southwest to northeast through the day.
The sub-freezing air will have completely scoured out across southern and central New England, leading to plain rain showers. Across interior portions of northern New England, particularly New Hampshire and Maine, cold air will remain at the surface as Saturday morning's front will wash out before reaching them. This will allow for freezing rain Saturday night through Sunday.
The cold surface temperatures will be scoured out from south to north through the day, but the higher elevations could see periods of freezing rain/drizzle for much of the day Sunday. The setup will be the same as it was for southern New England Saturday morning with a cold air dam holding the sub-freezing temperatures at the surface while warmer air advects aloft. How much ice accumulates will be dependent on how long the cold air at the surface holds on for. As of now, probabilities remain low for more than a tenth of an inch.
Below: Current Probabilities for at least a glaze of ice from Saturday evening to Sunday evening:
Throughout the day, the warmer air working into the region will surge northward at the surface. This warmer air will take time to push northward, leading to a sharp temperature gradient that will slowly move northward during the day. Sunday afternoon, temperatures will likely surge into the low to mid 50s across southernmost New England at some point while temperatures remain in the upper 30s to low 40s a bit farther north and colder still well north.
Below: Potential temperatures early Sunday afternoon:
The frontal system to the west of New England will push across New England later Sunday into Monday from west to east. By Sunday night, all of New England will likely have warmed above freezing, allowing for a soaking from plain rain. A half inch to inch and a half of rain is possible across New England. Across northern New England, conditions will be primed for plenty of snowmelt, with as much as an inch of additional water from snowmelt possible. At this time, signals for a flooding event remain low, but trends will need to be watched.
Going back to the potential for icing across New England over the next couple days, the setup is conducive for some icing to occur. With a temperature inversion that has been in place for a few days and frontal systems coming along to provide the precipitation and increased warm air advection, the setup is there for frozen precipitation. With a deep layer of warmer air aloft with a rather shallow layer near the surface, this supports freezing rain over sleet.
Currently, it looks like Tuesday will see a lull in this unsettled stretch of weather as New England will be between systems. Depending on how the timing shakes out for the frontal system discussed above and an incoming coastal storm (which will be discussed next) will determine exactly when the lull between systems will come, but New Year's Eve may be able to thread the needle for a mainly dry day.
Heading into New Year's Day, another system will likely work through the northeast. This system will likely involve a northern stream low tracking to the north of New England with a coastal low potentially forming near New England or the Mid-Atlantic. The timing of a secondary low development and its track will determine precipitation types in New England. This does look like a rain event for low elevations, but some mixing and snow will be possible across the higher terrain, depending on the coastal low's track.
