The system's primary low continues to look like it will move well north of New England, dragging its cold front across the region after a warm frtaontal passage. The main slug of rain will occur along a line ahead of the system's cold front. A secondary area of low pressure will form near New England along the front, allowing a blossoming of rainfall over southern New England through Monday.
After a mainly cloudy, but mainly dry Sunday, the first showers will likely break out across western New England in the evening before beginning to slide eastward. Guidance has continued to trend later with the arrival of the main shield of rainfall ahead of the cold front. This slug of rain may not arrive until after midnight for many.
Below: HRRR showing potential weather this evening (1st image) and overnight tonight (2nd image):
The cold front will slow down on Monday due to a ridge of high pressure offshore, allowing for periods of rain and showers to continue through Monday morning and into the afternoon. The secondary area of low pressure has trended northward, likely moving over New England.
This will allow showers to continue over much, if not all, of New England through midday. Rain will continue to shut down from west to east through the afternoon hours. It may take through Monday night to shut down all showers across Downeast Maine and Cape Cod, but elsewhere will likely completely dry out by Monday evening.
Below: Potential weather around sunrise Monday (1st image) and around midday Monday (2nd image):
There hasn't been much change in thought over the past couple days regarding the impacts from this system. New England is poised to see a widespread half an inch to inch and a half of rainfall by Monday evening.
The potential formation of a secondary low pressure near southern New England and the fact that the cold front will be slowing down as it moves eastward will help boost totals across southernmost New England and Downeast Maine. Northernmost New England will stand to see the least as the rainfall will be more progressive compared to farther south and east in New England.
Overall rainfall amounts may be a bit varied across the region due to the scattered nature of heavier downpours. There could certainly be locally higher amounts within the totals above due to the aforementioned scattered downpours. The overall coverage of convection will play a large role in overall totals.
The storm will be capable of creating periods of heavy rain Sunday night into Monday as a very strong southerly flow develops. This flow will allow for significant moisture from the deep south to work into New England on basically a conveyor belt working up the eastern seaboard. Precipitable water (PWAT) values may exceed 1" and potentially approach 1.30+". This indicates a moisture-rich environment given the time of year (PWATs are generally higher in the summer and take higher values to be considered moisture-rich).
Below: PWAT values overnight Sunday into Monday, showing a moisture conveyor belt from the deep south:
While rainfall totals on their own won't be too much to handle for most, it's the fact that significant snowmelt will occur across the mountains which will lead to potential runoff issues. Overall, 1-2.5 inches of runoff from snowmelt will combine with total rainfall. Total runoff across northern New England may come out to around 2-3.5 inches. Areas that have already completely melted out their snowpack will see a much lower chance of runoff/flooding issues.
River ice will continue to rot and break up as well. Rivers in southern and central New England will likely see river ice flushed out (if any ice still remains at this point), however, the thicker ice across the north may present some ice jam issues as it breaks up. Overall, ice jams are more likely across New Hampshire and Maine while open water river flooding is more likely in Vermont.
As of Sunday morning, the threat for minor flooding remains focused over Vermont, where a flood watch is in place. While minor flooding remains the going forecast, rivers will need to be monitored for isolated pockets of moderate flooding by Monday morning. Currently, the Missisquoi River in North Troy, VT; Mad River near Moretown, VT and Otter Creek at Center Rutland, VT are forecast to reach minor flood stage by Monday morning.
A handful of other rivers are forecast to reach action stage across all six New England states (which are seen in yellow on the river forecast map below), so rivers will need to be monitored through Monday across the region.
Below: Current river forecasts across New England that are expected to reach at least action stage. River gauges not shown are not expected to have any issues:
A strong low-level jet of 60-75mph will also cross the region Sunday and Sunday night. Naturally, not all of this wind will make it to the ground, and southerly winds can often have trouble mixing to the surface. This time will be no different as a strong inversion will be in place, which will help keep the strongest winds aloft. With that said, gusty winds will be likely during this time. They may be at their strongest Sunday afternoon, when mixing is deepest.
At this point, maximum gusts of 35-45mph appears to be the most likely outcome. This is below the threshold for damage and wind alerts. There will be the potential for occasional stronger gusts given the stronger low-level jet, but most of this jet's momentum is expected to stay above the surface.
This cold front is not going to usher in a much colder air mass. Ridging will build in after the system for Tuesday and Wednesday with seasonable to mild conditions for most. The next system looks to enter the picture next Thursday into Friday. A primary looks to move north of New England with potential secondary low development near the Mid-Atlantic. This storm currently doesn't look as moisture-packed as tonight's storm. The main area to watch will be the potential secondary low development in regards to strength, track and timing.
The potential development of this secondary could result in much colder air wrapping around the system, allowing rain to switch to snow by Friday for the northern third of New England. For southern and central New England, this appears to be another rainmaker at this point. Interior Maine would be the most likely to be affected by a switch to snow. Overall, a significant spread in outcomes and totals are present, so it's not worth trying to go into more detail (this may already be trying to get too detailed at this point).
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