A mature frontal system over the Great Lakes will push north and west of New England Wednesday into Thursday. A deep southerly flow will develop ahead of this system on Wednesday, allowing temperatures to continue to climb now that this current bout of Arctic air has ended. This will inject plenty of moisture into the storm, allowing for a soaking rain event region-wide.
Strong warm air advection will also allow showers to break out well ahead of the system's arrival. Showers will fire up through the day Wednesday, gradually increasing in coverage as the day goes on. Showers will continue to fill in through Wednesday afternoon region-wide. The steadier and more widespread rain will move into New England from southwest to northeast from Wednesday evening through the overnight hours. The peak of the storm will likely be Wednesday night.
Below: RGEM showing potential weather Wednesday afternoon (1st image) and Wednesday night (2nd image):
The initial batch of showers may be light freezing rain across portions of the northern tier of New England as cold air damming (CAD) will initially be in place. CAD is often stubborn and slower to scour out than what guidance shows ahead of time, so some freezing rain may be possible Wednesday across the north country. Given the overall setup of the system, this shouldn't last very long and accumulations would be fairly light before melting, but a period of slippery surfaces will be possible during the day Wednesday across the north.
A band of steady rain with embedded downpours will continue to push across the region Wednesday night into Thursday morning. Overall, the main band of rain seems to be rather progressive as it pushes into eastern Maine by Thursday morning. Much of the day Thursday will be spent in the warm sector of the storm, meaning rainfall will become much more scattered in nature through the day (most of the day could end up being dry) with warm temperatures (Thursday will be the warmest day of the week).
Below: RGEM showing potential weather Thursday morning (1st image) and Thursday afternoon (2nd image):
When all is said and done, it looks like a widespread half an inch to inch and a half of rain is expected across all of New England. Overall, confidence on exact rainfall totals remains lower than normal at this stage in the forecast. This is mainly due to the likelihood of embedded downpours within the steadier rain.
At least a half inch is likely for just about everyone; exactly where the potential for over an inch sets up is the main question. It still looks like portions of southernmost New England, Downeast Maine and the White Mountains could see the highest totals.
The storm will have a clear connection to high Gulf moisture. With a strong southerly flow and warm advection, this plume of moisture will have little trouble making it to New England. This will allow for widespread moderate rain with the potential for briefly heavy downpours to develop as forcing ahead the storm's cold front increases Wednesday night. The positioning of this plume in relation to the cold front favors a decent slug of rainfall, however, the more progressive nature of the system will help limit overall rainfall amounts.
Below: Precipitable water values Wednesday evening, showing a conveyor belt of moisture moving into the northeast:
With steady rainfall, a period of warm, gusty winds and increasing dew points, conditions will be primed for additional runoff across the north from snowmelt. With that said, the snowpack is still quite cold and deep across the higher terrain and northernmost areas. This means it will take time for the snow to "ripen" to the point of rapid melting. This may help limit the overall snowmelt and runoff coming off the mountains and north country. Latest guidance shows an inch or less of runoff from snowmelt through Wednesday night. The snowpack will eventually ripen and be ready for faster melting by Thursday.
Areas that have a much more ripe snowpack going into the storm will likely see much more runoff by way of snowmelt. These areas include Downeast Maine and the higher elevations of southern and central New England. These areas will have the best chance to see 1-2 inches of additional runoff, though these areas should be better equipped to handle it versus the mountains.
Below: Current snow-water equivalent across the region. Areas in the red are more ripe and ready to melt out. The lighter shades represent a colder snowpack that will absorb rainfall initially rather than completely melt out:
With all of that said, the overall risk of widespread flooding issues remains on the lower end region-wide. Rivers are forecast to rise as the ground is frozen, so all of this water will run off. At this point, the rises are not forecast to reach flood stage on any rivers. Currently, the Mad River, Otter Creek (Vermont) and Hoosic River (Massachusetts) are forecast to reach action stage, so forecasts will need to be monitored.
The greatest flood threat will come from potential river ice breakup and ice jams, though even this threat appears to be more isolated and localized in nature. As of now, river ice across the north ranges from 6-12+ inches. Areas that have a thicker ice cover (12+ inches) will have a lower chance of ice movement and potential ice jam issues. Areas that have around a foot of ice thickness would need river rises of 2-3 feet to move the ice.
Across southern and central New England, drought conditions remain in place from this past fall. With the overall flooding threat here low to none, this will likely end up being a more beneficial soaking than anything.
A robust low-level jet of 70-80mph will also cross the region Wednesday into Wednesday night, resulting in a period of gusty winds. At this point, a strong inversion looks to limit the wind threat, but gusts of up to 40-45mph will be in the cards. A rule of thumb is when a strong inversion is in place, to generally expect 50% of the winds from the low-levels to reach the surface. It will be difficult for stronger winds to mix to the surface.
With a sustained and decently strong southerly wind occurring throughout Wednesday night, there will be a chance for splashover or minor coastal flooding during the overnight high tide Wednesday night. This threat is low overall, and minor coastal flooding appears to be a worst-case scenario.
The storm's cold front will cross later Thursday, bringing cooler temperatures back to New England, though another Arctic Blast appears unlikely at this point. New England is more likely to drop back to near seasonal averages. As per usual with the day after storms, it will be gusty on Friday. Winds will likely be stronger on Friday than they will be Wednesday night. The storm will likely continue to strengthen as it moves away, and could swing some snow showers into eastern Maine during the day. Other than that, it will be dry.
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