A storm system will be pushing north-northeast from the south into the Great Lakes during the day today. This will lift a warm front across New England on Thursday. As the system continues to lift to the north of New England tonight into Friday, rainfall will fill in and become steadier for tonight. This will lead to a widespread half inch to inch and a half of rain, with the most falling across areas that will see rapidly melting snow, adding to runoff.
ALERTS
TIMING
Thursday during the daylight hours will see more by the way of scattered, light showers as the warm front lifts northward. Many areas may see plenty more dry hours than wet hours. With that said, showers will be scattered around throughout the day. The shield of light showers will overspread New England from southwest to northeast starting this morning and reaching eastern Maine by this evening. Overall, showers are likely to be most numerous in western New England during the daylight hours Thursday.
Expected weather early this afternoon:
After this initial shield of light rain, there will likely be a lull in more widespread rainfall, though isolated to scattered showers will hang around. By this evening, increasing moisture from a strong southerly flow will allow rain to fill back in across New England and become steadier.
Expected weather this evening:
The bulk of the rain is expected for most to fall from the early morning hours Friday through about midday Friday. During this time, moderate to at times heavy rainfall will occur across New England, with the most frequent downpours likely occurring across the White and Maine Mountains.
Expected weather Friday early morning (1st image) and Friday mid-morning (2nd image):
While the heaviest of rain will move out by midday Friday (except for eastern Maine), showery, damp and dreary weather will continue for much of the day. With the upper low still moving north of New England, Saturday will likely turn out to be a mainly cloudy day with some showers around, mainly in the mountains.
RAINFALL/FLOODING
Rain will be able to become moderate to heavy tonight into Friday morning as the atmosphere becomes moisture-rich. A widespread half inch to inch and a half of rain is likely across New England. Precipitable water (PWAT) values will shoot well over an inch for most of New England, with some areas seeing PWAT values near 1.5 inches.
PWAT is defined as the amount of water vapor contained in a column of air if it were to be condensed and collected. Basically, the higher the PWAT value, the higher the potential rainfall rates can get. The PWAT value does NOT correspond to how much rain is expected to fall, meaning that a PWAT of 1 to 1.5 inches does not automatically mean 1 to 1.5 inches of rain is expected.
GFS showing expected PWAT values Friday morning:
While PWAT values pushing well over an inch isn't uncommon in New England in the summer, it is highly elevated for mid-April. With all that said, there is plenty of moisture to be wrung out from the atmosphere. This will allow periods of scattered downpours. The big factor with this storm is that it will be progressive, so while downpours are possible, the time frame for them to occur isn't that large. This will help keep the highest of rainfall generally under two inches. Areas that see convection will likely be able to eke out higher totals.
As for the flooding risk, it will be greatest across the White and Maine Mountains. These areas will see the greatest rainfall with the potential for upslope enhancement, leading to higher totals across the mountains. Along with this, the mountain snowpack is ripe for rapid melting. This melting snow could add up to 2-3 inches of additional runoff. Full snowpack melt is expected below 2,000 feet. A still partially frozen ground will aid in immediate and nearly complete runoff into rivers.
Forecast 48 hour snowmelt:
All of this could lead to minor to moderate river flooding along with the potential for flash flooding across the White Mountains and western Maine. The Pemigewasset, Saco, Androscoggin, Kennebec, Piscataquis, Sebec and Pleasant Rivers are highlighted as rivers that may reach flood stage. Smaller rivers and streams may experience flash flooding.
Outside of these potential problem areas, more isolated and minor urban and river flooding is possible. The typical problem rivers in southern New England (Wood and Pawtuxet Rivers mainly; flood watches have been issued for these rivers) may reach minor flood stage as grounds are saturated across the region. Again, the window of heavy rain is brief for most which will help keep rainfall amounts down. The Green Mountains are unlikely to see enough rain to cause anything other than isolated issues in Vermont.
WIND
A strong 75-80mph low-level jet will cross eastern New England Friday morning, however, a strong temperature inversion will likely keep much of this wind from reaching the ground. A temperature inversion is an area of warmer air aloft with cooler air trapped below. This layer of warm air can prevent the strongest winds from reaching the surface. With that said, gusts of up to 50mph will be possible along the coast. It will be gusty farther inland, but should remain below impactful levels.
COASTAL FLOODING
Minor coastal flooding is possible, mainly along New England's south coast. A surge of 2+ feet is possible, though there is uncertainty in when exactly the peak surge will be. This will determine the extent of coastal flooding. Whenever the peak surge happens, anything greater than minor flooding or splash over is unlikely. Coastal flood advisories have been posted for the south coast.
Elsewhere in New England, scattered, minor issues are possible. A coastal flood statement has been issued for the eastern Massachusetts coast and could be extended farther north into New Hampshire and Maine later today or tonight.