Weather for New England and the Northeast

**Area Forecast Discussion, National Weather Service Boston/Norton MA, 247 PM EDT Tuesday, September 15, 2026**

**WHAT HAS CHANGED**

No significant changes to the forecast.

**KEY MESSAGES**

1. Dry and cool tonight. Heat and humidity begin to build for Wednesday.

2. Shower chances Thursday evening into Friday morning, followed by drier conditions Friday.

3. Next chance for rain returns Sunday, though confidence in exact timing remains low at this time.

**DISCUSSION**

**KEY MESSAGE 1, Dry and cool tonight. Heat and humidity begin to build for Wednesday.**

High pressure continues to dominate through Wednesday, although it will begin to weaken and move offshore tonight and early Wednesday. This will cause winds to turn more south to southwesterly, bringing warmer temperatures and higher dew points into southern New England Wednesday.

With relatively clear skies and light winds tonight, another round of radiational cooling is expected. Radiational cooling occurs when the ground loses heat efficiently under clear, calm conditions, although tonight should not be quite as strong as last night. Overnight temperatures should fall into the upper 40s to lower 50s.

Southwesterly winds Wednesday should push dew points back into the lower 60s. With temperatures climbing into the upper 70s to around 80 degrees, humidity will increase, producing a muggy and sticky feel.

**KEY MESSAGE 2, Shower chances Thursday evening into Friday morning, followed by drier conditions Friday.**

Thursday should be mainly dry, but a northern-stream shortwave trough will arrive late in the day and move out early Friday, bringing another round of showers to southern New England.

A shortwave trough is a smaller area of lower pressure and upward-moving air within the larger weather pattern. Clouds will increase Thursday morning as additional moisture moves into the region. Precipitable water values, which measure the total amount of moisture available in the atmosphere, are expected to be around 2 inches. This is near the upper end of what is typically observed based on atmospheric soundings from Chatham.

The main uncertainty is how much forcing will develop. Forcing refers to the atmospheric mechanisms that cause air to rise and can help produce clouds and precipitation.

The 8 AM EDT NAM model with 7.5-mile resolution and, to some extent, the 1.9-mile version, show stronger forcing near the New York City metropolitan area. The 1.9-mile model guidance does not extend beyond 8 PM EDT Friday. Farther northeast, however, the forcing over southern New England appears less favorable.

The ECMWF favors central New England for the stronger forcing, while the GFS and Canadian models show very limited forcing across central New England.

Another limiting factor is dry air in the middle levels of the atmosphere, which could reduce the amount of precipitation that reaches the ground.

Even so, a subtle shift in the forecast guidance farther northeast needs to be monitored, especially given the impressive swath of 2 to 4 inches of rain, with locally 5 inches, that occurred this past Sunday. More confidence would be helpful with this part of the forecast given that the event is only a few days away. However, when atmospheric forcing is weak, it can be difficult to pinpoint the exact location and amount of precipitation.

Current thinking favors the stronger forcing across central New England. There has been reasonable consistency from one model run to the next during the past 24 hours, as well as agreement across multiple models.

The precipitation forecast uses a blend of the current forecast, the National Blend of Models and the Weather Prediction Center forecast. Precipitation amounts are generally expected to be a few hundredths of an inch along the southern coast and perhaps around one tenth of an inch along the northern borders with Vermont and New Hampshire.

The NAM solution should not be completely discounted yet, so this portion of the forecast will continue to be monitored. Stay tuned for updates.

**KEY MESSAGE 3, Next chance for rain returns Sunday, though confidence in exact timing remains low at this time.**

A transient high pressure system moving out of the southern Canadian Prairies should bring a dry stretch of weather Friday and Saturday.

Seasonably mild temperatures are expected Friday, with highs in the middle 70s and dew points initially in the lower 60s. Dew points should fall into the 50s by Friday afternoon.

Saturday will bring a shift toward more fall-like conditions, with highs in the middle to upper 60s and crisp air as dew points fall into the 40s to lower 50s.

The weekend looks split, with rain returning Sunday. It is possible that the early hours of the day will remain dry, with conditions deteriorating during the afternoon and evening.

There remains some disagreement among the individual forecast models. However, the AI GFS and ECMWF are in fairly good agreement that unsettled weather will develop as an 850 mb open wave tries to close off across western New York.

An 850 mb open wave is a disturbance several thousand feet above the surface that has not yet developed into a closed circulation. If it closes off, it can help strengthen and organize the weather system.

This would lead to increasing southwest winds and higher atmospheric moisture moving into the region, with precipitable water values around 1.5 inches.

It is also worth noting that strong high pressure of more than 1030 mb is expected to build to the north during this period. Depending on how the overall weather pattern develops, this high pressure could push Sundays system farther south. This will be another feature to monitor.

Beyond Sunday into early next week, there is considerable variability in the forecast, mainly involving the arrival of high pressure expanding out of Quebec. This could affect how quickly the moisture associated with Sundays system moves away Sunday into Monday.