Scattered Strong to Severe Storms Possible Today Across New England

Today's cold front has entered into western New England this morning and will very slowly crawl eastward as the day goes on. Areas east of this front this afternoon will stand the best chance for widespread thunderstorms this afternoon. Westernmost and northernmost New England will see less storm activity and more by way of cooler showers as the front crosses before daytime heating can occur.

Scattered showers and a few thunderstorms will likely break out across western New England starting around midday or early afternoon. As the afternoon goes on, storms will likely gradually increase in coverage and intensity as they slowly slide eastward. By mid to late afternoon, a broken line of storms will likely have developed right across New England. These storms will likely peak in coverage and intensity in the late afternoon and evening as they push from the Connecticut River Valley to the coast.

Below: HRRR showing potential weather early this afternoon (1st image) and early this evening (2nd image):

The showers and storms from this cold front will likely bring a widespread half inch to inch of rain across New England. The storms have been trending toward being more widespread and longer-lasting across the region, which has led to an increase in total rainfall. An area of frontogenesis will likely occur, which will lead to a band of 1-2 inches across a rather narrow portion of the region.

Frontogenesis refers to the strengthening of a front, which can often lead to a band of heavier snowfall during winter storms. Since we're in early September, the band of heavier snow is replaced by steadier rain and stronger thunderstorms. This looks most likely to set up across interior Maine through southern New Hampshire and into central Massachusetts.

The overall severe storm threat has continued to trend up over the last 24 hours. This steady trend toward stronger storms has mainly been driven by an increase in instability this afternoon and evening. Yesterday, we wrote about the likely upgrade from level 1 of 4 to level 2 of 4 by the Storm Prediction Center. This did occur in yesterday's afternoon update. The best chance of severe storms will be in a band from southern New Hampshire into central and western Massachusetts. Storms will likely weaken as they push closer to the coast later in the day.

The greatest threat will be damaging wind gusts. Large hail will be a lesser threat due to poor lapse rates, but will still be possible (just not as widespread as the potential for strong winds). A quick spin-up tornado can't be ruled out given the potential for discrete rotating supercells (especially before the storms "congeal" into one unified line later in the day) and the possibility for low-level instability to aid in rotation given the strong shear values in place. There's also the chance for localized flash flooding given the high amount of moisture and the potential for torrential downpours.

Looking at the four main ingredients for severe storms, they are all in place for New England to see at least scattered severe storms today. There will be ample lift from the robust cold front this afternoon and evening. There's a rather impressive amount of effective shear, with values upwards of 50-70mph. This is plenty to allow storms to strengthen. Moisture is also ample with high humidity streaming north ahead of the front. The main uncertainty for severe storms all week has been the potential for limited instability.

The trend toward more severe weather has followed an uptick in forecast instability. CAPE (Convective Available Potential Energy) values are favored to reach the 1,500 to 2,000 joules/kilogram range. This is enough to create a favorable atmosphere for storms to develop. The issue for instability to climb to these values will be cloud cover. Should clouds rule much of the day, instability will get capped. Overall, this setup favors scattered severe storms today; more than isolated but we wouldn't go as far to say "widespread" across New England.

Below: HRRR showing potential CAPE values this afternoon:

Today will likely feature the most favorable setup for severe storms New England has seen this summer. This isn't saying much as this summer has featured much less thunderstorm activity than past couple years. In fact, New England has not yet seen a tornado this year. For perspective, the last time New England didn't see a single tornado in a calendar year was 1952.

Today's secondary threat will be the potential for isolated flash flooding. PWAT values may push 2 inches this afternoon, giving storms ample moisture to create torrential downpours. This threat is higher across Connecticut and western Massachusetts as these areas are less entrenched in drought conditions. Unfortunately, areas currently seeing a severe drought in New England will see less rainfall from this system. All of New England has been dry, so this will be beneficial in the long-term, with some short-term impacts possible.

Below: Current New England drought status:

Dew points will likely be well into the 60s on Saturday with widespread 80s for high temperatures where the front won't cross until the afternoon or evening. Interior northern New England will see the front cross earlier in the day, leading to much cooler weather. There could be a decent temperature gradient across New England Saturday afternoon.

This cold front is looking like it may take on more of an anafrontal setup. When most cold fronts cross New England, they advect cooler and drier air into the region, allowing precipitation chances to diminish (this setup is known as katafrontal). When an anafrontal setup occurs, the cold air rapidly pushes against the warmer air ahead of the front.

This causes the warmer air to rise over the colder air behind the front. This results in clouds and precipitation behind the front rather than clearing. This setup will likely allow for clouds and showers to linger on Sunday, which is why Sunday has been trending in the wrong direction when it comes to nice weekend weather.