Heat, Storms on Tap for New England Thursday

Today, a weak system will pass just to the north of New England. This will lift a warm front through the region, allowing warm air advection to occur at the surface. This will bring both temperatures and humidity up a notch from Tuesday. This system will allow for scattered shower development throughout the day and plenty of clouds around. The main question this afternoon will be how much cloud cover lingers, which will impact temperatures.

New England will remain in the warm sector on Thursday, with conditions more favorable for higher heat and humidity. Temperatures aloft will peak on Thursday, allowing for the hottest day of the week. A stronger frontal system will cross New England during the day, allowing for a stronger southwest flow, which will mitigate any sea breeze. Widespread highs in the 80s to near 90° are possible for southern and central New England. The northern tier will be kept cooler with more clouds and precipitation closer to the center of the system.

Thursday's frontal system will also likely trigger a line of scattered thunderstorms in the afternoon and into the evening. This line will come ahead of the system's cold front, expected to cross the region later Thursday into the overnight hours. The timing of when the cold front crosses the region will determine coverage and timing of storms as well as how potent they’ll be.

As of now, the western half of New England will have the best chance of seeing storms erupt in the afternoon. The ingredients for strong to severe thunderstorms will be in place. Western New England will have the highest chance for severe weather Thursday. How far east the main line of storms can push in the late afternoon and evening before fizzling out will be the biggest question. The main storm threat will likely be in the 3pm-7pm range for western New England and the 5pm-9pm range for eastern New England.

Strong to damaging winds gusts will be the greatest threat within storms as strong winds in the upper-levels will be present. Downdrafts within the storms will help bring these winds down to the surface. Both large hail and brief tornadoes are secondary threats. While conditions aren't really primed for large hail development, ample wind shear is present. With linear storm motions and elevated winds shears, it's possible New England sees its first tornado of the season, but this remains a low threat overall. Downpours and localized flooding will be possible as well.

Taking a look at the main ingredients for severe storms, all four will be in place. Wind shear will be on the order of 35-55mph, with the strongest values located across western New England (mainly west of the Connecticut River). This is ample shear to allow storms that develop to strengthen, and rather quickly. Moisture will be very high, with dew points well into the 60s, toward 70° within the warm sector. Precipitable water values will be in the 1.5+" range, indicating a moisture-rich atmosphere. The warm sector will be supportive of a moderately to strongly unstable atmosphere, with 1,500 to 2,000+ joules/kilogram of CAPE.

Below: NAM showing CAPE values (Convective Available Potential Energy, a measure of instability) Thursday afternoon:

The main limiting factor for strong storms will be in regards to the timing of the cold front. This front will be the main source of lift, and will likely not cross New England until later in the day, most likely in the late afternoon through the evening. Most guidance shows much of New England being capped on instability until the cold front arrives. The eastern half of New England may not see the cold front begin to arrive until later in the evening, and by then, storms will begin to weaken.

This is why the best chance for severe storms will be across Vermont, western Massachusetts and western Connecticut with chances gradually dwindling moving eastward. Some areas in eastern New Hampshire, eastern Massachusetts, Maine and Rhode Island may not see much of any storm activity until very late in the day.

Below: NAM showing potential weather in the early evening hours, showing the main line of storms still across western New England:

This late timing of the cold front is the main reason why a full-on severe weather outbreak is not expected despite favorable conditions. Scattered severe storms are likely, but plenty of areas across New England will likely see little to no activity as well. Pre-frontal storms are expected to be isolated in nature, but any storm that manages to form in the early afternoon before the cold front arrives will likely become strong to severe quickly.

Heading into next week, a strong ridge of high pressure looks to become centered to New England's southwest. This ridge is currently advertised with a height of 590-594 decameters extending into New England (the higher the decameters, the stronger the ridge and therefore, the higher the temperature potential), which would support highs well into the 90s in late June for New England's typical hot spots (mainly the river valleys).

Below: Euro showing geopotential heights early next week, with high heights being shown around a strong area of high pressure in the east:

The main question for New England when it comes to how hot it will get will have to do with the northern extent of the high pressure. The system will be set up to New England's south and west. The issue will be sorting out how far north the ridge manages to extend. Should it extend far north, it would help keep New England dry and scorching hot. If it gets suppressed farther south, it would open the door for showers or thunderstorms to round the top of it and enter New England. This would help tamp down temperatures. Should this occur, temperatures early next week will trend lower.