New England has seen a sweltering first half of July. Compounding on this has been the persistently high, tropical-like at times, humidity levels. This oppressive humidity is being replaced today by much lower dew points as a cold front works through the region. By this afternoon, New England will be under a much more comfortable west-northwest flow. An expansive area of high pressure will set up thereafter, continuing mainly dry weather and low humidity through the weekend.
Today will be the transition day from what we've been entrenched in over the last week to what will be coming to wrap up the week. The cold front will be slow moving, so humidity levels will start off muggy, especially for southern New England, but they will be dropping during the day. All of New England will be relinquished from the high humidity by Friday morning.
Euro showing expected dew points late this morning (1st image) and late this afternoon (2nd image):
Air temperatures will also be lower than what the region has seen. Temperatures won't drop off too much as there is a lack of really cool air across Canada to spill into New England. This, along with ample sunshine, will keep temperatures into the 80s. Many areas of southern and central New England may see temperatures well into the 80s with some areas potentially reaching 90°. Regardless, it will feel much better without the humidity.
The root cause of New England's persistent weather this month, besides the fact that it's the middle of summer, has been a strong ridge of high pressure over the Atlantic Ocean. This ridge of high pressure, sometimes known as the Azores or Bermuda high, is considered semi-permanent, but its exact location and intensity can vary. When this system is near Bermuda, it helps to pump tropical humidity up the east coast. The ridge has been very strong and expansive this month, helping to pump the heat and humidity into New England.
Map showing pressure and wind direction. This snapshot is from yesterday morning, but the general setup has been in place for much of July with the strong high pressure around Bermuda. You can see the southwest flow riding up the east coast:
This strong ridge of high pressure has caused cold fronts to slow down, weaken and stall over, or just offshore, of New England throughout this month. This has prevented these fronts from bringing relief to the region, which would be more typical of summer in New England rather than constant high humidity.
In some places, the constant heat is very noticeable. Manchester and Concord, New Hampshire have set New all-time records for longest heat waves. July 17th was Manchester's 13th straight 90°. The previous record was 9, set in 2002. The city will continue to be in the mid to upper 80s, perhaps touching 90°, through the weekend. Concord has seen 12 straight 90° days. The previous record in Concord was also 9.
Of course, it's not just the heat, it's the humidity, as the old weather adage goes. In Boston, the maximum dew point has topped out at 70° or higher since July 4th, 14 days in a row. The average dew point for the entire day has hit 70° or higher for nine of the 14 days. These are truly tropical-like levels. These high dew points keep the overnight lows warm. The overnight low has stayed above 70° for 10 of these 14 days. It has not dropped below 70° in Boston at any point during the day since July 8th.
This ridge has also aided in the recent heavy rainfall and flooding events seen over the last week or so. The deep southwest flow led to an extended period of a moisture-rich atmosphere being in place. The stalled front mentioned before provided forcing for heavy downpours. Last week, Hurricane Beryl was forced to the north of New England due to the high. This lifted a stalled front back into northern New England, leading to 3-6+ inches of rain falling amid a heavily moisturized environment, leading to a major flood in Vermont.
After this weekend, the high pressure near New England will dissipate, opening the door back to a more southerly flow. This will allow humidity to increase into next week, though the extreme levels are not expected at this time. A system will likely rotate up the east, bringing unsettled weather back for the middle of the week. Temperatures are currently looking to remain around seasonable levels for much of next week, with 80s continuing to replace 90s.
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