New England has been dominated by domes of high pressure for the better part of two weeks now. More recently, this high pressure has resulted in a Rex Block (high pressure directly over low pressure, blocking the low pressure's northward advancement) since the middle of last week. This has brought very stagnant, warm weather to New England along with a growing dry spell. This will finally be changing going forward, but it won't be an abrupt change. It will be a positive change for those who want more fall-like temperatures.
Today will be more of the same with plenty of sunshine and warm, summer-like temperatures. Wednesday will see the strong dome of high pressure begin to weaken. As this happens, low pressure will slowly work northward up the coast. Wednesday will be another warm day, but some clouds will be able to develop ahead of the approaching system. On Thursday, clouds will be much more common across southern and central New England and showers will move through southern New England.
Just how far north the showers get will remain an uncertainty until the system actually pushes into the region. Dry air will eat away at these showers moving northward. The strength of what's left of the ridge will be a determining factor in the northern extent of showers. The best chance for more consistent showers Wednesday night through Friday morning will be south of the Mass Pike, with the most rain falling across Cape Cod and the Islands.
Up to an inch of rain will be possible across Cape Cod and the South Shore. Amounts and frequency of showers will drop moving both north and west. For most of New England, this will not be much of a rain maker, though any water from the sky will be beneficial. The greatest uncertainty in rainfall amounts will be across eastern Massachusetts (north of the south shore) and into southern New Hampshire. This area will be the battle zone between the wet weather and dry air.
On Friday, a backdoor cold front will help shunt this system and its showers to the south of New England. While there's always a chance for scattered showers with a frontal passage, this one does look to be a mainly dry one. This front will likely keep plenty of clouds around for the day Friday. The front has sped up in recent trends, with it clearing New England by Friday afternoon. This front will help usher in a cooler air mass.
By the weekend, another ridge of high pressure will likely build into New England. This one will be from the north, however. This does mean dry weather will take back over, but a cooler air mass will linger over New England. Widespread highs in the 60s for New England will be in place this weekend into early next week before temperatures likely begin to moderate.
New England has been very dry to start off September. All of New England is running a deficit when it comes to normal precipitation for the month. Portland, Providence, Worcester and Concord have all seen 5% or less of usual September precipitation with less than a tenth of an inch of rainfall so far this month. Boston has seen 0% of usual September precipitation with no measurable rainfall this month. Boston has not seen measurable rainfall since August 20th.
This upcoming system will not be the answer to this deficit, but will help usher in a change in pace from the weather we've had for a while. The main change with this shakeup will be the cooler temperatures. With high pressure positioning itself to the north of New England, it will keep a more northerly flow around, allowing for cooler temperatures by this week's end. Overall, a large, blocking high will remain in place near Greenland. This will be New England's New driver of the weather.
The following is the 500mb anomaly map. This helps show where troughing and ridging will be. The first is for Tuesday afternoon, showing a ridge over New England. The second is for Saturday afternoon, showing a strong ridge building to the north of New England, allowing for cooler temperatures to seep into New England:
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