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Boston may be in for some record-breaking humidity

Thursday will be sticky.

David L. Ryan/Globe Staff

Now that we are past Labor Day, you’d be correct to assume that the humidity will start to be less of a factor in our daily forecasts.

However, don’t be too quick to celebrate; we can still see humid conditions and for some of us, Thursday may bring record-breaking humidity.

The highest humidity levels in September have dew points in the low and mid 70s in Boston.
The highest humidity levels in September have dew points in the low and mid 70s in Boston.NOAA

A quick reminder: the dew point is an important variable telling us how much moisture is in the air and when that number gets above 70 degrees, we know the air is tropical, and when we reach the low and mid-70s we’re into very uncomfortable territory.

If you look at the highest dew point ever recorded in September we find readings in the low 70s. Notice also that since 1945, the most humid days in September have occurred in the past 20 years, another indication of our changing climate and warming fall weather.  September is now very much a fourth summer month. A 75-degree dew point Thursday would be the highest so late in the season.

Very high humidity sweeps into New England Thursday followed by dry air early Friday.
Very high humidity sweeps into New England Thursday followed by dry air early Friday.COD Weather

A cold front is approaching the area and will eventually sweep the high humidity out to sea for Friday. Before the front arrives, some tropical moisture to the South, associated with what we call a tropical wave, will become part of this frontal system. Because the air will be so saturated with moisture, any precipitation that does fall could come in the form of heavy downpours.

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The amount of water in the atmosphere is very high. Another variable that meteorologists use called precipitable water, is at a level more than double what it typically would be this time of the year. All of this means that we could get quite wet Thursday afternoon and evening.

Measurable rainfall is likely for all areas Thursday into Thursday evening.
Measurable rainfall is likely for all areas Thursday into Thursday evening.NOAA

I’m not expecting any flooding rainfall, although there could be some urban street flooding south of the Mass Pike in areas hit with the heaviest downpours. Behind the frontal system skies will clear and it will turn much drier and much cooler for Friday. If your air conditioners are running to make yourself more comfortable amid the humidity, you certainly will not need them by the time we get to Friday morning.  Refreshingly, by Friday night and Saturday morning it will actually be crisp and cool, perfect September sleeping weather.

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Presently it looks as though the first part of the weekend will have the best weather, with the chance for a couple of showers on Sunday along with an increase in the humidity once again.