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Relief from wet, humid pattern to come at price in Northeast

AccuWeather logo AccuWeather 7/20/2021 Alex Sosnowski

AccuWeather meteorologists warned on Tuesday that relief from heat, humidity and flooding issues that accompanied a wet pattern plaguing the Northeast since last week will come at a price in the coming days. Drier air will arrive, but the exchange of air masses will trigger severe weather with the possibility of strong wind gusts.

The tumultuous weather will develop as quick shots of moisture arrive. However, in general the pattern will be changing up as the week progresses. A northwest flow of drier air will prevail, rather than the setup that the region has been stuck in. A constant flow of air from the Gulf of Mexico has led to "very high humidity and plenty of fuel for slow-moving storms with torrential downpours," AccuWeather Senior Meteorologist Brett Anderson explained.

The Philadelphia area is among those that faced repeated flooding downpours amid the stormy weather. A month's worth of rain in one day triggered a flash flooding emergency early last week.

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On top of flooding concerns, humidity levels were very much like that of the tropics across a wide zone of the mid-Atlantic over the last week.

Storms may still erupt as cold fronts press through the region occasionally, but the storms will not be as intense or "prone to lingering and repeating for hours and days" on end as the weather changes, Anderson added.

The setup through Wednesday will allow a cool front to drop southeastward from Canada and through the Northeast states.

Showers and heavy, gusty thunderstorms pivoted southeastward from southeastern Ontario and southern Quebec to upstate New York and northwestern New England into Tuesday night.

By Tuesday evening, the storms had strengthened enough to bring localized high winds, spurring dozens of preliminary wind reports from the National Weather Service's Storm Prediction Center (SPC). Numerous trees and power lines were noted to have been knocked down amid the strong winds, especially from the northern part of upstate New York.

Roughly 30,000 customers lost power Tuesday night across New York and Vermont combined, according to PowerOutage.US. A few of the counties in northern and central New York such as Franklin, St. Lawrence, Herkimer and Oneida saw the highest number of power outages as the storms tore through the area.

The northern tier areas can handle more rain due to long-term dry conditions, compared to areas farther south, where the ground is moist. Soil conditions range from abnormally dry in coastal Maine to moderate drought in northeastern New York state and severe drought in northwestern Maine, according to last week's United States Drought Monitor report.

However, even though the storms will tend to move along and carry the risk of strong wind gusts and even small hail, the more far-reaching concern will be from heavy rainfall.

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Areas from western New York to western Massachusetts, southern Vermont and northern Pennsylvania are some locations where only an inch of rain in an hour's time can trigger flash flooding. This was the case on Tuesday evening when a state of emergency and travel ban were briefly declared due to flooding in Niagara County, New York.

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The risk of disruptive and potentially damaging storms will not end Tuesday night. As the front continues to advance southward and eastward, thunderstorms are forecast to re-fire Wednesday.

The storms at midweek will erupt in a heavily populated and traveled zone along the Interstate 95 corridor from Massachusetts to Virginia during Wednesday afternoon and evening.

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As will be the case into Tuesday night, the ground over much of this region is moist to saturated and cannot handle the worst-case scenario of more than an inch of rain in an hour.

However, that type of rainfall seems unlikely in much of the thunderstorm zone on Wednesday. But, where downpours occur in major urban areas with vast amounts of paved and concrete surfaces, such as around Boston, New York City, Philadelphia and Baltimore, flash flooding is still a concern.

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In the wake of the front Wednesday, humidity levels Thursday will be slashed, and temperatures will be trimmed a bit, especially at night, over the northern tier, near the coast and over the central Appalachians and part of the Ohio Valley.

Well before the end of this week, people may be able to turn off the fan and air conditioner and let some cool, fresh air in at night. Lows Wednesday night are forecast to range from the upper 40s over the mountains to the upper 60s around New York City and near 70 in Washington, D.C.

Adding to that relief will be a drop in humidity. Meteorologists look at the dew point temperature as one way to gauge how humid the air is from day to day. This is the temperature to which the air must be cooled to become saturated. The higher the number, the more humid the air is and the slower perspiration evaporates off of skin and clothes -- and less natural cooling can take place in turn.

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Last week, there were times when the dew point temperature hovered in the middle to upper 70s, compared to a dew point in the upper 60s to near 70 which is closer to average for the middle of July.

Dew point temperatures have been fluctuating during the first part of this week and are forecast to continue to do so into this weekend. During Monday and Tuesday, dew point temperatures were generally close to average for the time of the year, but they are forecast to slide later in the week to almost fall-like levels in some areas. In much of the region, dew point temperatures are forecast to dip into the 50s on Thursday and Friday. However, in a few places, such as over the central and northern Appalachians, dew point temperatures can dip as low as the middle 40s.

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With the lower dew point temperatures, there is much less potential moisture in the atmosphere. So, even though there may still be a brief shower or thunderstorm on Friday with the next frontal passage, flooding downpours will be unlikely.

It may take until the second half of the weekend before the dew point temperatures rise enough with the next storm to raise the risk of significant downpours. But despite that inevitable rise in dew points later this weekend, there is no sign of tropical air returning next week.

Strong late-July sunshine will tend to negate the effects of cooler air originating from Canada. As a result, little change in day-to-day afternoon temperatures is likely, and highs are forecast to be within a few degrees of average for the balance of the week and into this weekend. Average highs in late July range from the upper 70s in northern Maine to the upper 80s to near 90 around the Chesapeake Bay.

Keep checking back on AccuWeather.com and stay tuned to the AccuWeather Network on DirecTV, Frontier, Spectrum, FuboTV, Philo, and Verizon Fios.

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