Fall temperatures finally in the forecast for Midwest and Northeast

A blast of fall-like weather is coming to the Midwest and Northeast early this week as summer’s grip begins to loosen across the country.

A pool of cold, dry air pushed into the Northern Plains and Upper Midwest Saturday night and will sweep through Chicago, Detroit and the rest of the Midwest this evening through Monday morning, reaching the Northeast and Mid-Atlantic by Tuesday morning.

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Temperatures dip into the 50s overnight, bottoming out in the 40s across the interior Northeast and Upper Midwest.

That cold air will stick around during the day and limit the afternoon warmup, holding high temperatures to the mid-70s across the regions.

Chicago will stay in the 70s today, dropping to the upper 50s Monday night as the cold air pushes on.

New York City will follow, falling to 60 degrees overnight Monday and barely climbing back into the 70s on Tuesday. Boston and Providence will be even cooler, in 50s overnight and staying below 70 degrees through Wednesday morning.

The cooldown is accompanied by a sharp drop in humidity — especially along the East Coast as cool, dry air pushes south into the Mid-Atlantic and lands on the eastern side of the Appalachians.

The mountains will help lock the cold air in place through a process called cold air damming. Cold air is too dense to climb west back over the mountains, which act like a dam in a river, so it piles up against the eastern side of the slopes.

The South and central US will stay locked in the heat that’s gripped both regions for weeks, with highs still reaching into the 90s and 100s. The Southeast likely won’t get its first real taste of fall weather until later in the week, possibly stretching into next week.

But cold outbreaks like this one will become more common as the Northern Hemisphere tips into the darker half of the year.

Major cities including Chicago, Minneapolis, New York, Washington DC, and Boston have all already lost upwards of two hours of daylight since the summer solstice, the longest day of the year. They are now losing daylight at close to their fastest pace of the year, over 17 minutes per week.

The longer nighttime hours give the atmosphere more time each day to cool down, allowing cold air masses to expand across North America.

本周初，一股秋季般的凉爽天气将席卷美国中西部和东北部地区，夏季的余晖开始在全国范围内消退。

周六晚间，一股寒冷干燥的空气涌入北部平原和中西部北部地区，并将于今晚至周一早晨席卷芝加哥、底特律和中西部其他地区，于周二早晨抵达东北部和大西洋中部地区。

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夜间气温降至 50 多华氏度，东北部内陆和中西部北部地区最低气温将降至 40 多华氏度。

这股冷空气将在白天持续存在，限制午后气温回升，使各地区最高气温保持在70华氏度左右。

芝加哥今天气温将保持在70华氏度左右，随着冷空气持续推进，周一晚上气温将降至50多华氏度。

纽约市也将步其后尘，周一晚间气温将降至60华氏度（约16摄氏度），周二也仅勉强回升至70华氏度（约21摄氏度）。波士顿和普罗维登斯的气温则更低，夜间将降至50华氏度（约10摄氏度），周三早晨之前都将低于70华氏度（约21摄氏度）。

气温下降的同时，湿度也急剧下降——尤其是在东海岸，因为凉爽干燥的空气向南推进到大西洋中部，并在阿巴拉契亚山脉东侧登陆。

山脉会通过一种称为冷空气阻滞的过程，将冷空气阻挡在外。冷空气密度太大，无法越过山脉向西上升，山脉就像河中的水坝一样，因此冷空气会堆积在山坡的东侧。

美国南部和中部地区将继续遭受持续数周的高温侵袭，最高气温仍将高达90至100华氏度（约32至38摄氏度）。东南部地区可能要到本周晚些时候，甚至下周才能真正感受到秋季的凉爽天气。

但随着北半球进入一年中较黑暗的半年，像这样的寒潮将会变得更加频繁。

自夏至（一年中白昼最长的一天）以来，包括芝加哥、明尼阿波利斯、纽约、华盛顿特区和波士顿在内的主要城市，日照时间均已减少超过两小时。目前，这些城市日照时间减少的速度接近一年中最快的速度，每周减少超过17分钟。

更长的夜间时间使大气每天有更多的时间冷却，从而使冷空气团能够扩散到整个北美。