Maps show the cities about to experience extreme humidity and heat

Over the next two weeks, extreme levels of humidity are forecast to hit around 40 states, with the first wave of very muggy weather building in central and eastern states through Thursday. Across the country, around 170 million people will also experience temperatures above 90 degrees.

Burgeoning humidity contributed to deadly downpours in West Virginia last weekend, and it will fuel severe storms that expected to hit the Plains and Midwest through Wednesday before reaching the East Coast on Thursday.

The high humidity and temperatures this week are being driven by above-average ocean temperatures, and southerly winds blowing from these areas. And there’s more to come. A second, stronger surge of potentially dangerous heat and humidity is expected this weekend into next week as a heat dome is likely to form.

Surging ocean temperatures in the Gulf of Mexico and western Atlantic are expected to fuel a summer of higher-than-average humidity across eastern states, including the nation’s capital.

Extreme levels of humidity are forecast across around 40 states over the next two weeks.

Summer humidity is trending higher across most of the United States, but the month with the biggest trend varies by region.

Parts of Florida, including near Tampa, experienced record humidity levels during May, fueling a marine heat wave. The trend shifted into Texas during June.

Where and when humidity will peak

What feels humid to one person may not to another, so it’s important to recognize that the stickiness of the air is somewhat subjective.

The table below describes the humidity level associated with dew-point thresholds — and how each feels. Dew points indicate the temperature at which air becomes saturated with water vapor.

Based on the above dew-point thresholds, here’s when eight cities will experience very high or extreme humidity levels, and the expected weather impact.

City-by-city forecasts

Washington, D.C.

Maximum humidity level: Extreme

Humidity will become very high Tuesday afternoon before nearing extreme levels Thursday, when severe thunderstorms are possible. Humidity will decrease slightly Friday, but it will quickly rise to very high or extreme levels over the weekend and into next week, when a prolonged period of excessive heat arrives.

Philadelphia

Maximum humidity level: Extreme

High humidity on Tuesday will become very high Wednesday and Thursday, coinciding with strong-to-severe thunderstorms. After a short break Friday, heat and humidity will escalate over the weekend, reaching very high to extreme levels early next week.

New York

Maximum humidity level: Extreme

Very high humidity is expected Wednesday and Thursday, breaking Friday, then returning late in the weekend. Very high to extreme humidity levels and excessive heat are expected from Sunday through next week.

Up to a dozen or more very humid days are possible in most central and eastern states over the next two weeks.

Cleveland

Maximum humidity level: Very high

High to very high humidity will last into early Thursday, when showers and storms will temporarily wash it away. The weekend will come with the arrival of very high humidity and excessive heat, which is forecast to last through next week.

Chicago

Maximum humidity level: Very high

High humidity will develop Tuesday, becoming very high Wednesday, when strong-to-severe storms are possible. After a brief break Thursday, humidity levels will surge from Friday through next week as a period of dangerous heat arrives.

Minneapolis

Maximum humidity level: Extreme

Moderate to high humidity will increase to very high levels Friday, before extreme humidity and heat arrives this weekend. Frequent rain and thunderstorms early next week will add to the tropical feel.

St. Louis

Maximum humidity level: Extreme

High to extreme humidity levels show no signs of letting up through next week. This will fuel strong-to-severe thunderstorms on Wednesday. A period of excessive heat is forecast to arrive on Saturday.

Houston

Maximum humidity level: Extreme

While extreme humidity is common along the Gulf Coast during June, its unrelenting nature this month could break records.

How hot it will get

Around 170 million people across 45 states are forecast to experience temperatures above 90 degrees over the next week.

Temperatures will reach or exceed 90 degrees for around 170 million people across 45 states through Monday.

Parts of the Southwest will also have a major risk for heat-related impacts from Tuesday to Thursday, before the risk moves into the Plains and Midwest from Friday through the weekend — where it may reach extreme levels.

The extreme category, which may be reached in parts of the Midwest on Sunday and Midwest, Mid-Atlantic, Northeast and Southeast on Monday, is NOAA’s highest of four levels, described as “rare and/or long-duration extreme heat” likely to affect health systems, heat-sensitive industries and infrastructure.

The risk for heat-related impacts is expected to range from major to extreme in central and eastern states from this weekend into next week.

Summer humidity trends

For most of the United States, July is the most humid month of the year on average. That’s unsurprising because July is also the warmest month — and warmer air can hold more water vapor.

There are a few exceptions, including Florida, where August is the most humid month, related to the seasonal peak in ocean temperatures, and parts of the West, where monsoonal moisture boosts humidity in August.

For most of the United States, July is the most humid summer month.

But how is summer humidity changing?

Over the past 40 years, September has been the summer month with the largest trend toward higher humidity across much of the United States. This is probably linked to the warm season becoming longer and more intense because of climate change, ocean warming and other patterns of climatic variability.

September is the summer month with the largest increase in humidity for much of the United States.

In parts of the Northeast, including Philadelphia, New York and Boston, July is the month with the largest humidity trend — meaning that the month with the biggest trend also happens to be the most humid one, which can lead to more humidity extremes.

The same can be said for parts of the Southeast and Florida, where August is both the most humid month and the one with the biggest humidity trends.

As peak humidity season arrives, significant relief across Central and Eastern states is unlikely in the weeks ahead.