Dew points at noon in the Lower 48. (Weather.com)
Dew points at noon in the Lower 48. (Weather.com)

An extremely moist, tropical air mass has taken the entire eastern United States hostage.

From Houston to Marquette, Mich., and from Miami to Bangor, Maine, humidity is oppressively high.

This is no ordinary deep tropical air mass. Levels of atmospheric moisture streaming into the Lower 48 from the Gulf of Mexico are among the highest ever recorded.

The superfluous atmospheric moisture pouring into the eastern United States has two primary effects: 1. It is unleashing suffocating humidity, and 2. It is fueling torrential downpours.

The humidity

Dew points, which are direct measures of humidity, are unusually high over such a large area. When they exceed 70 degrees, they’re indicative of uncomfortably humid conditions. Almost every city east of the Great Plains posted dew points above 70 early Thursday (see top map).


(The Washington Post)

When dew points exceed 80 degrees, the humidity is difficult to tolerate. Several locations in southeast Texas, including Houston, were reporting 80-plus dew points Thursday morning.

Dew points are forecast to remain in the 70s over large portions of the eastern United States through the weekend and even hanging on into the middle of next week along the East Coast south of New England.

When conditions are this humid, nighttime temperatures don’t cool off much. For the next several nights, record high overnight minimum temperatures are likely — particularly in the Mid-Atlantic and Northeast.


Locations circled are forecast to be within one-degree of the record warmest low temperature Friday in the lower forty-eight. (WeatherBell.com)

During this spell of high humidity, afternoon temperatures are generally not forecast to be record-breaking. However, highs are forecast to be in the 90s in many areas. Factoring in the humidity, it will feel another 5 to 10 degrees warmer — up to 100 to 105 degrees.


Forecast heat index values at 2 p.m. Eastern Friday from the Global Forecast System model. (WeatherBell.com)

Heavy rain threat

The plume of deep tropical moisture streaming into the eastern United States will feed thunderstorms developing near an area of low pressure in southern Mississippi and along a cold front pushing into the Midwest.

The National Weather Service has declared a moderate risk of excessive rainfall along the Gulf Coast from southern Alabama to southeast Louisiana and in eastern Iowa.


Risk of excessive rainfall determined by the National Weather Service on Thursday.

Rainfall rates along the Gulf Coast could reach 3 to 4 inches per hour. “Significant runoff problems and/or flash flooding will be possible given these rainfall amounts,” the Weather Service cautioned.

Over the next five days, widespread rainfall amounts of 3 to 6 inches are predicted along the central Gulf Coast, with localized 6 to 10 inches or more possible.


Five-day rainfall forecast for the eastern United States. (National Weather Service via WeatherBell.com)

To the north and northeast, a broad ribbon of 2 to 4 inches is forecast through the Midwest and toward the Northeast.

Behind the sopping wet air mass

A very large area of high pressure centered east of Bermuda is the primary culprit for this unusually warm and humid air mass. Clockwise flow around this high pressure system is transporting air from the tropical Atlantic Ocean into the Gulf of Mexico and then northeastward.


High pressure over the western Atlantic Ocean is circulating warm, moist tropical air into the eastern United States (WeatherBell.com)

Water temperatures in both the tropical Atlantic and Gulf of Mexico are substantially warmer than normal, intensifying the heat and moisture streaming into the eastern United States.


Sea surface temperature difference from normal over the Atlantic and Gulf of Mexico. (National Oceanic and Atmospheric Administration)

Along the central Gulf Coast, a measure of atmospheric moisture (from the surface up to the jet stream level) known as precipitable water is in the top percentile (3 to 3.5 standard deviations above normal) of historical values.


Global Forecast System model simulation of precipitable water Thursday early afternoon. (WeatherBell.com)

In New Orleans, the precipitable water was determined to be 2.78 inches Wednesday, which ranks among the top-five highest levels on record in August. “That is higher than some soundings [weather balloon measurements] observed here during past tropical cyclones,” the National Weather Service said.