Humidity reaches obscene range, heat breaks records - Washington Post (blog)

The summer's longest, most extreme heat wave extends to its fifth day, cooking the Great Lakes, Mid-Atlantic and Northeast.  For Washington, D.C., under an excessive heat warning, today may well be the most brutal, with heat indices approaching 110 degrees.

141 million people under heat advisories and warnings

USA Today reports 23 states and 141 million people are under advisories and warnings. The most intense heat and humidity coincides with the I-95 corridor from Richmond to Boston.

Heat advisories shaded in orange, excessive heat warnings in magenta. (National Weather Service)

From Richmond to New York City, the humidity levels are particularly unbearable, with dew points in the upper 70s to around 80 degrees

Dew points at 11 a.m. (National Weather Service)

These extreme dew points are more typical of humidity levels found in the Persian Gulf. This is a major excessive heat event.

— Gary Szatkowski (@GarySzatkowski) July 19, 2013

While the core of the heat has focused in the eastern U.S., the entire nation has been steamy. On Thursday, at least one location in each of the Lower 48 states hit 90 or higher.

WOW! Today, at least one spot in every state in the #CONUS made it to 90F. #USwx pic.twitter.com/GXfCHgxpCg

— Anthony Sagliani (@anthonywx) July 19, 2013

 

New York City's JFK Airport soared to 100 degrees, setting a  record for the date.

In Washington, D.C., in the wake of a thunderstorm that dumped over an inch of rain between 5 and 6 p.m., humidity levels spiked with the dew point hitting 80 degrees at 7 p.m., just two degrees below its highest known dew point ever recorded.

Why is it so humid?

We've explained why it's so hot: an usually strong area of high pressure a few miles up (although it has weakened a little) has sat over the eastern third of the U.S. much of this week, causing the air to sink and heat up.

The source of humidity – which became particularly unbearable Thursday – is an area of high pressure off the southeast U.S. coast, pumping in extremely moist air from the south. Whereas the prevailing wind direction was from the north earlier in the week, since Thursday the air flow has come from the tropics.

Left: Pressure difference from normal at high altitudes (deep reds indicate big positive difference). Right: pressure difference from normal at the surface (yellows and oranges indicate positive differences). (WeatherBell.com)

When you combine hot high pressure and moist tropical flow, it's a dangerously hot and humid situation.

Dew point temperatures at 9 a.m. Friday (Unisys Weather)

More record high minimum temperatures set in D.C.

While day time high temperatures have held in the mid-to-upper 90s in the D.C. area – several degrees below daily records, we've set or tied record high minimum temperatures the last three mornings.

This morning the low at Reagan National Airport was just 81, breaking the record high minimum temperature of 79 last established in 2011 (assuming it does not drop to 79 or lower before midnight, which is doubtful).

Low temperatures the last four mornings? 80 (Tuesday), 80 (Wednesday), 80 (Thurday) and 81 today (Friday).

Through 11 a.m. today, Washington, D.C.'s temperature has not fallen below 80 for 99 straight hours. If D.C. fails to drop below 80 before 4 p.m. Saturday, we will tie our longest stretch on record above 80. And, if we make it to 5 p.m. above 80, we will set a record of 129 consecutive hours above 80.

Showers and storms associated with an incoming cold front – that will end this heat wave – may not arrive until evening, so we have a legitimate chance to challenge or break this record.