How Unusual is this Heat & Humidity?

The air and dewpoint temperatures at National Airport were 85° and 76°, respectively, which produced a heat index or “feels-like” temperature of 95° for Washingtonians at one point on October 9.  Dewpoints in the mid-70s are common along the U.S. Gulf Coast and sometimes during the summer in the Mid-Atlantic Region, but are quite rare this time of year. 

The remnants of Hurricane Nate helped usher in a tropical air-mass that many would describe as downright sultry.  The dewpoint of 76° yesterday afternoon rivaled the dewpoint of 78° that Washingtonians briefly experienced in October 1995 when the remnants of Hurricane Opal passed through.  Similar to Nate, Opal made landfall on the U.S. Gulf Coast and tracked north. 

From September 7 through October 7, 2017, only 0.04” of rain was observed in the Nation’s Capital.  While there aren’t any drought concerns following a very wet July and August, some rainfall would be beneficial after 23 consecutive days without measurable rain at National Airport.

Aside from the drier than average weather over the last month, October picked up where September left off with lots of above average temperatures.  Since 1993, Washington, D.C. has averaged nearly 3.5 October days of 80°/+ warmth.  Today is poised to be the sixth consecutive day in the 80s.  Longtime Washingtonians may recall that October 2007 was the warmest on record, with an average monthly temperature (combining daily high and low temperatures) of 67.1°.  This was largely due to 11 days of 80°/+ heat in addition to 3 days of 90°/+ heat.

Yesterday’s high temperature of 86° was the warmest temperature of the month so far.  While not record setting, 86° is DC’s average high temperature on August 19!  Yesterday’s weather was also significant since it was the second consecutive day that Washington, D.C. set a record for a high-low temperature.  The Sunday, October 8 low temperature of 75° was the highest low temperature on record to occur during month of October.  Meanwhile, yesterday’s low temperature of 74° tied the previous record for warmest October low temperature. 

It is highly unusual to have low temperatures in October in the mid-70s, given the fact that today’s average high temperature is only 71°.  Having daily low temperatures that are warmer than the average high temperature is truly remarkable.  Having such a tropical air mass in place helps make that possible since temperatures cannot fall below the dewpoint.  So, when the dewpoint temperatures are in the low to mid-70s, that’s an indication of how mild overnight temperatures will remain.  When the air temperature cools to the dewpoint temperature, than the air becomes saturated, meaning relative humidity is 100%

NOAA’s outlook for the rest of October is for continued above average temperatures.  They also expect October to be a drier than average month for the DC Metro Area.