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.

An Active vs. Destructive Hurricane Season

Hurricane Nate nearing landfall on the U.S. Gulf Coast  (Source: NOAA)

As Hurricane Nate continues its track toward the U.S. Gulf Coast, it’s important to highlight the relationship between a busier than average hurricane season and one that’s destructive and/or deadly.  They’re not always the same thing.  This season has been active, as well as deadly and destructive.  

Nate is rapidly approaching landfall along the U.S. Gulf Coast as a strong Category 1 hurricane, just under Category 2 intensity.  It will be the fourth hurricane to make landfall in the United States (including Puerto Rico) this season.  None have made landfall on the Gulf Coast since Isaac in 2012.  Prior to Nate, were Hurricanes Harvey, Irma and Maria that brought unspeakable devastation to parts of contiguous United States, Puerto Rico and the Caribbean.  Never before had there been three Category 4/+ hurricanes that made landfall in the United States for the first time on record.

Although not as strong as Harvey, Irma or Maria, Nate will still have significant impacts well inland with heavy rain, gusty winds and a risk of severe weather.  Some good news is that the remnants of Nate will help end a dry streak in the Nation’s Capital.  No measurable rainfall has occurred in Washington, D.C. since September 13 but that’s poised to change tomorrow afternoon and Monday.

Hurricane season runs from June 1 through November 30 – with September being the busiest month.  So far this season, there have been 14 tropical storms and 9 hurricanes, 5 of which became “major” Category 3/+ hurricanes.  That’s already surpassed NOAA’s seasonal average of 12, 6 and 3.  This season has had five major hurricanes for the first time since 2010 and two Category 5 hurricanes for the first time since 2007.

There have been active hurricane seasons, though, where the United States largely avoided serious consequences.  Such was the case in 2010, when a total of 19 tropical storms, 12 hurricanes and 5 major hurricanes developed.  By comparison, the 1992 hurricane season was quieter than average with only one major hurricane.  However, that major hurricane was Andrew, which was only the third Category 5 hurricane to make landfall in the United States.  

This illustrates that storm track is more important than the overall number of tropical storms and hurricanes in a given season.  A variety of factors influence storm track and they vary on a storm-by-storm basis.  That’s why listening to your favorite meteorologists whenever a tropical storm or hurricane develops is important.  

With the expansion of social media, there is increased exposure to sensationalized content that can be misleading and confusing.  It’s vital to understand the basics and rely only on reputable sources.

What Can Washingtonians expect in October?

Great Falls, MD

October is a fine month in the Nation’s Capital.  Average temperatures range from a high/low of 74°/56° on October 1 to 64°/46° by Halloween.  Washingtonians can also expect an average of 3.4” of rain in October.  NOAA’s forecast for October is for a warmer and drier than average month.

Such a forecast doesn’t differ much from recent years.  Four of the last five Octobers have been warmer than average.  The warmest October on record in the Nation’s Capital occurred ten years ago in 2007.  Washington, D.C.’s hottest October temperature on record was on October 5, 1941 (96°).  90° temperatures in October are relatively rare in the Nation’s Capital.  No 90° heat has occurred in October in the Nation’s Capital since October 6, 2013 (91°).  The latest in the season Washingtonians have had 90° heat is October 11 (90° in 1919).

By comparison, the coolest October temperature on record in Washington, D.C. is 26° that occurred twice (on October 30, 1873 and October 31, 1917).  Official weather observations for the Nation’s Capital have been made at National Airport since it opened in 1941.  Prior to that, they were made downtown.  Washington, D.C.’s coolest October temperature at National Airport occurred on October 24, 1969 (29°).  The earliest in the season measureable snowfall has occurred in the Nation’s Capital was on October 10, 1979 (0.3”).

Some current or former tropical systems have impacted the Mid-Atlantic Region in October.  Memorable ones in recent decades include the remnants of Hurricane Opal in 1995 and the remnants of Hurricane Sandy in 2012.  By the time Sandy made landfall, it was considered a “post-tropical” storm since it had lost its tropical characteristics over cooler waters.  However, the gusty winds and heavy rainfall that occurred with the remnants of Sandy were virtually identical to that of a tropical storm.

The DC Metro Area could actually use some rainfall.  No measureable rainfall has occurred in the Nation’s Capital since September 13 (0.04”).  Washington, D.C.’s current streak of 18 consecutive days without measureable precipitation is significant, but not record-setting.  DC’s longest streak of dry weather is 34 days (ending October 18, 2007) according to the National Weather Service.  The current streak would have to reach 27 days just to crack the Top 5.  

Although NOAA expects a drier than average month, there is some precedent for a wetter than average October to follow a drier than average September.  September 2005 was DC’s driest on record (0.11”) and that was followed by the wettest October on record (9.41”), largely because of the 7.3” of rain on October 5-6.  For perspective, the record 7.3” of rain that occurred during that two-day period is more than rain than occurred during all of August and September 2017 combined (6.01”).