Weather Quiz

 

Hurricane Intensity is Measured on the Saffir-Simpson Scale  (Source: WUSA9)

True or False. 

2021 is only the fourth Atlantic hurricane season on record with at least 20 tropical storms.

Quirks of the 2021 Atlantic Hurricane Season

 

Hurricane Grace, August 2021 (Source: NASA)

The 2021 Atlantic hurricane season has already been a busier than average season with 19 tropical storms, of which seven developed into hurricanes.  Four of those seven became major hurricanes with Hurricane Sam being the only currently active system in the Atlantic.  A “major” hurricane is defined as a Category 3 or greater on the Saffir-Simpson Hurricane Wind Scale, with sustained winds of at least 111 mph.

The first major Atlantic hurricane of the season was Grace that peaked as a Category 3 storm.  Hurricane Ida made landfall in Louisiana late last month as a powerful Category 4.  Its remnants brought heavy rain and severe weather to the Mid-Atlantic Region.  Fortunately, Hurricane Larry had no direct impacts on the United States despite being a named storm for more than a week.  Hurricane Sam isn’t expected to have any direct impacts in the United States.

With Hurricane Sam dominating recent tropical weather headlines, it was easy to miss Subtropical Storm Teresa.  Not a fully tropical system, “Teresa” was named a subtropical storm on September 24 near the island of Bermuda.  However, it was declared a “remnant low” the next day having been a named system for less than 24 hours.  Teresa was hardly the only weak, short-lived system this season.

According to Dr. Philip Klotzbach of Colorado State University, 2021 has tied the 2007 Atlantic hurricane season with nine tropical storms that have lasted less than two days.  Some of this season’s other short-lived systems that didn’t impact the United States include: Tropical Storms Ana, Julian and Odette.  Meanwhile some others did make landfall in the United States: Tropical Storms Claudette, Danny and Mindy.  Fortunately, none of these systems were particularly destructive being relatively weak and fast-moving systems.

Having weaker storms that remain over the open ocean isn’t much different than a stronger hurricane that remains out to sea.  Prior to the satellite era, neither type of storm was detected unless a ship happened to be in its vicinity.  Unless a ship were in the path of a tropical storm or hurricane over the open ocean, the only other way to tell if a storm existed was if it made landfall.

Since a hurricane season could have a high number of weak systems that produces a higher overall number of named storms, scientists also look at “Accumulated Cyclone Energy” or “ACE.”  NOAA more formally measures ACE on the “ACE Index” that is a measure of how active the hurricane season is based on the overall “wind speed and duration of each tropical cyclone.”  An average amount of ACE during an Atlantic hurricane season is 96.7 units (10⁴ kt²). Last season’s final ACE Index value was 180.37 10⁴ kt² while this season’s current ACE Index value is 97.80 10⁴ kt².  ACE is independent of whether or not a particular hurricane season is a destructive one. 

D.C.’s Streaky September Weather

 

Avenel Park, Potomac, Maryland

Residents of the D.C. Metro Area are in store for a prolonged stretch of mostly sunny and stellar fall weather.  That follows yesterday morning’s heavy rain that’s becoming an increasingly distant memory for Washingtonians.  Rainfall totals of 1”-2” were common across the Nation’s Capital and nearby suburbs with higher totals further north and west of town.  That rain has helped guarantee that September 2021 will finish as a wetter than average month in Washington, D.C.

D.C.’s two-day rainfall total for September 22-23 of 1.63” helped push D.C.’s monthly total to 4.03”.  NOAA updated the temperature and precipitation averages for the United States earlier this year.  NOAA’s weather “averages” through last year were based on the 30-year average for the period of 1981 – 2010.  However, they now reflect the period of 1991 – 2020.  The updated weather averages in the Nation’s Capital are generally warmer and wetter because the 2010’s were a largely warmer and wetter decade than the 1980s.

Washington, D.C.’s new September rainfall average of 3.93” is greater than the previous average of 3.72”.  That’s somewhat contingent on the influence of rainfall from decaying tropical systems as was observed earlier this month.  Washingtonians saw widespread 1”-2” rainfall totals with the remnants of Hurricane Ida on September 1.

Following the wet start to September, D.C. residents had a relatively dry two-week period with a combined rainfall total of only 0.26” from September 2 – September 15.  Temperatures were also cooler than average on 7 of those 14 days.  That was followed by a streak of 11 consecutive warmer than average days from September 12 – September 22.  High temperatures reached 90° on two of those days (September 13, September 15).  This month’s first 90° temperature on September 13 made for D.C.’s latest first September 90° temperature since 2017 (September 24). 

With no more 90° heat expected during the next week, Washingtonians would have consecutive Septembers with two or fewer 90° days for the first time since 2003-2004.  Last September was a cooler than average month that also had only two 90° days and an average monthly temperature (combining daily high and low temperatures) of 70.2°.  However, September 2020 was only 0.6° cooler than average using D.C.’s average September temperature of 71° for the period of 1981-2010.  It wasn’t until this year that D.C.’s average September temperature increased to 72.4° with NOAA’s updated climate data. 

This month has been 1.3° warmer than average in the Nation’s Capital through September 23.  However, monthly temperatures will finisher closer to average with several days of below average temperatures expected for the D.C. Metro Area during the final week of September.