Weather Quiz

True or False.  In 7 of the last 11 years since 2007 (including 2017), the Atlantic Ocean has seen a tropical storm develop before the official start of hurricane season on June 1.

2017 Hurricane Outlook

Tropical Storm Arlene over the north Atlantic Ocean in April 2017  (Source: NOAA)

NOAA just issued its forecast for the upcoming hurricane season that starts Thursday, June 1.  They expect 11 to 17 tropical storms, of which 5 to 9 will become hurricanes and 2 to 4 of these will intensify into “major” hurricanes.  A “major” hurricane is defined as Category 3 or higher on the Saffir-Simpson scale, with sustained winds of at least 111 mph.  

NOAA’s forecast is significant because it represents their first forecast for a potentially busier than average season since 2012.  An “average” hurricane season has 12 tropical storms, 6 hurricanes and 3 major hurricanes.  NOAA’s forecast is busier than Colorado State University’s forecast that called for only 11 tropical storms, 4 hurricanes and 2 major hurricanes.  However, CSU’s forecast was last updated before Tropical Storm Arlene developed in late April and before the latest El Nino outlook was available.  Scientists at CSU will update their forecast this week to coincide with the start of hurricane season.  Some other groups also issue seasonal hurricane forecasts but the two most famous ones are issued by NOAA and Colorado State University.

NOAA scientists said that “‘Weak or non-existent’ El Nino is a factor” in its seasonal forecast.  During an episode of El Nino, conditions in the tropical Atlantic Ocean tend to be less favorable for tropical storms and hurricanes to develop and intensify.  An El Nino event is characterized by an above normal amount of wind shear – winds that change direction with height – and near to below average sea surface temperatures in the tropical Atlantic.  

A hurricane does not have to be a “major” storm to be a damaging or deadly one.  Hurricane Isabel in 2003, Ike in 2008 and Irene in 2011 were only Category 2 storms when they made landfall, but all rank among the costliest hurricanes in the United States.  Even tropical storms and their remnants have the potential to be damaging and deadly.  For example, Tropical Storm Allison in 2001 was so destructive that its name was retired.

Whether or not the hurricane season is busier than average is also less important than if the tropical storms and hurricanes impact land areas.  There have been several busier than average hurricane seasons in the last decade, but no “major” hurricane has made landfall in the United States since Wilma in 2005.  Ironically, the 1992 hurricane season was quieter than average but was also the year of infamous Hurricane Andrew. 

Memorial Day Weekend Weather

As people celebrate the unofficial start to summer this Memorial Day weekend it’s a good time to reflect upon some of unique weather that’s occurred across the Mid-Atlantic Region this month.  It has been DC’s first wetter than average month since last May with 5.25” at National Airport through May 26.  There have been 15 days with rain this month that’s also had an effect on temperatures.

 

Given the frequent rainfall and cloud-cover, this month could become DC’s first cooler than average month since May 2016.  Average temperatures this month are 0.6° cooler than average through May 26.  The same weather pattern that has brought such frequent rainfall has also helped keep temperatures near or slightly below average.  Given the early start to this year’s allergy season in the DC Metro Area, not everyone is disappointed by this month’s cooler and wetter than average conditions.

Although this month has featured more rain than any month in a year, May 2017 is still short of how wet last May was in the Nation’s Capital.  May 2016 saw 5.65” of rain at National Airport and was also 2.1° cooler than average.  According to the National Weather Service, there was a total of 22 days of rain last May in the Nation’s Capital (that includes a streak of 15 consecutive days with measurable rainfall from April 27 – May 11, 2016). 

While last May had a higher overall number of rainy days, this May has had more days with heavier rainfall with four days of at least 0.5” of rain compared to three such days a year ago.  This month’s rainfall has helped eliminate the drought conditions that had developed across much of the Mid-Atlantic Region. 

Despite the predominantly cloudy and damp conditions this month, there have already been two days of 90° heat so far this month and three so far this year.  The first 90° day of 2016 didn’t occur until June 11.  Also, last May had five days where high temperatures remained in the 50s (more characteristic of March), this month has had only one such day. 

While such temperatures are unusually cool for May in Washington, D.C., no record lows have been set at National Airport.  The last time Washingtonians experienced record lows during the month of May was in 2002, when the temperature fell to 42° on May 20.  Meanwhile, Dulles Airport, where the data pool is considerably less than downtown, several record low temperatures were set last May.  Dulles Airport has also had 6.61” of rain so far this month making this its wettest month since June 2015 (7.44”).  There are several more rain chances across the Mid-Atlantic Region during the Memorial Day weekend.