Feeling Like Fall

Temperature Outlook for the End of October 2018  (Source: NOAA)

This October got off to the warmest start on record in the Nation’s Capital.  However, while this week will feature plenty of sunshine, temperatures will actually be cooler than average for most of the week in the DC Metro Area.  The expected high temperatures in the mid to upper 50s on Wednesday, Thursday and Friday are more characteristic of mid-November than late-October.

October 2018 has been a unique weather month so far.  High temperatures were 81° or warmer on nine of the first 11 days of the month, including DC’s first 90° temperature in five years on October 4^th.  But, high temperatures have been cooler than average on six of the nine days since October 13.  Consequently, October’s average monthly temperature (combining daily high and low temperatures) in the Nation’s Capital has dropped from 74.7° on October 13 to 66.4° today.  

With the exception of near average high temperatures today in the mid to upper 60s, temperatures will be cooler than average the rest of the week.  In fact, NOAA expects below average temperatures to continue for much of the next six to 10 days.  Nevertheless, it’s worth pointing out that four of the last five Octobers (2013 – 2017) have had monthly temperatures at least 2.9° warmer than average.

Given the warm September and warm start to October, leaves have been slow to start changing color.  However, the cooler than average temperatures and decreasing amount of daylight should hasten the fall colors.  The fall foliage doesn’t normally reach its peak until early November in the immediate DC Metro Area and there should be plenty of color by then.  

Washington, D.C. has had a total of 1.81” of rain since October 1.  Although the bulk of this week will be dry, there is an increasing potential for appreciable rainfall on Saturday due to an early-season Nor’Easter.  Any additional rainfall would put DC closer to its October average of 3.40”.  Ten of the last 20 October’s have been wetter than average, but that hides how streaky October rainfall has been in the Nation’s Capital.  Ten of the 13 Octobers from 2002 – 2014 were wetter than average, while the last three have been drier than average.

DC’s Warmest Octobers (Source: National Weather Service)

1.  67.1° - 2007
2.  65.2° - 1984
3.  64.9° - 2017
4.  64.7° - 1971
5.  64.5° - 1947
6.  64.4° - 1941
7.  63.4° - 1949
8.  63.3° - 1973
9.  63.2° - 1975, 1919
11.  63.1° - 2016
12.  63.0° - 1879
13.  62.9° - 2014, 1881
15.  62.8° - 1990
16.  62.5° - 1970

59.5° - Average

Major Gulf Hurricanes

Category 5 Hurricane Katrina on 8-29-05  (Source: NOAA)

Some of the Atlantic Ocean Basin’s strongest hurricanes such as Michael reached their peak intensity in the Gulf of Mexico.  The Atlantic Ocean Basin includes the Atlantic Ocean, Gulf of Mexico and Caribbean Sea.  Warm ocean water, often at least 80° Fahrenheit to a depth of at least 200 feet, is required for a tropical storm or hurricane to intensify under ideal atmospheric conditions. Some of the warmest water anywhere in the Atlantic Basin can be found in the Gulf of Mexico.

Smaller areas of water that features higher sea surface temperatures can sometimes influence hurricane intensity.  The Gulf of Mexico’s Loop Current is a smaller area of warmer ocean water than can influence tropical systems.  There is also something known as a “warm core ring,” which is simply an eddy or localized current of water that breaks off from the main ocean current (like the Gulf Stream or Loop Current) and can linger over an area of a few to a few hundred kilometers.  These warmer waters can provide additional fuel for an intensifying hurricane

2005: Both Hurricanes Katrina and Rita grew into two of the strongest hurricanes on record while over the Gulf of Mexico.  At peak intensity, Katrina had sustained winds of 175 mph and a minimum central air pressure of 902 millibars (mb).  Standard sea level air pressure is approximately 1013 mb and the lower the pressure, the stronger the storm is.  A few weeks later, Hurricane Rita became even stronger with sustained winds of 180 mph and minimum air pressure of 895 mb.  Fortunately, both weakened below Category 5 status but still made landfall as major hurricanes.  Rita remains the strongest hurricane on record in the Gulf of Mexico. 

  

2004: Hurricane Charley underwent a period of rapid intensification in the southeast Gulf of Mexico.  Its air pressure dropped 18 millibars and its sustained winds increased 35 mph in just a three-hour period.  This powerful Category 4 hurricane made landfall near Punta Gorda, Florida with sustained winds of 150 mph.  It was the strongest hurricane to make landfall anywhere in the U.S. since Andrew in 1992 and the strongest hurricane in southwest Florida since Donna in 1960.

1995: Hurricane Opal underwent a period of rapid intensification while over a warm core ring in the Gulf of Mexico.  In less than 24 hours, Opal went from being a minimal hurricane to a strong Category 4 hurricane with sustained winds of 150 mph and a minimum central air pressure of 916 millibars.  Although Opal weakened to Category 3 storm before its landfall in the Florida panhandle, it was nevertheless destructive and deadly.

Hurricane Opal was actually very similar to this month’s Hurricane Michael in that both occurred in October and reached virtually identical peak intensities.  Michael, however, intensified up until it made landfall with sustained winds of 155 mph and a minimum air pressure of 919 mb. 

Weather Quiz

True or False.  2018 has already become one of the wettest years on record in the Nation’s Capital with more than 51” of rain since January 1.

How Wet Has 2018 Been in DC ?

A total of 1.51” of rain fell on October 11 due to the combination of a cold front and remnants of Hurricane Michael.  It was the 19^th day since January 1 that the Nation’s Capital got more than an inch of rain.  It was also DC’s first measurable rainfall since September 28 and highest daily rainfall total since September 9 (1.59”).

Washington, D.C. has had a total of 51.55” of rain since January 1.  That’s nearly 12” more than the yearly average of 39.74”.  As of October 15, DC has an annual rainfall surplus of 19.77” of rain.  Making this rainfall total especially impressive is that January and March were each considerably drier than average in the Nation’s Capital.  Washingtonians saw a combined total of only 2.86” of rain in January 2018 and March 2018, less than half of the average total of 6.29”.  

Although February 2018 was wetter than average, it wasn’t until April that the weather pattern took a significant shift in the Nation’s Capital.  For the first time since 2003, Washington, D.C. has had at least six consecutive wetter than average months.  According to NOAA, the months of May, July and September each had more than 8” of rain.  That’s the first time on record that a calendar year has had three such wet months.  Weather records in Washington, D.C. date back to 1871.

July 2018 is another good example of how unique the rainfall has been in the Nation’s Capital. Although the month finished with 9.73”, roughly 55% of that occurred during the two-day period of July 21-22.  In fact, July 21 was one of DC’s wettest July days on record with a daily total of 4” and the month finished as the fourth wettest dating back to 1871. 

September 2018 had the most days with rain (21) in the Nation’s Capital of any month so far this year.  The period from July 15 through September 30 was exceptionally wet with rain on 48 of the 77 days resulting in a total of 24.65”.  It’s hard to imagine that DC had more than 60% of its annual average amount of rainfall in only two and a half months.  It’s also remarkable since the DC Metro Area only saw limited effects from the remnants of Hurricane Florence last month.

Washington, D.C. only needs to have average rainfall during the rest of the year for 2018 to finish among the Top 5 wettest on record.  Making that even more unusual is that 25 of the 32 months from August 2015 to March 2018 were drier than average in Washington, D.C.  The Nation’s Capital accrued a rainfall deficit of 16.42” in that time, but has a rainfall surplus of 21.03” since April 1, 2018. 

Washington, D.C.’s Ten Wettest Years (Source: NOAA)

1.  61.33” (1889)

2.  60.83” (2003)

3.  60.09” (1878)

4.  58.17” (1886)

5.  57.54” (1948)

6.  54.49” (1937)

7.  52.95” (1891)

8.  52.92” (1906)

9.  52.59” (1877)

10.  52.03” (1942)

Fall has Finally Arrived

NOAA expects cooler than average temperatures to linger in the Mid-Atlantic

Although fall officially arrived last month, September 2018 certainly didn’t feel like autumn.  Last month was one of DC’s warmest September’s on record and finished tied with 2010 for fifth warmest.  September’s warm weather lingered into October, with D.C.’s first 90° October day (October 4) since 2013.  Moreover, the first 12 days of October were the warmest on record with an average temperature (combining daily high and low temperatures) of 74.7° according to the National Weather Service.

However, a major shift in the weather pattern occurred with the arrival of a cold front on three days ago.  The combination of the cold front and the remnants of Hurricane Michael brought a total of 1.51” of rain to the Nation’s Capital on October 11.  Area residents know that September 2018 was a very wet month (D.C.’s fifth wettest on record) with measurable rainfall on eight of its final 10 days.  By comparison, the precipitation on October 11 was D.C.’s first day with measurable rainfall since September 28.

In fact, 2018 has been a very wet year in the Nation’s Capital, with a total of 51.36” of rain since January 1 (compared to the annual average of 39.74”).  January and March were each drier than average this year, so had even average rainfall occurred in those months, DC would be closer to setting a record for wettest year (1889 is the current record-holder for D.C.’s wettest year with 61.33”).  Merely having average rainfall the rest of the year would give DC a total of 59.43”, making 2018 DC’s fourth wettest year on record.  

The warm and dry start to October has come to an end.  That’s because a shift in the jet stream has enabled cooler, fall-like temperatures to take control of the weather pattern for much of the eastern United States.  Temperatures this weekend have been cooler than average.  Average high temperatures in the Nation’s Capital in mid-October are in the upper 60s.  DC’s high and low temperature of 62°/49° on October 13 was the coolest day in the Nation’s Capital since April 29 (58°/49°).  Today’s high/low temperature of 60°/48° was actually more characteristic of early November than mid-October.

NOAA expects cooler than average temperatures to continue through the remainder of the month.  However, given the record warm start to October, having cooler than average temperatures over the second half of the month should help October finish with near average monthly temperatures.