Much Needed Rainfall

Latest Drought Conditions  (Source: U.S. Drought Monitor Index)

Washingtonians are finally seeing appreciable rainfall today.  Widespread rainfall totals of 0.5” to 1”/+ are likely.  That’s impressive since the last time DC had at least 0.5” of rain was over two months ago on August 7 (0.9”). 

Although July 2019 finished with an above average monthly total of 6.49”, the bulk of that occurred during the first half of the month on July 8 when a daily record 3.44” occurred.  But, for the three-month period from July 15 through October 15, Washington, D.C. had a rainfall total of only 3.05”.  That’s more than 7” less than the average of 10.19” of rain for that three-month period in the Nation’s Capital.  

Having such a high rainfall deficit in conjunction with above average temperatures for late-summer and early-fall have combined to produce moderate drought conditions for much of the Mid-Atlantic Region.  There have been other instances in recent decades where a very dry late-summer and early-fall have produced similar drought conditions.  For example, the six-month period of July 1998 – December 1998 saw a combined rainfall total of only 7.45” in the Nation’s Capital (12.55” below normal).  That caused 1998 to finish as a drier than average year in the Washington, D.C.

The two-year period of 2001-2002 produced a combined total of 64.28” of rain in Washington, D.C.  That’s 15.2” below average and led to drought conditions in parts of the DC Metro Area by the summer of 2002.  Fortunately for DC Area residents, the drought conditions were short-lived as the last three months of 2002 were wetter than average.  Similarly, 2012 was also an exceptionally hot and dry year with 10 months that were drier than average.

The Nation’s Capital has a history of going through stretches that can be significantly wetter or drier than average.  For example, the 32-month period from August 2015 through March 2018 DC had a combined rainfall deficit of 16.42”.  That was followed by a wetter than average 15-month stretch through July 2019, when DC had a rainfall surplus of 33.12”.  Although today’s rainfall won’t completely make up the deficit that’s developed over the last three months, it will provide some short-term relief.

DC’s Ten Wettest October Days (Source: National Weather Service)

1.  3.98” – October 4, 1877
2.  3.85” – October 29, 2012
3.  3.67” – October 8, 2005
4.  3.63” – October 7, 2005
5.  3.36” – October 14, 1995
6.  3.31” – October 23, 1878
7.  3.27” – October 14, 1953
8.  3.12” – October 25, 1872
9.  3.11” – October 23, 1890
10.  2.97” – October 20, 1910

DC’s Ten Driest October’s (Source: NWS)

1.  Trace – 1963
2.  0.02” – 2000
3.  0.28” – 1930
4.  0.29” – 1874
5.  0.34” – 1892, 1896
7.  0.40” – 1920
8.  0.44” – 1924
9.  0.53” – 1882
10.  0.59” – 1998

Drought Conditions Return

The Mid-Atlantic Region could use some rain  (Source: NOAA)

Moderate drought conditions have returned to the DC Metro Area for the first time since March 2018.  An exceptionally dry period over much of the Mid-Atlantic Region began this summer.  A total of only 2.26” of rain occurred in the Nation’s Capital since August 1.  That’s DC’s driest 2.5 month period since November 14, 2017 – January 31, 2018 (1.53”).

DC Area residents may recall that 2018 finished as DC’s wettest on record with an annual rainfall total of 66.28”.  That’s an astounding 26.54” above DC’s yearly average of 39.74”.  The exceptionally rainy weather pattern continued during the first seven months of 2019.  The Nation’s Capital had a rainfall surplus of 33.12” for the 15-month period from April 2018 – July 2019.  During that span, three months finished among the Top Ten wettest on record in addition to DC’s wettest November on record that occurred last year.

However, that rainy 15-month period followed an even longer stretch of abnormally dry weather.  Washington, D.C. accrued a rainfall deficit of 16.42” for the 32-month period from August 2015 through March 2018.  During that time, DC had four months that ranked among its Top 5 driest on record.  There was a 180° shift in DC’s weather pattern from exceptionally wet back to very dry.

With only 1.99” of rain for the entire month, August 2019 was DC’s first month to finish with less than 2” of rain since March 2018 (1.92”).  September 2019 finished as DC’s fourth driest with 0.25” and that was the driest overall month in the Nation’s Capital since March 2006.  With only 0.02” of rain so far this month, October has been exceptionally dry as well.

Since Washington, D.C. had an annual rainfall surplus of 5.32” through the end of July, this year won’t finish among DC’s driest.  The last two and a half months have evoked memories of other dry years, such as the 32-month period from mid-2015 through early-2018 or 2001-2002 when DC had a combined 64.28 of rain (15.2” below normal).

A total of 31.05” of rain has occurred in the Nation’s Capital since January 1.  It’s quite possible DC will ultimately finish with at least 39.74” of rain, making 2019 an “average” year.  That would mask how streaky the rainy and dry periods have been.  NOAA expects wetter than average conditions for the central and eastern United States in its extended forecast for October 20 – 26.  That’s definitely good news for the Mid-Atlantic Region. 

Weather Quiz

True or False.

More rain fell in the Nation's Capital in July 2019 than in the nearly 2.5 months since.

October’s Hurricane Power

Hurricane Michael, October 2018  (Source: NOAA)

Although September is the busiest month of the Atlantic hurricane season, the tropics often remain active well into October.  Some of the most powerful hurricanes on record have occurred in October, including several Category 5 hurricanes.

2018 – Hurricane Michael became only the fourth Category 5 hurricane to make landfall in the United States.  It was the first to do so since Hurricane Andrew 26 years earlier and it remains one of the few hurricanes that intensified up until its landfall.  Michael reached its peak intensity as it made landfall along the Florida panhandle with sustained winds of 160 mph and a minimum central air pressure of 919 millibars (mb).  That part of Florida had seen several Category 3 hurricanes, but never a Category 4 or 5 storm.

2016 – Hurricane Matthew was the first Category 5 hurricane to develop in the Atlantic Ocean since 2007.  That’s not to say there weren’t multiple destructive and deadly hurricanes during that time frame such as Ike, Irene and Joaquin, but none had reached Category 5 intensity.  Matthew had sustained winds of 165 mph and a minimum central air pressure of 934 millibars at its peak intensity.

2005 – This season broke virtually every hurricane record that was on the books for the Atlantic Ocean.  Prior to 2005, no more than two Category 5 hurricanes had developed in a single season.  Hurricane Wilma was not only the fourth, but it became the strongest hurricane on record for the Atlantic Ocean Basin.  At peak intensity, Wilma had sustained winds of 185 mph and a minimum central air pressure of 882 millibars.  That broke Hurricane Gilbert’s 1988 record of 888 mb.  Wilma made landfall on the southwest coast of Florida as a Category 3 storm.

1998 – Hurricane Mitch set several important benchmarks during its existence.  It ended a modest six-year stretch without a Category 5 hurricane in the Atlantic, being the first since Andrew in 1992.  Mitch was also the Atlantic’s first October Category 5 since “Hattie” in 1961.  At peak intensity, Mitch had sustained winds of 180 mph and a minimum air pressure of 905 millibars.  Mitch caused devastating flooding in Central America that claimed more than 10,000 lives. 

Feeling More Like Fall

Although October got off to a sizzling start in the Nation’s Capital with four hot days, this weekend has felt much more like autumn.  Nevertheless, Washingtonians have had an exceptional start to October with a temperature spread of nearly 50° in the span of three days.

The high temperature in Washington, D.C. reached a July-like 98° on October 2.  That not only obliterated the previous daily record high of 89°, but it also set a new record for DC’s hottest overall October temperature.  It was also DC’s 62^nd day of 90° heat of 2019.  That puts this year solidly in second place for highest annual total of 90° days behind only the total of 67 days that occurred in 2010 and 1980.  The Nation’s Capital averages only 36 days of 90°/+ in a given year.  By comparison, DC had an above average but much lower total of 45 days of 90° heat last year.

Yesterday’s high temperature of 67° at National Airport was DC’s coolest high temperature since May 14 (64°).  The low temperature of 49° yesterday was also the coolest temperature in the Nation’s Capital since April 29 (47°).  Those temperatures are common in the DC Metro Area in the fall, but not for a few more weeks.  For example, DC’s average high/low temperature on October 19 is 67°/49°.

Although Washingtonians often see the final day of 90° heat of the year in September (13 of the last 26 years), October has now had 90° heat six times since 1993.  The first frost of the fall season doesn’t typically occur within Washington, D.C. city limits until the second week of November.  However, the northern and western suburbs often see frost in mid-to-late October.  Fall allergy sufferers actually look forward to the first frost of the fall since it eliminates many environmental allergens such as ragweed and grass pollen.

Measurable snowfall has also occurred in the Nation’s Capital on more than one occasion during the month of October.  DC’s earliest accumulating snowfall of the season occurred on October 10, 1979 (0.3”).  However, having a significant amount of snowfall before December 31 doesn’t always correlate to a cold and snowy winter.  A good example of this occurred during the 1989-1990 winter season.  Washingtonians had a combined 12.5” of snow in November and December but less than 3” occurred the rest of the winter.

Temperatures will be milder today compared to yesterday with high temperatures in the 70s.  NOAA’s Climate Prediction Center expects near average temperatures to continue in the DC Metro Area through the middle of the month.  DC has a rainfall deficit of nearly 5” since August 1 and that’s contributed to moderate drought conditions for a significant portion of the DC Metro Area.  While some showers are possible tomorrow evening, no significant rainfall is expected.

Record Heat Coincides with October’s Arrival

Fall is a great time of the year in the DC Metro Area

September 2019 finished as DC’s third warmest on record with an average monthly temperature of 76.5° (combining daily high and low temperatures).  Five of DC’s 10 warmest September’s have now occurred since 2005.  Last month was also poised to finish tied with September 2005 for DC’s driest.  However, 0.14” of rain on September 30 caused last month to finish as DC’s fourth driest with 0.25”.  Monday’s rainfall also ended a 17-day stretch without measurable rainfall in the Nation’s Capital.  Last month was DC’s driest overall month since March 2006 (0.05”).

October has picked up where last month left off with record heat on tap today with highs in the low to mid 90s for the DC Metro Area.  There’s also a chance DC could set a new record high today for the entire month of October.  DC’s hottest October temperature on record is 96° and occurred way back on October 5, 1941.  Although Washingtonians experienced one October day with 90° heat last year, there have been only five Octobers with such unusual heat in the last 25 years.  That includes October 2007 when DC had a highly unusual heat wave with three days of 90° heat from October 7 – 9.  

Average high and low temperatures fall throughout the month of October from 74°/56° on October 1 to 64°/46° on October 31.  DC’s coldest October temperature of 26° occurred twice, most recently on October 31, 1917.  There has been a nearly even split with 11 of the last 20 Octobers that were warmer than average in Washington, D.C.

The Nation’s Capital averages 3.4” of rain during the month of October.  Although the last four October’s have been drier than average in Washington, D.C., there has been an even split with 10 of the last 20 Octobers having been wetter than average.  While last month fell just short of tying DC’s 2005 record for driest September, many comparisons have been made since both Septembers were significantly warmer and drier than average.  It’s worth pointing out that October 2005 featured a major shift in the opposite direction from the previous month and finished as DC’s wettest on record.

NOAA’s outlook for October 2019 doesn’t call for a significant deviation from normal rainfall in the Nation’s Capital.  It shows an equal chance for above or below normal rainfall with an elevated potential for below average rainfall in parts of southern Maryland and Virginia.  Significantly warmer than average temperatures are expected for the month.  That doesn’t mean there won’t be some chilly, fall-like weather at times this month; but October as a whole is expected to finish as a warmer than average month in the Nation’s Capital. 

October 2 Record Highs (Source: NOAA)

National Airport, Washington, D.C. (DCA): 89° (1986, 1881)
Dulles Airport, Sterling, Virginia (IAD): 89° (1986)
BWI Airport, Baltimore, MD: 89° (1986)

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All-Time October Record Highs (Source: NOAA)

DCA: 96° (October 5, 1941)
IAD: 94° (October 9, 2007)
BWI: 97° (October 5, 1941)