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Posted at 10:10 AM ET, 09/15/2011

What is Washington, D.C.’s weather like in September? Breaking down norms and extremes


Clouds from Hurricane Earl are illuminated by the setting sun on September 2, 2010, in Manassas, Va. Photo by CWG photographer Kevin Ambrose. See larger image.
Fall arrives in September. If you’re a meteorologist, the whole month “counts,” and if you follow the astronomical calendar, autumn becomes official in the final third of the month. While some remnants of summer often linger into September, there is little question about the cool season’s advance as high temperature averages dip well into the 70s by the end of the month.

On average, it’s the 4th wettest month of the year, but September rain often come in buckets over a few days. The month also frequently delivers fairly nice weather, with plentiful sunshine, lowering humidity levels and of course those much-needed — after a long hot D.C. summer — cooler days.

Keep reading for more in this sixth installment of a year-long series of posts detailing specific statistics on climate norms, extremes, and averages for the each month in Washington, D.C…

PRECIPITATION

September’s average* precipitation is 3.72”. This is an inconsequential change from the old 1971-2000 normal of 3.79”, which was recently updated to the current 30-year average occurring from 1981-2010. This most recent September change in precipitation is among the smallest of the months.


Monthly rainfall (in inches) in Washington, D.C. from 1871-2010. See sources for data.

September’s maximum daily precipitation is 5.16” on the 2nd in 1922. There have been eight September days in total with daily values at or above 4” and 39 days with 2” or greater rainfalls.

The most rain that has fallen during one month was the incredible 17.45” in 1934. This is the highest monthly total of any month during the year in D.C. In total, four Septembers have reached or surpassed 10 inches of rain on the high end.

On the low end of monthly totals, September has seen as little as 0.11” in 2005 and there have been 15 years with less than 1” of rain during the month. When taking the lowest monthly precipitation “top 5” and averaging them, September finishes only slightly behind October for the driest of the dry totals. Seems it’s “drought” or deluge?


Rainiest months on record, showing the #1 wettest year for each calendar month.

On the side of deluge, especially since we’ve seen them in 2011: What made 1934 so wet? Well, weather records are not superb back this far into history and this is especially true with tropical systems impacting the area thanks to satellites not being invented yet!

We do know the month started with an opening volley of about 1.5” over a two day stretch. This was shortly followed by a storm that looks strangely similar to Irene of 2011, both of which dropped around 4” of rain downtown.

Then it got really wet. 8.21” of rain fell from the 12th through the 16th as a tropical system spun well offshore. Interestingly enough, some parallels to 2011 (see Katia) appear here again, this time with early September’s historic rainfall.

The Washington Post, on the morning of the 13th in 1934 reported: “More rain fell in Washington in one hour yesterday than during any previous 60-minute period in the 63-year-old records of the Weather Bureau, turning streets into rivers, sending tons of water into basements in every section of the city and paralyzing street railway and electric power services in many communities.” (see 2011’s 1000-year rainfall in Ft Belvoir).

Re-analysis tools indicate this period featured a similar (to 2011) road block in the jetstream, featuring a “trough” of upper-level low pressure settled in just to our west (a location favorable for sending moisture and “rain trains” into the area). At the same time, strong high pressure also controlled the central United States, as well as much of the Atlantic Ocean. Nothing moved fast, except the rain gauges.

Before September 1934 was over, two additional significantly rainy events impacted the region, dropping 1.96” on the 21st through 22nd and then almost another 2” to close the month with a one day break on the 28th.

TEMPERATURES


September temperature averages throughout recorded history (in degrees F) at Washington, D.C. See sources for data.

When not interrupted by a tropical deluge or increasingly frequent (but often relatively dry) cold fronts, perhaps the most two pleasant months in D.C. get underway during September. Pleasant, that is, if you enjoy plentiful sunshine and cool morning temperatures followed by mild daytime readings.

The average overall temperature in September is 71.0 degrees. This represents a +0.5 degree change from the old 30-year normal. It is a drop of over seven degrees off the average of August. Feel the cool air rushing in?

The record warmest September came in 1980 when the monthly average was 77.1 degrees. The coldest September in modern history occurred in 1871 when the temperature averaged 62.3 degrees. Though the whole record — since 1871 — is considered one, the coolest monthly average seen at the current observation location (National Airport) was 66.5 degrees in 1963.

Daytime temperature averages fall from the mid-80s on September 1 to below 75 by month’s end. At the same time, morning lows in the upper 60s start to dip into what some may consider the chilly mid-50s as October takes over. Time to dust off the jackets and pull out some sweaters.

A breakdown of those often oh-so-sweet September days during the most recent 30-year climate period reveals highs reaching the 80s 42% of the time, the 70s 38% of the time, the 90s and 60s making up right around 10% each — the rest in the 50s. Lows ranged from the 60s about 54% of the time, to the 50s 27%, to the 70s 16%, and finally the chilled 40s came in at 3%.


Number of 90-degree or higher days in Washington, D.C., during September, by year since 1872. See sources for data.
For those who dislike summer heat, perhaps the best news is that 90-degree days are almost out the door in September, if not already done entirely. Averaging three such days a year, it is the last month we would typically expect to see these temperatures. While 90 degree or higher days can happen into October, it is so infrequent as to barely register an average of just above zero.

The hottest temperature any September day has delivered was the 104-degree scorcher back on the 7th in 1881. The hotttest National has been during September was the toasty 101 on the 2nd in 1980. And as recently as 2010, D.C. saw its latest reading as high as 99 degrees. When it comes to overnight lows, the warmest was 79 on the 3rd in 1993.

September’s coldest day in D.C. came on the 9th in 1984, when the thermometer only reached 52 degrees. The chilliest overnight reading was the near-freezing 36 in 1904, and it’s been as low as 39 at the airport during the month.

DATA

30-year (1981-2010) Averages / Overall (all history) Averages...

Highs 80 or above: 15.6 / 14.5
Highs 90 or above: 3.0 / 3.0
Highs below 75: 7.5 / 8.8
Highs below 70: 3.0 / 3.8
Lows below 55: 4.1 / 6.5
Lows 60 or above: 20.8 / 17.7
Days with a trace of rain: 11.4 / 11.4
Days with .1” of rain or more: 5.5 / 5.3
Days with .5” of rain or more: 2.4 / 2.2
Days with 1” of rain or more: 1.0 / 1.1

Days with thunder: 2.4 / NA
Days with hail: 0.0 / NA
-This count is done through records on Weather Underground, and does not include the entire historical period.

All-Time Records (High / Low)...

Highs 80 or above: 28 (1881) / 3 (1876)
Highs 90 or above: 14 (1980) / 0 (35 years)
Highs below 75: 21 (1924) / 0 (1881 and 1968)
Highs below 70: 12 (1928) / 0 (15 years)
Lows below 55: 16 (1879 and 1916) / 0 (6 years)
Lows 60 or above: 30 (1881) / 6 (1917)
Days with a trace of rain: 20 (1888) / 4 (1871 and 1884)
Days with .1” of rain or more: 12 (1876) / 0 (1977 and 2005)
Days with .5” of rain or more: 9 (1876) / 0 (17 years)
Days with 1” of rain or more: 6 (1934) / 0 (46 years)

Days with thunder: 5 (1992 and 1998) / 0 (1986 and 2010)
-Same as above. (30-year high / low listed)

SOURCES

Daily climate records for Washington, D.C., 1871-present
- See selected and additional statistics compiled for this post (Excel)

D.C. climate norms 1981-2010

DC September daily norms and records

DC monthly temperatures

DC monthly precipitation

NWS Baltimore/Washington climate information

NOTES

*All averages, unless otherwise noted, refer to the 30-year period of 1981-2010. Unlike prior months, a full set of new climate norms is now available and will be used throughout. Past months will be edited to reflect the new norms.

ABOUT THIS SERIES

While numerous local climatology statistics including daily, monthly, and seasonal normals and records are available from National Weather Service Baltimore/Washington, there are myriad other pieces of information which help frame the story. Rather than attempt to recreate what is already available, this series of posts will act as a companion to most information presented there.

This is the sixth installment of a series of living documents. Through early 2012, each month will be broken down in a similar or evolving way. As input is taken from the community, some items may be added to or changed in older postings. These postings will be updated as necessary to reflect changes when needed. Other articles may also become part of this series.

Feel free to share thoughts or additional data you may be interested in seeing.

Learn more about each month:

-April weather in Washington, D.C.
-May weather in Washington, D.C.
-June weather in Washington, D.C.
-July weather in Washington, D.C.
-August weather in Washington, D.C.
-September weather in Washington, D.C. (viewing)

Article last updated: September 15, 2011.

By  |  10:10 AM ET, 09/15/2011

Categories:  Local Climate, Latest, History

 
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