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Posted at 01:30 PM ET, 10/05/2011

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


October 31, 2010 — Fall foliage in Washington, D.C. Photo by author. See image larger.
October is often one of Washington’s finest months of fair weather, though the trend toward the cold season is evident by its conclusion. As the month begins, high temperatures above 70 are still common, but by the end, readings typically only rise into the mid-60s.

Average precipitation for October ranks 6 (out of the 12 months), highest in D.C. after the “warm season” through spring. However, like September, rainfall in October is often heavy over a short period, thanks usually to tropical moisture or stalling weather systems in the region. It also can be a really dry month.

Keep reading for more in this seventh 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

The average* monthly rainfall for October is 3.40”, which is a modest increase from the previous 1971-2000 average of 3.22”. October is one of seven months which saw precipitation increases relative to that previous base period. It’s also closest to the monthly mean (3.31”) when factoring in the entire calendar year.


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

The most rain that has fallen in one day in October in Washington is the 3.98” which fell way back in 1877 on the 4th. Notably, the number two and number three October days came back-to-back on October 7 (3.63”, #3) and 8 (3.67, #2) 2005, when the remnants of Tropical Storm Tammy interacted with subtropical storm #22 to cause severe flooding across parts of the Northeast. For context, 25 days in all Octobers since 1871 have seen rainfall in excess of 2”.

Not surprisingly given the daily records for rain, the wettest October on record came during 2005. That high mark stands at 9.41” and — while impressive — the first monthly record below 10” for D.C. since April. As we see moving forward in the year, monthly averages bottom out in winter when less atmospheric moisture is available due to colder temperatures and a more progressive pattern.

Perhaps more interesting is the low end of October rainfall. It is the only month of the year that has a record low monthly precipitation of a trace, which occurred in 1963. The months of April and March don’t fall much off that mark with 0.03” and 0.05” respectively. When adding together the five driest years for each month, October finishes substantially below nearly all other months with September a somewhat close second.


An average of the top five driest years for each month display the “driest of the dry trend” over a period of time. Numbers are in inches of rainfall.

So what has caused this dry intrusion in October’s of past? Using some re-analysis tools, similarities between 1963 (T) and 2000 (0.02”) emerge. In both cases, the pattern is dominated by a surface high pressure centered over the eastern half of the country. Many October’s feature relatively tranquil patterns as the speedy winter jet stream has yet to migrate fully south and tropical systems are less frequent than earlier in the year.

One thing to keep in mind is that the origins of the high pressure during October differ from summer. Rather than a humid Bermuda high off the coast, October highs typically follow frequent cold fronts coming down into the United States from Canada. Since the origin is not off the ocean initially, the airmasses tend to be dry compared to summer and early fall.

Though quite dry, neither October 1963 nor 2000 were much above average temperature-wise. From the standpoint of temperatures, a dry October does not mean the same thing as a dry July. 1930 provides further evidence that dry does not necessarily mean warm in October, as it was about 3-degrees below average by today’s standards and stands as the third driest October on record.

And while it remains a footnote for at least one more month, it can snow during October in Washington. Accumulating snow has fallen on five days (out of 4,340 in the record through 2010). The earliest accumulating snow of the month — and on record — fell on the 10th in 1979 (yep, at National!) when 0.3” was measured. The most October snow in one day was 2” on the 30th in 1925 (with another 0.2” the day after). Ten other days have witnessed a trace of snow in the month.

TEMPERATURES

October is a month we can usually count on to be pleasant on the whole, as averages fall squarely into the “comfort zone” for most folks. Mild temperatures become ever cooler, but not too often are they continually chilly to the point of needing to bundle up for extended periods of time. However, lower sun angle can mix with clouds and anomalously cold air to produce some memorable cold shots even quite early.


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

October’s average temperature of 59.5 degrees is up (just like every other month) 0.7 degrees from the old normal, but the change between September’s average and October’s average is the largest of the year in D.C. A difference of 11.5 degrees separates the two months as the mercury falls downhill rather rapidly.

The warmest October in D.C. came in 2007, when the average was 67.1 degrees. In 1876, the monthly average was a chilly 50.7 degrees, the record lowest for October. As with many cold records, the chilliest result at National Airport is a good deal warmer (even if still much cooler than normal). The 54.4 degrees recorded in 1988 comes in as the lowest there.

Average daytime maximum temperatures fall below 70 degrees on the 13th. By month’s end, the average dips all the way to 64 degrees, a loss of 10 degrees over 31 days. Similarly, the average low falls from 56 degrees down to 46 degrees during the month. If you have not had to grab a jacket by the 1st, you almost certainly will by Halloween.

The breakdown of high temperatures during the current climate period (1981-2010) shows it to be quite favorable for comfortable outdoor time. Almost 3/4 of the month, or 72%, featured highs in the 60s and 70s, with 15% of days in the 50s, 11% in the 80s and 1% each in the 90s or 40s. 80% of the lows were in the 50s and 40s, with 12% in the 60s, 7% in the 30s and only 1% in the balmy 70s.


Number of occurrences of a high temperature equalling 70 degrees or greater on each day in October during the 30-year period from 1981-2010. See sources for data.

For anyone looking to maximize shorts and t-shirt weather, those critical 70-degree or higher days are on a steady decline, and for the first time since April they trend toward outlier rather than norm. When the month begins, we can historically expect about a 68% chance of hitting 70 degrees. The probability falls to around 23% to close.

The highest temperature recorded in D.C. during October came on the 5th in 1941, when it was 96 degrees. The hottest it’s been at National is 94 degrees, most recently occurring on the 9th in 2007. The warmest low temperature for the month is 74 degrees, which has happened on 3 dates, most recently in 1941.

There have been 25 days in October with a high temperature of 90 degrees or higher. This means the odds of seeing a 90-degree day are not all that much better than seeing a trace of snow, and having a day with a low below freezing is almost 3 times as likely as a 90-degree day.

Like snow, it’s still a footnote, but cold air starts to make itself at home in October. The lowest high temperature for the month was the 40 degrees on the 23rd in 1889. At National, the lowest high recorded was 41 degrees on the 30th in 2002. It’s been as cold as 26 degrees at night back in 1893 and 1917, each on the 31st. National has seen a temperature as low as 29 degrees on the 24th in 1969.

DATA

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

Highs 70 or above: 14.2 / 13.5
Highs below 60: 4.9 / 5.7
Highs 80 or above: 3.6 / 3.1
Highs 90 or above: 0.2 / 0.2
Lows below 50: 13.9 / 16.6
Lows below 40: 2.0 / 4.2
Days with a trace of rain: 11.2 / 11.1
Days with .1” of rain or more: 4.8 / 4.7
Days with .5” of rain or more: 2.5 / 2.0
Days with 1” of rain or more: 1.0 / 0.9

Days with thunder: 1.0 / 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 70 or above: 25 (1973) / 4 (3 years)
Highs below 60: 17 (1925) / 0 (1881, 1971)
Highs 80 or above: 15 (2007) / 0 (26 years)
Highs 90 or above: 5 (1941) / 0 (123 years)
Lows below 50: 28 (1909) / 2 (1984)
Lows below 40: 16 (1895) / 9 (22 years)
Days with a trace of rain: 22 (1906) / 3 (1874)
Days with .1” of rain or more: 10 (4 years) / 0 (1963, 2000)
Days with .5” of rain or more: 7 (1885) / 9 (20 years)
Days with 1” of rain or more: 4 (1937, 1942) / 0 (59 years)

Days with thunder: 4 (1992) / 0 (15 years)
-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 October 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 seventh 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.
-October weather in Washington, D.C. (viewing)

Article last updated: October 5, 2011.

By  |  01:30 PM ET, 10/05/2011

Categories:  Local Climate, History, Latest

 
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