Most Read: Local

Posted at 10:45 AM ET, 10/12/2012

First freeze of fall in the Washington, D.C. area is typical by mid-October


First freezes — grouped by period — at National, Baltimore, and Dulles for the current (1981-2010) climatological norm, plus 2011.
An autumnal chill has certainly arrived in the D.C. area this week, which means it’s time to think about the first fall freeze. Last year, we discussed how the date of the first freeze varies widely across our region. Today we’ll recap some of that key information and also look at how the first freeze has changed in the past half century.

While downtown Washington and some urban areas inside the Beltway will likely not see freezing temperatures until November, the month of October is fair game for a large part of the region. Tonight, for example, D.C.’s north and west suburbs might see their first fall frost or a full-fledged freeze as cold high pressure settles over the mid-Atlantic.

If Dulles Airport (IAD) dips to 32 degrees or lower tonight, it would hardly be unusual. The current average first freeze at that location is October 15. Over the past 30 years, Dulles’ five earliest freezes have occurred by the first week of October. One of those was a record-early-tying Sept. 24 in 1983. The graph above shows that Dulles gets its first freeze by October 15 roughly every other year. So if you live in D.C.’s typically colder suburbs, this might hold true as well.

Link: Map and in-depth statistics on area first freeze information


Earliest freezes at D.C. area climate stations during the current climatological period.
Reagan National Airport (DCA) is one of the last locations in the D.C. area to record a freeze. Only once in the last 31 years has DCA dropped to 32 or lower in October, and in six years the first freeze was not until December. D.C.’s “official” first freeze comes quite late, not until November 18 on average. Because temperatures at National Airport often run warm this time of year, it’s not the best point of comparison on a freeze metric – sometimes even if you live downtown. Generally speaking, though, many locations inside the Beltway stay frost-free until around the beginning of November or later.

D.C.’s lower-elevation inner suburbs might best look to Baltimore-Washington Airport (BWI) for a gauge to plan on their first freeze. It comes about half the time in mid-to-late October and about a third of the time not until November. This gives BWI a current average of October 29 for first freeze – almost right in between Dulles and National Airport’s averages. Wide ranging results at locations not too far apart.


Latest freezes at D.C. area climate stations during the current climatological period.
How much has the first freeze date changed over time?

Looking at the past four 30-year climatological periods (1951-1980, 1961-1990, 1971-2000 and 1981-2010) at DCA, we see a nine-day shift in the average first freeze, from November 9 to November 18. A linear trend would suggest it’s closer to a two-week shift since the mid-1940s, when observations began there. Though there is some decade-to-decade variance, the climatological periods exhibit a steady later trend each in each bucket.

At Baltimore, as measured at BWI since 1950, the signal is much less noticeable. In fact, if looking at those same four climatological periods, we see a change over time of only one day. The average first freeze could be expected on the 28th of October from 1951-1980 compared to the 29th today. A linear trend for Baltimore over the entire period shows a similarly small change in first freeze dates, with a gain of about two days.


First freeze dates since 1963. Dulles Airport’s first full year of records was in 1963, so we’ve pulled truncated records of the other two airports to examine the same period. Both DCA and IAD exhibit more of a trend toward later first freezes as compared to BWI which is rather steady.

IAD’s climate data for first freeze did not come along until 1963, so the sample period is inherently less informative than the overall records for D.C. and Baltimore. Using one skewed 30-year climatological period (1963-1992) in addition to the last two as above, Dulles has seen a change in first freeze greater than Baltimore but less than D.C. IAD averaged its first freeze on October 13 from 1963-1992, and on October 15 from 1981-2010. A linear trend shows a more extreme change of closer to 10 days.

This is just a snapshot of the shifts in first freeze we’ve seen over the past half century or so. It would at least seem that the change in first freeze seen at D.C. is a bit of an outlier in the area. Its location right near water that retains heat may play a role in this given a coinciding trend of warmer summers and warmer water temperatures. Baltimore may also be an outlier on the other end. Something to examine further in the future.

By and  |  10:45 AM ET, 10/12/2012

Categories:  Latest, Local Climate

 
Read what others are saying
     

    © 2011 The Washington Post Company