About two weeks ago our overnight lows were in the balmy 50s, but since then we’ve flirted with the freezing mark twice. Freezing temperatures may again visit the far northern and western reaches of our region tonight (freeze warning in effect) and even creep close to the beltway Friday night (freeze watches in some areas already).
You might be wondering when is the risk of freezing temperatures finally over in the region? In some places, it won’t be until some time in May.
As we might expect, the average date of the last freeze* is largely elevation dependent. Compared with the first autumn freeze, a location’s proximity to water usually plays less of a role due to relatively cool water temperatures in early spring (this year being an exception).
Let’s review the final spring freeze statistics for the D.C. area’s three major airports to get a better sense of what is typical for our region…
Last freeze in downtown Washington
At Reagan National Airport (DCA), the average date of the last freeze (32ºF or lower) is around March 28 per 1981-2010 climate norms. If we look at D.C.’s entire temperature record since 1871 (note: prior to 1945, the observing station was located at 24th and M. St rather than Reagan National Airport), the average date of the last spring freeze is April 2. The 5-day earlier shift in our new averages is most likely a combination of increased urban heat island effect and greenhouse warming.
Washington’s latest spring freeze on record was on April 29, 1874. At National Airport, the latest spring freeze was on April 21, 1956 when the temperature dropped to 32 degrees. More recently, DCA recorded 31 degrees on April 9, 2007. Prior to this, the last freeze to occur in April was on April 5, 2004.
On the other end of the spectrum, D.C.’s earliest final spring freeze was on February 27, 2010. The second earliest was March 5, 1945 and the third earliest was March 9, 2008.
When was the latest spring freeze at National Airport in the last 30 years? Interestingly, it was April 13, 1990. Exactly a month earlier, the mercury hit a record-breaking 87 degrees on March 13 (and an even warmer 89ºF on March 12). Gardeners and warm weather lovers beware: among D.C.’s ten warmest Marches prior to 2012, four were followed by an April freeze inside the Beltway!
Last freeze outside the Beltway
As many of us are aware, overnight low temperatures at National Airport are usually not representative of the greater metro area. So if you live outside the Beltway – particularly north and west of D.C. – the last spring freeze at Dulles or Baltimore-Washington Airports is probably a more useful metric.
Dulles Airport (IAD)
At Dulles Airport, the 1981-2010 average date of the last spring freeze is April 21. This is about two days earlier than the average last spring freeze in IAD’s entire climate record since 1963. The earliest final spring freeze at Dulles was on March 24, 1986, when the temperature dropped to 24 degrees. The second-earliest final freeze was on April 1, 2003.
Unlike DCA and BWI, Dulles holds the distinction of recording several final spring freezes in May. In fact, Dulles has seen its last freeze during May in 6 of the last 30 years. The latest freeze on record was a reading of 31 degrees on May 22, 2002. The most recent May freeze at Dulles was on May 3, 2005.
Baltimore-Washington Airport (BWI)
For simple comparison, we’ll only consider the average date of Baltimore’s last spring freeze according to new 30-year climate normals. In the 1981-2010 period, the average final freeze at BWI is on April 11 (about two weeks later than DCA and ten days earlier than IAD). The earliest final freeze in this period was March 23, 2006 and the latest was on April 26, 2001. Due to Baltimore’s slightly more northern location and easier exposure to backdoor cold fronts from the northeast, it is not unusual for a freeze or frost to occur around the Baltimore region during the second half of April.
The important lesson here is that weather can behave in unpredictable ways. A warm spell in March or early April doesn’t guarantee that freezing temperatures are over and done with until fall. Consider this: in 2010 it hit 93 degrees on April 6 at Dulles Airport. This was Dulles’ earliest 90-degree day on record, yet less than two weeks later the temperature dropped to 32ºF on April 18!
What does that mean for 2012? There is a good chance that National Airport already saw its last spring freeze on March 6. But areas far enough removed from the D.C. urban heat island… well, you’re probably not fully in the clear until mid-to-late April.
Useful gardening links
* The average minimum temperature on the date of the last freeze was about 30ºF at all three local airports.