September 2013 may be remembered most for its plethora of pleasant days, but also may have marked the early stages of a drought.
The month offered the highest number of Nice Day qualifiers in recent memory with two-thirds of the high temperatures falling in the pleasant 70s to low 80s range. This came at the expense of precipitation with measurable rain on only five days this past month. Monthly rain totaled a mere 1.22 inches, 2.5 inches below normal.
Temperatures in detail
The average temperature of 71.3F was identical to the 2011 result and was just barely warmer (0.3 degrees) than the 30-year normal. These two years tied as either the 8th warmest or 6th coolest of the 2000s (take your pick!).
High temperatures ranged from 69F to 94F, but were mostly in the 70s low 80s. Low temperatures ranged from 51F to 76F, but were mostly in the 50s and 60s. Of the three airports, we only had one record of note:
* National saw a record high minimum of 76F on September 11th that bested the record from 1900
The graphic below shows the day-by-day details for the month. You can see the warmest differences from average fell early on (more of a distant memory, with somewhat cooler weather more recent):
A sequential look through the 2000s does not reveal much in the way of trending, but the coolest year was 2000 and warmest was 2010. 2013 is right around the middle.
September was dry, but it wasn’t the driest in recent memory. 2005’s measly 0.11 inch result was actually the driest of all time. Examining all years, 2013 falls in as the 20th driest (since the late 1800s).
For the 2000s, it was the 3rd driest, but it was twice as wet as the second driest. No major trends in precipitation are detected through the period with our driest being 2005 and wettest being 2011.
A daily view of precipitation totals through the month reveals how sparse our rains were with only five days of measurable precipitation and only one above a half inch. Both Dulles and Baltimore saw more rain than National, but all three airports deficits exceeding 2 inches.
The dry August and September period has now put 2013 at 0.58″ below normal for the year at Reagan National. This is not a big deal yet, but if we see a dry October too, drought concerns could turn more serious.
The weather pattern
The pattern in September was quite variable with both periods of impressive warming and cooling, but we had more ridging in the North Atlantic toward the latter half of September that favored extended periods of high pressure and cooler, comfortable readings. And all that high pressure also kept us on the drier side.
The most amazing aspect of September was the exceptionally quiet scene in the tropical Atlantic. Usually, September rainfall is enhanced by tropical events that can deliver healthy rainfall totals to the area. But this year saw above normal wind shear, more than normal dust/dry air, and weaker tropical waves across the Atlantic basin.
Our new month is the middle of the meteorological autumn. The normal high starts the month in the middle 70s but ends in the middle 60s. Normal lows start in the middle 50s and end in the middle 40s.
Like September, we are starting the month with warm compared to normal. Instead of early September 2013 90s for highs, we are seeing low to middle 80s. But, in both instances, these readings are around 10 degrees above normal.
Despite the warm start, we may still see more variability through the balance of the month given an unstable Pacific pattern. The best thinking right now is to favor a warmer-than-normal result. Temperatures could end up a few degrees warmer than normal, but I believe the variable pattern should give some strength to occasional cool fronts which will boost precipitation compared to last month.
The National Weather Service agrees with the warm outlook here. But they do not offer any guidance on precipitation with EC meaning Equal Chances of anything happening.
For further information
The National Weather Service publishes nice monthly assessments usually within a week of the close of each month (should be available shortly):
You can click on your closest airport location here: