According to National Weather Service records, all three of our region’s major airports reported “cloudy” conditions for 10 days in a row ending Monday. On Tuesday, we only just fell into the “partly cloudy” realm with a late-day save.
There were a lot of happy people in the District late Tuesday afternoon, gawking at the blue sky and sunshine.
This dark and dreary period is, of course, coming in a month often remembered for its plentiful blue skies. While September is rarely that chilly, it is the month when the average high temperature falls into the 70s and the average low dips to the 50s. Here at the Capital Weather Gang, that’s squarely in “Nice Day” territory (e.g., lots of sun, minimal wind, highs in the 65-to-85 zone).
While the weather can often become boringly pleasant by now, that certainly hasn’t been the case this year. It has been more floods, more storms, and even some tornadoes.
Sheldon Kusselson, a meteorologist now retired from the National Oceanic and Atmospheric Administration, points out that at his location in Colesville, Md., the eight-day streak of cloudy conditions (note: The method here and above may not be quite the same as highlighted in the graphic above) he recorded tied for the longest at his location. The tie comes with two months in winter. Winter is, of course, a time when we might expect to be stuck with clouds for a while. Now, not so much.
Long-term cloud-cover stats are difficult to obtain, but we can see the same general idea looking at data compiled by the Iowa Environmental Mesonet and synthesized in the graphic below. I’d be remiss not to point out that these values were gathered before the sun returned. Today and more sunny days in the future are likely to bring readings down before the month ends. Nonetheless, a graph of our daytime cloudiness gives a good indication of where we’ve been this month, so far.
During most waking hours, it has been very cloudy. We have been well above normal in the cloud-cover realm overall, with frequency values by hour as much as two to almost three times the normal. This data goes back to the 1950s, although there are a few gaps in there, but it is a long enough period to have some certainty in its conclusions.
Looking only at noon data, I found that through Tuesday it was overcast at lunchtime for almost three-quarters of days. That was the most for any September in this database.
As you might expect, with a bunch of clouds has come rain.
Through Tuesday, D.C. had eight days with measurable precipitation this month. That was good enough for a tie for the seventh-most on record. The three days with one inch or more is enough for a tie for second-most, and, of course, we remain near the top for an annual tally there as well with 18 days of one-inch or more rainfall, to date. It’s the same general story at Baltimore, where 11 days with .01 inches of rain this month is the second-most on record since the 1870s.
This is all something of a continuation of the exceptionally warm and humid air mass that overtook the Northeast in August. And at this point, I think we can go ahead and say this is also a continuation of an unusually humid warm season overall.
When it rains a lot, it is often humid. September is crushing dew-point values seen in recent times. In addition to reaching and surpassing records for hours above various higher-end thresholds in September, we are now pushing and exceeding similar values on an annual scale. There’s a reason the mushrooms outside are the size of trees!
A major cause is a very strong and persistent high-pressure zone that has been locked in across the international border region from the Great Lakes to off New England through much of the warm season. Winds around it have kept moisture coming in off the Atlantic and out of the tropics. It is what helped guide Hurricane Florence into the East Coast during a year in which many expected comparatively low hurricane activity.
Until this feature and others around it break down further, it’s hard not to lean at least somewhat toward persistence, although it is also increasingly true that it can’t hold on forever as winter approaches.