Humberto... Stunningly close to a record

This morning, the National Hurricane Center issued an advisory stating that Humberto, a tropical system in the Atlantic Ocean, had officially become a hurricane, with winds reaching above the 74mph (65kt) threshold. There is nothing remarkable about this hurricane. It's a Category 1 storm, it isn't a threat to land over the next 5 days, and it is forecast to be a hurricane for a short amount of time. I guess it's fitting, because this storm is just barely missing out on having its own unique place in history!
If Humberto had gained strength just a little bit later in the day, it would have set a new record for the latest first hurricane in a calendar year during the satellite era. What do I mean about the satellite era? As I mentioned in an earlier article, the first weather satellite wasn't even launched until 1961. That means we couldn't see tropical systems from above, and it's pretty easy to spot most tropical storms on satellite imagery because of their circular structure and rotation (see picture below).

A classic hurricane structure is evident when viewed from above,
using modern satellite imagery. (Courtesy of USAToday)

It's not just the satellite technology that helps us spot storms. Also, the system of using reconnaissance aircraft to study hurricanes was first used in the 1940s, which is decades after we started keeping official records of tropical storms and their impact on the United States. The aircraft's structure, and its data collection methods, are much more sophisticated now than they were back in the '40s. Today, we fly airplanes into developing systems to get accurate measurements for central air pressure, core wind speed, and other parameters. It's possible that, because of our more advanced technology, there are storms that were out there in the ocean and were never acknowledged as being tropical storms or hurricanes!
Based strictly on the data we have, the "official" record for the latest hurricane belongs to Hurricane Alice, going all the way back to 1954. As mentioned above, this is before the satellite era. Alice became a Tropical Storm on December 30th, and a hurricane on December 31st! The Atlantic Basin almost got through an entire calendar year without a single named storm... as far as we know.

Is the tornado risk growing in the USA?

 So far, 2013 has been a pretty calm year for tornadoes. It might not seem that way, because of the disasters in the Oklahoma City area in May, and our own tornado outbreak in the DMV back in June. But, surprisingly, this year's tornado count is actually below average. This is illustrated in the graph below, which I got from the Storm Prediction Center's website. 

Tornado counts, based on local storm reports from weather offices around the country

As you can see, the tornado count is below normal, and it's also way below the count for 2011, which was arguably the worst year for tornadoes in American history. Here's a map of the tornado reports for 2011: 

2011 tornado reports, courtesy of NOAA. Triangles are color-coded for a tornado's
strength; lines indicate long-track tornadoes.

In comparison, the tornado reports were a lot sparser 50 years earlier. 

1961 tornado reports, courtesy of NOAA. Triangles are color-coded for a tornado's
strength; lines indicate long-track tornadoes.

Tornadoes are reported via eyewitness accounts by trained storm spotters, and/or through rotation signatures on Doppler radar. Tornadoes are then confirmed via damage assessments, relying on a path of destruction to determine if a funnel touched down. So here's the problem-- how do we know that 2011 was the worst year for tornadoes? There are a few factors that should be taken into consideration. 

Not as many people lived in the Midwest and the Plains states in 1961 than in 2011, so there were fewer people who could potentially see a developing funnel. More people means more buildings, so it's also possible that it's easier to see and assess tornado damage now than it was 50 years ago, simply because there are more buildings and structures that could be damaged by the storm's winds. 

Technology improvements are a big factor. In 1961, the weather radar was primitive compared to the tools we have today, and 1961 also happens to be the year when the first weather satellite was launched into space. Before then, we could not look at storm systems from above. (I can't even comprehend what it was like to make a forecast without looking at satellite data!) 

Finally, it has occurred to me that the "interest factor" should not be ignored! As technology has improved and weather coverage has expanded in the media, people's interest in the weather has skyrocketed. The phenomenon of the storm chaser is not new, but its recent popularity has led to a huge increase in the number of storm spotters and other people who follow thunderstorms in anticipation of a developing funnel. The increased number of tornadoes could be partially attributed to a larger number of trained storm spotters, amateur weather watchers, and everyone in between! 

The Fall Forecast Looks Much Drier in the DC Metro

Well, how's this for unfortunate timing? Now that meteorological summer is over and the kids are back in school, our weather pattern is turning much more pleasant. The long-range forecast looks sunnier and drier, in stark contrast to the soggy summer weather we had to deal with.
As I talked about in a previous post, the summer was a bit cooler than normal, and much wetter. If we're just considering meteorological summer- which is the months of June, July and August- we had 5.3" more precipitation than in a typical 3-month summer period. Statistically, the summer didn't look that cool on paper- our average temperature was 0.57 above normal- but that number takes both the high and the low temperature for the day into account. Since it was so humid this summer, our overnight low temperatures were much milder than average. The humidity not only prevented temperatures from cooling down overnight, but it also kept us from getting too terribly hot during the day. The humidity is partially responsible for our small number of days with 90+ temperatures (only 23 days in 3 months). As a result, most of the summer was very warm and uncomfortably humid.
Of course, summer is typically more humid than fall, but this pattern shift in the forecast is more than just a seasonal variation! We're also finally seeing a breakdown of the huge ridge out west, which gave portions of the Rocky Mountains one of the hottest and driest summers on record. An upper-air map from September 1st shows the ridge in place out west, and our hot and humid pattern from the Labor Day weekend:

Model data, such as the chart below from the European model, suggests that the ridge will subside a bit and slide to the east. Here's a look at the forecast upper-air map for this upcoming Sunday: 

Notice how the ridge is now in the center of the country! When a ridge is situated just to our west, it blocks strong and/or moisture-laden cold fronts from moving through the DC Metro. A general northwest flow will keep the humidity nice and comfortable, and should lead to some nice, crisp fall nights. The ridge is expected to slowly progress eastward during the month of September. If that happens, we can expect a period of dry weather with temperatures above normal. I think we'll see lots of 80s for high temperatures in September, with only a scant few days with highs in the 70s. The 90s that we had to begin the month might be it for this year! Let's hope I didn't speak too soon. :-)

Hurricane Season Update

Recently, a viewer asked me if the forecast for this year's Tropical season was going to be updated to reflect the fact that it's been a slow year in the tropics thus far. NOAA's prediction for 13-19 tropical systems this year might sound high, since we haven't felt the impact of a tropical storm or hurricane in the United States since the first one, Andrea, made landfall in early June. That said, there have been six named storms so far in the Atlantic Basin in 2013. Last year at this point in the season, we'd had seven named storms. By the end of the year, there were 19 tropical storms and hurricanes on the books, including the most infamous from last year, Sandy.
As the summer season winds down, hurricane season usually heats up. Believe it or not, the peak of hurricane season is right around Labor Day! This graph shows very clearly how the number of hurricanes peaks in early September:

Courtesy of NOAA (National Oceanic & Atmospheric Administration)

The map below shows the point of tropical storm formation for all cyclones between 1859-2009 in the first 10 days of September (typically, this includes all of Labor Day weekend).

Courtesy of the NHC (National Hurricane Center)

As we head into the fall, the water in the Atlantic will slowly cool, and the number of potential storms will start declining. The map of tropical storm formation for October 1st-10th looks vastly different, with far fewer storms in the open waters off the coast of Africa, but more storms in the warm, shallow waters of the Caribbean:

Courtesy of the NHC (National Hurricane Center)

So, this is the time of year when we expect to see tropical systems popping up in the Atlantic Ocean, off the coast of western Africa. Lo and behold, the National Hurricane Center (NHC) says that there are two areas of potential development that they're keeping an eye on.

Courtesy of the NHC (National Hurricane Center)

The yellow shaded area has a 10% potential for becoming a tropical storm in 48 hours, and the orange area has a 30% potential of formation in the same 48-hour period. The Atlantic Basin is starting to get more active, right on cue with climatology. So, even though the season has gotten off to a somewhat slow start, we need to remember that it is far from over!