Hi, it's Rebecca here with another blog installment. This entry will look at some of the parameters that will shape the Summer of 2012; it will also touch on the upcoming hurricane season. I will try to keep it general and also leave the math out of it. I will just mention percentages along with a general overview of where things stand and look to be heading. However before I get into the Summer outlook. I want to look back on the Winter of 2011-2012 and what the jet stream is.
What is the Jet Stream:
I've touched on jet streams in past blog post, however, I will give a brief explanation on what the jet stream is.
Jet streams are bands of strong westerly wind around the 300 mb level of the upper atmosphere. This would place it above 20,000 feet, or between 6 and 9 miles above the Earth's surface. The jet marks where the strongest temperature contrast can be found at the surface. Jet streams are formed by the temperature differences between two air masses. These are the warm and cold air masses, along with areas of high and low pressure you hear about.
The polar jet stream is the one that has the most impact on our weather in the U.S. It marks the divide between cold arctic air and mild mid-latitude air. Because its position is a result of temperature contrasts, it's position shifts throughout the year, depending on the season. In the U.S. the jet pushes weather systems from west to east.
The winter of 2011-2012:
I'm sure most of you will agree that the past winter was quite unusual. There were quite a few underlying reasons for this. However, the primary reason was the position of the polar jet stream. Below are a couple of images of the jet. The first image shows how the polar jet behaves on average during the winter. The second shows how the polar jet more or less looked like during the winter of 2011-2012; outside a couple of dips south, it more or less stayed in this configuration for most of the winter.
Image 1
Image 2
The reason I'm showing you this is to show the impact the jet has on any season. As for winter 2011-2012, the pattern of the jet, locked the cold air up in Canada. Therefore, the lower 48 saw the 4th warmest winter on record.
The jet stream during the summer:
Meteorological Summer is almost here. During mid to late spring temperature and precipitation patterns start to get locked in. As I said above, the jet plays a very important role in how the summer season will behave. During a typical summer the polar jet will move back to the north. Image 3 shows how the jet looks on average during the summer.
Image 3
During the summer the temperature difference is less extreme between the air masses and the jet stream is weaker. When the jet is in this configuration, the warm air surges northward. During the summer, the jet orientation often prevents the large scale synoptic storm systems we see during the winter. As a result, our rain in the Northeast is often the result of short-waves and cold fronts. This is why the summer months are normally drier than the other seasons of the year.
La Nina is dead, but El Nino is stirring:
El Nino is defined as warmer than normal sea-surface temperatures (SSTs) in the tropical Pacific ocean.
La Nina is the opposite with colder than average Pacific SST's.
A large change in the ocean temperature patterns is underway in the equatorial waters of the eastern Pacific Ocean, The SST has warmed to the point that in April, La Nina was officially declared to be over. Presently the SST's in the Pacific are near average. This would place us into a El Niño/La Niña-Southern Oscillation (ENSO) neutral state. Many are basing their summer outlooks on the idea of the ENSO being/staying neutral. However there are model trends that are showing the ENSO heading into El Nino. Based on everything I've seen, I feel we will be ENSO neutral for the rest May into the first part of June. Then for the rest of June onward we will under El Nino conditions. Who is right will have huge impact on how the summer and hurricane season will play out. I think the summer will feature a weak El Nino perhaps becoming a bit stronger heading into next winter. Now that I've set the stage let's move on to the Summer outlook.
The Summer of 2012:
My summer outlook will be based on the idea of a weak ENSO along with the recent temperature/precipitation trends across the Northeast. Before I begin, it's important to say.....a seasonal outlook is just an educated guess ...and as we saw with my winter outlook....they can go wrong. This is not a forecast per say. Instead it's an attempt to see how the general temperature/precipitation should go. It will not talk about individual areas/municipalities. Also, it will not go into detail about the severe threat over the summer.
Precipitation:
As most of us are aware, the Northeast is having precipitation issues. In-spite of the recent few weeks of rain, many of us are still experiencing mild drought conditions ( 2.5 to 4 inches below normal, with more than that in spots ). I think the current tread of a few days of rainy weather followed by a week or so of drier weather will continue. Therefore, drought conditions will most likely get a bit worse as the Summer swing into gear. However, I see no reason to fear an extreme drought. That said, some areas will have a bigger drought problem than others.
Image 4 Northeast drought conditions
Temperatures:
The lack of prolonged heat in the Northeast has been an issue the last 4 to 6 weeks. This has been because of blocking over Greenland or northern Canada. I think this pattern will continue into at least the first part of summer. If we don't see a lot of heat before the El Nino starts to get its act together we might not see much in the way heat. I'm not saying there won't be a few days were the temps get very warm. But the kind of heat-waves we've seen over the last few years will be less common. Any severe weather we see this summer will be during those days where we see a bit of warmth. The storms could involve low level convection and be more cold core in nature
So, in general this summer looks like it will feature slightly above (above normal temperatures for some) Along with a chance for below normal precipitation for a big chunk of the Northeast.
The Atlantic hurricane season is officially from 1 June to 30 November.
A few things I took into consideration,
The ENSO pattern, as I said, right now the ENSO is neutral. However, I feel it will become a weak El Nino around mid summer.
Past years, I looked at several years that had very similar setups; the two that matched the closest where 1994 and 2006.
The Forecasted Sea Surface Temps (SST's) in the Atlantic, Caribbean, and Gulf of Mexico
Image 5 sea surface temps
As you can see for most of the SST's for most of the Atlantic, Caribbean, and GOM are near or slightly below normal, Now add in the wind shear from a weak El Nino, and hurricane development should be below average.
La Nina not only effects areas like the Northeast; it also had a large influence on the last two Atlantic hurricane seasons . La Nina provides more favorable wind conditions over the regions of the ocean where storms spin up. However, El Nino is just the opposite, the wind patterns are aligned in such a way that the vertical wind shear (change of winds with height) is increased over the Caribbean and Atlantic. The increased wind shear helps disrupt hurricane formation.
2011 was not very far behind. 20 tropical cyclones, 19 tropical storms, 7 hurricanes, and 4 major hurricanes.
2012 and 2011 with their back-to-back 19 named storm seasons were well above average; both years stacked up to 97.9% above the 1950 – 2000 average of 9.6 storms per year.
A few statistics:
During the past 114 years, no El Nino event has had more than one major hurricane making a US landfall.
A hurricane that makes at least one landfall or impacts the coast with hurricane force winds is considered a U.S hurricane. with this in mind, the mean annual number of U.S. hurricanes during El Niño years is 1.04 and 1.61 during neutral years. You can see there is a big difference.
Strike probabilities for this season:
(There is a difference between a yearly strike probability and a climatologically based probability)
For a large part of the Northeast the stats are,
For one or more named systems making landfall it's 11.8%, land falling hurricane is 7.0%, and a major hurricane is 3.3%. Keep in mind this isn't just one state or area. For individual areas the odds go down. For example Maine has a 2.9% chance for a hurricane strike and <0.1% for a major strike.
How I see the numbers stacking up:
10 to 12 named storms
Four to five named storms will obtain hurricane status
One or two of those hurricanes will obtain major hurricane status (CAT 3 or greater).
Remember, while the stats show a much lower chance for a season like last year; it only takes one landfalling hurricane in your region to make things terrible. So please take heed of any hurricane watches or warnings.
Remember, while the stats show a much lower chance for a season like last year; it only takes one landfalling hurricane in your region to make things terrible. So please take heed of any hurricane watches or warnings.
Hurricane Return Periods:
Hurricane return periods are the frequency at which a certain intensity of hurricane can be expected within a given distance of a given location. For example, a return period of 20 years for a major hurricane means that on average during the previous 100 years, a Category 3 or greater hurricane passed within 58 miles of that location about five times.
I thought some of you would find these maps interesting. So I thought I would throw them on here.
Image 6 (hurricane return period)
Image 7 (major hurricane return period)
Image 9 ( Atalantic basin coastal strikes)
National Hurricane Center
I hope you found this interesting and maybe learned something in the process. As always, feel comfortable to ask me any question you may have, either in the comments or as an email.
Rebecca
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