Friday, April 26, 2013

2013 Official Atlantic Hurricane Outlook

Hello everyone, and welcome to The Weather Centre's Official 2013 Atlantic Hurricane Outlook. We begin with my analog package.

We start now with the analog package. I utilized the Pacific-Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) for this outlook's analog years. The PDO involves a positive and negative phase, where each phase depends on the prominent water temperature anomaly in the Northeast Pacific. A glance at the sea surface temperature chart for the northeast Pacific reveals a swath of above normal water temperatures in the offshore regions of the Gulf of Alaska, with below normal temperatures immediately offshore of North America and Alaska. This is the typical signature of the negative PDO. As for the Atlantic Multidecadal Oscillation, we also see a positive and negative phase with this index. The positive AMO signifies warmer than normal water temperatures across the waters off Greenland and in the far north Atlantic Ocean. In the same sense, the negative AMO allows below normal water temperatures to encompass the North Atlantic waters.

After choosing specific years from these two indices, I combined my chosen years and used the ones that had both the PDO and AMO in phases similar to what we are currently anticipating. As a result, I came up with the years 1951, 1952, 1955, 1956, 1999, 2000, 2008, and 2011. All of these years had a clear negative Pacific-Decadal Oscillation and positive Atlantic Multidecadal Oscillation, which is what we are experiencing now and what we should experience moving forward into the spring and summer months.

Let's take a look at an archive of hurricane tracks during the aforementioned years to see if we can see a trend in my analog years that could assist in helping us find a common track for the upcoming season.

In 1951, we saw a storm season that had many storms going out to sea rather than towards the US Mainland. At least 3 of those storms didn't even have a chance of making it to the United States. However, there were a few storms that did have a close brush with the Carolinas, as well as one landfall in Florida. A couple storms did make a threatening track towards the Gulf, but for one reason or another they did not hit the US. One storm (Hurricane Charlie) did hit Mexico, the Yucatan Peninsula, and Jamacia. All in all, the trend for this year was a slight Gulf threat with multiple close calls on the East Coast.

1952 was not unlike 1951 in terms of where tropical systems went. While the Gulf of Mexico was considerably quieter with no systems actually in the heart of the Gulf, there was one landfallig system in the Southeast US, another landfall in Florida, and at least 3 storms that ended up recurving out to sea. The system that struck Florida, which formed on Groundhog Day (yes, in February), was never named. Hurricane Able was the system that hit the Southeast. The trend in this season was a threat to the East Coast and a rather quiet Gulf of Mexico.

1955's Atlantic Hurricane season involved a pretty darn active season in terms of landfalling tropical systems. We saw multiple impacts on the Gulf Coast, especially in Louisiana and Mississippi. We also saw possibly more than 2 separate landfalls on the East Coast, all of which struck the Mid-Atlantic (one also sideswiped the Northeast). Tropical Storm Brenda was one of the systems that hit the Gulf Coast, and it was Hurricane Connie that slammed into the East Coast. Tropical Storm Five followed Brenda into the Gulf, while Hurricane Diane took a hint from Connie and sped towards the East Coast to make landfall. The clear trend here is an active landfalling season in the Gulf and East Coast.

1956 continued the idea of storms running dangerously close to the US Mainland, even making landfall a couple points along the way. We saw a rather unusual season in that the majority of the storms took almost a due-north path out to sea. Usually, storms will form off the coast of Africa and curve west before doing a 180 and going out to sea. In this season, the storms just formed and made their intentions clear as day. Tropical Storm One, the first named system of the season, impacted the Gulf with a landfall. Both Hurricane Anna and Tropical Storm Dora hit Mexico, while Hurricane Flossy did a double-take by hitting the Yucatan Peninsula and the Gulf Coast. An unnamed subtropical cyclone hit Florida and skirted into the Mid-Atlantic. The trend in this season is clear: Tropical cyclone threats were highest in the East Coast and Gulf Coast.

1999 was no different than previous years. We saw multiple tropical cyclones skirt immediately offshore of the Eastern Seaboard, and the Gulf of Mexico came under fire. We saw extreme southern Texas take a landfall from Hurricane Bret, while Florida was surrounded by several near-misses and at least two landfalling systems. It was Hurricane Dennis that took one of the most awkward tracks I have ever seen a tropical cyclone take. It began to curve out to sea after narrowly missing eastern Florida, but suddenly make a hard turn south, then a hard turn west before making landfall in the Mid-Atlantic as a weak tropical cyclone. The trend continues; East Coast and Gulf Coast were threatened in this season.

The year 2000 was a bit different in that the number of threats to the East Coast was reduced. We saw more of a tendency for storms to curve out to sea earlier than storms in, say, 1999. The state of Florida did bear the brunt of at least one landfall (courtesy of Hurricane Gordon), while Alabama and Georgia were affected by a tropical cyclone known as Tropical Storm Helene. A tropical depression also made landfall in the western Gulf Coast, although it is not shown in the map above. The Gulf Coast definitely made headlines as the most affected area this season, with the East Coast in a not-so-close second.

2008 brought an absolutely chaotic Atlantic hurricane season. We saw over half a dozen tropical cyclones hit the Gulf Coast, with one cyclone hitting the East Coast. Tropical Storm Cristobal affected the East Coast, while Hurricane Dolly and Tropical Storm Edouard hit the Gulf Coast. Tropical Storm Fay zig-zagged through Florida, while Hurricane Gustav ravaged the Gulf. Hurricane Hanna hit the Eastern Seaboard, and it was Hurricane Ike that dominated the Texas coast. The trend is probably the clearest we have seen in all of the analog years: The Gulf experienced the largest threat, with the East Coast also getting in the action.

My last analog year, 2011, echoed previous analog years, but also raised the recurring theme of storms curving out to sea. We saw the heavy majority of tropical cyclones curving out to sea in 2011, with little to no damage coming between them. However, there were two Gulf Coast landfalls that were weak, as well as one Mid-Atlantic landfall. The final analog year continued the idea that the US mainland was threatened.

The next piece of evidence I want to show you is a chart of the latest observed sea surface temperature anomalies over the Atlantic Ocean. I outlined three regions of anomalies- an above-normal sea surface temperature (SST) area in and just south of Greenland, a below normal SST area to the east-northeast of the Mid-Atlantic coastline, and a large swath of above normal SST values from the coast of Africa and west into the Caribbean. This type of arrangement of SST anomalies is called an Atlantic Tripole. Years that had an Atlantic Tripole in place had a positive correlation with the monthly hurricane total from June to July and Accumulated Cyclone Energy (ACE) values. This positive correlation means that, since the Atlantic Tripole is in its positive phase right now, we could reasonably expect more tropical cyclones than normal to form this season, and those cyclones could have more energy than normal, which would then raise the ACE index.

I am monitoring the El Nino-Southern Oscillation at this time, and current sea surface temperatures suggest we are wavering between an El Nino and neutral-ENSO conditions. The CFS long range model forecast shown above projects any hint of an El Nino dying off going through the summer months and rest of spring months. A variety of ensemble members from the CFS forecast system average out to produce a neutral-ENSO situation through the hurricane season. The lack of a solid El Nino or La Nina means more weight is placed on smaller-scale factors that we will look at later on in this article.

Based on all the factors listed above, and after observing tropical outlooks from other sources, I have composed a map detailing the areas where I believe a tropical cyclone of any strength (tropical storm or stronger) could strike. Areas outlined in green show the probability of a landfall as 'Low'. This means that climatology and projected atmospheric factors don't especially portray the outlined area as prone to a tropical cyclone landfall. Areas in yellow are denoted as having a 'Moderate' risk of a landfalling cyclone. The 'Moderate' risk means that climatology and multiple atmospheric values tell me the risk of a landfall is above the historical norm, but not by much. Another way to look at it is just over 50-50, favoring a landfall (55-45, if you like). Finally, the red areas depict a 'Fairly High' chance of a landfalling tropical cyclone. I would put 'High', but you can't really put that out unless the outlook is made days before the season starts and your confidence is over 100%. To play it on the safe side, I define the 'Fairly High' category as regions where climatology and multiple atmospheric factors are contributing to what could be the hotbed of activity for tropical cyclone threats to land. It should be noted that this 'Fairly High' region could be extended further west and maybe a tad north if my analysis continues to be favorable for landfalling cyclones. That subject will be addressed further in coming updates as we inch closer to the hurricane season.



Anonymous said...

Andrew thank you so much for the extraordinary posts today! I knew you were going to do two major posts today but this topped what I expected. Thanks!, Justin

alan decker said...

i have a question, with the current temperatures being very low here in America what kind of impact does that have on hurricane season?

Andrew said...

Alan: The cold has more or less ended across much of the country, and shouldn't have a significant effect this season.