Masters student in tropical meteorology at FSU. Raised in Alaskan blizzards, but drawn toward tropical cyclones by their superior PGF.
By: Levi32 , 4:05 PM GMT on July 03, 2009
Tropical Tidbit from 12:00pm EDT Friday, July 3rd, 2009:
There are no areas of imminent threat to develop through the end of this week.
However, there is one item of interest which the NHC has mentioned in its tropical outlook. A non-tropical area of low pressure in the NE Atlantic WSW of the Azores has become non-frontal, and has the potential to try to become a sub-tropical system. This low is embedded in the base of an upper shortwave trough that has become cut-off from the main westerly flow on the east side of an upper ridge in the SW Atlantic. This is imparting strong NW shear on the system, which is pushing its convection to the east of the center. If the low is to become sub-tropical it needs to become warm-core at the low-levels, and it can only do that if convection is allowed to build near and over the center. The system is over SSTs of between 25C and 26C, which is well above the 23C threshold for sub-tropical development. The models are hesitant to take this warm-core, but they keep it moving slowly eastward over waters greater than 23C for about 3 days before they become too cold for sub-tropical development. This system is not particularly likely to become sub-tropical but it will be monitored for any changes.
The GFS and ECMWF are still hinting at mischief off the Carolina coasts in 4-8 days. An upper shortwave trough is forecast to lag behind the main westerly flow over the southern states and help keep a front stalled off the SE coast in 4-5 days. With the upper pattern a little more zonal than it has been, the troughs off the eastern seaboard become more positively tilted (oriented SW to NE) and tend to be slow to drag out, especially the surface fronts that accompany them. In this situation you can get home-brew mischief from lows that form along the tail-end of the front between the SW side of the upper trough and the ridge over the Gulf of Mexico. The latest run of the GFS shows a weak closed low forming off the SE coast in 7 days. I will be continuing to keep an eye on this situation as it evolves.
There continues to be a massive build-up of heat in the eastern Pacific, evidenced by the very large area of convection that continues envelope the entire area. There is a broad area of low-level vorticity between 100W and 90W at around 10N. Somewhere in this area, likely further west than east, a surface low is forecast to form during the next couple days, and may end up developing into a tropical storm if the energy can bundle enough. If a storm does form it will most likely be steered NW and away from the Mexican coastline.
We shall see what happens! And Happy Independence Day to all!
Caribbean Visible Satellite: (click image for loop)
Below I have written up a presentation about the analog years for this year's hurricane season. My goal here is to provide some insight into what this season may have in store for us. If you find this kind of stuff interesting, you may want to stay tuned for a video I intend to make tomorrow about El Nino, how it works, and how it affects the Atlantic hurricane season. I hope you enjoy.
Tropical Tidbit Topic: 2009 Analog Years
I haven’t yet taken the time to do an extensive analysis of what this year’s hurricane season may have in store for us in terms of where the storms may go and how many there will be, although I have voiced my thoughts. While things are still quiet, now seemed like a good time to do more research and find evidence to back up my forecast.
My main focus here will be historic analog years, which are one of the greatest tools at our disposal for long-range and climate forecasting. Analogs are past years that had similar conditions to the present. Looking at what happened in the past often gives extremely valuable insight into the future. With all the talk about El Nino’s potential impact on the season, I decided to use the SOI (Southern Oscillation Index) as my base parameter for finding analogs. There are many other factors to consider, but the ENSO has one of if not the most influential impacts on Atlantic hurricane seasons. I looked for years with similar SOI patterns to this year (going negative in May and June, with predominantly positive values in all the prior months January-April). This particular pattern of monthly values doesn’t happen as often as you may think, especially in the last 60 years. Keep in mind that weather data starts becoming rapidly more sparse and unreliable the further back you go before the 1940s. I would have stuck to analogs for the past 60 years but as I said there are very few of them in this time period so I included older years as well.
The analog years I found were 1894, 1898, 1899, 1908, 1911, 1925, 1928, 1935, 1939, 1949, 1963, 1972, 1982, 1984, 2001, and 2006. 1963, 1982, and 2001 are less-than-perfect analogs, but they are still worth putting in. Instead of cluttering up the blog with a big map and description of every year, which are non-essential details, I’m going to put them all in a smaller wall so you can see the "big picture". They’re too small to see the dates but they start with the oldest year at the top left and the latest year at the bottom right. You can view the best track data here, courtesy of Unisys Weather.
Figure 1. Historical hurricane tracks in 2009 analog years based on January-June SOI behavior.
Notice where the greatest concentration of tracks is. In most of these years there is a congregation of tracks passing through or near to the north and east of what I like to call the "Bermuda Box". This box is formed off the SE US by connecting the points of Cape Hatteras, Bermuda, Turks/Caicos Islands, and south Florida.
Figure 2. The Bermuda Box.
The pattern in these analog years sticks out to me, with tracks focused more north and east and a lot of hurricanes affecting the US east coast, New England, Canada, and Bermuda. Also notice the considerable lack of tracks in the tropical Atlantic east of 55W, and even in the Caribbean. There are a couple of the years that were exceptions to this, but even most of those storms were very weak and didn’t ramp up until they reached 55-60W. This supports my thinking that this year will see a less active Cape Verde season due to low SSTs in the MDR (Main Development Region), anomalous downward air motion, and dry air. This is when you look for tropical waves coming off Africa to remain mostly weak in the eastern Atlantic but then try to develop out west closer to the Caribbean and SW Atlantic.
Not surprisingly, most of these analogs are either El Nino or neutral years (although there are only 6 years after 1950 when SST monitoring began). This is a good example of what El Nino does to the Atlantic hurricane tracks. It creates more downward motion, low SSTs, and higher wind shear in the deep tropics which limits storm activity south of 20N. This moves the bulk of the storm tracks north of 20N and closer to land, which makes home-brew systems more common and true deep tropical-born systems less of a majority.
The currently developing El Nino is already enforcing most of these factors with the MJO tending to focus downward motion in the tropical Atlantic, and wind shear has been mostly higher than normal since May. SSTs have been running below normal in the MDR this year, although they are starting to warm up and may recover to normal levels due to a weaker Azores high in the NE Atlantic, which causes weaker trade winds over the eastern/central Atlantic and therefore less upwelling of cooler water. There is a significant area of cold anomalies in the SW Atlantic as well, but the Gulf Stream and New England waters are starting to get above normal.
Figure 3. SST anomalies for the Atlantic and equatorial Pacific on July 2nd.
The upper pattern across North America and northern Atlantic also plays a big role in where storms will go. The CFS is forecasting an upper trough to stay locked off the eastern seaboard of the United States throughout the late summer and early fall, and a predominantly negative NAO (North Atlantic Oscillation) would support this in keeping the Bermuda High weaker than normal. This pattern would tend to steer most approaching hurricanes to the north and east, protecting the Gulf of Mexico from getting a lot of storms crossing over Florida. That doesn't necessarily mean it won't happen, as a short-term change in this pattern could easily steer a storm more towards the west.
Figure 4. The CFS 500mb height forecast for September, 2009.
Due to all these factors I expect the bulk of the storm tracks this year to be further north and east than last year, and less activity in the deep tropics and eastern Atlantic. That would put the areas of Bermuda, SE canada, the Bahamas, and the SE US coastline at higher risk this year. I expect several storms to recurve well east of the US, but the SE coast and New England has a good chance of being threatened when the Bermuda High extends far enough west to steer storms towards the eastern seaboard. This doesn't mean I think ALL the tracks will be in this area. There will likely be a few through the Caribbean and Gulf of Mexico as well, and everyone, as always, should be prepared to get hit.
So how many storms am I forecasting? Well, the average number of named storms each season for this set of analog years was about 9 storms. There were a few "non-seasons" in there with 5 years not making it past 6 named storms. This is what El Nino is usually expected to do to our hurricane season, however, there were also 4 years of 11-15 named storms. This proves that the ENSO is not a magic wand that makes years active or inactive, and there are many other factors that determine what the season is like. Any year can have a large number of storms, and even years with a low number can have a high impact season on the coastline. The slogan around here is "it only takes one". I am still forecasting 12 named storms for this hurricane season, which is on the high end of most of the forecasts out there. This is no different than my June 1st forecast, and the same goes for the tracks forecast.
Please keep in mind that I am not a certified Meteorologist and this forecast is unofficial. I hope everyone is prepared for the hurricane season!
We shall see what happens!
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