Average 20 year old weather nerd. Plymouth State University Meteorology, Class of 2018. Interning this summer at NWS Boston.
By: MAweatherboy1 , 6:11 PM GMT on July 13, 2013
The Atlantic basin is quiet right now in terms of active tropical cyclones, as is every other major tropical basin in the world, as Soulik, a once very powerful typhoon, has dissipated over mainland China after an original landfall on Taiwan. The remnants of this storm will still pose a threat for flooding rains as the move NNW over the next day or so. There are no major areas of interest in the Atlantic right now, however both the remains of what was once Tropical Storm Chantal as well as an area of disturbed weather originating from an old frontal boundary in the northern Gulf of Mexico are being given a 10% chance of development in the next 48 hours. I would put the odds at near 0% for ex-Chantal and about 20% for the Gulf disturbance, but I do not see either of these features developing. In addition, none of the reliable computer models are consistently developing a tropical cyclone in the next week, and the GFS model is not showing consistent development within its range of about 2 weeks. Still, there are encouraging signs for the Atlantic. One is that the Saharan Air Layer (SAL), a layer of dry, dusty air that can come into the east Atlantic from the deserts of west Africa, is pretty much gone. There has been significant SAL, as usual, for much of the season so far, which has prevented any tropical waves from really getting going. Now, however, as we get a little deeper into the season, we are seeing the erosion of the SAL, largely thanks to the series of tropical waves that have moved through the east Atlantic. While another outbreak or two of SAL is possible before the end of the month, it is clear the overall trend is to remove this dry layer of air, giving tropical waves a better chance to develop.
Figure 1: SAL map. Notice the lack of widespread reds and pinks over the Atlantic, with just one fairly minor and weakening area of concentrated SAL in the deep tropical Atlantic.
Another good piece of news for Atlantic development is that wind shear is generally low between 5N and 15N east of 50W, an area that is watched for tropical development in the peak months of the season. If this low shear continues, waves should be in a good position to develop after they leave Africa. This will be especially true a few weeks from now, as climatology would tell us, since sea surface temperatures are still pretty marginal for tropical development at 15N and higher. These waters should steadily warm as we get farther into the season.
Figure 2: North Atlantic wind shear.
While the declining SAL and low shear is good news for Atlantic development, there do remain a few problems. One is that water temperature anomalies in the Atlantic main development region (MDR) are not as warm as expected. In fact, they are coming off of a multi-month low, and are barely above 0C. Many preseason forecasts, including the CSU forecast, pointed out that warm anomalies in the MDR were expected to persist or increase, which would leave plenty of fuel to ignite some strong tropical cyclones. We have in fact seen the opposite occur, however, which has already led some agencies, including Tropical Storm Risk (TSR) to lower their seasonal expectations, and CSU will probably be next. In addition, some long range pattern forecasting models, including the ECWMF, continue to predict hostile conditions (higher shear and mean sea level pressure) across the Atlantic for the peak months of the season, though this may be due to the fact that some of these models still appear to have the idea that an El Nino may form this season, which would tend to suppress tropical activity in the Atlantic. While some of the Nino regions have shown occasional temperature increases, all of their anomalies remain at or below 0C, more indicative of a La Nina than an El Nino. To me, and to most, an El Nino still appears quite unlikely for this hurricane season, especially considering we are only less than two months from the climatological peak of the season.
Figure 3: Atlantic MDR sea surface temperature anomalies, struggling to stay above 0C and attempting to bounce back from long time lows.
A final feature that I and many others have noticed so far this year is the strength of the Bermuda high in the Atlantic. This ridge of high pressure appears in some form or another every year in the Atlantic, and is often the dominant steering influence on tropical cyclones that form in the deep tropics off the coast of Africa. A stronger Bermuda high will steer storms farther west, such as what happened with Chantal, while a weaker ridge gives storms more room to recurve out to sea, such as what we have seen a lot in the past few seasons. This high doesn't really mean much for seasonal numbers, but it has everything to do with the odds of a long track Cape Verde hurricane impacting areas in the Caribbean as well as the United States. The strong, persistent high we have seen so far this year is definitely a worrying sign, as it would indicate predictions that seemed to put the Caribbean and U.S. in the line of fire for multiple hurricanes this year are becoming more likely to verify. I continue to expect a busy year for landfalls, and a good chance that the U.S. will break its long drought of no official landfalling major hurricanes.
So, now that all this is on the table, what do I personally expect for the rest of July and the rest of the season? Well, for July, not a whole lot. We are in a transitional lull right now. Our odds of development close to home from a passing MJO pulse are pretty much gone, so the Gulf and Caribbean are pretty much on lockdown for the next 2 weeks at least. Meanwhile, the MDR has yet to really open for business this year, as waves coming off Africa now are still not strong enough to develop. Overall, I would put the odds of further development from tropical origins in July at about 30%, and the odds of development at all at about 40%. Meanwhile, for the season, my thoughts from before the season remain unchanged, as I continue to forecast 15 total named storms, 8 hurricanes, and 3 major hurricanes, and continue to expect a high probability of landfalls in the Caribbean and United States, particularly the central and eastern Gulf Coast and the southeast coast.
Thank you as always for reading; I will be away for a few days early this week, and with no tropical development seeming imminent, I will likely not have a new blog up for awhile, so I hope you enjoy the rest of your weekend and have a great week!
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