BMS Tropical Update – Mid-August

By: - August 22nd, 2017

It has been 45 days since I last provided a tropical update, mostly because there has been little to write about in term of U.S threats. However, with the peak of the season approaching, I would like to provide an assessment of the seasonal outlook that was outlined at the start of .the season. I will also provide some initial thoughts on a threat to the Texas coastline with remnants of Harvey expecting to come back to life over the next 24 hours.

Harvey’s Return

My general philosophy for posting tropical updates on specific tropical systems is to not post anything until there is a real concern of a U.S. landfall and subsequent impact to the insurance industry. I think we have now reached that threshold with the system formerly known as tropical storm Harvey, which is now located over the Yucatan Peninsula. It is expected that this system will be reclassified as a tropical storm Harvey as strengthening is expected during the next couple days, and it could even reach hurricane status as it approaches the U.S coastline by the end of this week. I expect the NHC to start issuing watches and warnings for the Texas coastline within the next 24 hours.

Harvey is currently forecast to make landfall somewhere between the Texas / Mexico border and central Louisiana, so there is obvious uncertainty with the forecast track guidance until Harvey moves back into the Bay of Campeche and reconnaissance flights get a better idea of where the exact center of the system is. The computer models don’t have the best read on this weak system at this time.

Current Track Ensemble Guidance for Harvey

Currently most of the model intensity guidance keeps Harvey as a strong tropical storm until it approaches the Texas coastline. However, systems that enter the southern Gulf of Mexico can strengthen rapidly and likely will do so until they make landfall. The water temperatures in the southern Gulf of Mexico are some of the warmest in the world and this means there is plenty of fuel for Harvey to strengthen rapidly and become a hurricane at landfall, which cannot be ruled out at this point. The further north the storm tracks up the Texas coastline, the longer the opportunity for the storm to stay over warm water and become stronger. This needs to be watched carefully.

The tropical cyclone heat potential, is a measure of the integrated vertical temperature between the sea surface temperature and the estimate of the depth of the 26°C isotherm. These regions have been associated with the sudden intensification of tropical cyclones.

Regardless of Harvery’s ultimate strength and track, it almost certainly looks like Harvey will have a slow forward motion and dump as much as 8” – 12″  of rain along the Texas coastline over the next week and if it stalls out like some models forecast the amount of rain could be even higher.  This will likely cause areas of flooding and past rainfall of this amount in the Houston area have not been good.

Current NWS Accumulated Rainfall over the next 7 days.

Peak of the Season Approaching
Several tropical systems have been monitored for formation by the NHC since my last update, of which five of them have been officially tracked. As I stated in my last update, tropical systems moving off the coast of Africa in the Main Development Region (MDR) will struggle to develop, but the overall environment becomes much better for development once those system get closer to the U.S. mainland. So far this season we have seen very little development in the MDR. This is mostly due to dry dusty air that has been a common deterrent the last few seasons. It’s not impossible for a named storm to develop off the coast of Africa, but the environmental conditions are limiting the opportunities this season.

The development that we have seen has come in waves this season, and without the major influence of an El Niño or La Niña as mentioned in the July 7 update, the MJO would likely influence these waves of named storm activity. I mentioned “The next phase of the MJO which could possibly enhance Atlantic named storm activity would occur near the end of July or first part of the August.” Five systems have been tracked by the NHC during this period, with Gert actually reaching category 2 hurricane status as it moved away from the east coast of the U.S. However, most of this year’s tropical systems have been weak and short lived with no major hurricane yet in the Atlantic basin. The Accumulated Cyclone Energy (ACE) is running about 84% of normal for the year, with almost half coming from Hurricane Gert. However, we have already reached the letter H in regards to named storms. This typically wouldn’t occur until around September 24, so we are far ahead of normal in regards to named storm count, which is in line with many of the seasonal forecasts.

 

Typically the Atlantic basin would be coming alive with activity with the peak of the season approaching on September 10, but the MJO’s expected move into a negative phase over the Atlantic Basin should suppress tropical convection and make it more difficult for named storm development during the peak of this season. Although named storm formation could still occur over this period, environmental factors such as dust and the MJO are pointing to less activity than normal for the month of September.

Currently the Red Area is Suppressed. This means less activity. The Pink and Blue is Active Phase of the MJO which can enhance convection and named storm activity. The three week forecast suggest this suppressed phase will move over the Atlantic during the peak of the Atlantic Season.

With the understanding that the North American weather pattern could change over the next month or so, we have seen two distinct tracks take shape this tropical season. One of these tracks is for tropical waves to take a southern route across the Southern Caribbean and influence the Yucatan, similar to Franklin and the current path of Harvey. The other path is for storms to track north of the Leeward Islands and influence Florida and the east coast. We have seen a few tropical waves influence south Florida (currently Invest 92L being watched by the NHC) and of course Gret strengthened along the east coast. This follows the general thinking for the season that as tropical waves move off Africa, they won’t strengthen into named storms until they are close to the U.S.

Without a major North American pattern change and implementing the ideas of clustering which is built into catastrophic risk models, it appears likely that we will see tropical waves take similar paths yet this year. However, as we approach the end of September and into October, which is when the next positive phase of the MJO would likely move back over the Atlantic, the current North American weather pattern will likely change.

Although our named storm count is high, overall ACE for the season so far is below normal. However, on the 25th Anniversary of Hurricane Andrew’s formation, it may be worth the reminder that it only takes one storm to have a major insured impact.

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