LRandyB's Tropical Weather Discussion

Tropical Weather Discussion - Monday, July 9th 2013

By: LRandyB, 4:46 PM GMT on July 09, 2013

We're watching TS Chantal this week as it makes its way into the Caribbean. There are a lot of factors going into the long range forecast! We'll talk about that this morning! Let's go over the tropics!

Gulf of Mexico …

Over the past few days, the sub-tropical jet stream has been slicing from southwest to northeast across the northwestern Gulf with shortwave troughs working along the flow resulting is a lot of rain over the northern Gulf coast states. This pattern is starting to break up now as a broad area of high pressure builds in over the southeast US. This area is producing a broad scale mid and upper level flow out of the east over the northern Gulf from FL to TX. South of this flow is a series of upper level lows. The first is over northern Mexico, the second is actually in the process of developing over the central northern Gulf, and the third is in the Bahamas. All three of these lows are tracking west in the flow around the broad high centered in the TN River Valley. Over the last 24 hours, the rainfall has finally subsided over the northern Gulf though the abundant moisture still persists and thunderstorms can be expected to pop up in the day time heating this afternoon over most of the northern Gulf States. 

Caribbean Sea ….

There isn't a lot to talk about here. High pressure centered over the southwestern Caribbean is producing generally good weather across much of the region. A monsoonal-type trough has been producing rain showers and thunderstorms over the land areas of Central and northern South America. Daytime heating under the upper level low in the Bahamas has been producing scattered showers over the Bahamas and Hispaniola. And of course, Tropical Storm Chantal is tracking this morning through the Lesser Antilles bringing rain and gusty winds to that area. 

Atlantic Ocean  ……

The bulk of the tropical Atlantic is being dominated by a broad strong area of high pressure centered in the eastern Atlantic. The weather is generally good across the entire tropical Atlantic. Visible satellite loops do show a region of Saharan dust over the western Atlantic. The ITCZ is generating showers and thunderstorms south of 7-8N across the Atlantic. 

Tropical Storm Chantal

Chantal is moving at a fast pace (near 25kts) to the west-northwest through the Lesser Antilles at this time. This fast track is being driven by the strong easterly flow around the high pressure ridge in the central Atlantic. As of the latest discussion from NHC, the intensity is set at 50kts based on aircraft recon. However a number of factors will work against future intensification. The presence of Saharan dust around the storm is capping convection in the periphery of the storm and relatively dry air around the system isn't helping either. Wind shear is expected to rise as Chantal enters the eastern Caribbean caused, at least in part, by the upper level low over the Bahamas. The eastern and central Caribbean is an area of low level divergence. The air there accelerates westward leaving a vacuum of sorts at the surface which is filled by air from above. Descending air warms, dries, and kills convection. The current forecast track brings Chantal over Hispaniola, which would inhibit, if not outright dissipate the storm.  Lastly, Chantal's own rapid forward speed makes it difficult for the storm to consolidate its energy. 

The long term intensity forecast is dependent to a great extent on the track. Models currently bring a trough of low pressure into the eastern US which is expected to pull the upper level low in the Bahamas more northward, resulting in a weakness on the strong Atlantic ridge driving Chantal to the west. This is expected to turn Chantal more toward the northwest by about the 72 hour point (Thursday). This track will bring the storm over Hispaniola and the interaction with the mountains there may spell the end to the storm. Beyond that point, the models diverge. It looks like the ridge will build back in over the western Atlantic which would induce a more westward track in the remains of Chantal again. But a lot will depend on whether or not that trough/upper low combination will indeed be able to pull Chantal north and how much the storm dissipates over Hispaniola. That will depend on whether the track does indeed go across the island.  

The models are indicating another system developing late in the week about where Chantal did in the eastern Atlantic and following a similar course so we'll have to watch and see how that unfolds as well.



Updated: 4:52 PM GMT on July 09, 2013


About LRandyB

I was an AF aviation weather forecaster for 12 years, then 15 years as a dropsonde systems operator with the AF Reserve Hurricane Hunters.

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