So many unknowns in the tropics…for everyone…

Good afternoon.

The tropics are still giving us headaches this afternoon as the Caribbean wave continues to menace all of us. At the same time, wind shear is quite high so the wave cannot close off a low level circulation or sustain convection. Considering we are much closer to any significant tropical event and the oceans are very warm off the Florida coast and in the Gulf of Mexico, rapid development is certainly not out of the question. Most current models, however, keep the storm weak or do not develop it at all.

In addition to that low which has all of our attention, the remnants of Fiona (which are not expected to redevelop) continue to roam not far from Bermuda, while Tropical Storm Gaston is also an issue and should re-intensify. Fortunately, Gaston should not be a US threat.

Current satellite image of the broader Atlantic (image from NHC/NCDC) annotated showing current systems as of 3:45 pm EDT August 25, 2016.

Current satellite image of the broader Atlantic (image from NHC/NCDC) annotated showing current systems as of 3:45 pm EDT August 25, 2016.

Tropical Storm Gaston

Gaston looks less organized than it did 24 to 36 hours ago, due to higher wind shear in the area. A Global Hawk research aircraft flew out to the eastern Atlantic on an unmanned mission yesterday and in the evening, after it had weakened, confirmed that Gaston indeed reached hurricane intensity via a dropsonde. Hence, it likely peaked early Wednesday with sustained winds around 85 mph. Right now, the NHC estimates that Gaston has weakened to about 65 mph, which appears reasonable. A bit more weakening is likely through tomorrow, then during the weekend into next week it is likely it will restrengthen into a hurricane. In fact, it is certainly possible it could become a major hurricane by Monday or Tuesday.

When it comes to track, Gaston looks to continue moving westward for a while. The models are split on when recurvature will occur, and Bermuda may be at risk early next week. No reliable model brings it anywhere near the US coast though. As a result, the only significant threats will be high surf, rip currents and waves on the beaches.

Disturbance/Invest 99L

This disturbance that we’ve been tracking all week does not look good today. The tall mountains of Hispaniola are disrupting the circulation, plus wind shear is quite high. Working in its favor are very warm oceans (88-92 degrees) and the fact that shear should decrease into the weekend. The NHC has decreased the development potential to 40% through Saturday and 70% through Tuesday. I believe those are quite reasonable. The GFS and the GFDL fail to develop 99L at all.

Right now, I believe there are four reasonable scenarios for the track and intensity should 99L develop into Tropical Depression Eight or Tropical Storm Hermine. Note that the probabilities add up to 100% but such is only valid if this develops, which I say is about 70% likely. They are all dependent on what the strong ridge over the Southeast will do over the next few days.

Scenario 1: Ridge holds firm (20% chance)

12Z CMC August 25, 2016 model run for Tuesday afternoon (2:00 pm EDT). Hurricane Hermine, a compact storm, is making landfall near Cameron, LA.

12Z CMC August 25, 2016 model run for Tuesday afternoon (2:00 pm EDT). Hurricane Hermine, a compact storm, is making landfall near Cameron, LA.

This is a status-quo scenario, which assumes that the ridge over the Southeast remains locked over the next few days. It is supported by the CMC and some ensembles. In this scenario, Hermine is forced west or west-northwest near South Florida or the Florida Straits into the Gulf of Mexico. The warm waters (especially in the central Gulf) makes this an ominous scenario with the potential for rapid deepening. Landfall would likely be along the Texas or Louisiana coast.

Impacts to the Northeast and Mid-Atlantic would be limited to inland after the storm recurves and becomes extratropical (if it doesn’t dissipate by then). It would be bad especially for Louisiana who is still recovering from catastrophic flooding earlier in August.

Scenario 2: Ridge shifts east (20% chance)

06Z CMC August 25, 2016 model run for Tuesday morning (8:00 am EDT). Intense Hurricane Hermine (likely category 3 or 4) is turning northward towards the central Gulf Coast.

06Z HWRF August 25, 2016 model run for Tuesday morning (8:00 am EDT). Intense Hurricane Hermine (likely category 3 or 4) is turning northward towards the central Gulf Coast.

This is the preferred solution from the ominous 06Z HWRF and from an ECMWF run yesterday, but has little current support other than from a few ensembles. The ridge over the Southeast largely remains intact but shifts eastward offshore. That allows Hermine to recurve northward somewhere between southeastern Louisiana and the Alabama coast. Similar to the previous scenario, the potential for rapid intensification exists particularly in the central Gulf. Like the previous scenario, the Northeast and Mid-Atlantic should be largely spared here. The only impacts will be after landfall as it heads northward.

 

Scenario 3: Ridge breaks down (50% chance)

12Z ECMWF August 25, 2016 model run for Wednesday morning (8:00 am EDT). Tropical Storm Hermine is making landfall near Tallahassee, FL and quickly moving out to sea afterward.

12Z ECMWF August 25, 2016 model run for Wednesday morning (8:00 am EDT). Tropical Storm Hermine is making landfall near Tallahassee, FL and quickly moving out to sea afterward.

Most models have converged on this solution and it is probably the most likely track. However, I don’t believe it is a foregone conclusion at this time as it requires the Southeast ridge to weaken or a weakness to exploit itself. In this scenario, Hermine tracks near or over South Florida then turns northward immediately, perhaps due to a new ridge in the western Gulf. That allows it to turn northward, either making another landfall on the Florida Panhandle or running up the peninsula. It should then immediately turn out to sea, avoiding the Mid-Atlantic and Northeast for the most part. Land interaction should help hold its intensity down at least somewhat as well.

Scenario 4: Gaston interferes, resulting in a Fujiwhara (10% chance)

This complicated scenario is not supported by any models at this time, but some earlier runs had it. It assumes that Gaston continues westward and is blocked from turning out to sea from the eastern ridge (or the weakness north of Bermuda closes). As Hermine moves close to Florida, it tracks similar to Scenario 3 (perhaps on the eastern envelope of those possibilities) except it is blocked and the two storms interact. Where they end up is unknown – they could shear themselves apart or could move in unusual directions. It is still worth watching, but it is not a likely scenario.

Conclusion

Meteorologists have been mentioning this system all week and it continues to be a nuisance. Really, anyone along the east and Gulf coasts should always be preparing for storms as it is hurricane season (with the peak near). Uncertainty remains very high, but confidence is fairly high that it will not be a significant issue in the Northeast or Mid-Atlantic. Nonetheless, we must continue to be vigilant for any surprises.

Forecaster Craig Ceecee – @Ceecee_Wx

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