Concern Increasing For Tropical Troubles
In SC Early Next WeekWhile there remains considerable uncertainty about the forecast for early next week, the trends over the last 24 hours or so have not been favorable for the Palmetto State. Tropical Storm Humberto formed at 5 p.m. Wednesday, as expected. We continue to monitor the progress of the tropical wave we’re calling Invest Area AL94. 
This loop of visible satellite imagery shows the features of interest across the Atlantic Basin around midday Thursday, including Humberto, Invest Area AL94,
and Hurricane Gabrielle approaching the Azores. Image Source: University of Wisconsin RealEarth You can also see Hurricane Gabrielle over the eastern Atlantic marching at double time toward the Azores, but it’s not a threat to South Carolina. Humberto is also unlikely to threaten South Carolina directly. However, the National Hurricane Center’s (NHC) forecast calls for Humberto to become a Category 3 Hurricane over the western Atlantic before it gradually weakens and splits the Bermuda and Hatteras uprights around the middle of next week. 
Humberto won’t have any direct effect on South Carolina. However, the swells it will generate will bring rough surf and rip currents to our beaches in the coming days, along with potentially hazardous marine conditions on our coastal waters. Humberto’s behavior going forward is one of the several variables that bring uncertainty into the forecast for how AL94 might affect us early next week. Its track and intensity will influence AL94’s future track and intensity. AL94, the feature that could impact South Carolina early next week, remains a disorganized tropical wave that’s moving through the Dominican Republic and Haiti today. A circulation is trying to form just north of Haiti this afternoon, but the thunderstorms are hanging back over the Dominican Republic. NHC’s current forecast calls for a low-pressure area to form over the southern Bahamas or just north of Cuba by Saturday morning, which will quickly develop into a tropical cyclone. Their outlook from this afternoon indicates an 80 percent chance for it to be a tropical cyclone by Saturday afternoon. 
Most computer model guidance has come in line with a tropical cyclone forming in this area by Saturday night, so we have at least moderate confidence in that part of the forecast. Confidence is somewhat lacking right now because AL94 remains disorganized. Models tell us to trust the process, but that will be hard for me until I see thunderstorms erupting near the developing circulation center. The longer it takes for this to happen, the greater the chances are that AL94 will slip farther west than expected and end up over Cuba or Florida. Assuming that AL94 consolidates and becomes a tropical cyclone over Bermuda … the next name on this year’s list is Imelda when (“if?” seems less of a question now) it reaches tropical storm intensity … there appear to be two scenarios for where it goes. Which scenario comes to pass will depend on how quickly a storm develops, the behavior of Humberto, and the behavior of a storm system moving into the Southeast this afternoon from the Mississippi Valley. The first scenario, and the one we hope will work out, is that AL94 becomes Imelda over the Bahamas, but Humberto to its east is strong and close enough that it can pull Imelda to the east out to sea on Sunday into Monday. In this scenario, Humberto’s outflow aloft cases shear over Imelda, which limits Imelda’s strength. The other scenario, the one we hope doesn’t happen, is that AL94 becomes Imelda, but Humberto is too far away or too weak to have a significant influence on Imelda. That would allow Imelda to move northward toward us instead of getting pulled out to sea. It would then become caught in a developing upper-level low over the Southeast (the storm now located over the Mississippi Valley) that would pull it ashore on Monday or Monday night. Additionally, the usual effects of having an upper-level low nearby to the west would apply, as seen with Helene: the upper low would provide the storm with a more favorable environment to strengthen and maintain itself after landfall. Therefore, it could be a hurricane when it reaches us if this scenario unfolds, and the stronger winds could affect areas well inland. It might also be a slow-moving storm that causes widespread heavy rainfall. However, even in this scenario, there is uncertainty about the storm’s track; it’s possible that the worst impacts could occur in North Carolina instead of South Carolina. A hybrid of the two scenarios is also possible: Imelda moves close enough to our coast that our coastal areas see damaging wind, a storm surge, and extreme rainfall. The wind and rain could persist for an extended period as the storm may move slowly for a time. Then it eventually feels the influence of Humberto and gets pulled out to sea. The worst-case scenario at this point is for us to be dealing with a hurricane hitting us on Monday or Monday night, with impacts lingering into Tuesday. Rain impacts may even linger beyond Tuesday if the storm becomes stuck over us after landfall. It’s hard to put odds on that right now; I estimate that there is a 20-30 percent chance of that happening. That’s high enough that you need to prepare this weekend if you’re in one of our coastal counties, unless the forecast changes and we become confident in a low-impact or no-impact forecast. If you’re in one of our inland counties, you should closely monitor the situation. This is not going to be a Helene or Hugo, but the worst-case scenario includes locally damaging wind gusts over inland areas. As always, SCEMD has the hurricane.sc website to provide you with hurricane prep advice and evacuation zone info. Plan on preparing for the worst this weekend, and let’s hope and pray that a low-impact or no-impact scenario is what we face early next week. While I have your attention, the storm system moving our way from the west brings us some potential hazards through Friday. Thunderstorms affecting the state through this evening have a hot and juicy, summery air mass to feed upon, so there is a low-end risk for severe storms. 
There is a low-end risk of damaging winds associated with the more intense storms in the level 1 of 5 risk area shown on the Storm Prediction Center’s outlook map. The damaging hail and tornado risks are near zero, but ‘never say never’ applies. There is also a risk for isolated minor flooding from repeated downpours. Yes, it’s barely rained across South Carolina over the last 30+ days, but the rain could come hard and fast through Saturday.  
Keep this in mind if you’re traveling across the Upstate and vicinity through tonight or anywhere in the state Friday. If you live in a flood-prone area, you might need to move to higher ground and motorists may have to avoid a flooded road (turn around, don’t drown). Frank Strait
Severe Weather Liaison
S.C. State Climate Office | 
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