Potential Tropical Cyclone 9 looking like it’s going to be a serious rainstorm for the eastern Carolinas

In brief: Invest 94L was reclassified as Potential Tropical Cyclone 9, meaning it is likely to become a tropical storm within the next 24 to 36 hours and produce tropical storm conditions in the Bahamas. We discuss the rain risks there and in Cuba. We also take a closer look at the current forecast goalposts of rainfall for coastal South and North Carolina.

We wanted to add a post this evening to highlight that Invest 94L has been designated as Potential Tropical Cyclone 9, meaning that the National Hurricane Center expects it to develop within the next 36 to 48 hours, requiring tropical storm watches and warnings. In this case, those warnings are presently limited to the Bahamas.

5PM ET Advisory for PTC 9, soon to be Imelda. (NOAA/NHC)

PTC 9 is expected to become a tropical depression tomorrow and a tropical storm tomorrow night as it moves into the Bahamas. Heavy rain is likely in the Bahamas, along with increasing winds and seas as the storm moves through.

Total rainfall of 4 to 8 inches is expected across much of the Bahamas, with isolated much higher amounts over the eastern tip of Cuba. (NOAA WPC)

Some intense rain is likely over the eastern tip of Cuba as well. Rain totals may reach as high as 16″ or more in the higher terrain there leading to dangerous flooding and landslide risks.

From there, you can see the National Hurricane Center track above. It shows a hurricane and eventually a tropical storm approaching the coast of South Carolina by Tuesday night or Wednesday. There is a tremendous amount of uncertainty on exactly where this system will go, and if you look under the hood and read the NHC’s discussion, they make this clear as well. It’s possible that a landfalling hurricane or tropical storm hits the South Carolina or North Carolina coast early to mid-next week. It’s also possible that this thing just taps the brakes and meanders offshore for a few days. Both scenarios deliver impacts, including torrential rain to the Carolinas, particularly in the Coastal Plain between the Lowcountry, Pee Dee and southeastern North Carolina.

Rainfall scenarios

So let’s lay out a couple of the rainfall scenarios. First, here’s the current official forecast. Consider this the best estimate of how much rain may fall, on average, across the region over the next 7 days.

The official WPC forecast from NOAA for the next 7 days, showing anywhere from 4 to 8 inches of rain over a wide area of the coastal plains in the Carolinas, extending into the Piedmont and parts of Virginia as well. (Pivotal Weather/NOAA WPC)

So, officially at least, the current thought is 4 to 8 inches with isolated higher amounts at the coast.

Let’s say the system never really makes landfall and approaches the coast but probably stays offshore and drifts eastward. The GFS model shows this fairly well today. Its rainfall totals are still impressive.

The GFS model from late this afternoon shows the heaviest rain primarily on the South Carolina coast, with 4 to 8 inches in the Charleston area and lesser amounts north of there. (Pivotal Weather)

But they’re also notably lower than the current official forecast. This feels like a nice “floor” for how much rain could occur. I would anticipate at least this much. There will obviously be ways this could change, but given the scenarios in play, that’s where we are right now.

Now, let’s say the storm comes ashore or just hugs the immediate coast for a couple days. This is sort of what the European model showed this morning. How much rain falls in that scenario?

European model rainfall forecasts are much worse than the current outlook or the GFS forecast due to the storm sitting on the coast for a couple days. (Pivotal Weather)

Well, the Euro represents an extreme example at the moment but one that cannot be entirely ruled out. In this scenario, rain totals of 20 inches would be possible on the coast between Charleston and Wilmington with a wide area of 5 to 15 inches in the Pee Dee and southeastern North Carolina. This would bear some similarities to Matthew or Florence in that scenario in terms of the rainfall. Both of those storms were terrible flood producers in this region. To be clear, no one is forecasting a Matthew or Florence redux. But on the higher end of the spectrum of realistic possibilities, we can’t adequately rule the Euro model’s scenario out yet. Same goes for the less problematic GFS, of course.

But in addition to the “there’s a storm coming” mindset, we want folks on the coast and inland in the coastal plain to prepare for the potential for a long-duration rain event and the potential for flooding as well. There will be much to monitor this weekend, and we’ll be back in the morning with the latest.

Tropical disturbance nearing the Bahamas, and it may threaten the southeastern United States early next week

In brief: There is a lot to talk about in the Atlantic tropics this morning, but the headline remains an as-yet unformed storm that is likely to become a tropical depression near the Bahamas soon. After that there is an increasing possibility that the tropical system will approach the southeastern United States by next Monday or Tuesday.

Invest 94L

We start with the storm most people will care about the most: an area of low pressure now moving away from Hispaniola toward the Bahamas. The system does not yet have a closed circulation, but it will encounter favorable conditions as it pulls away from the Caribbean landmasses toward the open waters of the southwestern Atlantic Ocean.

A satellite image captured on Friday morning. (NOAA/The Eyewall)

We are fairly confident about the evolution of the storm through the weekend, as it should become a depression and then probably Tropical Storm Imelda. By Monday, when we look at a broad range of traditional and AI model solutions, we see that the system should have moved to the north of the Bahamas, and be located somewhere off the southeastern coast of the United States. The map below shows the model outputs for 8 am ET on Monday morning.

AI and traditional model forecasts for the location of Invest 94L on Monday morning. (Google/The Eyewall)

After this time frame we see a pretty broad divergence in the models. A majority of the solutions, but not an overwhelming majority, bring the system toward the Carolinas by around Tuesday. However a non-trivial number of models peel the system back to the east, into the open Atlantic, due in part to some complex interactions with Hurricane Humberto. Anyone having confidence in what, precisely, will happen at this point is probably not being honest with themselves (or you).

In terms of intensity the models are not super bullish on this blowing up into a major hurricane. In fact, the majority of solutions keep the system at a tropical storm or Category 1 hurricane level. Again there is a lot of uncertainty here, so something to continue watching.

What I am reasonably confident in saying is that there is real potential here for a rainmaker in the Carolinas, and particularly North Carolina. Readers there will certainly and understandably be concerned that this could become another Hurricane Helene-like event. For now, we think that is unlikely, as the strongest rains appear to be east of areas most impacted by Helene, and the overall signal for prolonged, heavy rainfall is lower this time. For now NOAA is predicting 6 to 10 inches of rainfall for areas hardest hit, which is no picnic, but also far from reaching Helene-levels.

NOAA rain accumulation forecast for now through Wednesday evening. (Weather Bell)

We are also monitoring storm surge and wind threats, which of course will be highly dependent on the track and intensity of the storm.

Hurricane Humberto

Humberto has become a hurricane, with 75 mph winds. Fortunately this storm is further out to sea than Invest 94L, because it is highly likely to blow up into a major hurricane over the next several days. The storm’s biggest threat will likely be to the small island of Bermuda.

National Hurricane Center forecast for Humberto as of Friday morning.

At present Humberto has about a 40 percent chance of bringing tropical storm-force winds to Bermuda early next week, likely on Tuesday. There will also be the potential for storm surge, but my hope is that the hurricane is weakening and remains far enough west of Bermuda to spare the island more than a glancing blow. We will keep a close eye on the track over the weekend.

Gabrielle

This storm is no longer tropical, but it still managed to bring significant winds and waves to the Azores. It could also bring tropical storm-force wind gusts into Spain by Sunday, along with some much needed rains for the Iberian Peninsula.

Invest 94L remains worth watching for the East Coast, while Gabrielle slams the Azores tonight

In brief: Invest 94L’s forecast is no clearer today, but we have a couple things to discuss related to it. It remains worth watching closely on the East Coast. Meanwhile, Humberto is now expected to become a major hurricane while passing southwest of Bermuda. Then we have Gabrielle which will impact the Azores with hurricane conditions tonight and make a brush with Portugal and Spain later this weekend.

Invest 94L

We’ll get to the named storms shortly, but let’s take stock of Invest 94L this morning.

Invest 94L has plenty of thunderstorm activity, but the presumed center is quite displaced from most of the storms. (Weathernerds.org)

Right now, the Invest is not terribly well organized, though it does give off a rather healthy-looking appearance. The surface “center” is presumed to be somewhere just north of Hispaniola, while thunderstorm activity is displaced mainly south and east of there, closer to a mid-level low pressure. So, we continue to see everything related to 94L right now looking rather disjointed.

As the wave moves into the Bahamas, it will encounter a slightly more hospitable environment to begin to try and organize. This probably won’t occur until late tomorrow or Saturday. If we see organization, it will probably begin later Saturday or Sunday.

The forecast location of 94L on Sunday evening ranges from just north of Cuba to all the way at the northern tip of the Bahamas. (Weathernerds.org)

By the time we get to Sunday evening, we actually have some growing model agreement in the general track of 94L, which is to say it will track off to the north-northwest. However, there is a massive difference in where along that path Invest 94L will be. Some European and GFS ensemble members have it passing just off the North Carolina coast. Others keep it locked up down near Cuba. There is some cross-track spread and a lot of along-track spread. The Google AI look above represents a happy medium of everything right now I think. And you can see how uncertain the placement is. This is important because where it sits and how close to Humberto it is will have implications on future track.

A combination of all the traditional physics-based ensembles shows a pretty good consensus on track for 94L up to about 30°N latitude. Thereafter, it gets unruly. (Polarwx.com, Tomer Burg)

But in general, I think what we’ve seen since yesterday is probably a slightly riskier track for the East Coast, as the model guidance has trended a little to the west. Of course, Humberto’s track has also trended a bit farther west too. Confidence remains low, and most of what I would say beyond Sunday is just rampant speculation at this point.

Bottom line: Continue to monitor Invest 94L if you live on the East Coast, particularly in the Carolinas and Virginia. But we have a long, long way to go.

Heavy rain seems likely regardless in the Carolinas and Virginia. We’ll continue to update that into the weekend.

Tropical Storm Humberto

Humberto’s forecast track and intensity, showing it becoming a major hurricane by Sunday evening. (Polarwx.com/Tomer Burg)

Humberto is getting its act together this morning. Slow strengthening is expected to continue, and Humberto should become a strong tropical storm by tomorrow morning and a hurricane by Saturday. The track of Humberto takes it over some of the warmest water in the southwest Atlantic outside the Caribbean islands, so it will have a good deal of energy to work with.

Humberto’s trend over the last 8 to 10 hours has been to get a bit better organized with a big blow up of thunderstorms near the center. (Cyclonicwx.com)

Humberto will remain well offshore, but the question becomes whether or not it will impact Bermuda. As of now, with the westerly trends we’ve seen since yesterday, this appears to build confidence that Humberto should remain to the west of Bermuda. However, depending on its size and intensity, some degree of direct storm impacts are possible in Bermuda as it passes. But it’s too soon to say anything with much confidence. We’ll also see how much intensity Humberto can gain in the next 72 hours. SHIPS model guidance shows a 2 to 3 times greater than climatology chance of rapid intensification, slightly lower odds than we saw with Gabrielle but still a bit higher than is typical. We’ll watch through the weekend.

Hurricane Gabrielle

Gabrielle is still a hurricane this morning, though it is clearly interacting with colder water, causing it to lose a bit of steam.

Hurricane Gabrielle is slowly losing intensity, but it will get a shot in the arm later tonight from jet stream interaction near the Azores, bringing hurricane conditions to the islands. (Weathernerds.org)

Despite this, Gabrielle is an imminent threat to the Azores, and hurricane warnings are posted there. If you look closely at the European model forecast of winds today through tomorrow, you can see how Gabrielle weakens in the near-term but then fires back up hurricane-force winds (red) as it crosses the Azores tonight and early tomorrow.

Gabrielle will ramp back up tonight as it begins to transition to an extratropical storm with some help from the jet stream. (Weathernerds.org)

This extratropical transition process can be deceptive. In theory, the storm is losing hurricane characteristics, but often times, that transition to a more jet stream dominated storm rather than a tropical/warm water dominated storm can lead to a burst of intensification. Think about Hurricane Sandy in 2012. That was technically no longer a hurricane, but a potent jet stream made it behave identically to one. Same goes for the Azores tonight; Gabrielle will be making that transition, but the impacts on the ground will be the same: Hurricane-force winds, rough seas, heavy rain, and surge.

From the Azores, Gabrielle will continue to track toward Europe, where it shall retire in leisure. Let’s extend that wind forecast above out through the weekend.

Gabrielle’s remnants will likely approach Portugal at low-end tropical storm or tropical depression intensity before it falls apart completely near or in the Gulf of Cadiz. (Weathernerds.org)

You can see that Gabrielle’s tropical storm force wind field (in green) disintegrates as it approaches Portugal. Thus, we don’t expect tropical storm impacts in Europe. We could still see some heavy rain or gusty winds and rough seas, particularly in southern Portugal. Current European model forecasts call for about 15 to 30 mm of rain (0.6-1.1″) in parts of southern Portugal and southwest Spain.

Rainfall on the order of 15 to 30 mm with perhaps some higher amounts will be possible through early next week in Portugal and southwest Spain. (Pivotal Weather)

Rain would be most welcome in northern Portugal, which is in significant drought at the moment.

Wednesday evening update: Humberto cast as lead actor with the dueling southwest Atlantic disturbances

In brief: Invest 93L was upgraded to Tropical Storm Humberto this afternoon, and it is not currently expected to threaten land. Invest 94L’s forecast remains directly related to Humberto and has a very high degree of uncertainty. This will continue to be a low-predictability situation for another couple days at least.

I think this is the first time all hurricane season we’ve done two posts in a day? We’ll call that a win. Anyway, this will be brief, but I want to catch you up on some changes today.

93L is now Humberto

Humberto will move and strengthen quite slowly over the next few days before perhaps threatening to become a major hurricane by early next week. (NOAA/NHC)

In a case of again dispensing with formalities, we took Invest 93L and skipped the tropical depression status and jumped right to tropical storm classification. Tropical Storm Humberto has winds of 40 mph. It’s certainly not the worst looking tropical storm we’ve ever seen.

Tropical Storm Humberto slowly gathering itself northeast of the Islands. (Weathernerds.org)

Thankfully most land will not likely need to worry about Humberto. However, interests in Bermuda should continue to monitor this storm’s progress. Various tropical models and most ensemble guidance curves Humberto on a trajectory similar to Erin earlier this summer, which would keep it west of Bermuda. However, depending on how well organized the system is, it could produce some meaningful impacts by early next week in Bermuda. Stay tuned.

Invest 94L

Meanwhile, the one we’re watching a little more closely, Invest 94L remains in the Caribbean. It was initialized this afternoon very near the southwest tip of Puerto Rico.

Invest 94L looking very disorganized but very stormy this evening. (Weathernerds.org)

But you can see that there are clusters of thunderstorms both north and south of the island. The system is quite strung out at the moment. While the surface reflection may be near Puerto Rico, there is clearly some sort of mid-level and upper-level reflection elsewhere. You’d like to see a system better vertically stacked before it can organize, but at this moment in time, 94L is anything but that. So, don’t expect this to pull a Humberto we wouldn’t think.

We have a good 2 to 3 days before this clears Hispaniola and makes it into the Bahamas. At that point, assuming it is not shredded by the mountains of Hispaniola, we would likely begin to see an environment more conducive to organization. Much like this morning, the frustrating reality is that we have no real clue what happens next. The operational model guidance is definitely dropping options though.

The GFS seems to favor a Fujiwhara interaction between 94L and Humberto, which would potentially pull 94L toward Humberto and pull Humberto a little farther west (offshore of the East Coast) before it all heads out to sea.

You can see how Humberto basically pulls the weaker 94L into its orbit on the GFS model, a scenario that while plausible isn’t any more likely than other scenarios. (Weathernerds.org)

This scenario is a realistic one for sure, but I’m not sure it’s any more likely than other possible scenarios. The European model basically leaves 94L behind once Humberto heads out to sea and does weird things with it, which again is plausible but not necessarily likely. Other options include literally nothing, a storm going up the East Coast, or just a storm that follows Humberto out to sea. Put all that together, and we’re still very much in “choose your own adventure” mode with regard to 94L’s future. Stay tuned, but I don’t expect much clarity on this for at least another day or two.

Notably, regardless of what transpires here, there will begin to be some heavy rain risk in the Carolinas and Virginia in about 3 or 4 days. After Helene last year, I know this won’t give people warm and fuzzies, but just bear in mind that this is a much different scenario than Helene from an entirely different direction. Certainly, keep tuned to the forecast, particularly near the coast, but we just want to offer a little reassurance that anything remotely like that is not currently in the cards on any model guidance.

The rainfall forecast over the next week calls for 1 to 3 inches, with isolated 4-to-5-inch totals in eastern Virginia and interior North Carolina, east of the mountains. (Pivotal Weather)

Rain totals of 1 to 4 inches are broadly forecast across Virginia and North Carolina down to about Columbia, SC over the next week. Notably, this is east of the mountains.

More tomorrow.