Tropical Depression 9 remains a major rainfall threat to coastal Carolinas and a big headache for forecasters

In brief: Tropical Depression 9 remains a royal headache of a forecast. While the risk of an East Coast landfall seems to have diminished some since yesterday, the threat of heavy rain on the Carolina coast remains. But there are still many questions left to be answered. We try to explain what those are today.

Tropical Depression 9

Forecast track from 11 AM ET Saturday. (NOAA/NHC)

Good morning! Tropical Depression 9 has formed off the north coast of Cuba this morning. It is no longer Potential Tropical Cyclone 9. We do expect the system to acquire a name as a tropical storm later today, which would be Imelda.

Tropical Depression 9 looking fairly healthy off the north coast of Cuba. (Tropical Tidbits)

TD 9 is unlikely to rapidly intensify in the near-term, though probabilities are around 2 to 3 times climatology (normal) for the extended timeframe (days 2-3). So watch for TD 9 to slowly organize today and tonight into tomorrow. Modeling has been pretty consistent on the intensity front. For as much uncertainty as this storm has had, we’ve pretty much known that this would probably slowly form into a tropical storm and eventually a hurricane. From the NHC discussion today, there is some hints that wind shear will impact TD 9 eventually, which means it probably won’t look as healthy on satellite as its compatriot to the east, Humberto.

Let’s talk track. If storms had sponsors, TD 9’s would be Excedrin because it’s a headache inducer. The hurricane models are actually in pretty good agreement this morning, certainly the best we’ve seen in this system’s life cycle.

12z hurricane model guidance shows the majority keeping TD 9 offshore of the Carolinas. (Tropical Tidbits)

This is an improvement over last night. Recall, in yesterday evening’s post we discussed rainfall scenarios, and the one that hurt the most was the operational Euro which sat the storm right on the coast for a couple days. It appears that the Euro backed off significantly overnight, It keeps the system offshore and backpedals on rain totals by multiple inches. Good news.

That said, the European ensemble’s spread of solutions is a bit less confident in what ultimately happens to TD 9.

Density forecast of the European model ensemble showing a rather bifurcated track, with several members going toward the coast but several out to sea. (Polarwx.com/Tomer Burg)

To understand how this will all play out, you almost need a football play diagram. There are four players on the field, including TD 9 itself. Here’s a forecast map for Sunday morning that shows how each feature will be trying to impact TD 9.

The trough over the Southeast is going to try and nudge it toward the coast, while the Bermuda high will impart a south to north motion on TD 9. Meanwhile, because Humberto has become a bit of a beast, it too will leverage influence on TD 9, probably canceling out the south to north motion as it passes to the east, as well as weakening or completely shelving any impact of the Bermuda high. Why? Since Humberto is a large storm, the wind around it is counterclockwise, as it passes TD 9 to the east, imparting north to south winds on the steering of TD 9. In that case, Humberto will be trying to push TD 9 back south. The end result of all this depends on the strength of each feature, but in general it likely means that TD 9 will meander or stall for a day or so as Humberto passes, which is expected to occur on Tuesday. Exactly where this stall or meander occurs will determine what impacts are experienced on the Carolina coast.

The question after that point becomes whether TD 9 or Imelda follows Humberto out to sea or if it gets left behind to meander in the western Atlantic. The tropical models tend to think it follows Humberto, and the NHC track follows that. But you can see how on last night’s 6z European model. TD 9/Imelda gets left behind as Humberto exits. You can also see how Humberto impacts TD 9 as it passes.

The complicated interplay of the various features that will impact TD 9 is making for a very, very difficult forecast. (Tropical Tidbits)

Let’s exhale for a moment. That’s a lot. It’s complicated meteorology. That’s the reality of tropical systems. They aren’t easy. We say at least once per hurricane season that “we can’t remember such a difficult forecast.” Well, welcome to tropical weather forecasting, kids. It’s not for the faint of heart.

Alright, let’s look briefly at the rainfall situation. Obviously, this entirely depends on how close to the coast TD 9/Imelda gets. A storm closer to the coast will deliver more rain, while a storm that hangs offshore but not totally away from the coast will produce slightly less rain. Either scenario produces significant rainfall. It’s just that one is far worse than the other.

Rainfall forecasts for the next 7 days across the Carolinas and Mid-Atlantic. (Pivotal Weather)

For now, the WPC forecasts above represent the likely rainfall outcome based on the NHC forecast track at the top. Again, this will vary depending on where the stall occurs. It will also vary based on whether the system gets left behind as Humberto exits, which could potentially raise the stakes some. Questions we cannot yet answer.

In addition to the heavy rainfall and likely flooding risk in the coastal plain of the Carolinas, there will be rough surf, rip currents, beach erosion, and the potential for coastal flooding. Currently, tropical storm watches are posted from the Palm Beach/Martin County line to the Flagler/Volusia County line in Florida. Expect those to be expanded northward either later today or tomorrow. Please consult your local NWS office for more specific details for the coastal communities of interest.

In addition to tropical storm conditions emerging in the Bahamas this weekend, rain totals of 4 to 8 inches are likely. (NOAA WPC)

In the Bahamas, heavy rainfall and tropical storm conditions are likely as the weekend continues.

We’ll continue to track TD 9 closely through the next few days.

Elsewhere

Hurricane Humberto? Big time. Humberto is a strong category 4 hurricane with 145 mph maximum sustained winds.

(NOAA/NHC)

Humberto is expected to pass west of Bermuda only grazing the island with fringe impacts. I wouldn’t be shocked to a watch or warning issued for tropical storm conditions by tomorrow, but for the most part, Humberto is not a huge deal for Bermuda. The combination of Humberto and TD 9 will kick up surf along the entire East Coast.

Humberto is quite a beautiful meteorological feature in the open Atlantic. (Tropical Tidbits)

Also, off to the east, the remnants of Gabrielle are heading into Portugal. It is now expected to come ashore there later today, bringing locally heavy rain.

Gabrielle’s remnant low bearing down on Portugal and Spain. (Weathernerds.org)

Nothing else out there to monitor for now.

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.