Matt Lanza

Tropical Depression 9 trends are favorable, but heavy rain and rough marine weather still likely off the Southeast coast

In brief: Tropical Depression 9 is still expected to become Imelda by tonight. The track forecast has shifted farther south and east, keeping it farther off the Southeast coast. Still, heavy rain and marine impacts are a good bet on the coast of the Carolinas, but some “stand down” measures can be taken now it appears. Also, today we discuss Humberto’s journey to category 5 intensity and the broader context of a marine heat wave.

Tropical Depression 9

11 AM ET forecast for TD 9 (NOAA/NHC)

Trends since yesterday have generally been a continuation of trends since Friday, which is to say that threat of a high-end impact to the Carolina coast is diminishing. Friday’s forecasts were far more foreboding in terms of a stalling rainfall event for the region. But since then, all trends seem to have nudged the center of TD 9, presumably Imelda farther from shore. To be clear, this doesn’t mean there will be no impacts, nor any significant ones. It just means the risk of severe, widespread flooding and higher-end wind has dropped a lot in 2 days.

Tropical Depression 9 is a bit jumbled over the Bahamas, but in general it’s trying to push toward tropical storm intensity. (Tropical Tidbits)

Over the next 24 hours, we do expect to see TD 9 begin to “find itself” in the Bahamas, as many of us probably wish we could also do. Expect a slow strengthening in the next 24 hours, followed by a slightly more aggressive intensification from tomorrow morning into Tuesday, taking TD 9 to Imelda and then a hurricane, possibly a category 2 type storm by later Tuesday or Wednesday. Assuming the wind shear it will likely fend off doesn’t become a prohibitive limit on the ceiling, like we saw with Humberto (more below), we could see this push beyond current consensus intensity forecasts.

Various model intensity forecast for TD 9 with most in tropical storm or category 1 range. (Tropical Tidbits)

You can see how the HWRF (HWFI) and HAFS-B hurricane models bring this into category 2 or 3 range. With the lumbering Humberto in the vicinity, there’s certainly uncertainty in this, but generally speaking with hurricanes, it’s been like a good old fashioned Big 12 slugfest, and the “over” for intensity has generally won this season.

Model agreement on the track of TD 9 has also increased significantly since yesterday.

There is still a very small subset of ensemble members bringing TD 9 closer to the Carolina coast than we’d like, but the vast majority take a hard right just before it gets to 30°N. (Polarwx.com/Tomer Burg)

We have far fewer outliers closer to the Carolina coast. Most make a hard right now south of 30°N latitude, allowing it to exit without a ton of consternation. You can also see this reflected in the “trend” of the consensus model since yesterday morning, continuing a trend from Friday.

Consensus guidance for TD 9 has been trending south and east with time in the next 72 hours of the forecast, keeping the system farther from the Carolinas. (Brian Tang/SUNY Albany)

The lighter colored tracks above are older forecasts, while the darker ones are newer forecasts. You can particularly between hours 30 and 72 that there is a sharp deviation to the southeast from older runs to newer runs. Basically, with respect to the Carolinas, TD 9 continues to trend farther away, good news to be sure.

Could this change? Of course. But we really have not seen any reliable model guidance continue to threaten the Carolinas severely. So you should obviously keep monitoring the forecast, but it’s ok to exhale sometimes.

What of the rainfall forecast? Well, the official forecast looks like this:

The rainfall forecast for Tropical Depression 9, through Wednesday morning. (NOAA/WPC)

At least through Wednesday, which should cover about 90 percent of the storm, rain totals are expected to be around 2 to 4 inches in Charleston, Myrtle Beach, and Wilmington, with lesser amounts surrounding that. Some higher amounts are possible, and slight risks (2/4) are in place for flash flooding tomorrow and Tuesday. The NWS office in Charleston has a helpful chart outlining the impacts for their area. Suffice to say, conditions to the north and south of there won’t be terribly dissimilar.

An impacts chart from the NWS in Charleston. (NWS Charleston, SC)

The most significant overall impacts will be marine-related with rough seas from both TD 9 and Humberto churning up high waves, beach erosion, and extremely dangerous rip currents. This will expand up the East Coast in time over the next day or two as well. “Locals season” as we called it on the Jersey Shore should probably involve more observation from land than going into the water. If you’re a surfer, exercise extreme caution.

Interests in Bermuda should continue to monitor TD 9 over the coming days. You’re now in the higher uncertainty part of the forecast, with a track spread mostly south of the island, but close enough to keep you sitting up and paying attention. More to come on this through the week.

Humberto becomes our 2nd Category 5 storm

Since 1980, the era of decent satellite observation, we have had 25 category 5 hurricanes in the Atlantic Basin. Almost half of those (12) have occurred since 2017. Make of that what you will. The newest entrant is Humberto, which surged to category 5 intensity yesterday.

A satellite image of Humberto near peak intensity on Saturday. (College of DuPage)

Humberto is back down to a category 4 today, as the eye has filled in some, and it appears to be undergoing an eyewall replacement cycle. Category 5 storms don’t usually stay there for too long, though some have sat there for 3 days (most recently Irma in 2017).

Thankfully, Humberto is still expected to push out to sea without much fanfare other than the wave action it will continue to produce for Bermuda and the East Coast and Bahamas.

(NOAA/NHC)

Humberto is not currently expected to impact the Azores.

Warm ocean context

I sort of jokingly said “make of that what you will” up top. But the reality is that the Atlantic subtropics, where both Erin and Humberto popped to category 5 this summer are near the top of the scale of warmest on record.

The subtropical North Atlantic is challenging 2024 for warmest on record. (Kim Wood/University of Arizona)

The Caribbean and the Gulf are doing the same, but mercifully we have not yet had a meaningful system there this summer and autumn. And it’s not just the Atlantic. Global water temperatures are about 0.5°C warmer than normal on average. You can definitely pick out some cooler pockets here and there, but the reality is that most of the global oceans are running a fever.

Global sea-surface temperature anomalies. (Cyclonicwx.com)

You can see that this is especially true in the North Pacific, eastern Indian Ocean, Barents Sea, Norwegian Sea, Gulf, Caribbean, and subtropical Atlantic. The only meaningful cool anomalies are in the western Indian Ocean, El Niño region (Hello, La Niña?), and far north Atlantic. The North Pacific marine heat wave (colloquially called “the blob”) is a big story, and one that may have further implications on weather heading toward winter. It’s already notable in California.

The reality is that the oceans are continuing a heat wave that began a few years ago, and it’s almost certainly been having some impacts on the intensity of Atlantic storms — and beyond.

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.

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.