The Eyewall

One-sentence summary

Potential Tropical Cyclone 16 (which will likely take the name Ophelia later today) will deliver tropical storm impacts to a broad swath of the Mid-Atlantic coast between the Jersey Shore and North Carolina, with heavy rain, gusty wind, surge and tidal flooding all likely regardless of naming or classification.

Potential Tropical Cyclone 16 (likely to become Ophelia)

Here are the key points on what we’ll refer to as PTC 16:

• Tidal flooding will be a serious issue for portions of the Lower Chesapeake and Delaware Bays, as well as along the coast between the Outer Banks and Jersey Shore. Locally major tidal flooding is expected from Delaware south to the Virginia Tidewater.
• Tropical Storm Warnings are posted between Cape Dear, NC and Fenwick Island, DE, including Albermarle & Pamlico Sounds and much of the Chespeake Bay roughly south of Easton, MD. Storm Surge Warnings are posted from Duck, NC through Chincoteague, VA, the Lower Chesapeake Bay, the Neuse & Pamlico Rivers in North Carolina, as well as portions of the sounds. Storm Surge Watches cover additional ground beyond there.
• PTC 16 will have a broad wind field, meaning a wide swath of 30 to 50 mph winds should be expected from the Carolinas through Long Island. The strongest wind will likely be confined to a small area near the center, which should come ashore between Cape Hatteras and Cape Fear in North Carolina.
• Widespread moderate to heavy rain is expected between North Carolina and Southern New England with flash flooding potential highest in coastal North Carolina near the center of PTC 16.

Potential Tropical Cyclone 16 is gradually organizing this morning off the coast. Despite having maximum sustained winds of 50 mph, it is still considered a “potential tropical cyclone” because it has not yet shed the technicalities (ie: fronts) that make it a non-tropical entity. For all intents and purposes this is already a equivalent to a tropical storm and should be prepared for as such. The NHC says that this will likely take on a subtropical or tropical tag once the frontal boundaries are shed, meaning it would become Ophelia.

Let’s walk through the expected impacts from PTC 16.

Coastal impacts

In my view, this is the most important aspect of this storm, and it’s the one that may cause the most problems. Current tidal forecasts show a significant number of stations in the Mid-Atlantic expecting moderate or major tidal flooding from the storm this weekend.

Let’s pick out a couple gauges to talk about real quick.

We’ll start in Norfolk, VA which is expected to see moderate tidal flooding, with a couple gauges nearby pinging major flooding in their forecasts.

For the James River at Sewell’s Point, the 6.4 feet forecast tomorrow morning would be the highest tidal reading in Norfolk since Hurricane Joaquin in 2015. Historically, tides at these levels are likely to inundate portions of the waterfront in Norfolk.

For Lewes, DE on the Delaware Bay, this storm will likely produce the highest tides since January 2016 (the blizzard that was dubbed by The Weather Channel as “Jonas.”). Although Lewes will fall a solid foot or more shy of the record, this will still be borderline major flooding for the Delaware Beaches.

Flood levels of this nature will tend to cause mostly roadway flooding for Sussex County, DE but some structural damage is possible, particularly if tides come in higher than forecast.

Storm surge flooding is likely into North Carolina as well, in the Storm Surge Warning area.

In addition, we are expecting waves of 10 to 15 feet just offshore. The combination of high tides and rough surf will likely lead to beach erosion issues up and down the East Coast. I imagine some beachfronts are a bit more vulnerable after dealing with rough surf from Franklin, Idalia, and Lee already this summer, so erosion issues will get a bit worse.

Wind

PTC 16 is expected to top out near 60 mph winds as it comes ashore this weekend. However, over a rather broad area, we should expect 30 to 40 mph winds, with gusts over 40 to 50 mph, basically from Myrtle Beach, SC to Montauk, NY. A smaller area near the center should see the 60 mph winds on the coast of North Carolina, should the system develop as expected.

These winds will be capable of doing mostly minor damage, though scattered to numerous power outages are absolutely possible.

Rainfall

Total rain will be generally 2 to 5 inches, with isolated higher amounts up and down the coastal plain, extending inland in spots, particularly in Virginia and southeast Pennsylvania. Obviously this could cause flash flooding in spots. I would especially be aware of this rain on the coast, particularly in developed, more urbanized areas (Virginia Tidewater, Ocean City, MD, etc.) where the heavy rain could exacerbate tidal flooding by slowing how quickly it recedes at low tide. Some more considerable flash flooding risk probably exists near the center of this storm for Surf City to New Bern to Cape Lookout.

The bottom line is that this will be a notable storm on the East Coast with widespread impacts that will be disruptive and locally damaging.

Deep Atlantic wave (Invest 90L): Out to sea?

Yesterday, the Cabo Verde wave we’ve been watching was tagged as Invest 90L. We are slowly honing in on a compromise between the GFS and Euro solutions that I laid out in yesterday’s post.

We will watch trends today and tomorrow, but at this point, the odds will heavily favor a track that probably constitutes a “near miss” for the islands. It still needs to be monitored for obvious reasons, but the trends over the last 24 hours have been mostly positive for Puerto Rico and the islands. In this scenario if the system makes it into those areas, it would almost certainly be a weaker system.

Elsewhere, there is nothing of note out in fantasyland. For our Gulf of Mexico readers, yes, it still looks quiet.

One-sentence summary

We discuss that the system off the East Coast this weekend will produce some notable coastal impacts, particularly from heavy rain and tidal flooding, and today we will assess the players that will shape the next Atlantic system near the Cabo Verde Islands.

Awaiting the next round of Atlantic action

Cabo Verde wave

Over the weekend, I noted how Hurricane Lee was emblematic of hurricane season, a marathon. You need to pace yourself to make it to the finish without losing your mind. Discussion about Lee lasted over three weeks, and it feels like we’ve already been sort of discussing this next Atlantic wave for about a week, and it’s just now awaiting the invest designation. This one has been a bit of a head scratcher, and we’re not a ton closer to resolution today on what will come of this. Let’s talk about what we do know.

Sometimes it makes sense to look forward before looking backwards. In other words, as a forecaster, sometimes I want to see what a model’s outcomes are before I go back and assess why it has that outcome. This is especially true when two of our more reliable models show two relatively different outcomes.

In the map above, you can see the forecast IQR, or interquartile range difference of sea level pressure in the Atlantic from the GFS ensemble (L) and the Euro ensemble (R). Explain this to you like you’re three years old? You got it. You have operational models, which are the ones that are deterministic; one run, one solution. Then you have the ensemble models which are the ones that run the model roughly 30 (GFS) to 50 (Euro) different times with tweaks each time to produce a range of outcomes. The maps above show you what the ensemble model outcomes say for sea level pressure of the 75th percentile minus the 25th percentile. In other words, how spread out are the outcomes within the majority of ensemble solutions? Or are they spread out at all? This will illustrate key differences and/or areas of higher confidence.

The European model is fairly tightly clustered east or northeast of the islands next Tuesday afternoon with the tropical wave, or whatever it is at that point. The GFS at left is slightly less tightly clustered, more spread out, and also faster, with a couple members even almost in the Caribbean next Tuesday. Interestingly, the IQR differences are not substantially higher on the GFS than the Euro, which may be a bit of a red flag for the GFS. But we’ll note that in our back pocket and operate under the idea that there are noteworthy differences.

So, we look at this and say, “Ok…the GFS is quicker and potentially near the islands, while the Euro is slower. Why the difference?”

So we’ll next look up, about 20,000 feet at the 500 millibar level to get a sense of what is steering the wave.

If we look at the forecast at 500 mb next Monday morning, we see the Euro and the GFS differ in terms of location and size of the high pressure system in the central Atlantic. The Euro is stronger and maybe actually holding the system up a bit. But the high pressure center moves out of the way, and an exit door into the open Atlantic awaits. Thus, the risk of the system making it into the Caribbean on the Euro is low.

But if you look at the GFS, the high is weaker, and a hair farther north, which probably allows the system to move a little faster, and it gets to the islands in some ensemble member cases before it can curve out to sea.

We can take this a step further to see which model develops the system faster. The GFS tends to be quicker to the punch than the Euro (forecast for Sunday morning shown here)

So what does all this tell us? The GFS is quicker to organize the wave, moves it faster west, and by the time the escape route north opens, the system may be bearing down on the islands. The Euro is slower, organizes the wave slower, and when the escape route opens, it’s still probably several hundred miles east of the islands and likely to hook northwest or north. Those are fundamental differences hinging on how the high develops and how the wave develops and interacts with other things surrounding it, questions we cannot adequately answer yet. But at least we have a cheat sheet of sorts now that we can apply to this wave. And we should know in a couple days if the islands are seriously at risk or if the northward solution is more likely. Stay tuned.

East Coast subtropical sloppiness

We continue to see a good chance that a surface low is going to develop well off the coast of Florida or Georgia tomorrow or Saturday. Look for that to wobble generally north up the coast and either into the Outer Banks or just ride up the East Coast toward Delmarva or the Jersey Shore as a subtropical storm or strong nor’easter.

A subtropical designation just has to do with how the storm forms, and it has no bearing on the impacts. What will those impacts be? Well from the upper coast of South Carolina into most of North Carolina, coastal Virginia, Delmarva, and the Jersey Shore, we can expect rough surf, gusty northeast winds, moderate to locally major tidal flooding, beach erosion, and rip tides, in addition to heavy rain. This may be a step above nuisance status in some places now, akin to a rather strong nor’easter.

We are looking for a wide swath of 2 to 5 inches of rain on the Coastal Plain from Myrtle Beach through Long Island. This may cause flooding, particularly in lower-lying and urban areas. Coastal communities may see high tides exacerbated because of heavy rain, so this could slow the drainage process for a tide cycle or two this weekend.

We will be keeping you posted on the East Coast storm, as well as the progress in the forecast of the Atlantic system through the weekend.