Matt Lanza

Tropical Storm Debby forms, poised to track into Florida and then unleash copious amounts of rain on Georgia & South Carolina

Headlines

  • Tropical Depression 4 is now Tropical Storm Debby.
  • Debby is expected to track toward the Panhandle or Big Bend of Florida as a strong tropical storm or intensifying Category 1 hurricane Monday.
  • Copious amounts of rain will follow Debby into Georgia and South Carolina where significant flooding may unfold.
  • Debby’s exact track beyond Monday or Tuesday is highly uncertain.
  • There is another wave beyond Debby worth monitoring next week.
Debby’s latest forecast at 5 PM ET shows a broad northward move, followed by a shift to the east and a sharp slowdown in forward speed. (NOAA NHC)

Tropical Storm Debby: 40 mph, NW 15 mph

Well, here we go again on the west coast of Florida, with another system that will generate some headaches in terms of track. This afternoon, newly minted Tropical Storm Debby looks reasonable. The center shows up nicely on visible satellite, and there is plentiful thunderstorm activity surrounding all sides of the system. As this lifts northward into the Gulf and gets out of the way of Cuba, I suspect we’ll begin to see some consolidation and steady intensification of this one.

Tropical Storm Debby late on Saturday afternoon shows lots of thunderstorm activity and a pretty well defined circulation. (Tropical Tidbits)

The current intensity has it sitting around 40 mph maximum sustained winds, hence the upgrade to a tropical storm. Debby is large in size, (not quite a “Little Debby”) which means it may take a little effort to strengthen. Debby should become a formidable tropical storm, if not a category 1 hurricane before coming ashore. Hurricane warnings are now posted from the Suwannee River to the Ochlockonee River on the Florida Gulf Coast. Tropical Storm Warnings are posted for the west coast of Florida into the Keys. Storm Surge Warnings are in effect between Aripeka north to Indian Pass.

Debby’s track

Debby will be steered north and northeast in between two ridges and in the direction of a trough over the East Coast. Arguably, the track forecast for Debby has the least uncertainty over the next 48 hours or so, and most modeling is in good agreement on that.

European ensemble members show strong agreement on Debby’s general track north and northeast-ward over the next 48+ hours, coming ashore in the Big Bend or Panhandle of Florida. (Weathernerds.org)

After Debby comes ashore somewhere in the Big Bend or Panhandle, that’s when forecast confidence begins to nosedive a bit. A generally slow east or northeast drift to Debby’s motion should set in. But exactly whether or not that gets Debby back offshore and with an ability to try and restrengthen off the Georgia or South Carolina coasts remains to be seen. Expect a very erratic motion to Debby after Monday.

Debby’s intensity

As noted above, there is some chance that Debby can become a hurricane before it comes ashore in the Big Bend or Panhandle Monday morning or so. The obvious comparison for this storm will be Idalia from last year. Idalia made landfall as a weakening category 3 hurricane with 125 mph maximum sustained winds. Debby is likely to make landfall not far from where Idalia did but as a strengthening tropical storm or category 1 hurricane. Much like we witnessed in Houston and coastal Texas last month with Beryl, there are elements about strengthening category 1 storms that are often underplayed ahead of time. Folks in the Panhandle and Big Bend should be taking Debby seriously and listen to local officials in terms of evacuation orders or preparedness recommendations.

After landfall, Debby will lose some of its intensity, though some locally strong winds are possible across south Georgia or North Florida as it turns east. Again, the big wild card right now is what Debby does when it approaches the Georgia coast and makes an effort to get offshore.

Debby’s rainfall

While folks on the Florida coast need to be preparing for a strengthening category 1 hurricane, people across North Florida, Georgia, and the Carolinas need to be preparing for a potentially major rain and flooding event.

Debby is likely to bring copious amounts of rain, flooding, and flood damage to Georgia and South Carolina heading into next week. (NOAA WPC)

The very latest rainfall forecasts is suggesting anywhere up to 12 to 20 inches of total rain from Debby on the coast of Georgia and South Carolina, including Savannah, Hilton Head, and Charleston. This has the potential to also produce a wide swath of 8 to 12 inch rains in the Lowcountry of South Carolina and much of southeast Georgia (as well as parts of North Florida) also. Combine this with rough seas and high tides, and we could be looking at an especially dangerous, damaging flooding scenario in coastal spots and a significant, locally damaging flooding event away from the coast. Heavy rain will extend up to North Carolina at times as well, but we are really concerned with South Carolina and Georgia first in this scenario. Slow moving tropical systems loaded with abnormally high amounts of atmospheric moisture are never a good combination. Please take this threat extremely seriously in those areas.

How long the rain lasts and how far north it gets remain questions to monitor over the next few days.

So, in summary:

  • A strong tropical storm or intensifying category 1 hurricane is likely to turn into the Big Bend or eastern Panhandle of Florida by Monday.
  • Debby will slow down as it tracks across Georgia and North Florida, delivering copious amounts of rain to Georgia and South Carolina.
  • A major flooding threat exists, particularly along the coast, including Savannah, Hilton Head, and Charleston.
  • Debby’s future remains a bit uncertain beyond that, and folks in North Carolina should continue to monitor the system’s progress in the days ahead as well.

Some quick miscellany

First, we have people wondering why we are on Debby and not Chris. Well, if you blinked, you missed Tropical Storm Chris. It formed for about 8 hours before coming ashore in Mexico in early July. This occurred while Beryl was hammering the Caribbean, so it obviously got lost in the shuffle.

Second, we are monitoring another tropical wave behind Debby. This system has shown up on various deterministic/operational model guidance at times as a formidable Gulf storm. Remember, we try not to use deterministic guidance 10 days or so out, as it is prone to wild and extreme variability run to run. Looking at ensemble guidance is more useful, and in this case, we have decent European ensemble agreement on something in the western Caribbean or near Cuba again in about 6 days.

Behind Debby, another tropical wave is beginning to generate buzz, but it’s too soon to say much of anything coherent about this one’s future. (Weathernerds.org)

For now, we’ll wait for Debby to get out of the way and then focus on that as well.

Tropical wave with decent chance of development this weekend continues plodding toward the southwest Atlantic

Headlines

  • Slow development of the Atlantic tropical wave is likely this weekend near the Bahamas.
  • The track of the system is highly uncertain with some plausible outcomes ranging between the eastern Gulf and offshore of the East Coast.
  • Interests from Louisiana to Florida to the Carolinas should continue to monitor the progress of this system, even though current expectations are not for a major storm.

It’s a slow go from the Atlantic wave

We continue to track a tropical wave in the open Atlantic this afternoon that the National Hurricane Center continues to gradually beef up development odds on. This afternoon they stand at 60 percent over the next 7 days (near zero the next 2 days).

Development odds in the Atlantic have increased to 60 percent over the next week. (NOAA NHC)

This is a larger, lumbering wave with slowly increasing thunderstorm activity. Larger waves are generally slower to develop, and given how much dry air this thing still has to shed, we likely won’t see any real movement toward organization until this weekend. But compared to Monday it certainly looks better I guess.

The thunderstorms (blue/green) are still few and the dry air (red/orange) still plentiful in and around the tropical wave in the Atlantic. (Weathernerds.org)

We should see a touch more in the way of storminess around this wave tomorrow and again on Thursday. Once we get to Friday or Saturday and this approaches the Bahamas, that’s when we could finally begin to see organization.

From that point, this wave’s future is cloudy. If you dig into the European ensemble’s 51 members, you can actually get a good picture of the possible outcomes. No ensemble spread is perfect, but this gives you a very high level understanding of the generic possibilities that exist with this wave. I’ve broken them into 3 camps below.

Three general possibilities with the upcoming tropical wave as it approaches the Bahamas and Florida. (Weathernerds.org)

On the right, we have the more aggressive solutions, which include the Euro operational and ICON models at present. These quickly strengthen the wave in the Bahamas such that it gets pulled north by a trough, generally remaining offshore, away from land and probably not a huge deal for anyone. This solution seems a bit overaggressive to me, so I’d say odds of this are relatively low. But it’s a possibility.

In the middle, you get the general consensus view of the Euro ensemble which is a storm that only slowly strengthens and organizes, still enough to turn it north, but not before it gets closer to Florida and the Bahamas and potentially close enough to eventually threaten the Carolinas. In this case, you still probably wouldn’t have a particularly strong storm, but you’d have something a little better organized than just a wave or depression. European A.I. modeling also supports a hybrid of this outcome and the previous one, closer to the coast but still offshore.

Then, on the left, you have a small cluster of a couple ensemble members that do not develop the wave at all and allow it to continue west or west-northwest into the eastern Gulf of Mexico. Eventually it would probably still turn north, but because the upper air pattern steering this thing would be fairly convoluted next week, any confidence in where that happens would be quite low. Still, this would likely only gradually organize in this scenario. The GFS operational model supports this outcome at present. Notably, that model also places a little mid-level system ahead of the tropical wave which would probably boost wind shear a bit and could help direct it north a little faster. Maybe. It’s complicated.

What does it all mean? They’re scenarios for a reason. Is one more likely than the other? Well, the middle one seems to have more support among modeling. The GFS is sometimes a bit wonky with these things, so I’m not sure I entirely buy the Gulf scenario. And it’s noteworthy that despite the expected slow organization of this system, between warm water and extremely low wind shear, conditions look pretty good for some strengthening near the Bahamas this weekend.

Wind shear is expected to be minimal over the tropical wave this weekend in the Bahamas, with most meaningful shear well east of it on Saturday. (Tropical Tidbits)

The meteorologist in me favors some combination of the Euro op’s quicker strengthening and the middle majority cluster right now. This would yield minimal direct impacts. But the public communicator in me must tell you that interests from Louisiana through the Carolinas should continue to monitor the progress of this tropical wave. Even if it’s not currently expected to be a significant storm, it will have a few things going for it as it organizes.

In summary:

  • Slow organization is expected this weekend near the Bahamas.
  • The track is uncertain, with some modeling quickly strengthening the wave and turning it out to sea, while other modeling brings it closer to Florida or the Carolinas and even a few not forming it at all, bringing it into the Gulf.
  • At the least, a tropical system should form, but where and when exactly remains to be seen.
  • Interests from Louisiana and the Carolinas should continue monitoring the progress of the wave in the coming days.

I will be out of pocket the next few days for some medical stuff, but Eric has you covered going into the weekend!

As we await the Atlantic’s awakening, a tropical wave without thunderstorms is what we’re watching

Headlines

  • The tropical wave in the Atlantic is unlikely to develop before at least Friday.
  • Development after Friday should be slow as it moves toward the Bahamas.
  • From there, numerous questions about which will determine if this system can up in the eastern Gulf or turn north off the Florida coast and where it goes from there.
  • Interests between eastern Louisiana and the Carolinas should check in on this again in a day or two.

A dry tropical wave for now

You hear the term “tropical wave” tossed around by meteorologists and others all hurricane season long. But what is it, exactly? By definition, a tropical wave is “an inverted trough (an elongated area of relatively low pressure) or cyclonic curvature maximum moving east to west across the tropics.” Nothing in that definition says thunderstorms or rain or anything like that. So, yes, in theory some tropical waves can be dry.

When we look at the central Atlantic today on the traditional satellite imagery, we don’t see a whole lot that’s perceptible. There’s a wave there, sure; you can see some cyclonic curvature. When you look at water vapor satellite imagery, you can see it even a bit better, but where you see browns, reds, and oranges, that indicates the presence of a lot of dry air. Use the slider below to toggle between a visible and water vapor image of the central Atlantic wave this afternoon.

Visible satellite imagery in black and white, and water vapor imagery with lots of red, orange, and brown indicating the presence of dry air in the central Atlantic. (weathernerds.org)

What does this tell us? It says that the tropical wave in the Atlantic is not going to develop anytime soon into a formal tropical entity. However, the presence of a tropical wave indicates that once it can shed some of this dry air, perhaps then we could see slow development. That may not be til Friday or the weekend.

Modeling continues to indicate the potential that the slow development noted above could occur as the wave comes west northwest toward the Bahamas or Turks and Caicos late this week or weekend. Clearly, it’s dealing with dry air and Saharan dust right now, but once it gets to the west, north of Puerto Rico by Thursday morning it will begin to likely increase moisture presence.

The overnight run of the European model shows this tropical wave adding moisture by the time we get to Thursday morning when its north of Puerto Rico. (Tropical Tidbits)

For the Lesser Antilles, Virgin Islands, and Puerto Rico, this system is unlikely to cause much heartburn. After Thursday, the system will continue to the west-northwest into the Bahamas. The overall atmospheric setup by Saturday as forecast by the European model is fairly complex, and as usual the ultimate path of this wave, depression, storm, whatever it is will be contingent on its strength as well as the exact location of these features. But at this point, I can count three big players: High pressure in the Atlantic, high pressure over Texas and the Plains, and a sharp trough of low pressure digging into the Ohio Valley and Appalachia.

Tropical wave over the Bahamas on Saturday will be steered west and northwest around high pressure in the Atlantic, tugged north by a sharp trough in the Eastern U.S., and likely slowed a bit by high pressure over Texas and the Plains. (Pivotal Weather)

The interplay that the Euro is “seeing” here right now appears to be that the wave slows on approach to Florida. It is being steered by the high to the east, but it slows as it runs into the influence of high pressure over Texas (the arrow pointing down) and also as it “feels” the trough over the Eastern U.S. trying to pull it north. It’s tough to speculate much on exactly what happens here. A weaker system would probably keep drifting west into the Gulf, whereas a stronger one would be more apt to turn northward in the Bahamas and possibly stay off the Florida coast. Most European ensemble members have supported this stronger idea, and you can see today’s 12z probabilities of a tropical depression from the ensemble favoring the “off the East Coast of Florida” idea.

The bulk of the European ensemble model guidance seems to favor a track of the tropical wave east of Florida, but sketchy beyond that as it comes north. (Weather Bell)

Beyond Florida, even in a faster turn scenario, all bets are off because the upper pattern may get a bit convoluted next week. An out to sea turn is not inevitable by any means in that scenario. Likewise, if a weaker storm sneaks under the trough and ends up in the eastern Gulf of Mexico, interests between Louisiana and the Florida Big Bend may be on notice too.

So, to summarize:

  • Tropical development is not expected before Thursday or Friday at the earliest.
  • Development is likely to be slow, though with exceptionally warm waters, any risk of development should be treated with respect.
  • The best guidance we use favors a slowing in the Bahamas, followed by a turn north near or off the coast of Florida.
  • However, given the very high uncertainty with regard to the speed of development and the precise location of steering features, any track forecast is low confidence and interests between Louisiana and the Carolinas should continue to monitor this system’s progress.

Elsewhere, no further tropical development is expected before mid to late next week, but we continue to look toward mid-August as the Atlantic inflection point, and activity may begin apace by then.

Central Atlantic tropical wave and disturbance will take a minimalist approach to development this week

Headlines

  • A tropical disturbance and wave in the Atlantic may very slowly develop this week or next weekend as it comes west.
  • There is no imminent concern of rapid development or risks to land at this time.
  • Still, check back in on this through the week to ensure nothing has changed.
  • No other development risks are seen at this time.

Tropical wave in the dust

Rumors are swirling and hype is beginning around a tropical wave in the Atlantic that several models have shown developing into a system later this upcoming week or weekend. In fact, after my Friday post, the National Hurricane Center had it on their tropical outlook. Today, they’ve got chances bumped up to about 40 percent over the next 7 days.

A disturbance and approaching tropical wave may help trigger some development in the northern Caribbean or southwest Atlantic later in the upcoming week or next weekend. (NOAA NHC)

So what is it that we’re watching? Well, it’s this wave in the Atlantic, just southwest of the Cabo Verde Islands.

The Atlantic shows a tropical wave encased in dust to the west of the Cabo Verde Islands. This may slowly develop through the week. (College of DuPage)

Admittedly, this doesn’t look like much at all. It’s being choked by Saharan dust right now, and it has little to no thunderstorm activity surrounding it for the most part. In fact, the wave emerging off Africa to its east looks far more impressive, though that one is likely to fizzle in the coming days. Meanwhile, this wave will migrate westward. By Tuesday evening, it should be just east of the Leeward Islands, still unlikely to have developed much at all.

The tropical wave will approach the Leeward Islands Tuesday night or Wednesday, unlikely to be developed much, if at all. (Tropical Tidbits)

The system will still be essentially surrounded by dry air. It will also have to contend with a bit of wind shear. All in all, through Wednesday, it seems unlikely that this wave will develop much, and for the Lesser Antilles, Virgin Islands, and Puerto Rico this will not be one to create significant impacts.

Beyond midweek there will be a little less dry air and slightly more favorable atmospheric conditions for development as it approaches the area between the Bahamas and the Florida Straits. Through hour 144 (Friday night), about half of the European ensemble’s 51 members develop this system into a formal low pressure, and none do so significantly.

About half of the European ensemble’s 51 members develop this tropical wave by early next weekend between about Cuba and east of the Bahamas. None do so in major fashion. (Weathernerds.org)

So based on all this information we have, here’s what we can say about this tropical wave:

  • Development should be slow to occur and probably wouldn’t even begin until very late in the week, if not the weekend.
  • The wave should track generally harmlessly through the northeast Caribbean and end up somewhere between Cuba and east of the Bahamas by the weekend.
  • Dry air is likely going to play a significant role in the development (or lack of development) of this wave.
  • The 40 percent odds as posted by the NHC is a perfectly fine place to be right now.

Beyond the weekend, bets are off. We’ve seen some models track this toward the Southeast, some out to sea, and some not even develop the system at all. The GFS ensemble currently favors the latter outcome, with few to no ensemble members developing this. So for now, it’s just good to keep an eye on this one and wait and see. There’s clearly nothing here yet that is definitive, and there’s no reason to get too worked up about things right now. More to come.

Beyond this one, there are no specific signs of anything to follow. Perhaps after August 10th we’ll see our next potential candidate in the Atlantic.