Matt Lanza

Flash flooding risk in SoCal today, as the Atlantic continues to put on an exercise in futility

September 18, 2025 at 8:17 amSeptember 18, 2025 at 8:17 am by Matt Lanza

In brief: Gabrielle is struggling mightily this morning, with 50 mph winds but very little thunderstorm activity. It looks unlikely to threaten Bermuda but should still be watched. Not much else on the horizon. Locally heavy rain could produce flash flooding from southern California and southwest Arizona north into the Sierra today and tomorrow.

Tropical Storm Gabrielle

Well, we finally got our “G” storm! Gabrielle formed out of the slop that was Tropical Depression 7 yesterday. And if there ever were a storm to typify this season, Gabrielle may be the one. It’s essentially a naked swirl this morning. Looking at it on satellite, you can literally see the wind shear impacting the storm.

Gabrielle’s low level circulation is evident, spinning right to left, while significant wind shear is evident in the clouds moving from bottom to top on the loop above. (Tropical Tidbits)

In addition to the shear, Gabrielle is also fighting dry air. Overall, this is about as pathetic as it gets for a tropical storm. It seems as though this is going to struggle mightily for the next 2 to 3 days. I wouldn’t be shocked to see it even be reclassified to a depression again at some point. Beyond that, the environment looks to get a little less hostile. This should support strengthening as the storm turns north and (likely) passes east of Bermuda. Some models still drag Gabrielle close to the island, but over 90 percent of the suite of solutions keeps it to the east.

The suite of a bunch of Google AI models (blue, green, purple) and the Euro ensemble (orange) show good agreement on Gabrielle passing east of Bermuda. (Google Weather Lab)

Given Gabrielle’s disorganization, it probably remains worth watching for Bermuda, because weird things can happen track-wise when you’re dealing with a system like this. Water temperatures in the subtropical Atlantic are quite warm, so assuming this gets to an environment that is less hostile, whenever it does get its act together (if it does), it could come together rather quickly. But at least for now, it’s not a real serious threat.

What comes next?

Well, the NHC took the 20 percent area off the board in the Atlantic and replaced it with a more generic 20 percent area for the next wave behind that one in the pipeline. This seems to be the season of “well, let’s watch the next wave.” Truth be told, when I look out beyond days 5 to 7 right now, I cannot point to much of anything other than vague “well, it’s September so we’re watching here and here maybe I guess.”

The background state of the Atlantic has been hostile all season, and it remains hostile, and I’m not sure I see where that’s changing. A lot of folks have mentioned the MJO, or Madden-Julian Oscillation, which is a periodic pattern of favorable conditions for thunderstorms (rising air) that works around the globe in the tropical regions. There is a fairly good correlation between supportive MJO activity and increased tropical risks. Much of this season has been spent in the unfavorable side of the MJO, with sinking air dominating the Atlantic. And it seems that every time the models suggest this may change…

So here we are in mid-September, without really much of anything showing up over the next 7 to 10 days. This gets us to about September 26-28, give or take. It’s important to recognize that October can still do bad, bad things. Very bad things. But at this specific moment in time we’re in good shape.

California rain

A slight risk of excessive rainfall and flash flooding is in place today for much of southern California’s mountains and deserts.

Flooding is possible in spots across SoCal today. (NOAA WPC)

Flash flood watches are posted from Yuma, Arizona through Yosemite National Park. While any flooding risk today is more or less isolated in nature, there could be some healthy rainfall rates with any storm. We could see some places with a tenth of an inch or less, and others that pick up 1 to 2 inches of rain in short order with flooding. A good day to be weather aware across SoCal, particularly north of LA up into the Central Valley as well as between Los Angeles and Las Vegas or east of San Diego to Yuma. The slight risk (2/4) focuses more on the Sierra on Friday.

Friday’s flooding risks are more focused on the Sierra. (NOAA WPC)

Another round of moisture increase could occur Sunday into early next week, bringing more rain chances to parts of California.

Tropical Depression 7 forms, worth watching in Bermuda but not a major concern

September 17, 2025 at 7:59 amSeptember 17, 2025 at 7:59 am by Matt Lanza

In brief: The Mid-Atlantic coastal storm is winding down, so we debate whether it was subtropical or non-tropical. Tropical Depression 7 has formed in the Atlantic, which is not currently a land threat but should be monitored in Bermuda. Meanwhile, the forecast in California is a little wetter, which is good news mostly.

Mid-Atlantic coastal storm

The coastal storm in the Mid-Atlantic is now inland and winding itself down. Rain totals were impressive, with 36-hour values up to nearly 8 inches around Virginia Beach.

36-hour rain totals ending this morning across the Tidewater and parts of northeastern North Carolina. (NOAA)

Additional rain today should be manageable, but a couple pockets of heavier rain can’t be entirely ruled out.

The question about this system will persist into the offseason. Was this a subtropical storm? Did it have characteristics of both a nor’easter-type storm (extratropical) and a tropical storm? The answer is very much “yes, it’s plausible.” If you look at what is known as a cyclone phase diagram, constructed by Dr. Robert Hart at Florida State University, you can see on the map inset how (based on yesterday’s European model guidance), the disturbance originated in the tropics (A) and lifted north to the Mid-Atlantic coast (C) where it will basically dissipate.

The cyclone phase diagram suggests that the Mid-Atlantic storm may have been subtropical in nature, meaning it had both characteristics of a tropical storm and a nor’easter-type storm. (Florida State University)

But then if you look at the “phase” part of the diagram, notice how it went from cold core (extratropical) to warm core (tropical). This is where the “sub” comes in with subtropical. It began as a frontal low pressure system, basically just a disturbance organizing on a cold front, something we see many times each year across the country. Since it was over warm water, the disturbance gradually took on *some* (not full) tropical characteristics as it came north, and now it will retreat back to non-tropical phase. This feels like the type of system that will get an extra look in the offseason, and it may turn out that this was our first September storm.

More notably, I think this really underscores that the technicalities don’t much matter in terms of impacts. Did the system produce heavy rain? Yes. Did it produce significant tidal flooding? Yes. Did it produce winds of 45 to 55 mph? Yes. Did it have a name? No. Should we have written more about this before yesterday? Probably. I think this is a good example of a storm where impacts are more clear-cut than the “what” it is. The “what” is all too often an academic exercise that means nothing to the vast majority of people. Remember, focus on impacts.

Tropical Depression 7

Overnight, we saw Invest 92L get an upgrade to Tropical Depression 7!

Tropical Depression 7 will head northwest over the next few days, likely passing comfortably north of the Caribbean islands. This morning, TD 7 looks…well, it looks.

Tropical Depression 7 has plenty of storms, but it remains very loosely organized. (Tropical Tidbits)

The thunderstorm activity is plentiful around the depression, so that signals that it’s got some good underlying bones. However, it’s not particularly clear that this has a closed circulation right now. Regardless, we’re again debating technicalities. This is a system that is ready to intensify once the bones become better organized.

The only land we’re focused on right now for a potential threat is Bermuda. Models seem split right now on whether it tracks toward Bermuda or curves back into the Atlantic east of the island. We’re about 6 days out from that happening, either way. Interests in Bermuda, be it for travel or otherwise should monitor Tropical Depression 7 over the coming days.

Other news and notes

Next wave up

Beyond Tropical Depression 7, there remains another wave with about a 20 percent chance of development. For now, this seems unlikely to be a big deal.

Mario & West Coast risks

The remnant moisture from Tropical Storm Mario will move north today and tomorrow, bringing the risk of heavy rain to California. The overall track of the moisture seems to have shifted a little west, which means much of California will see scattered showers and thunderstorms, especially west of the deserts. The good news is that it appears most of these thunderstorms in this setup look wet, with lesser dry lightning risk than initially feared. This should keep fire risk relatively low.

Scattered showers and thunderstorms are likely across southern and central California, with rain amounts from a couple tenths of an inch to perhaps as much as 1 to 2 inches. The heaviest rain passes offshore. (Pivotal Weather)

Rain totals will be very erratic in nature, as some places may only see a couple tenths of an inch or less, while others may see flash flooding concerns and 1 to 2 inches of rainfall. Follow your local NWS office for the latest here.

The next Atlantic depression may soon form but is not a land threat, while a coastal storm pounds the Mid-Atlantic coast

September 16, 2025 at 9:46 amSeptember 16, 2025 at 9:46 am by Matt Lanza

In brief: Invest 92L is on the cusp of becoming a tropical depression later today or tomorrow in the Atlantic. Neither it nor the wave behind it appear to be land threats right now. A coastal storm will deliver wind, tidal flooding, and heavy rain to the Virginia Tidewater and northern Outer Banks today and tonight. Mario’s moisture will play a role in a very, very active pattern for California beginning tomorrow.

Invest 92L

We got the invest overnight. The Atlantic tropical wave is now designated Invest 92L, and it looks as though this is on its way to getting an upgrade at some point in the next 12 to 36 hours.

Over the next week or so, 92L will track west-northwest or due northwest in the Atlantic, tracking over gradually warming water.

Tropical model forecast tracks for Invest 92L with sea-surface temperatures underlaid show that the system will be encountering at least favorable water temperatures over the next 5 to 7 days. (Cyclonicwx.com)

The current forecast track shows minimal concern for any land mass. Bermuda can’t be entirely ruled out here, but that’s at least 6 to 7 days out. For now, this isn’t one to worry about, but we’ll continue to watch to be safe. The next name on the list remains Gabrielle.

Behind 92L

The NHC has identified the next wave as another area of interest. This one is smaller in size than Invest 92L, and it also has a little less model support for development than 92L had at this stage.

The next wave off Africa has about a 20 percent chance of developing over the next 7 days. (NOAA NHC)

The initial odds of 20 percent feel right at this point. We’ll keep an eye on this as well, but it seems less likely to be a development issue than 92L should be. No threat to land at this point either.

Gulf/Western Caribbean

We continue to see some “noise” from the usual suspects (cough, the GFS model) far out in time in the Gulf or western Caribbean. This is a normal model bias for late September and October, so get used to seeing spurious tropical systems on that specific model guidance.

That said, there is some very, very modest support for some potential “noise” in that area on other guidance. We can’t latch onto a specific disturbance right now, nor can we say something will come of this with any confidence whatsoever. But the reality is that it’s the time of year we watch there, and the models are producing noise there to a small extent. In other words, we’ll be looking there to see if anything can emerge to close September or start October. For now, no one need be worried at all.

North Carolina & Virginia coastal storm

A storm system deemed non-tropical is moving ashore in North Carolina and Virginia today. We’ve had wind gusts since midnight as high as 50 mph in spots, including Duck, NC and Cape Henry, VA.

Wind gusts in excess of 45 mph have occurred in several spots since midnight in northeastern North Carolina and the Virginia Tidewater. (NOAA)

In addition to the wind, we’re seeing coastal flooding, with the highest tide occurring later today.

Tidal flooding is likely, with major tidal flooding this evening along the tidal James River. (NWS Wakefield, VA)

None of the current tidal forecasts look like records, but they should at least be near the higher values seen with passing storms this year and last year. Rain won’t help matters, with heavy rainfall compounding drainage issues in areas with tidal flooding. A slight risk (2/4) for flash flooding is in place today in southeast Virginia and northeast North Carolina, along with flood watches.

Flood watches remain in effect today and this evening south and southeast of Richmond. (NWS Wakefield)

Definitely a day to take it easy in this part of the world, watch for flooding, and never drive through flooded roadways. The Virginia Tidewater is one of the most vulnerable locations in the country for coastal flooding, and with a combination of sea level rise and land subsidence, this will continue.

Mario’s impacts to California

With Mario slowly lifting north, the overall weather pattern off the West Coast will favor tropical moisture being funneled northward around the east side of a dominant upper level low pressure system. As this happens, moisture will surge into California later this week bringing a good chance of localized heavy rain, thunderstorms, or possibly dry lightning.

Atmospheric moisture anomalies (PWATs) over California and Arizona and Nevada through the week increase dramatically. (Pivotal Weather)

While the rain could produce localized flash flooding, it will be interesting to see if dry thunderstorms play a role here also. The risk of dry lightning looks highest Wednesday night into Thursday, especially north of Los Angeles, up into San Luis Obispo and Santa Barbara and northward into the Bay Area.

There may be more to come heading into next week with Pacific Invest 96E possibly playing a role in that. For more details on the situation in California, I strongly encourage checking out (and subscribing to) Daniel Swain’s Weather West blog.

Modest Atlantic development possible, while Mario will aid in SoCal storm chances later this week

September 15, 2025 at 7:45 amSeptember 15, 2025 at 7:45 am by Matt Lanza

In brief: The tropical wave in the central Atlantic is becoming more likely to develop over the next few days as it moves west northwest across the Atlantic. No land concerns are expected right now. Meanwhile, Pacific Tropical Storm Mario is likely to spread thunderstorm chances across Southern California later this week. We also check in on the Dakotas which had a mini tornado outbreak on Sunday.

Atlantic tropical wave

The wave we’ve been talking about since last week in the middle of the Atlantic looks pretty decent this morning. It’s certainly better looking than the previous wave we were tracking out there.

This wave now has 80 percent odds of development per the NHC, and I imagine this will get tagged as an Invest within the next day or so. Whereas the previous wave we tracked out here had high development odds and failed to make it, this one has much less of an “all or nothing” chance to develop. There is strong model agreement in some development by as early as Wednesday or Thursday (shown).

Strong model agreement that by mid to late week we have a tropical depression forming in the central Atlantic. (Google Weather Lab)

Notice that this system is already at a fairly moderate latitude by mid to late this week, and with it continuing west northwest, we should see this pass fairly well north of the Caribbean islands. From there, it seems as though it will either curve back north and northeast out to sea, or it will continue west northwest to the north of the islands, sort of in the fashion of Hurricane Erin earlier this season, particularly if it’s on the weaker end of the spectrum. The most likely outcome is some loosely organized storm turning out to sea. But for folks in Bermuda, it probably makes sense to keep a side eye on this one through the week, unlikely as it would be to impact the island.

Generally speaking, while this is the most interesting item we’ve had in what seems like weeks in the Atlantic, it’s thankfully not a serious land threat at this point.

What’s next in the Atlantic?

The background state of the Atlantic may revert back to hostile again after about September 20th, meaning that we’re probably going to continue to see systems struggling for the end of September. Probabilities for tropical development are not exactly strong looking outside of that one tropical disturbance in the middle of the Atlantic.

ECMWF Weekly model odds of tropical cyclones for the end of September showing generally below average development odds beyond the central Atlantic wave. (ECMWF)

In fact, if anything, they’re below average off Africa and below average off the Southeast coast. I do suspect we’ll begin to see more noise in the southwest Gulf or western Caribbean, as is typical for late September and October, but aside from that, it’s becoming apparent that this is probably going to end up being one of the more subdued hurricane seasons in recent memory. Of course, one bad outcome in early to mid-October can change the whole perception, but we currently don’t see anything else out there.

Tropical Storm Mario

In the Pacific, Tropical Storm Mario was declared dissipated this weekend, but it turns out the princess was just in another castle. So, Mario is back to life.

Mario should continue off to the northwest, eventually succumbing to King Koolpa, as sharply cooler water awaits it to the north.

Mario’s tracks from various models plus water temperatures turning sharply cooler after the next 24 hours. (CyclonicWx)

So why am I taking up virtual oxygen talking about a storm that will remain offshore and dissipate. Besides the ability to add Mario puns to the post, the moisture from Mario is going to play a role in the weather in California later this week, with showers and thunderstorms likely to infiltrate from the south, particularly in SoCal. In addition to the potential for locally heavy rainfall in Southern California, there could also be some isolated dry lightning issues with respect to fire starts in parts of the interior. The entire region is under a marginal (1/4) risk for locally heavy rainfall on Thursday.

Some locally heavy rain in addition to dry lightning concerns will permeate SoCal later this week. (NOAA WPC)

Rainfall amounts will be minimal in most places, but there will be areas that see a solid half-inch to 2 inches of rain, which could cause some flash flooding in spots.

North Dakota ‘naders

Quite a day yesterday in the Dakotas. Historically, not a tornado-prone location in September (there have been only one or two historical noted reports of tornadoes in this corridor of the Dakotas since at least 1950 in September), there were nearly 20 reports of tornadoes on Sunday from at least 4 tornadoes. Reports ranged from near Mobridge, SD through Bowman, ND, Golden Valley County, and Hettinger, ND.

The National Weather Service office in Bismarck is looking for reports from yesterday across the corridor in northwest South Dakota and western North Dakota. Certainly, there were a number of chasers on these storms. But overall, this was a bit of an impressive, somewhat surprising September event this far north.