Weather Nerd

97L less likely to hit U.S. mainland, but still worth watching; flood threat in Puerto Rico

The tropical wave that I blogged about late Sunday night, “Invest 97L,” is continuing to remain fairly disorganized as it moves through the eastern Caribbean Sea. Things have gotten interesting this morning, as the wave has been looking a bit better — Dr. Jeff Masters writes that 97L “has a moderate amount of spin and heavy thunderstorms, with a respectable upper-level outflow channel to the north,” albeit “no sign of a well-organized surface circulation” — but the system also finds itself competing for moisture with a new area of thunderstorm convection off to its east (not designated as an “invest” yet):

Hurricane Tracker App posted a video this morning about the interaction issue, and even referenced the possibility that the two systems could “merge,” which would slow any development — but would also, I think, make things even more unpredictable for a while. I haven’t seen others discussing such a “merger,” but certainly, the interaction of these two areas of convection is significant. Dr. Masters calls this issue a “wild card,” writing that the “new tropical wave may compete for moisture, slowing development of 97L, and could also modify the track of 97L.”

For now, though, the big-picture reality is that 97L hasn’t developed much since my last update on Sunday. That’s roughly as expected; most forecasters figured that, if 97L didn’t develop before crossing the Lesser Antilles, it probably wouldn’t get its act together until it reached the more favorable central or western Caribbean later this week. The key change in the forecast, however, and the reason for my “less likely to hit U.S. mainland” headline, is that 97L is now expected by most computer models to “turn right” and head toward the Greater Antilles (i.e., Puerto Rico, Hispaniola and/or Cuba), instead of moving into the open waters of the western Caribbean:

That’s both good news and bad news. It’s bad news for Puerto Rico and Hispaniola, which could see significant flooding from the wave’s heavy rain. Even though 97L doesn’t have a name or an official tropical-cyclone status right now, it has plenty of moisture, and the mountainous terrain of those islands lends itself to flash flooding and mudslides. Here’s the San Juan radar. As Mark Sudduth notes, “Even tropical waves/disturbances can dump excessive rainfall and…the terrain of the Greater Antilles islands is mountainous and is just inviting disaster each time a heavy rain event comes around.” Dr. Masters writes:

Regardless of whether or not 97L becomes a tropical depression today, the major danger from this slow-moving storm will be heavy rains. Three to six inches of rain are predicted to fall over Puerto Rico and the Virgin Islands by Thursday morning, with 3-day rainfall totals of 5-10 inches expected along the south and southeast shores of Puerto Rico. These rains are capable of causing dangerous flash flooding and mudslides, and a Flash Flood Watch has been posted. Similar rainfall amounts will fall in the eastern Dominican Republic, and heavy rains of 3-6″ are also likely to affect the Southeast Bahamas and Turks and Caicos Islands later in the week. Since 97L is relatively small, Haiti may see lower rainfall amounts of 2-4 inches.

Conversely, on the “good news” side, 97L’s turn toward the Greater Antilles will likely delay strengthening, due to land interaction and to the mountains disrupting any nascent low-level circulation that 97L might try to form. Instead of a window for intensification beginning on Thursday or so, as had been predicted previously, 97L now probably won’t have a chance to seriously organize until the weekend. Again quoting Dr. Masters:

The models take 97L to the west-northwest, bringing the center over the eastern tip of the Dominican Republic on Thursday morning. This track will allow the high mountains of Puerto Rico and Hispaniola to disrupt the circulation of 97L, forcing the storm to regroup on Friday over the Southeast Bahamas and Turks and Caicos Islands. The best chance for development of 97L would appear to be on Saturday, after the storm has had time to recover from its encounter with Hispaniola. The UKMET model predicts that 97L will become a tropical depression just north of the Southeast Bahamas and Turks and Caicos Islands on Saturday, but the GFS and European models show little development over the next three days.

Here’s a chart demonstrating this wide spread among the computer models — some calling for significant intensification, others calling for little or none at all:

The smart money, though, seems to be on less pronounced intensification. Here’s Mark Sudduth again:

Once the sprawling area of low pressure gets past the large islands of the Caribbean, it should gradually develop somewhere in the Bahamas. … Fortunately, none of the traditionally accurate intensity models show much coming out of this system as far as strengthening goes. That being said, I would not be surprised to see 97L eventually become a tropical storm before it gets pulled out to the northeast and away from the Southeast U.S. coast and the Bahamas.

That last point — the track — is the other reason why a significant hurricane approaching the United States mainland, like some models were predicting over the weekend, seems less likely now. As shown in the computer-model spaghetti map earlier in this post, the models now generally favor “recurvature” — i.e., turning further right and out to sea — rather than movement toward the mainland. That isn’t the universal prediction, but it’s certainly the trend. Dr. Masters again: “There will be a strong trough of low pressure off the U.S. East Coast this weekend, and the models predict that this trough will be strong enough to turn 97L to the north and northeast by Sunday, keeping 97L well offshore from the U.S. East Coast, but with a possible threat to Bermuda next week.”

One other areas of disturbed weather, in the southern Gulf of Mexico, merits a brief mention.

Sudduth: “[A] tropical wave continues to move across the Yucatan towards the southern Gulf of Mexico with no solid signs of development just yet. As we saw with the evolution of Fernand several days ago, it is possible that we could see development from this system as it moves across the southern Gulf. This area seems to favor quick development so we’ll see what happens. Anything that does manage to get going should be short-lived as was the case with Fernand.”

Masters: “A tropical wave over Mexico’s Yucatan Peninsula and adjacent waters of the Gulf of Mexico is causing scattered disorganized heavy thunderstorms. This activity will move over the extreme Southern Gulf of Mexico’s Bay of Campeche on Wednesday and Thursday, then moving ashore on the Mexican coast between Veracruz and Tampico on Friday. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC put the 5-day and 2-day odds of development at 20%.”

Meanwhile, far off in the eastern Atlantic, waves continue to emerge off Africa. A notable run last night by the best American computer model, the GFS, developed one of them into an enormous, larger-than-Sandy hurricane in a week’s time — but safely over the open ocean, bothering only the fish:

But that’s extremely speculative, and I’m really only posting it because OMG PRETTY COLORS. :) The bottom line is, there’s nothing to worry about right now, in terms of waves emerging off Africa. If that changes, I’ll let you know, of course. Stay tuned to this blog and my Twitter account for the latest.