Will We Get Tropical Or Subtropical Storm Bonnie?

I started discussing the possibility of a subtropical or tropical system developing somewhere off the Southeast U.S. coast towards the end of this month into early June, and as the majority of you know by now, there is an area of low pressure that is currently being monitored. What we try to do at Firsthand Weather is look at the upcoming pattern and determine what implications that has later down the road. Often this time of year when a mid and upper-level ridge builds into the northeastern U.S. with a trough underneath that extends into the open waters of the western Atlantic, it’s usually time to watch for home-brew subtropical or tropical development. However, these systems generally stay on the weaker side, even if they technically do meet the criteria to be named. So, could we have tropical or subtropical Bonnie by this weekend?

Brief Discussion:

As mentioned above, an upper-level trough extends down from the eastern Gulf of Mexico up along the Southeast coast. On the east side of this trough where the dynamics are favorable, convection (stormy conditions) have developed over the open waters off the Southeast coast. Due to the favorable environment, an area of low pressure has developed in the region of interest. Everything that I just explained has been expected for a while now given the pattern.

Now, the forecast begins to get tricky when we actually get that region of interest like we have. The upper-level trough is primarily responsible for the development of the low pressure system; however, the trough itself actually creates a sheared environment (strong winds aloft). . .a condition unfavorable for the development of a fully tropical storm. Generally, convection needs to wrap fully around the center of circulation (around the low pressure system), but a sheared environment doesn’t often allow that to happen. In this case, storm activity is developing to the north and northeast of the center of circulation. Recently however, convection has begun to fire just to the west/northwest of the center also.

To give you a visual of all of this, this is the last satellite image as of 1:45 ET. The “I” designates the center of circulation, but actually that’s a little off. It’s a little northwest of that point now.

Over time, the area of low pressure will continue to drift northwestward towards the Southeast coast into a less sheared environment. It will be steered in this direction by a Bermuda ridge that extends all the way to the East Coast. So, I generally agree with the consensus of this system moving into the Carolinas.

Some Other Obstacles For This System:

I know the map below is in Celsius, so please bear with me. A tropical system needs greater than 26°C (79°F waters) to maintain its strength or strengthen further. As you can see, temperatures are still cool off the Southeast coast, with the exception being that tongue of warm water (which is actually the Gulf Stream). Subtropical systems do not need waters quite that warm, but keep in mind that the winds are typically weaker and more spread out from the center of circulation for subtropical systems.

Assuming that other factors become more favorable for this system to develop into a subtropical or tropical system, this system will likely strengthen most over the Gulf Stream waters. However, before we make that assumption, we need to consider yet another obstacle. See all of that orange in the water vapor image below along and just off the Southeast coast? That’s dry air. When these systems start wrapping around/ingesting dry air, they can weaken if they’ve previously strengthened or remain weak overall.

So as you can see, there are a few obstacles ahead for this system (along with a couple others I didn’t mention), but that doesn’t mean we won’t get a weak tropical or subtropical system out of this.

What You Should Expect From This:

Finally, I want to give you an overall idea on what should be expected. There will likely be a window this weekend that this system could strengthen. As mentioned, wind shear should decrease farther west, but it will take some time before this system moves into an area with substantially less shear. Even if this system gets a name later tonight or tomorrow, convection still may have a tendency to not wrap fully around the core of the system.

The timeframe to watch this most closely will be once it moves over the warm Gulf Stream, especially since this system is a slow-mover. If there is a period of some quick strengthening, it would occur then, but still, I expect this to remain a relatively weak (under hurricane strength). If we do get the Gulf Stream strengthening, the system will likely weaken as it moves over the cooler shelf waters along the coast, especially if this becomes purely tropical. Again, dry air could also hinder this system’s development.

The regions that should watch this most closely are those along the Carolina coast and the very northern Georgia coastal regions. Heavy rainfall with possible flooding will be the biggest issue this weekend, along with breezy/windy conditions and rip currents. Precipitation could eventually spread farther northward along the East Coast once this system pulls north. There are some uncertainties on overall timing, but Firsthand Weather will keep you updated on all of that.

Below is a 5-day rainfall forecast that I pulled off the WPC website:

Of course, stay up to date with Firsthand Weather. The National Hurricane Center currently gives this system a 90% chance of becoming a tropical or subtropical system storm within the next 48 hours, which I agree with. If anything changes, I’ll keep you up to date!

As an aside, keep an eye on the western Gulf of Mexico/western Caribbean in early June. There could be something to watch there around that time period, but it’s much too soon to give any details.