Large low pressure area will bring a lot of rain, some wind and minor coastal impacts this weekend

Invest area 90L, which is tucked away in the northwest Caribbean Sea, just off the coast of Belize, is keeping a lot of people guessing as to what its eventual outcome will be. This is fairly typical of late May or early June tropical systems, especially in the Gulf of Mexico – which is where this system is headed.

So let’s start with what we know as of now:

NHC says development chances are now at 60% over the next five days. This means we could see a subtropical  or purely tropical depression form before all is said and done. What ever it is classified as, it will have some negative impacts on Memorial Day weekend plans, including travel on the busy I-10 corridor and elsewhere.

Right now, the low pressure area is broad, diffuse and void of any solid thunderstorm activity. Dry air in the mid-levels plus strong upper level winds are keeping things in check for the time being.

As we head in to the weekend, the NHC mentions the fact that conditions are forecast to become more conducive for slow development and this is when we may see more organization of the system – perhaps enough so that it is classified as a depression. The subtropical label means it has a mix of mid-latitude storm features, a spread out wind field and a much broader area of overall impacts. On the other hand, a purely tropical depression or storm has a more compact wind and pressure field, tighter overall banding features and a smaller geographic footprint so to speak; in short, it’s more concentrated.

The label won’t matter – I am sure you have heard this a lot about this system already. It’s true – we name weather systems to give us clarity and a way to keep up with everything. The end result is what really matters and for 90L, and/or what ever it ends up being called, that means rain and a lot of it.

WPC's 7 day precipitation map showing a huge area of potentially very heavy rain as the low pressure area takes shape in the Gulf of Mexico.

WPC’s 7 day precipitation map showing a huge area of potentially very heavy rain as the low pressure area takes shape in the Gulf of Mexico.

I have again posted the Weather Prediction Center’s 7 days precip forecast and it shows the potential for widespread heavy rain across a good deal of the Southeast, including all of Florida. Now, this does not mean that this map will verify as depicted, it is a guidance tool to indicate the potential for heavy rain across the region as the low pressure area moves slowly north towards the central Gulf Coast.

If you have plans to travel across the I-10 corridor this weekend, please pay close attention to local weather info and radar updates. Driving at 60-75 mph in torrential downpours can be hazardous to your health – take it from someone who has been in more rain than most people will ever see! Slow down, leave early and take this seriously. Rain is a hazard, even if it’s not causing major flooding like Harvey did last August.

For areas that receive sustained onshore flow, the possibility of large wave action and some minor to perhaps moderate coastal flooding will present challenges as well. We will have to watch this potential problem closely as we get further along in time and the low pressure area develops (or not) more. I suspect that some coastal flood watches will be posted at some point over the coming days.

The wind won’t be too much of an issue overall but consider this: if we get gusts to 45-50 mph in any heavier downpours or thunderstorms, plus the saturated ground, well then, you can imagine the trees with their newly dense leaf system could be toppled in some cases. Obviously, we need to wait and see how strong the low gets but be aware that power outages due to downed trees may be a problem where the wind gusts are high enough to lay some trees over.

So the bottom line is this: we have a real mess on our hands for a very busy weekend ahead. Millions of people will be hitting the road in anticipation of the start of summer. This low pressure system, what ever it ends up being called, will be an issue to contend with. Understanding the totality of the potential impacts is important so I encourage anyone with interests along the Gulf Coast and in to the Southeast to keep up with the latest as we move towards the weekend.

I will post a detailed video discussion later this afternoon and will go over the most updated computer guidance as well as focus on what impacts to expect.

M. Sudduth

8:40 AM ET May 23

NHC showing likely development of “something” as we head in to the weekend

NHC 5-day outlook map showing the high chance of development for invest area 91L off the Southeast coast

NHC 5-day outlook map showing the high chance of development for invest area 91L off the Southeast coast

The area of interest off the Southeast coast, now known as “invest 91L”, is slowly getting better organized. The NHC has increased the chances of development in to the high category as we head in to the big holiday weekend.¬† But development in to what, exactly? That remains to be seen.

According to the latest statement put out this morning, a tropical or subtropical storm could form from the system as it approaches the coast this weekend. Let’s take a look at the differences between the two possible scenarios:

A tropical storm is what we are most used to hearing about. Winds are more or less concentrated around a well defined area of convection or thunderstorm activity close to the center.

A subtropical storm is more like a hybrid storm that has some tropical characteristics while also displaying some non-tropical signs as well – such as having winds and energy spread out over a larger area and more loosely defined convection. In other words, a subtropical storm hasn’t quite bundled all of its energy around a distinct, warm-core center like we are used to seeing with purely tropical systems, especially hurricanes. Subtropical storms usually transition completely in to classical tropical storms if they remain over warm water long enough.

In the case of 91L, right now, it remains spread out and not very concentrated, therefore, development has been slow. As long as this continues, we won’t see much more than a nuisance rain maker for the Carolina coast this weekend. However, water temps in the Gulf Stream, which is still to the west of the developing storm, are quite warm and it is possible that we will see a pure tropical storm form which would mean more wind, rain and rough surf conditions for the coastal areas that it impacts.

The good news is that none of the model guidance suggests anything too strong coming from this. After all, it is only late May, not September. That being said, we should never ignore a festering tropical feature that is so close to land. If you have plans along the beaches from Georgia to Cape Hatteras, keep them, but be aware of this feature and the potential for heavy rain and some gusty winds. The other hazard that would concern me is rough surf. Water temps along the beaches are still sub-80F but this will not keep people out of the water this weekend. Watch for local conditions to change including the chance of increased rip currents. Remember, tropical storms have the potential to be deadly if people don’t understand the local impacts. Keep an eye on the kids if they plan on heading in to the ocean in the affected area during the long holiday weekend.

I will post a video discussion concerning 91L later this afternoon followed by a blog post update here this evening.

M. Sudduth 9:50 AM ET May 26