Month: July 2020

This afternoon we’re issuing our first “flood scale” forecast for the Gulf disturbance, which is increasingly likely to move into Texas this weekend as a tropical depression or weak tropical storm. We are calling for a “Stage 2” flood event for coastal counties in the greater Houston area—Brazoria, Galveston, and Chambers as well as southeastern Harris County (i.e. the Clear Lake area). For inland areas we think this will be a Stage 1 event. As with all tropical weather, of course, this forecast is subject to change. (Read more about the scale, and why we created it).

Now let’s explain how we came to this forecast.

Satellite appearance

Let’s begin with the system itself, which at present is about due south of the border between Alabama and Mississippi, and moving nearly due west. The storm’s organization has improved today, adding westerly winds, which is a precursor to developing an overall circulation. Trends favor additional organization and we probably will see this named a depression or Tropical Storm Hanna later today or on Thursday. We expected this, but do not anticipate further strengthening to, say, a hurricane. Theoretically this is possible, but does not seem likely given the storm’s interaction with dry air, among other factors.

Track

The storm increasingly seems likely to move toward the central or southern Texas coast. The preponderance of ensemble members of the European and GFS models predict such a track. Certainly, without a well established center of low pressure it is important to not give too much credence to these predictions. But as we are perhaps just two or three days away from a Texas landfall, and there is good consistency in the track forecast, our confidence level in the track is moderate.

Rainfall

Unfortunately, knowing the track of a tropical system does not confer meteorological superpowers when it comes to trying to forecast rainfall totals. We believe the heaviest rains will be to the north of the storm’s center, but the “where” is difficult to say.

Some of the forecasting models we commonly use are starting to predict rainfall “bullseyes” of 10 inches for hardest hit areas, particularly along the coast. (This seems pretty reasonable for a tropical system like this). However, we do not have confidence in whether these highest totals will come in Corpus Christi, Galveston, or Port Arthur. It is because of this potential for 10 inches of rain in the Houston area that we are issuing a Stage 2 warning for coastal areas. We still think much of the greater Houston region will see 2 to 4 inches of rain over a couple of days, which is totally manageable.

Beyond this system

The National Hurricane Center may upgrade Tropical Storm Gonzalo to hurricane status tomorrow as it approaches the southern Caribbean Sea. Right now we have relatively low concerns about Gonzalo becoming a threat to the Gulf of Mexico. We will of course continue to watch it, but advise you to not lose any sleep over it. Beyond Gonzalo the tropics are going to remain active.

Today’s update will talk briefly about that Gulf disturbance coming to Texas this weekend, but more about what is behind that system as it appears the tropics are waking up, and they may not shut back down for a while.

Tropical outlook in a sentence

We expect a low-end system to form in the Gulf this week, another system will make an attempt to develop in the Atlantic, east of the Lesser Antilles, and with more waves emerging off Africa, more development will be possible heading into early August.

Gulf system

Eric has done a good job each morning discussing what we know and expect from the potential late week system in the Gulf. As of this afternoon, the system is a disorganized “blob” of thunderstorms over and around Cuba.

However, as that moves into the Gulf, it will find decreasing wind shear and fairly warm water to work with. It seems there are two things that will hinder this system’s development into anything worse than a depression or moderate tropical storm: 1.) It will enter the Gulf disorganized. And 2.) It will move at a steady clip, so it’s going to run out of time over water fairly quickly to develop. So the ceiling is constrained by time. Still, a strengthening tropical storm in the Gulf on approach to land is nothing to sneeze at or write off completely, so we’ll monitor this closely. Of note, the morning model data so far is not any further bullish on this system’s development prospects, and the National Hurricane Center maintains a 40 percent chance of development over the coming days. Regardless, we will see some locally heavy rainfall associated with this, be it as a disturbance or organized depression or storm. Thankfully, it is expected to keep moving at a steady pace. We will continue to monitor it for you, and Eric will have the latest in the morning.

Invest 99L

The NHC has given the disturbance classified as Invest 99L about a 90 percent chance of development out in the open Atlantic Ocean, just east of the Lesser Antilles.

This system is very, very close to being a tropical depression already (as of this writing at 1 PM Tuesday). Invest 99L is going to be interesting. It’s rather far south and small, which will help shield it from a rather significant plume of Saharan dust off to its northeast. It is moving into an area of warm water temperatures and pretty low wind shear. Theoretically, this is a good candidate for development over the next 3 to 4 days as it approaches the islands. The various intensity models are very split on what happens here. I wouldn’t take the ultra high-end ones literally, but you can clearly see two camps: One that keeps the system middling over the next few days and another that says this thing will strengthen as it approaches the islands.

Which camp is right? Great question, and one we don’t have an answer to at this time. There are good meteorological cases to be made for both a lower-end system, as well as one that perhaps organizes faster than anticipated.

The track & intensity of Invest #99L likely to be closely linked over the next 3-5 days.

Forecast low-level steering is faster & zonal but deeper-layer steering (850-200-hPa) is slower & more poleward. If 99L becomes a vertically deep vortex early (<48h), large impact on future. pic.twitter.com/TIFABN0CWz

— Philippe Papin (@pppapin) July 21, 2020

Either way, interests in the eastern Caribbean should be monitoring the progress of Invest 99L closely. For us in Texas? It would be unlikely that this would survive the trip here. In fact, no storm within 150 nautical miles of where Invest 99L was centered this morning has ever made it to Texas, with the exception of Hurricane Allen in the Rio Grande Valley in 1980. So, if you’re playing odds, you’ll take those odds. We’ll continue to watch it however.

Of note, the next two names, which both could be utilized this week are Gonzalo and Hanna. The earliest “G” storm on record was Gert on July 24, 2005. The earliest “H” storm was 2005’s iteration of Harvey on August 3rd. Both records could fall.

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