As one might expect at the end of August, the tropics are very active today. Here’s a Sunday roundup of everything that’s going on, with an emphasis on what might affect the Gulf of Mexico.

GOES-East image as of 2:47pm CT Sunday. (NOAA)

GOES-East image as of 2:47pm CT Sunday. (NOAA)

Hurricane Gaston

Gaston may be a beast, but we can only admire his eye and 105-mph sustained winds beauty from afar. A quick summary:

No one stays away from land like Gaston!

No storm is worth ignoring like Gaston!

Tropical Depression 8

This storm developed quickly on Saturday night and Sunday morning, and may become Tropical Storm Hermine during the next day or so. This system is going to move toward the North Carolina coast by Tuesday or so, but shouldn’t strengthen much further due to increasing amounts of wind shear. Should still be a rainmaker for the Carolinas and possibly Virginia this week before it moves back out to sea.

Invest 99L-Now Tropical Depression #9

Ahh, our old friend Invest 99L. It seems like we’ve been tracking this undeveloped tropical wave since about 2013. Matt joked that it might be the first Invest to ever get “retired” by the National Hurricane Center. With that said the system does look a little bit better on satellite today, and the hurricane center just upgraded it to a tropical depression. Here’s a look at the track models:

12z track models for Invest 99L. (NCAR)

12z track models for Invest 99L. (NCAR)

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Heading to the beach this weekend? I may not be your best friend after today’s post. After scattered showers and noisy storms on Thursday, we have some morning rain moving in today. Over the weekend, an upper level area of low pressure over the Gulf, sort of mimicking a disorganized tropical wave should help produce a good deal of rain and storms along the coast. This may keep beach weather at a bit of a premium going through the weekend, though there are still a couple question marks. The details…

Today

A batch of rain was sliding in from the east this morning, and we have also had some thunderstorms along the coast. Weather models hold this shield of light to moderate rain together this morning, fizzling toward midday.

HRRR model today shows rain diminishing, but scattered afternoon storms as well. (Weather Bell)

HRRR model today shows rain diminishing, but scattered afternoon storms as well. (Weather Bell)

 

As we go into this afternoon, we should see additional hit and miss showers and storms pop up. Like yesterday, these storms will probably have a good deal of lightning and be capable of producing locally heavy rainfall. Not all will see them, but you’ll be well aware when you do. Temperatures should be held down a few degrees with additional clouds around, so expect upper 80s at best, mid 80s where clouds persist longer.  Read More…

After two drier, warmer days in the mid-90s—typical weather for August—Houston will slip back into a mild, wetter pattern.

Today

As high pressure has retreated some, and moisture levels are fairly high, I’m expecting some fairly decent shower and thunderstorm coverage today, especially across southern parts of the metro area where precipitable water levels should be highest. Expect highs around 90 degrees, perhaps a bit higher to the northwest of Houston where rain chances should be lower. Showers may linger well into the evening and overnight hours.

Friday through Sunday—Mild temperatures

A series of disturbances seem likely to approach the region this weekend, beginning on Friday, and with a tropical airmass in place we can expect to see a fair amount of rain coverage. These three days should see partly to mostly cloudy skies, and on-and-off rain showers. I’m not expecting anything too extreme, and most of the Houston area will likely see 2-3 inches of rain or less. However we can’t rule out some locally heavy rainfall, and brief street flooding, where the strongest storms develop.

Highs will be blessedly cool for late August—likely only in the upper 80s each of the three days.

Temperatures on Friday could be a solid 8 to 10 degrees cooler than normal for August. (Weather Bell)

Temperatures on Friday could be a solid 8 to 10 degrees cooler than normal for August. (Weather Bell)

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By now most readers have heard about the tropical low pressure system in the Caribbean Sea, which is approaching Puerto Rico. It is not a tropical storm yet, as a reconnaissance aircraft on Wednesday failed to find a well-defined circulation. It nonetheless appears to be developing stronger thunderstorms, and is producing tropical storm-force winds. It likely will become a depression and then a storm later this week as it passes near the Bahamas and then tracks toward Florida.

The purpose of this post is to discuss what may happen after that, but before doing so, I want to discuss the fallibility of forecast models at this time, and why we don’t have that much confidence in where the system, Invest 99L for now, will go in five to seven days, if it does head toward the Gulf of Mexico.

Model inaccuracy—it’s considerable

Let’s focus on track models. They tell us where a storm will go. Although models can do a fairly good job simulating the large scale steering currents that guide storm motion, such as upper-level pressure systems, these forecasts remain sketchy when a tropical system is poorly defined. And as I noted earlier, Invest 99L has yet to develop a center of circulation.

With that said let’s look at track errors for National Hurricane Center forecasts for three classes of storms: hurricanes (which have well defined circulations), strong tropical storms (likewise) and depressions and weak tropical storms (not so much). Actually, the category we’re interested in isn’t even on this chart—”stuff that ain’t formed yet.” Thus at five days we can expect the track error to be at a very minimum of 350 nautical miles for undeveloped tropical systems, and likely much, much more.

(National Hurricane Center)

(National Hurricane Center)

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Good morning. Today we’re just going to take a quick look at the weather forecast, and then focus our attention on the tropics, where Invest 99L continues to be a system of interest.

Today

I expect another day like Tuesday, with only isolated to scattered rain chances and highs in the mid-90s (the temperature got to 95 degrees on Tuesday in Houston). Skies will be mostly sunny.

Thursday and beyond

High pressure is going to move off to the east later today or Thursday, and this should bring an increase in cloud cover and rain chances, likely keeping high temperatures to around 90 degrees. I do not anticipat any significant flooding, and most of the area should see only perhaps about an inch or so of rain over the next several days. However that does not preclude the possibility of some locally heavy rain. Essentially, expect our pattern of a cooler, wetter August to continue for most of the rest of the month.

Tropics

For a couple of days we’ve been discussing the possibility of a tropical system known as Invest 99L moving into the Gulf of Mexico, and that remains a distinct possibility. The system is not yet a depression or a tropical storm (it has an 80 percent chance of becoming one in the next five days, says the National Hurricane Center), but as it approaches Puerto Rico today it appears increasingly likely that it will reach Florida by Saturday or Sunday.

Possible track of Invest 99L. (NOAA)

Possible track of Invest 99L. (NOAA)

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