Month: August 2016

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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As promised, here’s another look at the tropics as we near the peak of the Atlantic hurricane season. Like with our post on Monday, today we’re going to focus almost exclusively on a tropical wave known as Invest 99L, rather than the rapidly strengthening Tropical Storm Gaston, which likely will become a hurricane later today or on Wednesday. This is because Gaston is unlikely to threaten land, and the same unfortunately cannot be said for Invest 99L.

What is Invest99L?

For now it’s a tropical wave nearing the Lesser Antilles. The system has had some difficulty organizing up until now because of dry air, which has helped to limit the development of thunderstorms. Here’s the satellite presentation from about 1:45pm CT on Tuesday afternoon.

Satellite appearance of Invest 99L on Tuesday afternoon. (NOAA)

Satellite appearance of Invest 99L on Tuesday afternoon. (NOAA)

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For once, the radar is mostly quiet this morning. The reprieve in showers won’t last too long, however.

Today and Wednesday

Pressures have risen over the Houston metro area as the region has fallen under the western edge of a large area of high pressure anchored over the southeastern United States. This is going to bring a couple of more typical August days to Houston, with highs in the low- to mid-90s, a fair amount of sunshine and some scattered showers during the afternoon hours.

High pressure is (sort of) in control of Houston's weather today and Wednesday. (Weather Bell)

High pressure is (sort of) in control of Houston’s weather today and Wednesday. (Weather Bell)

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