Last night about this time, the meteorological community was abuzz discussing then Tropical Storm Bonnie. However, since last night Tropical Storm Bonnie made landfall in the Miami, FL vicinity and has subsequently weakened to Tropical Depression. Because Tropical Depression does not appear to be very noteworthy at the moment (in fact, @ryanvaughan has described Bonnie as “Rain With A Name ©”) tonight I’ll mention another area of heavy rain.

Currently heavy rain associated with strong thunderstorms area developing to the south and southwest of southern Wisconsin. This area of showers and thunderstorms is moving northeast and will move into southern Wisconsin in the next several hours. This is important because this area experienced significant flash flooding last night as slow moving thunderstorms repeatedly moved over the same area. The city of Milwaukee, WI had almost 8″ of rain fall in a couple hours. Milwaukee, W joined Oklahoma City, OK; Nashville, TN; Louisville, KY; and Atlanta, GA as cities major cities experiencing record/historic flooding within the past 12 months. I hope the residents of southern Wisconsin heed all instructions from emergency personnel and remember to “Turn Around, Don’t Drown“!

Last night, I mentioned several factors that were inhibiting the development of a tropical depression/storm near the Bahamas. If you read that post before going to bed, I can only imagine your surprise this morning when the National Hurricane Center (NHC) issued a special forecast update (above). Last night the probability of development was 40%, which it was earlier today as well. It was then increased to 70% by mid-morning, before being upgraded to near 100% with the special update. And sure enough, during the day, tropical depression #3 developed and strengthened into tropical storm Bonnie (graphic below).

So what happened? Was I completely wrong in what I said last night? Not exactly. Tropical storm Bonnie is still experiencing a lot of southwesterly mid-and-upper-level shear. This is evident in the image below in that all the cold clouds (tops of the strongest convection) are located to the northeast of the center of circulation. Because of this shear, the latent heat release is still being blow away from the storm, which is a strong argument against rapid development.

So what did happen? The (short)wave trough moved far enough away from Hispaniola (i.e., out over open water) that a low-level center of circulation was able to develop. In other words, the higher terrain of Hispaniola is no longer disrupting the low-level air flow into the now tropical storm that a true “center” was able to develop. This is the first step to strengthening. Now, tropical storm Bonnie must develop thunderstorms encompassing the center of circulation before any significant strengthening can occur. I’ll be watching…

PS… I wonder how many times I’ll have to hear the song referenced in tonight’s title…

For those of you who don’t routinely keep up with the happenings of the National Hurricane Center (NHC), you might be a little baffled by my choice of title tonight. “Invest 97L” is the official designation of the tropical (short)wave trough (orange dashed line below) I’ve been discussing on this blog the past few evenings; it is currently being monitored by the NHC for possible development into a tropical storm.

The NHC has downgraded the Invest 97L’s chances of development into a tropical depression/storm in the next 48 hours from 60% to 40%. The reason for this is that the (short)wave trough is still poorly organized. There are two main reasons for this. The first reason is that the (short)wave trough is currently interacting with Hispaniola. The increased friction in the low levels is disrupting air flow into the (short)wave trough just enough to prevent a surface low-pressure from developing.

The second, and probably more important, reason for the downgrade is the interaction of the tropical (short)wave trough with an upper-level low (yellow circle below) just northwest of the (short)wave trough. The airflow around the upper-level low is responsible for increasing the mid-to-upper level shear (green arrow below) atop the tropical (short)wave trough.

I know previously in this blog I’ve discussed how an increase in vertical wind shear is a good thing for the development of severe thunderstorms, so if there is a decent amount of wind shear affecting the tropical (short)wave trough, wouldn’t this increase the chance of development? In this case, no. Here’s why.

Tropical Storms and Hurricanes are giant atmospheric heat engines. They rely on a process known as “latent heat release” to maintain their vigor. In general, the more vigorous the thunderstorm, the more latent heat released into the atmosphere. If this latent heat release is confined to a small area, the pressure at the surface beneath the latent heat release will decrease. This is because as the air aloft heats, it expands. As the air expands, it exerts less pressure with the region it originally resided in, which results in lower pressure at the surface.

If we have an endless supply of warm, moist air (such as over the tropics), the potential exists for a positive feedback to occur. As a result of the lower pressure at the surface, surface air in the areas near the low pressure will begin to move into the low-pressure area in an attempt to increase the surface pressure. This increase in air moving into the same location results in surface convergence, which forces the air to rise. As long as the rising air is warm and moist (which it typically is in the tropics) it will create additional thunderstorms. These additional thunderstorms release latent heat into the atmosphere, which decreases the surface pressure even more, causing an increase in surface convergence… (and we’re off to the races).

This feedback will continue until you remove the supply of warm, moist air at the surface (i.e., hurricane moves over land or into colder waters) or you remove the latent heat from the middle-to-upper atmosphere. Here is where wind shear is detrimental to hurricane development — it blows the latent heat away from the center of the tropical storm/hurricane. The wind shear tilts the thunderstorms so they are no longer straight up and down (similar to the Leaning Tower of Pisa). Because the thunderstorm is no longer vertically oriented, the latent heat is not released over the center of the storm, preventing the positive feedback mechanism from fully taking hold.

In severe thunderstorm environments, we don’t have an endless supply of warm, moist air. As such, thunderstorms developing in this environment won’t have the benefit of the positive feedback loop described above to help maintain themselves. These thunderstorms must find a way to keep the air being drawn up into the thunderstorm (the updraft) warm and moist. If by chance the thunderstorm downdraft rains into the air being drawn into the storm, the air being drawn into the thunderstorm will cool. This cool air weakens the updraft (remember, warm air rises, cold air sinks!) to the point that the updraft can no longer hold all the water in the atmosphere it was previously holding. This water then collapses to the ground and the thunderstorm dies. In this case, if wind shear is present, the updrafts and downdrafts actually get tilted in such a way that the thunderstorm does not rain into the updraft and the thunderstorm can survive.

The National Hurricane Center (NHC) is forecasting a 60% chance that the tropical wave discussed in last night’s blog post will develop into a tropical depression and/or storm. Is this Bonnie?

Well, I’ve made it to day 200; that’s 200 consecutive days of blogging. I should get a gold star or a cookie or something, right?!?

Anyways, a relatively quiet weather pattern persists across the United States tonight. A large, subtropical ridge is located across the southern United States, helping to keep the heat locked in place across the southern half of the US. To the north of this ridge, mid-level flow is a bit stronger on the periphery of the ridge. Embedded within this stronger flow, several minor shortwave troughs are racing through the flow, helping to initiate daily bouts of convection acorss Nebraska and Iowa. This convection then develops into an MCS overnight and pushes eastward and/or southward during the subsequent day.

However, I wanted to turn attention toward the tropics. A large upper-level trough is moving through the Gulf of Mexico currently; it looks like a giant “6″ in the water vapor image above. This upper-level trough is increasing shear throughout the Gulf of Mexico, which will help inhibit any tropical storm development in the near term.

Turning attention toward the Puerto Rico area, a large area of tropical showers and thunderstorms are ongoing just north of the island. The National Hurricane Center (NHC) currently gives this tropical wave a 20% chance of developing into a tropical depression/storm during the next 48 hours. This is partly the result of strong shear co-located with the tropical wave, owing to the mid-level low currently located just to the north and east of the tropical wave. Model forecasts show the shear weakening atop the tropical wave in the coming days, which will give the tropical wave a chance to develop into a tropical depression/storm, just about the time the wave moves into the Gulf of Mexico. It’s still a long ways off, but certainly is of more concern than anything else over the last week or two.

Day 197′s post (2 days ago) displayed the severe thunderstorm outlook from the Storm Prediction Center (SPC) valid on Friday. So what happened on Friday? Well, as you can see, there was quite a bit of severe weather across the northern plains.

As promised, the heat wave is returning to the southern and central United States. Heat advisories and Excessive Heat Warnings have been hoisted for a large area in the southern/central United States. Although this is bad news for those who dislike heat, the good news is that current forecasts show the heat should not reach the levels achieved last week.