Good morning! Earlier this week I alluded to the possibility of areas of interest in the tropics over the coming days; as each new model run comes in, the idea that there will at least be areas of disturbed weather in the western Atlantic and Caribbean Sea becomes more certain.
First things first, let's talk about the possibility of a subtropical system developing between the east coast, the Bahamas and Bermuda later this weekend or early next week. Most of the forecast model guidance supports the idea that an upper level area of low pressure will move from the Ohio Valley southward toward the Atlantic coast late this week and early this weekend. This is one piece of the puzzle (the extratropical piece -- the part of a subtropical storm that more resembles a "regular" area of low pressure that one would expect to see during the cold season). The other piece? Well, you felt it yesterday -- or at least noticed it as clouds got in the way of your beach day or any of your outdoor plans! We had a lot of high cirrus clouds around most of SWFL yesterday, with lower clouds and precipitation across southern parts of the area. Even heavier rain was located across the Florida Straits eastward toward Miami and the Bahamas. This tropical moisture is associated with a trough of low pressure in the low levels of the atmosphere and a strong subtropical jet stream.
Our models forecast a weak area of low pressure to develop this weekend over or just east of the Bahamas. Eventually, the upper level low off the east coast is expected to "link" with this weak surface low and potentially form a subtropical storm (if the National Hurricane Center acknowledges this, this could become our first named storm of the year -- Alex). This is not altogether unusual for May (in fact, subtropical developments are the "most likely" developments this time of year!). Any storm that does develop, subtropical or not, would meander for quite a while out in the western Atlantic with weak steering currents in place. This is why my weekend forecast is pretty dry across southwest Florida. Down the line, the question will be whether whatever develops is picked up by the next upper level disturbance or is, rather, guided back toward the southeast by an area of high pressure building to its north. Right now, this seems (at least initially) like the most likely outcome, meaning our shower and thunderstorm chance will increase by the middle of next week.
What else am I watching? There's a tropical wave right now located south of 10 degrees north latitude east of South America. This wave is likely too far south for any sort of development even though upper level wind is favorable and water temperatures are much warmer than average across this area of the Atlantic. Down the line, this wave is forecast to move into the southern Caribbean; this is where things might get interesting.
Above, I've included a map of sea surface temperatures in the Atlantic. Notice the amount of territory in the tropical Atlantic that already has temperatures at or above 80 degrees (shaded in yellow, anything south of the red line). Also included is a map of something known as Tropical Cyclone Heat Potential (TCHP); this is important because it measures not only the temperature of the water, but the depth to which the water is warm. The deeper and warmer the water, the higher the TCHP and the more fuel there is for tropical systems to develop. Notice the hot spot right now? South of Jamaica and Cuba.. the exact area where this tropical wave is expected to be drawn by the middle of next week. There is strong upper level wind located north of here (and this is expected to still be the case next week); so the wind shear present should preclude anything that develops in the Caribbean from surviving its trip north of the northern islands. That said, the environment seems favorable for some sort of development in the Caribbean next week.
Things may stay more active than average into the first couple of weeks of the Atlantic hurricane season as a pulse of upward moving air moves out of the eastern Pacific Ocean and into the western Atlantic Ocean (this is another feature that favors the idea of early season development in the Atlantic). This pulse is known as the Madden-Julian Oscillation and travels eastward across the tropics. Generally, you can expect to see a burst of upward motion (thunderstorms) in the tropics for a few weeks followed by the descending phase (fewer thunderstorms). Ever notice how tropical activity seems to come in bursts, and then its quiet for weeks? This is just one of the explanations for that observation.
Stay tuned over the next several days -- we'll be tracking the tropics a little more closely than we usually do this time of year!
Brian
