Tuesday update: TD4 intensified sufficiently to become tropical storm Colin overnight. Some additional strengthening is possible over the next couple days.
From Monday evening: Tropical depression four (TD4) formed over the tropical Atlantic near 41W earlier this morning, just as the disturbance developed a closed low-level circulation. No where near land, TD4 is roughly 2600 miles southeast of the Outer Banks of North Carolina. Maximum winds in TD4 are estimated at 35 mph and its movement is toward the west-northwest at 16 mph.
The satellite presentation for this fledgling system appears reasonably healthy with tall thunderstorms (bright white shading) near the middle of an otherwise reasonably symmetric cloud pattern. TD4's inner structure, as far as we can tell, may be getting better organized. Earlier today, the National Hurricane Center (NHC) noted that the low-level center was outrunning the mid-level swirl associated with the main thunderstorm cluster. This lack of vertical alignment is unfavorable for significant intensification. However, recent observations suggest TD4 may be more vertically stacked now.
Keep reading for more on TD4, including a discussion of its prospects for impacting land...
The latest hurricane track models nearly unanimously move TD4 toward the west-northwest for the next several days before potentially recurving it out to sea.
To estimate the likelihood that TD4 will become a problem for the U.S. coast down the road, which is right now beyond the forecast horizon the hurricane models are designed for, we look at the global weather models - the ones we use to predict the locations of the troughs (cold fronts) and ridges (heat waves) at our latitude. While not so useful for predicting temperatures at a particular location more than a week away, these weather models can give us some idea of what the big picture might look like in the 7-14 day period - the kind of picture that sheds light on whether an upper trough will set up shop near eastern North America and deflect approaching hurricanes out to sea. Right now the global weather models are indeed hinting that might be the case.
There are other indicators we can look at to assess the likelihood of a close approach to the Eastern Seaboard. In particular, forecasts for the MJO (Madden-Julian Oscillation) can help clarify that probability. The MJO is a tropical weather system the size of an entire ocean basin. It is intricately involved with varying the wind, sea surface temperatures, cloudiness, and rainfall in the hurricane development regions. It also tends to push the weather in midlatitudes toward certain configurations (like one with an upper-level trough near eastern North America, or another with an upper ridge similarly positioned).
Right now, many of the forecasts for the MJO push it along toward an area in the next 1-2 weeks where it favors an eastern ridge, and thus more likely allows tropical cyclones to affect North America rather than recurve. This may very well happen too late in the game for TD4 ... if it even survives the often hostile tropical Atlantic trek during the next week. But it will be interesting to see if the global models' predictions for the mid-latitude trough/ridge pattern will gradually shift in the coming days toward one more conducive to tropical cyclone strikes.