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Posted at 7:00 PM ET, 08/ 2/2010

Atlantic tropical storm a distant threat to land

By Greg Postel

* PM Update: Cool for now, heat cranks up tomorrow *
* New hail record in South Dakota | Rediscovering a classic D.C. photo *
* Outside now? Radar, lightning, temps & more: Weather Wall *

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.

Satellite image of of tropical depression 4 - spinning in the open Atlantic. Courtesy NOAA.

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...

Model track projections for tropical depression 4. Courtesy South Florida Water Management District.

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.

By Greg Postel  | August 2, 2010; 7:00 PM ET
Categories:  Tropical Weather  
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Greg, thank you for an informative post on the impacts to hurricane tracks. I spend all summer & early fall worrying whether we'll duck these storms here in the metro DC area, so I appreciate the explanation.

Posted by: --sg | August 2, 2010 9:02 PM | Report abuse

Hi Greg, great post. I've been watching this one on models for a few days and some have consistently moved the storm in pretty close to the area (if nothing else, implying an East Coast threat). One thing I am keying in on lately is that we've spent a fair amount of time in between two 'centers' of subtropical high pressure (one over the continent and the other in the Atlantic). This seems like it might put us at greater risk than normal to see some impacts of a storm if it were to hold into the heart of the season -- and persistance says it might?

"Colin" seems tricky so I don't feel like there is super high potential with it. The models are starting to spit out a good bit of activity though. The 18z GFS brought a nice cyclone up through the Gulf and NHC has highlighted it since. Needs to get away from land, but could be one to watch.

Posted by: Ian-CapitalWeatherGang | August 2, 2010 9:11 PM | Report abuse


you are very welcome. I think you're pretty safe from hurricanes there in D.C. Not just in this case, but landfalls in general are rare that far north.

fear not,


Posted by: gregpostel | August 2, 2010 11:10 PM | Report abuse

hi Ian-CapitalWeatherGang,

thank you for the kind words. The global models are indeed spinning up the tropics.

MJO may become favorable in the next two weeks as well for development in the Caribbean.

We shall see.

take care,


Posted by: gregpostel | August 2, 2010 11:13 PM | Report abuse

glad i just returned from the caribbean!

Posted by: nativetexan2 | August 3, 2010 9:31 AM | Report abuse

Hi Greg,

Great post! I love the hyper-local, scientific approach you and the gang articulate to us lay-folks on the reg. I was wondering, when covering tropical weather, if you could include predictions or notes on the marine effects (wave hieght and surf forcast) of a given system. I know DC isn't really a coastal place, but some of us like to hit the water when there is some summer size. Do you think Colin will produce any waves for New Jersey beaches in the coming weeks?


Posted by: DexPolizzi | August 3, 2010 10:46 AM | Report abuse

Colin is dead (for now). Remnant low as of 5p.

Posted by: Ian-CapitalWeatherGang | August 3, 2010 4:53 PM | Report abuse

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