Wednesday, July 24, 2013

Tropical Storm Dorian Forms

Tropical Storm Dorian has formed overnight in the eastern Atlantic as a result of the vigorous tropical wave I discussed a few days ago getting its act together at sea.

[Image Removed]

The latest model suite for Dorian has the system placed directly east of St. Lucia in the Caribbean, moving at a generally west-northwest path over coming days. The model guidance above does appear to have a consensus with taking Dorian along the open Atlantic waters towards Hispaniola and Cuba. This is where things get iffy. Some models begin to curve Dorian to the north to allow the system to make a u-turn and move away from the US Mainland and out to sea again. Some other models attempt to push Dorian towards Florida, possibly as a bid for a landfall on the Sunshine State. Still other models (not shown on this image) would appear to want to push Dorian through Cuba and into the Gulf of Mexico, while our last category of models would try to do a re-curve out to sea, but would curve north too late and pose a real landfalling threat to the Carolinas.

The latest depiction of the atmospheric flow for a system of Dorian's strength (1000 millibars or higher) is rather straightforward, but has no model support. Going by this image, Dorian (shown as the 'L' on the bottom right corner of the picture) would go with the flow in a west-northwest pattern, in similar fashion as the model suite. From there, it falls apart. While the atmospheric flow will certainly change in 48 hours, Dorian would likely encounter two options of this overall pattern in the above image were to remain intact:

1) It sees the gap between the two anticyclones in the open Atlantic and pushes through, recurving well out to sea and posing no threat to land.

2) The gap between the anticyclones closes and Dorian is forced south, which could lead to a threat to some northern Caribbean regions, including Hispaniola, Cuba and some other land masses in that area.

As of now, after analyzing the full model suite and some other factors, I would be more inclined to believe the second option, mainly out of both model agreement on a more WNW track over a NW track, and also the tendency for weaker systems to maintain a more westward track.

I bring up the topic of weaker systems because we will see Dorian go through more ups than downs in the next 120 hours. Shown above is a chart of current wind shear values. Dorian is not shown here, but you can see approximately where the system is, shown by a sudden drop in shear values in the bottom right corner of the image.

If we look ahead on this chart and follow the track proposed by the model guidance at the top of this post, we see Dorian heading directly into 40-50 knots of wind shear. That's a lot of wind shear, folks. Dorian will have a very rough time battling this, and we will likely see the system struggle. No matter the track, Dorian is likely to enter that large swath of 40 knots of wind shear extending from the northern fringes of South America well out to the eastern Atlantic.

As of now, I prefer the current model suite in the top image to the extent of when it begins to approach the waters north of Hispaniola. Beyond that time frame, the models become too muddled and uncertainty grows to a point I believe a forecast will be unsuccessful.


1 comment:

Anonymous said...

How likely do you think it is that dorian gets completely ripped apart and dissipates due to that shear?