Bob Henson
Posted by cypresstx on 8/18/2015, 4:21 pm
on Masters blog:  http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=3075

The outlook for TD 4

As the first Cape Verde depression of the year, TD 4 formed well east of the Caribbean, where El Niño has been producing record amounts of vertical wind shear this summer. Even though the Atlantic tropics have been largely suppressed, as expected with this year's strong El Niño event, TD 4 appears to be finding a window in time and space where some development is possible. Wind shear near the system is quite low, only around 5 knots (see Figure 3), and the region of stronger upper-level westerlies to the north of TD 4 could end up helping to support development by serving as an outflow channel. A massive area of Saharan dust and dry air lies just north of TD 4, but as mentioned in Tuesday's morning's NHC discussion, it appears that the depression is surrounded by enough moisture that it may be able to intensify even if it ingests some of this dry, dusty air.

The 11 am EDT outlook from NHC brings TD 4 to hurricane strength by Friday morning and to Category 2 strength (sustained winds of 100 mph) by Saturday. This long-range forecast is consistent with the statistical models that show more skill than dynamical models at intensity prediction beyond 3 days. Of the two dynamical models most trusted for intensity forecasting, the HWRF has consistently called for TD 4 to develop into at least a strong Category 1 hurricane, while the GFDL has failed to develop TD 4, so the recently upgraded HWRF may end up closer to the mark in this case. Intensity prediction is still very challenging, so it is quite possible that TD 4 could be substantially weaker or stronger by this weekend than the current NHC forecast indicates. Over the last two years, the average 5-day error in NHC intensity projections was around 15 mph, or a bit more than half a category on the Saffir-Simpson Hurricane Wind Scale. This is substantially better than prior years: between 2000 and 2010, intensity errors at 5 days averaged more than 25 mph.

Looking ahead
TD 4 is many days away from any threat to the Leeward Islands. Most of the dynamical track models keep TD 4 moving west to west-northwest at a modest pace. The model consensus keeps TD 4 east of 50°W longitude until this weekend, when a building ridge to the north of TD 4 should help push it at a faster rate toward the islands. By that point, the system would draw on oceanic heat content that gradually increases along its path. It is far too early to predict with any confidence how much of a threat TD 4 might pose to the United States next week. Only a small change in trajectory this far out can have big implications for the track many days from now, and it remains to be seen whether dry air and dust will keep TD 4 from maximizing its potential for development.
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NHC Upgrade to TD4 - BobbiStorm, 8/18/2015, 10:39 am
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