24 August 2006

Debby struggling...

Debby is presently experiencing the coldest SSTs of its journey thus far, around 26C.  There is only minimal convection associated with the storm now, and it looks rather ragged.  Over the next few days, the SSTs will increase again (up to almost 29C), likely allowing the storm to reintensify, and PERHAPS finally reach hurricane status.

skip this part if you're just interested in the headlines...
The SST is an important ingredient in intensity forecasting, but far from the only one.  From a strictly thermodynamic viewpoint (disregarding shear, eyewall replacements, land, dry air entrainment, boundary layer humidity, other moisture-related issues), the outflow temperature is just as important.  If you assume the outflow of the hurricane to be at about 200mb, you can use the temperature there and find the difference between that and the temperature at the inflow layer (sea surface).  It's this difference that controls the lapse rate and instability in the hurricane near-environment.  Using Debby as an example (and taking the diagnosed SST along the forecast track from a statistical model called SHIPS), we get the following:
-- Thursday morning SST: 26.1C
-- Thursday morning outflow temp: -53.0C
-- Thursday morning's DIFFERENCE: 79.1C
-- Saturday morning SST: 28.5C
-- Saturday morning outflow temp: -54.1C
-- Saturday morning's DIFFERENCE: 82.6C
So, in a couple days, the temperature difference between the inflow and outflow of the storm will be 3.5C greater, which in the tropics is a big deal, because lapse rates typically vary VERY little there.  Taking this morning's data and tweaking it a little just to make a point, suppose the outflow temperature were -56.5C... you'd get exactly the same healthy lapse rate as what is forecast for Saturday morning!  In the tropics however, SSTs are of more interest because the outflow temperature typically changes so little.  There are subtleties involved with this explanation, but that's the basics!

The latest intensity estimate (15Z) for Debby is 45kts and 1000mb, and heading WNW at 17kts.  It is located at 20.4N 37.8W, basically in the middle of the basin, and heading for even more open ocean.  It is forecast to slowly strengthen as it turns to the NW then N, recurving by 55W.

The strong tropical wave that was near 11N 55W yesterday is near 12N 61W today, and not organized enough to be upgraded to a tropical depression, but it is still convectively active and showing signs of at least a mid-level circulation, if not low-level as well.  It's tracking W at 15kts and has a 1009mb Low embedded within it.  This could be upgraded to TD5 or TS Ernesto within the next 24 hours, and in the long run, stands a very good chance at reaching hurricane intensity as it cruises through the central and western Caribbean -- low shear, 30C SSTs, and significant oceanic heat content!

Please visit my tropical Atlantic headquarters.

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