#3727 Postby NEXRAD » Thu Sep 04, 2008 8:51 am
Hanna the "Ghost" Storm
Late Wednesday dry air intruded into Hanna's SW quadrant and subsequently filtered NE into the storm's convectively prefered eastern regions. Since then the dry air has greatly disrupted the storm, and newly formed deeper cells situated mostly well east of the low level circulation have shown limited longevity and very poor organization. Since about 11Z, new cells have begun firing nearer the storm's circulation center, but only one of these is showing much for deepening. With favorable divergence aloft, convergence near the surface and low level center, and more limited shear directly over Hanna's low level position, this convection has some opportunity to reorganize close to the storm center through midday.
The two biggest factors working against Hanna this morning are: 1) the ULL to the storm's SW, and 2) shear N of the storm.
Per IRWV, the upper low is situated farther east than was analyzed by the 12Z CIMSS or RUC/2. The ULL is situated just southeast of Andros Island, and this upper level feature has been pulling moisture northwest away from Hanna and has been dredging dry air Nward into the tropical storm. You can tell that the ULL is controlling the moisture fields near Hanna by the moisture band moving towards Florida (as of 9AM ET). The moisture band's curvature suggests that it is organized about the upper level feature. The shear generally north of Hanna is also helping to strip the tropical storm's moisture aloft, and on IRWV deep moisture formerlly associated with Hanna is streaming quickly north, now ahead of the trough over the West-Central Atlantic. A final possible longer-range factor is Ike; on the IRWV subsidence ahead of Ike seems to be eroding moisture fields NNE of Puerto Rico. Granted, this possible Ike-effect is likely quite minor compared to what's going on with the main factor, which is the ULL.
Two scenarios are likely for Hanna today. 1) The ULL pulls more west, as indicated by most guidance, and the resulting reduced influence on Hanna allows the tropical storm to rebound, or 2) Hanna's circulation becomes (or remains?) less influential over the deeper pattern over the SW Atlantic, which prompts the moisture fields to continue to organize more around the upper low and trough axis. This second scenario would yield a weaker tropical storm, and one that has more limited opportunity to restrengthen.
As for intensity, ship and buoy reports indicate sustained winds of 35 to 40 knots are still present at surface, though some of this might be a result of the tight pressure gradient between Hanna and a surface ridge to the system's north. The AM profiler over Nassau shows 50-knot winds in the mid-levels, but some of this might be related to shear rather than the TS.
- Jay
Last edited by
NEXRAD on Thu Sep 04, 2008 8:54 am, edited 1 time in total.
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I agree. Hanna may not be done yet, but comparing it to Hugo and Andrew? I mean, your body shakes just to hear those names.