Tri-agency Forecast Discussion September 2, 2010

Forecast Discussion for September 2, 2010

Large-scale features

The primary features of interest in the Atlantic basin are Hurricane Earl, Tropical Storm Fiona and Tropical Depression Gaston as are shown on this morning‚ NHC Graphical Tropical Weather Outlook page (Fig. 1). These features are also evident as areas of high total precipitable water (TPW) as depicted on the CIMSS TPW image valid for1200 UTC 2 September (Fig. 2). This figure shows the TPW associated with 4 pouches: PGI34L (Hurricane Earl), PGI36L (Tropical Storm Fiona), PGI38L (Tropical Depression Gaston), PGI39L, and PGI40L. Although most of the aforementioned systems appear to be presently situated in regions of fairly high TPW, regions of relatively low TPW are presently located to the west-southwest of PGI38L (Tropical Depression Gaston) and west of PGI39L. These regions of lower TPW can also be seen in the CIMMS Saharan Air-Layer imagery from this morning‚ 1200 UTC imagery (Fig. 3). The primary upper-level feature depicted in the CIMMS upper-level wind analysis (Fig. 4) from this morning is a large anti-cyclone over and to the east of Hurricane Earl that is producing relatively strong northeasterly flow over Tropical Storm Fiona which appears to be inhibiting this systems development. The upper-level flow over Tropical Depression Gaston is moderate and no longer diffluent and appears relatively unfavorable to additional development.

Hurricane Earl

At 11 AM EDT Hurricane Earl was located near 30.9 N 74.8 W (Fig. 5) with 120 kt maximum sustained winds and was moving north at 15 kt. The early model track guidance from 1200 UTC this morning are in fairly good agreement and show a north and then north-northeastward motion for the next 48 h followed by a northeast motion with an increase in forward speed beyond that time frame (Fig. 6). The intensity guidance (Fig. 7) is also in fairly good agreement indicating that Earl will likely start to weaken as it moves northeastward into regions of lower SSTs, lower instability and higher vertical shear. This morning‚ official NHC forecast brings Earl very close to the outer banks of North Carolina by late Thursday to early Friday morning as a Category 3 Hurricane before heading northeast and passing very near to Cape Cod by late Friday to early Saturday morning with an intensity somewhere between 70 and 100 kt.

Tropical Storm Fiona

At 11 AM EDT Tropical Fiona was located near 24.4 N and 65.8 W (Fig. 8) with maximum sustained winds of 45 kt and was moving north-northwest at 15 kt. Nearly all of the track guidance (Fig. 9) show Fiona continuing on a north-northwest track for about 48-72 h before turning more to the north and northeast with an increase in forward motion beyond about 48 h. BAM Deep takes Fiona toward the southwest after 12 h but this does not seem to be a likely scenario. As it continues this track it will only gradually distance itself from Hurricane Earl and its restrictive outflow. The intensity guidance (Fig. 10) indicates that Fiona should begin weakening to minimal tropical storm strength before perhaps re-intensifying near the end of the forecast period. The official NHC forecast calls for Fiona to move northward, passing close to Bermuda late Friday or early Saturday and weaken slightly before moving more northeastward and weakening to a tropical depression by 72 h.

Tropical Depression Gaston

At 11 AM EDT Tropical Depression Gaston was located near 14.0 N and 38.9 W (Fig. 11) with maximum sustained winds of 30 kt and was moving to the west at 6 kt, although from satellite loops it now appears almost stationary. Most of the 1200 UTC early model track guidance (Fig. 12) shows Tropical Depression Gaston moving to the west-northwest over the next 120 h. The intensity guidance (Fig. 13) generally show Tropical Depression Gaston strengthening to a tropical storm even a hurricane during the 120 h forecast period; the LGEM and SHIPS models suggest that Tropical Depression Gaston could attain minimal hurricane intensity in 72 to 84 h. However, we have witnessed a serious collapse in the convection near Gaston and these forecast seem too aggressive. It is understandable given that the SHIPS model guidance indicates that the upper ocean conditions (SST) should become increasingly favorable and the shear should remain relatively low throughout the forecast period (between 6 and 15 kt). T he atmospheric thermodynamic conditions will likely not be particularly favorable for intensification with relatively dry stable air surrounding the system and entering from the west. The official NHC forecast indicates that Tropical Depression Gaston should move generally westward over the next few days while strengthening to a tropical storm with 50 kt winds by 48 h and possibly an 85 kt hurricane within 120 h.


According to this morning‚ pouch products, PGI39 was centered near 12 N and 18 W (Fig. 2). The vorticity associated with PGI39L is rather elongated (Fig. 14) and the system is currently embedded in a region of fairly low shear (5-10 kt), so the prospects for this system have improved, but moving out over the marine environment west of Africa is a critical change. The pouch product output indicates that the GFS is able to track the system out to 120 h (Fig. 15) at which time it is relocated near 15 N and 28 W so it is possible that this system could be viable target for research flights sometime early next week.


This system now appears to be the northward part of PGI39L and tracks rapidly northwestward, making it of little interest for research flights.

Sim Aberson, Altug Aksoy, and Neal Dorst