Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 6:24 AM GMT on July 25, 2013
Tropical Storm Dorian continues to move across the open Atlantic Ocean. As of the most recent (0300Z) advisory from the National Hurricane Center, the following information was available on the cyclone:
Wind: 50 mph, with higher gusts
Location: 15.1°N 33.2°W
Movement: WNW at 20 mph
Pressure: 1002 mb
The satellite signature of Dorian is fairly impressive, especially considering the marginal sea surface temperatures that the cyclone is traversing. An AMSUB pass caught the circulation just before 0z, and showed tightly-coiled banding, but no evidence of an eye or eyewall; unfortunately, there have been no recent reliable microwave passes since then. Conventional satellite images also suggest that Dorian is a small tropical cyclone that possesses tightly-wound banding features.
Figure 1. Latest infrared satellite image of Tropical Storm Dorian. Image credit: NOAA's Satellite Services Division (SSD).
Since Dorian has a rather large moisture envelope and an accompanying upper-level anticyclone, it appears that it will effectively ward off the otherwise detrimental influence of the cooler underlying waters; in addition, the relatively fast forward speed of the tropical storm suggests that cold water upwelling -- normally quite significant over this area of the Atlantic -- will not be a factor. Given this, the most appropriate forecast at this time seems to be to forecast a steady-state storm until water temperatures rebound to 27C near the 72 hour mark. Beyond that time, some slight reintensification is shown, although I am choosing to be conservative with the intensity at longer ranges, since water vapor images still show a rather large area of dry air, albeit propagating westward in tandem with Dorian, over the central Atlantic; also, the global models still forecast an increase in westerly shear during that time as Dorian nears the mid-oceanic trough. Given these factors and the small size of the storm, Dorian could easily dissipate or be a hurricane at the end of the forecast period.
Dorian is embedded within a well-established mid-level ridge over the central Atlantic. This ridge is forecast to elongate and build westward ahead of Dorian during the next several days, which is not expected to cause immediate recurvature. Whether or not Dorian ultimately makes United States landfall at longer ranges is still extremely uncertain, and we probably won't have a better idea for at least another four or five days. The global models show a large and well-defined upper low amplifying over the Great Lakes in about 96 hours, which is anticipated to weaken the western extent of the subtropical ridge over the western Atlantic. However, that feature appears very fragile, and will not be enough to recurve Dorian. A second piece of shortwave energy is forecast to dive from Canada into the northern United States beyond 7 days, which will reinforce the weakness in the ridge. It is this secondary shortwave pulse that we will need to monitor over the coming days, as the amplitude and progression of that feature is critical to determining when, where, or if Dorian makes United States landfall at longer ranges, ranges that go far beyond the scope of my 5-day forecast.
At this time, the pattern seems more supportive of an east coast storm than a Gulf coast storm, and I am reminded of Hurricane Floyd in 1999, albeit at much lesser intensities.
Regardless, the Lesser Antilles are first in line, and, although Dorian is forecast to pass to the north of those islands and Puerto Rico, interests in those areas should continue to monitor the progress of this tropical cyclone.
5-day intensity and forecast positions
INITIAL 07/25 0300Z 15.1N 33.2W 45 KT 50 MPH
12 hour 07/25 1200Z 15.7N 35.1W 45 KT 50 MPH
24 hour 07/26 0000Z 16.3N 39.8W 45 KT 50 MPH
36 hour 07/26 1200Z 16.8N 43.5W 45 KT 50 MPH
48 hour 07/27 0000Z 17.5N 46.4W 45 KT 50 MPH
72 hour 07/28 0000Z 18.9N 53.7W 50 KT 60 MPH
96 hour 07/29 0000Z 19.5N 59.7W 50 KT 60 MPH
120 hour 07/30 0000Z 19.7N 65.8W 50 KT 60 MPH
5-day track forecast
Figure 2. My 5-day forecast track for Dorian.
A non-tropical area of low pressure is located several hundred miles east of Bermuda. Scatterometer and satellite data suggest that the circulation is poorly-defined, but there is a healthy burst of convection ongoing, likely assisted by upper-level diffluence associated with a small and elongated upper low to the west.
Figure 3. Latest infrared satellite image of Invest 99L. Image credit: NOAA's Satellite Services Division (SSD).
Despite the unimpressive nature of the circulation, waters appear sufficiently warm for some development during the next 24-36 hours, and there appears to be no significant vertical shear over the system; in fact, the upper flow might actually be southerly in connection with said upper low, a pattern which could support development of a northward-moving system like 99L.
This low is forecast to become absorbed in a frontal zone in about two days.
Probability of development in 48 hours: 20%
Tropical Depression Six-E
Tropical Depression Six-E formed on Wednesday evening from what was previously Invest 98E. As of the latest NHC advisory, the following information was posted on the system:
Wind: 35 mph, with higher gusts
Location: 15.2°N 122.0°W
Movement: WNW at 14 mph
Pressure: 1005 mb
An ASCAT pass from around 1800 UTC Wednesday showed winds already in excess of tropical storm force, so perhaps the NHC will recognize an earlier classification of this system during post-season analysis.
Satellite images show a large and expansive burst of very cold convection, with the center presumed to be roughly in the center of this activity.
Figure 4. Latest infrared satellite image of Tropical Depression Six-E. Image credit: NOAA's Satellite Services Division (SSD).
There still appears to be some northeasterly shear over the system, as the outflow pattern appears squashed in that direction. Other than that, conditions appear favorable for some strengthening, and the depression is forecast to become a tropical storm very soon, if it is not already one. If the system had a little more time, it would probably become a hurricane, but alas, the cyclone is forecast to move into colder waters in about 36 hours, so steady weakening is anticipated beyond that time. The models show westerly shear increasing as the system crosses the central Pacific on Saturday, so the weakening I am projecting is a little more rapid at days four and five than that of the National Hurricane Center.
Although the global models show this system reaching the Hawaiian Islands early next week, waters are not going to warm along the current forecast track, so if anything does reach that area, it is very likely to be nothing more than a showery remnant low capable of disrupting the local trade wind flow.
The dynamical models are in good agreement on an undulating west-northwestward track for the next 72 hours, followed by the possibility of a more westward turn beyond that time as the system comes under the influence of the low-level flow as it becomes a vertically shallower system.
5-day intensity and forecast positions
INITIAL 07/25 0300Z 15.2N 122.0W 30 KT 35 MPH
12 hour 07/25 1200Z 15.5N 123.5W 40 KT 45 MPH
24 hour 07/26 0000Z 16.1N 127.1W 45 KT 50 MPH
36 hour 07/26 1200Z 17.3N 130.5W 50 KT 60 MPH
48 hour 07/27 0000Z 18.7N 134.7W 45 KT 50 MPH
72 hour 07/28 0000Z 20.1N 140.0W 40 KT 45 MPH
96 hour 07/29 0000Z 20.7N 144.2W 30 KT 35 MPH
120 hour 07/30 0000Z 20.8N 149.2W 25 KT 30 MPH
5-day track forecast
Figure 5. My 5-day forecast track for TD Six-E.
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