Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 6:37 AM GMT on July 26, 2013
Tropical Storm Dorian continues to march across the central Atlantic. As of the 0300Z advisory from the National Hurricane Center, the following information was available on the storm:
Wind: 50 mph, with higher gusts
Location: 16.6°N 39.6°W
Movement: WNW at 20 mph
Pressure: 1001 mb
I am having a deja vu. In a fashion similar to Chantal a few weeks ago, Dorian's low-level center appears to be becoming exposed to the west of the convection, confirmed by satellite and microwave imagery. Since there does not appear to be any large-scale vertical shear nearby that would cause this, it is likely that the fast forward motion is creating some westerly to southwesterly shear over the storm. Although convection has been increasing a bit as the cyclone approaches the diurnal convective maximum, both the microwave data and satellite images suggest that this convection is not well-organized, with no evidence of banding features. Interestingly, a 0z ASCAT pass suggested that Dorian retains a very well-defined surface circulation, and I sincerely doubt the inner core has weakened rapidly in just a few hours since the pass.
Figure 1. Latest infrared satellite image of Tropical Storm Dorian. Image credit: NOAA's Satellite Services Division (SSD).
The intensity forecast is quite problematic, to say the least. Water vapor imagery shows dry air to the east and northeast of Dorian, and shows fast upper-level northeasterly to easterly winds associated with the unusually strong subtropical ridge anchored to the north of the system. This could cause an increase in dry air into the inner core, although there is no sign of this yet as I can tell. In about three days, as Dorian approaches the Lesser Antilles, the global models forecast an increase in westerly shear as Dorian approaches the mid-oceanic trough, which is quite well-defined on water vapor images at this time. Considering that the high is not forecast to abate anytime soon -- and may actually strengthen a little, Dorian is unlikely to slow down over the next few days. Given these factors, it is entirely possible that Dorian will lose its status a tropical cyclone at any point during the forecast period, possibly sooner rather than later given current trends. The global models generally do not initialize Dorian as a distinct tropical cyclone, with only the GFS doing so, and even that model sends Dorian southwestward into the eastern Caribbean at longer ranges, all the while losing its status as a tropical cyclone. Given the large uncertainty, I will maintain Dorian as a steady-state tropical cyclone throughout the forecast period.
Dorian remains south of a well-established mid-level ridge, rather anomalously strong for late July. Again, the global models maintain this ridge and even build it westward with time. Compared to 24 hours ago, the model consensus has shifted significantly southward, by at least 150 miles, and there have been even larger shifts within some of the individual global models, particularly the GFS.
With the GFS and ECMWF sending Dorian southward at longer ranges, and the model consensus moving southward as well, I have no choice but to go along with that. There is still considerable uncertain on Dorian's future track, and the models are clearly not handling this system very well.
Should the upper-level winds somehow improve as Dorian approaches the islands, it may very well survive beyond five days, counteracting the speed shear.
It should be noted that, unlike the typical case for tropical cyclones, the deep-layer steering does not differ much from the low-level steering, as denoted by analysis of CIMSS data and the global model forecast steering fields; this attests to the unusually strong nature of the subtropical ridge this year. Thus, Dorian won't gain appreciable poleward momentum even if it miraculously becomes stronger, and vise versa.
Interests in the Lesser Antilles should monitor the progress of Dorian in case it survives as a tropical cyclone.
5-day intensity forecast and positions
INITIAL 07/26 0600Z 17.1°N 40.1°W 45 KT 50 MPH
12 hour 07/26 1800Z 17.5°N 42.5°W 45 KT 50 MPH
24 hour 07/27 0600Z 18.0°N 45.0°W 45 KT 50 MPH
36 hour 07/27 1800Z 18.3°N 50.0°W 45 KT 50 MPH
48 hour 07/28 0600Z 19.0°N 54.9°W 45 KT 50 MPH
72 hour 07/29 0600Z 19.8°N 60.3°W 45 KT 50 MPH
96 hour 07/30 0600Z 20.3°N 65.3°W 45 KT 50 MPH
120 hour 07/31 0600Z 20.3°N 72.7°W 45 KT 50 MPH
5-day track forecast
Figure 2. My 5-day forecast track for Dorian.
Tropical Storm Flossie continues moving toward the central Pacific. As of the latest NHC advisory, the following information was available on the system:
Wind: 45 mph, with higher gusts
Location: 15.1°N 129.0°W
Movement: W at 17 mph
Pressure: 1002 mb
Convection had decreased earlier in the day, but has rebounded over the last couple hours. With the diurnal maximum yet to plateau in this area, Flossie could strengthen a little over the next 24 hours, and my forecast will reflect that. The center is difficult to locate, but appears to be on the southern edge of the convection. An ASCAT pass around 1800 UTC Thursday indicated the low-level center was not well-defined on the south side, with a uniform fetch of southeasterly winds; it is likely that this instrument is having difficulty resolving the large-scale easterly low-level flow within that region, so little weight was placed on it.
Figure 3. Latest infrared satellite image of Tropical Storm Flossie. Image credit: NOAA's Satellite Services Division (SSD).
Flossie is expected to cross the 26C isotherm in about 36 hours, concurrent with a slight increase in westerly shear as suggested by the global models. Thus, a weakening trend is expected by that time. By day five, Flossie will probably degenerate into a remnant area of low pressure as denoted by the guidance.
The tropical storm appears to be moving pretty much due west based on analysis of shortwave infrared satellite images this evening, which would also go well with the latest UW-CIMSS steering map. However, this motion is expected to be short-lived due to an upper low to the northwest of the cyclone, which is creating a weakness in the subtropical ridge to the north. The global models suggest a general west-northwestward motion until Flossie enters the central Pacific, at which time a more westward motion is shown as the cyclone becomes shallower and follows the low-level flow of the trade winds. While my forecast track takes Flossie very near the Big Island of Hawaii at five days, I strongly believe the cyclone will be a remnant low at that time. Regardless of its technical status when it reaches the longitude of the Hawaiian Islands, Flossie is expected to produce locally high surf, heavy rain, and strong gusty winds.
5-day intensity and positions
INITIAL 07/26 0600Z 15.5°N 129.7°W 40 KT 45 MPH
12 hour 07/26 1800Z 15.8°N 132.1°W 45 KT 50 MPH
24 hour 07/27 0600Z 16.5°N 135.2°W 45 KT 50 MPH
36 hour 07/27 1800Z 16.8°N 137.7°W 40 KT 45 MPH
48 hour 07/28 0600Z 17.8°N 142.8°W 35 KT 40 MPH
72 hour 07/29 0600Z 19.7°N 148.3°W 30 KT 35 MPH
96 hour 07/30 0600Z 19.9°N 151.1°W 30 KT 35 MPH
120 hour 07/31 0600Z 19.9°N 154.5°W 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
5-day track forecast
Figure 4. My 5-day forecast track for Flossie.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.
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