Tropical weather analysis - August 12, 2012
A tropical wave producing minimal shower activity continues over the eastern Atlantic about 500 miles west-northwest of the Cape Verde Islands. Save for a small area of convection west of the wave axis, 93L is devoid of significant shower activity within the large circulation.
Figure 1. Latest infrared satellite image of Invest 93L. Image credit: RAMMB
Upper-level winds are currently only marginally favorable for development -- UW-CIMSS analyses reveals 20 knots of shear over the disturbance. Additionally, the system remains over rather cool sea surface temperatures, although they are expected to gradually warm up as 93L gains longitude over the next few days. Some of the global models suggest long-term potential for redevelopment as the system recurves well east of Bermuda, but I am wary of that at this time given the persistent subsident pattern over the central Atlantic. However, this wave still retains a rather vigorous mid-level circulation, so the potential for long-term development is definitely there. I just don't see the dry air getting out of the way.
Model guidance suggests the onset of recurvature beginning on Tuesday near 50W. Indeed, a weakness in the low- to mid-level ridge is already evident there.
Probability of development in 48 hours: 10%
Remnants of Tropical Depression Seven
Tropical Depression Seven dissipated around midday on Saturday just as it was approaching the Windward Islands. Most likely it dissipated before then, but nothing beats state of the art reconnaissance aircraft. In any event, the depression, or more accurately the remnant tropical wave, is currently lashing portions of the central and northern Leeward Islands with heavy rainfall and strong gusty winds. Satellite and surface observations indicate that the surface wind field associated with the tropical wave is extremely disorganized, with hardly even a wind shift noted in the surrounding islands. Earlier scatterometer data also suggested this, with mostly northeast winds associated with the disturbance. The observations also indicate that surface pressures are not falling at this time. However, island reports indicate that this system is producing wind gusts to tropical storm force in heavy squalls.
The system is currently battling about 40 knots of southwesterly shear according to UW-CIMSS. This shear is being induced by an upper low centered within the mid-oceanic trough to the north of the system. The GFS does not see this shear decreasing until around Monday evening, when the system will be approaching Jamaica. Afterward, the wave should encounter a break in the western Atlantic ridge, which should cause a consequent slowdown. The question after that becomes: where will the tropical wave go next? That is where the models express their respective disagreements. For the last two runs, the GFS has been moving it into the central Gulf of Mexico, and then developing it into a tropical cyclone. The Euro had a similar solution at 12z, albeit much weaker and farther south. The remainder of the models take it into Central America and/or the Bay of Campeche. The GFS is particularly good at sniffing out storms in the long-range, and one week isn't that far out, so this needs to be watched.
For now though, the former depression poses little threat to develop, as environmental conditions over the eastern and central Caribbean are not favorable for tropical cyclogenesis. The wave should continue rapidly westward at about 20 mph for the next day or two, and begin to slow down after that.
Probability of development in 48 hours: 10%
Tropical Storm Hector formed just after 0000 UTC today from what was once Hurricane Ernesto. While this tropical cyclone was technically related to Ernesto, based on a revised naming policy instituted by the National Hurricane Center, if a tropical cyclone dissipates -- even if it retains some semblance of coherence on satellite or surface data -- should it form into another tropical cyclone in a secondary basin, it will garner a new name. This was the case with Tropical Storm Arthur in 2008, which formed from the mid-level remnants of Eastern Pacific Tropical Storm Alma. Tropical Storm Hermine in 2010 formed in a similar manner.
As of the latest NHC advisory, the following was posted on Hector:
Wind: 40 mph, with higher gusts
Location: 18.5°N 108.1°W
Movement: WNW at 13 mph
Pressure: 999 mb
The center is located along the eastern edge of a rather large ball of deep convection, as indicated by satellite and microwave fixes. This is due to about 20 kt of easterly shear induced by a ridge over the New Mexico/Texas border.
Figure 2. Latest infrared satellite image of Tropical Storm Hector. Image credit: NOAA
Given that the shear is forecast to continue, conditions do not appear supportive of significant intensification, and Hector is not expected to become a hurricane. The large size of the tropical storm is also an issue. By Wednesday, Hector is expected to begin moving over cooler water, although the SHIPS guidance suggests that it will still be over 27C waters by Wednesday evening. This shouldn't be significant to disrupt Hector unless it miraculously becomes a hurricane. By Thursday, a weakening trend is forecast to become well-established.
Water vapor and synoptic data show that Hector is south of a mid-level ridge. However, a weakness is evident in this ridge near 120W, and this is apparently what is causing the models to forecast a slowing of the forward speed over the next few days. Here I am faced with another forecast of considerable uncertainty. While the models pretty much agree on a slowing of the forward speed, substantial differences arise in the timing of this slowdown, as well as the ultimate track of the cyclone. The GFS and ECMWF keep Hector well offshore Baja, while the CMC shoots it into California. The latter scenario is considered unrealistic, and is likely reflective of a stronger system being pulled more strongly to the north. The GFDL also suggests a more northward track, but again, this is likely predicated on the idea of a stronger system. Since Hector is not expected to become a hurricane, it is not currently expected that any significant northward motion will occur in the long-term. Given the usual reliability of the GFS/Euro, I will go with a consensus of those models to mold my forecast track.
While Hector is expected to remain well offshore southern Baja, high surf and rip currents will likely afflict the southern portions of that peninsula for the next few days.
5-day intensity forecast
INITIAL 08/12 0600Z 35 KT 40 MPH
12 hour 08/12 1800Z 40 KT 45 MPH
24 hour 08/13 0600Z 40 KT 45 MPH
36 hour 08/14 1800Z 45 KT 50 MPH
48 hour 08/15 0600Z 50 KT 60 MPH
72 hour 08/16 0600Z 60 KT 70 MPH
96 hour 08/17 0600Z 55 KT 65 MPH
120 hour 08/18 0600Z 45 KT 50 MPH
5-day track forecast
Figure 3. My 5-day forecast track for Hector.