Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 3:28 AM GMT on July 24, 2013
A vigorous tropical wave and associated surface low pressure system is located about 150 miles southwest of the Cape Verde Islands. The second tropical wave in a row to defy Atlantic basin climatology this year, satellite, microwave, and scatterometer data show a small but well-defined surface circulation. Convection has increased directly over the center in a small thunderstorm burst. In addition, there is evidence of curved banding to the south, which may attempt to wrap up into the center overnight and increase the overall organization of the system.
Figure 1. Latest infrared satellite image of Invest 98L. Image credit: NOAA's Satellite Services Division (SSD).
Although upper-level winds are relatively favorable, satellite analysis this evening suggests that 98L has begun to move more toward the west-northwest, which is in contrast to 24 hours ago when the motion was pretty much due west; this is probably more related to a local weakness in the subtropical ridge rather than any large scale low pressure system signaling the start of recurvature. There continues to be no evidence that would suggest anything close to what the models were seeing a couple of days ago, when they were pretty much unanimously taking it above 20N before the system even got near 60W.
In the interim, however, the observed west-northwestward motion will take the system over the 26C isotherm in about 18-24 hours; the associated atmospheric stability triggered by those cool waters will likely halt the intensification process. A decent analog for a storm in this area would be Hurricane Bertha in 2008, which also formed in July, and did not immediately intensify due to a similar patch of cooler waters. It took Bertha until around 45 to 50W to begin ramping up as it escaped the influence of those waters, and that is what I would anticipate occurring with 98L, with significant intensification not beginning until then.
The SHIPS model and SST diagrams of the area indicate that 98L may gradually escape the influence of these cooler waters, the waters warming to 27C by 72 hours according to the 0z SHIPS file. If the system can survive the meager thermodynamic environment in the eastern Atlantic associated with the cooler waters, it will have an opportunity to intensify at longer ranges. The GFS is showing considerably less shear than a couple of days ago, particularly if the system remains south of 20N, which based on the guidance and current trends, I am expecting to happen; the 18Z GFS showed very little zonal shear over the system throughout the next five days, and the system only lost its anticyclonic support when it died a painful death over the mountains of Hispaniola.
It is far too early to speculate on the long-range track prospects, but interests in the Cape Verde Islands will experience gusty winds and locally heavy rainfall, even if the system does not become a tropical cyclone overnight. Interests in the Lesser Antilles should also monitor the progress of this disturbance, as it could pay that area a visit on Sunday or Monday, particularly the central and northern islands. The ECMWF and UKMET still suggest a trajectory out to sea, but there have been some hints in the remainder of the models that the ridge will amplify and build westward over the central and western Atlantic in tandem with 98L's westward progression, which seems to be more believable option lacking any real weakness in the Atlantic subtropical ridge at the present time. Given the reliability of the ECMWF, however, the out to sea scenario still cannot be discounted, although I expect this model will come southward over the next couple of days. I personally favor a more west-northwest track over the next 48-72 hours, with a gradual turn back toward the west at those ranges.
Assuming the current rate of organization continues, I expect a renumber in the ATCF file to Tropical Depression Four or Tropical Storm Dorian for the tentative 5 AM NHC advisory. While some have speculated on dissipation of this system amidst the cool waters, lacking any substantial vertical shear to assist the cool waters in dismantling the system, I honestly find the solution of this system dying pretty unrealistic; it'll more likely remain steady state for the next few days before reintensifying beyond 72 hours.
Probability of development in 48 hours: 80%
A broad area of low pressure over the Pacific located about 900 miles south-southwest of the southern tip of Baja California has not become any better organized over the last 24 hours. The circulation still appears to be poorly-defined, with no evidence of northerly or westerly winds along the western half of the circulation that would signify the beginnings of a closed surface circulation. There is more convection today than yesterday, but a lot of it appears to be associated with upper diffluence in connection with an upper low to the northwest, and the convection exhibits little curvature, and is actually rather shapeless at the moment.
Figure 2. Latest infrared satellite image of Invest 98E. Image credit: NOAA's Satellite Services Division (SSD).
Nevertheless, there are finally some signs that the storm-relative vertical shear is beginning to decrease a little, and the system could still develop a bit before it reaches cooler waters beyond 48 hours. I do not anticipate 98E ever becoming a tropical cyclone.
98E is not a threat to land, and the aforementioned upper low will take it a on west-northwest trajectory into the central Pacific, where it will ultimately dissipate.
Probability of development in 48 hours: 30%
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.