Reminder: the information and illustrations on this site are NOT INTENDED to protect property and people from cyclonic risks. For this, you must follow the information and recommendations of your prefecture for France and the competent authorities for other territories.
This article is not up to date. A more recent is here
THEInvest 98L continues on its way towards the West Indies, followed like its shadow by a second disturbance. It has been envisaged in the last 48 hours that these 2 systems could merge to create a larger and stronger one. The question then was which one was going to “eat” the other. So far, none of that has happened and to be honest, that's great news. Two such close systems interact widely with each other, which quite upsets the organization of each. In the end, neither of the 2 has a real well-defined internal circulation. This is confirmed by the latest ASCAT images (wind analysis by satellite / scatterometer or radar scatterometer) which show the poor organization of these 2 disturbances which seem to be “linked” by part of the ITCZ as shown in the analysis map opposite. on the other hand these same readings show stronger winds with 30 kts, especially on the 2nd system.
The slower build-up than expected despite rather favorable local conditions (I will talk about regional conditions at the end of the article) is good news. However a system with a rather high pressure (1009 hPa in this case) cannot easily push back high pressures to its north, which could lead the assembly to pass a little more south than envisaged at the longitude of the West Indies. We also note that theInvest 98L is still below 12N, which is more southerly than envisaged in recent days at this longitude. In general, these situations are a bit delicate when the conditions are favorable, since a late reinforcement in the immediate vicinity of the arc cannot be excluded. As long as it's depression, it's not too much of a problem, just don't get stronger at the wrong time.
Pour it part, le nhc balanced the risks of reinforcement at 48 hours and 5 days at most, ie 90%. They therefore bet on a classification in tropical depression by the passage to the longitude of the arc. Otherwise they do not get too wet as usual in these conditions. One can reasonably think that if the organization does not allow a passage in depression tomorrow there will be an intermediate classification in “potential cyclone” to initiate the procedures of warning on the north of the arc.
In the end, the models (apart from HWRF and known SHIPS) do not provide for reinforcement beyond the depression stage and a passage on or near the north of the arc. This is also the position of Météo France, which follows the forecasts of its Arôme and Arpège models, which are still very close to the European model. ECMWF extension. And so that also seems to be the position of the nhc. Almost everyone agrees and that's a very good sign. I am even quite astonished by this almost perfect consensus when the situation is extremely complex and could lead to diverging analyzes. Let us not forget, suddenly, the necessary (even imperative) caution in this kind of situation, with a rather imposing disturbance which arises in front of the West Indies.
I maintain 2 daily points unless there is a particular change.
EDIT: the last satellite image seems to show the first system in a strong convection boost while the second is a little off .. it may be nothing but it needs to be monitored.
I will quickly come back to this question of favorable conditions, which are often quite poorly understood. I will write a dedicated article in the coming weeks on the subject but I will summarize here (it's a bit complicated to summarize by the way). Basically there are 3 types of conditions; the Global (oscillation ENSO), Regional (MJO type oscillations, Kelvin waves and Atlantic oscillation) and finally Local (water temperature, air humidity and shear for the main ones). Oscillation ENSO is slow and mostly covers the entire season. We are in a neutral phase and it is quite stable. At the local level, we have seen that they are rather favorable to strengthening with a shear low, hot water and dry air pushed back by the strong convection of the 2 systems. On the other hand, in terms of regional conditions, it is less favorable. The Madden Julian Oscillation (MJO) which travels the globe from west to east is currently over the Eastern Pacific (this probably partly explains the very rapid strengthening of Hurricane Genevieve), which somewhat limits the updrafts on our pool. But it should pass through the Caribbean and the Atlantic from next week, which could add a layer favorable to cyclonic development. Another element could be added to this favorable situation, it is a Kelvin wave (atmospheric disturbance). These 2 elements are likely to significantly change the atmospheric circulation in the Atlantic basin and promote the emergence of more powerful cyclonic systems. For the moment, we are benefiting from regional conditions less favorable to strengthening, which may partially explain the fairly stable situation of theInvest 98L. Sorry for the complexity of this paragraph… but I will come back to it in more detail.
Image credits; Tropical tidbits, NHC / Noaa, UW-CIMSS
