Figure 1 shows the latest satellite imagery. The spiral banded structure has recently evolved to an eye feature. The development of an eye usually signifies an intensity which is at least category 1 strength (>74 mph). However as I explained in my previous blog post high latitude tropical cyclones tend to naturally have superior satellite presentations which can give misleading intensity estimates. Nevertheless the development of an eye structure signifies that Numa has become a very powerful medicane indeed, possibly the strongest, at least, since Quessandra in 2014. Numa is also, notably, a very small storm so is likely to have an extremely concentrated wind field. This makes gauging intensity even more difficult. Cloud top temperatures of the ‘eye wall’ are as low as -60C which is consistent with a category 1 storm. We do, now, have the ASCAT data which is very illuminating (see figure 2).
Firstly there can be no doubt that even 4 hours ago that this is a completely tropical storm. Unlike Yesterday’s ASCAT pass that showed an expanded windfield, this has a very clear and fully tropical wind field with a prominent eye and eye wall. The maximum wind speed, at this time, was given as 40kn. The satellite estimate at this time was probably DT3.0-3.5 based on a bit less than a single revolution log spiral which corresponds to around 50kn. If satellite intensity estimates are overestimating by 10kn then that would imply a current intensity of around 55kn which I think is realistic. 55kn is around 63mph making this a very strong medicane indeed.
The forecast for Numa is highly uncertain which is often the case for medicanes; even for very short time periods they are extremely difficult to predict. Medicane Trixie from last year was initially predicted to move south westward and landfall in Tunisia yet instead did the complete opposite and moved eastward to landfall in southern Greece and this was for a forecast only 48 hours before. The same issue is present with Numa. The only thing that can be said with some real certainty is that Numa will probably start moving again soon (at the moment it is almost completely stationary) but different models offer different solutions; the ECMWF takes it south eastward to eventually landfall in Greece on Sunday whilst the ARPEGE has it landfalling in Albania tomorrow. The ‘cone of uncertainty’ for Numa is given in figure 3 below.
I suspect the ARPEGE solution is incorrect and the ECMWF is closer to the mark and the Medicane will track southward or south eastward tomorrow before eventually turning on more of an eastward path to landfall in Greece on Sunday. Nevertheless Italy, Greece and Albania should all prepare for the possible impacts of Numa. Regardless of the exact location Numa eventually landfalls at (unfortunately I think a landfall somewhere as a fully fledged medicane is highly likely) heavy rain and flooding will be an issue for all three of these countries and affect a large area. The effects of wind damage are much harder to predict because the wind field is so concentrated so these problems are likely to be localized but potentially a complete, unpredicted shock for where it does effect. On the other hand, Numa may landfall in a remote area. The strongest hurricane ever, Hurricane Patricia, caused minimal damage as a result of not only its rapid weakening but its exceptionally concentrated wind field; by luck it landfilled in a remote location.
Intensification is somewhat easier to predict. Of course any landfall will rapidly extinguish the storm (and it will die far quicker than a conventional tropical cyclone due to its small size) but if its eye stays away from land then we can expect at least another 15-24 hours of intensification. From then on vertical wind shear is likely to increase to moderate levels which will limit any further intensification but is not expected to cause rapid weakening either. To be honest landfall is likely to have occurred at this point anyway unless the medicane moves much further south than expected.