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The West Indian Arc on the verge of a nervous breakdown
Social media saw another episode of weather fever last week. The reason is always the same, namely access to specific data which is misused. Some weather pages on social networks publish information at an extremely long timeframe without informing that the forecasts at this time are more than uncertain. These publications are taken at face value by an audience that is often unable to take stock and which tirelessly shares its concerns. In the end, we had another weekend totally irrelevant vis-à-vis an unproven threat.
It is becoming a real problem whose frequency has only increased for 2 or 3 years. And of course, when you're in an unusually active season like this year, it all takes on crazy proportions. It is everyone's duty to limit as much as possible the sharing of information that is not assessed. A gross model output at 180h is not information. It is a simple hypothetical data which is only of interest if it is confronted with other data. When it is used in affirmation in an alarmist tone (or the reverse in a “bisounorus” tone) it becomes fake-news which can have important consequences.
Understand the NHC charts to take a step back on the shares
Almost all of the people on social networks who live in a hurricane zone know the site of the nhc and its famous graphics. But how many know the specifics of each graphic representation? However, it is very important to understand what each zone corresponds to so as not to panic unnecessarily and not to panic the others.
As long as a disturbance is not classified as a cyclone (depression, storm or hurricane) it can be monitored by the nhc. It is assigned 3 specific data:
- A risk of cyclonic strengthening at 48 hours
- A risk of strengthening at 5 days
- The 5-day strengthening risk zone.
In the image on the left we see the monitored area leaving Africa. A risk of cyclonic strengthening of 14% at 50h and 48% at 70h is assigned to him on the 120pm bulletin. An area (red hatch) is defined in which reinforcement is most likely. This zone is not a cone of uncertainty! There is as much risk that the classification will be done in one place as in another. This zone is defined according to the speed of movement, and its current trajectory. If there is a consensus among the major models, the zone will take this trajectory consensus into account. In the absence of consensus, the area will be quite linear. It was the case for Teddy. The forecast zone, in the absence of consensus, was stretched towards the West Indies (as in the image above). As soon as it was classified as a depression, the zone was replaced by a trajectory and a cone of uncertainty with a largely marked inflection towards the north. In 10 minutes we went from a rectilinear area to a curved trajectory. There was no difference in forecasting between the 2, just a difference in method.
I repeat, it is very important to understand how it works to avoid worrying unnecessarily.
Finally, you will notice that the nhc only gives forecasts to 120 hours maximum. For disturbances, the zones are for the next 5 days and for cyclones the cone of uncertainty is at 120h. This is quite simply because the uncertainty beyond this deadline is too great to make relevant forecasts. Take this into account when you see a post that says anything beyond this deadline.
The Atlantic is still very rough
Even though we have passed the statistical peak of the season, the Atlantic basin does not seem to be particularly concerned. 2 hurricanes, 2 storms (one of which, Teddy, should quickly turn into a hurricane), 2 Invests and a monitored disturbance. No less than 7 disturbed areas monitored by the nhc for almost a week, it's totally new. The fact that we are at the letter V on September 15, and therefore only 1 letter (W) from the transition to the Greek alphabet is also a great first. During the 2005 season, the most active before this one, the name Alpha was used on October 22. In short, 2020 will probably go down in history as the most active season since 1850 in terms of the number of cyclones.
Note that the nhc monitors a disturbance on 45N. I don't remember having seen such a situation in the past, but the warming of the water in the Atlantic may make these situations more frequent in the years to come.
We also need the humility to assume that, for the moment, we have been extremely lucky in the West Indies. It is likely that without Hurricane Paulette to its north, Teddy would have threatened the West Indies much more… in warmer water. So let's be happy with this situation but let's not minimize the reality of the activity of the season.
The Invest 98L
A new disturbance out of Africa this WE is gradually being organized on the Atlantic. The nhc gives it a 70% strengthening risk at 5 days. For the moment neither the GFS nor theECMWF extension do not develop this disturbance. The GFS sees a small reinforcement in depression before a dissipation on a fairly NW trajectory (but with a great disparity between its members (sub-models) while theECMWF extension doesn't see much going on.
In the other models, it is essentially the CMC (global model) and theHWRF (regional model) which develop it into a cyclone. The CMC on a very northern trajectory (close to Teddy's) and theHWRF on a more tense course towards the west.
For the moment, its speed of movement of less than 20 km / h does not allow to have a precise vision of its future vis-à-vis the West Indies. The models are quite divergent in their 120-hour forecast, a bit like Teddy and for the same reasons in terms of trajectory. As I explained in a previous article, the presence of a hurricane in the NW poses a lot of problems for the models because the slightest deviation in speed or trajectory of the first can have significant consequences on the trajectory of the second.
I would do a specific point on it if it is classified as a cyclone.
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Image credits; Tropical tidbits, NHC / Noaa, UW-CIMSS
