Status Status:Ongoing analysisFinalised Material from: Linus, Ivan, Mohamed
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During different stages of Hermine, extensive dropsonde missions took place , e.g by using the NASA Global Hawk platform. The different plots below show the observation statistics for dropsondes for temperatures and wind vector difference for the days with most dropsondes. The panels in each plot shows the first guess departures (obs-fg, right) and analysis departures (obs-an, left). The top row is for all dropsonde observations and the bottom for used observations. Each panels include averaged statistics in the title bar. In all but one of the plot, the standard deviation of the departures is decreased from the first guess to the analysis.
25 August 12z
30 August 00z
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The plots below show the tropical cyclone activity for 31 August to 1 September. Already in the earliest forecast presented here (from 24 August, bottom-right plot), the probability for a tropical cyclone in the northern part of Gulf of Mexico was above 20% and in the forecast from 25 the probability increased to above 50%. However, in the subsequent forecasts the probability decreased again. This change was connected to the whether the cyclone should form already in the Atlantic, which it did not. But later Hermine formed in the same tropical depression.
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The intensification of Hermine that was in early forecast once it starts moving over the open sea Atlantic did not materialize. When Hermine made landfall on Friday, 2 September, it lost a lot of its intensity and more or less it's kept it later when it moved again over the ocean whilst some early forecasts suggested intensification especially the HRES. The good point is that the reported core pressure falls within the ENS range although at the upper tale of ENS distribution. It's also worth mentioning that the ensemble spread was quite large and the HRES was also within the range of the ensemble but at the lower tail. Regarding the track, Hermine moved north-eastwards at some point it changed the direction approaching the north-east US coast. That turn in thr the direction was given by a number of ensemble members as well.
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Two other global prediction centres, UKMO and NCEP, also provided good short-range forecasts for the rainfall after the landfall. It's worth mentioning that the resolution of HRES is high enough to capture structures in the rain field which are more realistic than lower resolution global models. In the very short-range forecast (T+12-36h) we can spot the eye of the cyclone with less rain than the surrounding area and spiral structures that were also observed although with some location errors.
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5. Good and bad aspects of the forecasts for the event
- Early capturing of the movement of the tropical depression.
- Forecast too confident about cyclogenesis in the Atlantic before Florida, connected to the convective system?
6. Additional
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