Sunday, April 19, 2009

Plans for 2009

ESTOFEX has had a meeting in Prague on March 27-28th 2009. There, ESTOFEX members have discussed several subjects, including a number of ideas that already existed for a few years. The most important issues were:

1) Excessive rainfall forecasting
2) Probabilistic forecasting

Excessive rainfall forecasting

Of all threats associated with thunderstorms, flash flooding causes the most casualties and can produce a lot of damage. We have decided to start including convective rainfall threat in our "threat level" system and will be focusing on the meteorological aspects only, since we cannot practically take into account local hydrological conditions. We will only consider convective events to be part of our forecast job. Like severe wind gusts, which can also occur in non-convective situations, we will use the presence of lightning near the place and time of an event to determine whether the event is convective.

The criterion to identify whether convective rainfall was severe has been set at 60mm in 3 hours (and, as said, lightning must be detected). The difficulty, of course, is that there are many different ways precipitation is reported, such as "flash flooding occurred", "90mm collected over 24 hours", or "20mm in just 10 minutes". Our criterion is chosen so that we can exclude the short duration events with small sums and focus on the significant events. We will not use an "extremely severe" criterion for precipitation as we do with the other types of severe convective weather.

With the addition of excessive rain to threat levels, they are likely going to be issued more often. Because there can be several reasons for issuing a threat level, we will, starting May 1st. 2009, add phrases above the forecast text indicating the primary severe weather threat for each threat level area.

Probabilistic forecasting

Thunderstorm areas

Our current forecasts for thunderstorms consists of one line (yellow), which, in the ideal case, separate the areas where thunderstorms will occur and where they will not. In practice, of course, thunderstorms do not always occur everywhere where the regional scale conditions are thought to be favourable. So, if you find yourself in a thunderstorm area, the chance that you will experience a thunderstorm is much lower than 100%. Similarly, outside the thunder areas, the chance is higher than 0%. Forecast verification has shown that on average, in central Europe in the summer season, the true chance to get lightning within a radius of 40 km of your location when you are included inside the area is about 50%. In northwestern Europe this chance is less than 20%.

We have decided to start drawing two lines for the chance of thunderstorms, in the near future. This will allow us to better characterize the true chance of storms. a thin yellow line will indicate where we forecast a low spatial coverage of thunder, or when we have lower confidence in the development of storms, while a thicker yellow line will indicate where confidence of higher storm coverage is large. This change will probably be implemented later in 2009.

Severe weather threat areas

The severe weather forecasts will, starting May 1st, also have a well-defined probabilistic interpretation. The lines will namely represent a chance percentage of severe weather occurring within 40 km of a point on the map. Using the data from the ESSL European Severe Weather Database, Pieter Groenemeijer has conducted a verification of severe weather for our levels. As a part of this procedure, the number of times severe weather occurred inside the different level areas was determined. With this information the threat level lines will now be labelled with a percentages indicating the probability of severe, or extremely severe weather (details in the FAQ soon).

A problem ESTOFEX faces is, however, that the reporting of severe weather to ESWD is still very low in certain areas of Europe, so that we basically had to rely on verification statistics from the part of the forecast area of which most data are available. Put briefly, we need your reports! So, to support ESTOFEX, please, submit severe reports to the ESWD. Note that an ESWD report has only five required fields: DATE, HOUR, LOCATION, DESCRIPTION, and SOURCE.

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