Impacts and likelihoods of rainfall from storm Dennis

It is a good job I documented how the warnings issued for the heavy rain for Dennis have changed during the last few days because it’s quite interesting to see what the Met Office have done. In their initial amber warning for heavy rain from storm Dennis for this weekend issued on Thursday it was for four distinct areas of England and Wales, with highest expected totals in a few location of 120-140 mm. The likelihood of this happening was set at level three as was any likely impact from it.

Thursday’s original amber warning

Yesterday they split off the amber warning for Wales and lowered the highest accumulations to 100-120 mm. They also dropped the likelihood of it happening to level two, but increased the likely impacts to level four.

Warning issued ~1030 UTC on Saturday

Early this morning they issued a red warning for a subset of the amber warning area to cover for the hills above the Welsh Valleys and upped that to totals of 100-140 mm, the same as the original amber warning. At the same time they also increased the likelihood and any likely impacts to four and triggered a red warning.

Red warning issued Sunday 0610

The question occurs to me why didn’t the Met Office issue a red warning yesterday afternoon when they seemed to tone down their warning?

  • Surely the likelihood of it happening must have been higher than level two on Saturday morning if not, certainly by late afternoon?
  • What do they mean by rainfall totals?

To verify any warning you need to know what time period the warning is for. All that you can assume is that it’s for the time period of the warning, in this case 1200 UTC on Saturday till 1500 UTC today, and that’s why in all my radar rainfall estimates that’s the period I use in my analysis. I notice in the latest red warning they use the phrase ‘event totals‘. To me that’s another way of them obfuscating any attempts at anyone verifying any rainfall warning they issue. I already posted an article about how late in the day the validity time for the start of the amber warning on Saturday was, 12 UTC instead of 06 UTC.

No one of course care’s about the minutiae of what the various warnings that were issued, in the final analysis it probably made little difference in alerting people to the chances of flooding, because the media have been all over this story for days, with flocks of TV reporters already vying for best positions along the river Taff in south Wales to capture all the details.

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