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Discussed in the following Daily reports: |
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1. Impact
A major storm was responsible for the floods in California on 17-19 February (strong winds were reported as well). This was not the first event of the season and California has been battered by anomalous wet weather. The week before more than 150K people were forced to abandon their homes due to a problem with a crack in Lake Oroville's dam as a result of excessive water. The anomalous wet season came after several years of drought conditions in California.
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The development of this AR in central equatorial Pacific seems to coincide with the propagation of a strong MJO event in western equatorial Pacific some days before. Looking at few products that monitor the presence and progression of the MJO we can seen that around 10 February the system, in terms of OLR anomalies was located in western Pacific (top panel). This is also confirmed by the Wheeler and Hendon index based on the forecast initialised on the 9th February (left panel). The extended range forecast initialised on the 26th January shows the progression of an MJO event in Indian Ocean. By day 20 (~15 January) a good fraction of the ENS members predict a strong MJO in the western Pacific.
The rainfall during the winter has put an end to the drought in California. The plots below show tha drought monitoring maps from February 2014, 2015, 2016 and 2017.
3. Predictability
3.1 Data assimilation
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The EFI signal of abnormal rainfall over the region 7 days before the event (and SOT values indicating an extreme scenario). As we get close to the event the EFI become higher.
The Atmospheric River event behind the extreme rainfall in some parts of California was indeed an abnormal case (see above) . David Lavers has been working on an EFI of the "vertical integrated of eastward water vapour flux" (see maps below). The maps show the forecast day 10 and the forecast day 1, both valid for the 15th February. An impressive large scale plume of v. integrated eastward water vapour flux dominates the North Pacific region (in true the largest pattern seen on a global scale). As expected the signal is stronger when we get close to the event but is remarkable also the amplitude of the EFI, the extension and phase (location) ten days before.
3.4 Monthly forecasts
The series of plot below show weekly precipitation anomalies for 13-19 February (note that the California event was in the end of the week).
As the winter 2016-2017 was very wet it is relevant to look at the seasonal forecast. However, there is very little trace of rainfall above normal in south-western U.S in the forecast from 1 November for DJF.
3.5 Comparison with other centres
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