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The extreme event was associated with a change of the weather pattern. The sequence of MSLP & 200hPa winds maps below are valid between 14 to 19 February shows the high pressure system, located NE Pacific, retreating (& weakening) westwards allowing the jet and disturbances to move towards the West Coast. A rapid cyclogenesis is taking place steered by the upper level jet as it moves SE towards the Baja California.
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Maps of total precipitable water based on an algorithm using microwave observations from polar orbiting satellites that are generated at Cooperative Institute for Meteorological Satellite Studies (CIMSS) are available to the public. Based on these maps we can detect the presence of Atmospheric Rivers atmospheric rivers (ARARs) which was were responsible for the large amounts of precipitation that occurred in California last weekend (see DR of yesterday). A stream of high values of precipitable water stretches from subtropical region, crossing Hawaii the Hawaiian Islands towards the coast of Washington stateState, sliding equatorwards along the coast and reaching South Southern California 2 days later, embedded in the extra-tropical cyclonic circulation.
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The development of this AR in the central equatorial subtropical Pacific seems to coincide with the propagation of a strong MJO event in western the western equatorial Pacific some days before. Looking at a few products that monitor the presence and progression of the MJO we can seen see that around 10 February the system, in terms of OLR anomalies was located in the 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.
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The rainfall during the winter has put an end to the drought in California. The plots below show tha that drought monitoring maps from February 2014, 2015, 2016 and 2017.
The plot below show shows the accumulated precipitation for 8 stations in northern California from http://cdec.water.ca.gov/cdecapp/precipapp/get8SIPrecipIndex.action
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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 closer to the event the EFI become higher.
The Atmospheric River 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 vertically integrated of eastward horizontal water vapour flux" (see maps below, as shown in the maps below (note the title on the maps is incorrect). The maps show the EFI on forecast day 10 (left plot) and the forecast day 1 (right plot), both valid for the 15th February. An impressive large scale plume of v. integrated eastward integrated horizontal water vapour flux flux dominates the North Pacific region (in true fact, the largest pattern seen on at a global scale). As expected the signal is stronger when as we get close closer to the event, but it is remarkable also how well the amplitude of the EFI, the extension and phase (location) are captured 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 at 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.
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