...
The animated OPERA radar below shows no clear evidence of any orographic triggering or enhancement of the rainfall rates. Instead there are several examples of quasi-stationary triggering points over the ocean, which are probably maintained, temporarily, by back-building (where convective outflow meets inflow, gives convergence, which fuels more convection). These seem to have fed the prolonged extreme rainfall rates in the afflicted area. There was evidence on satellite imagery of a very similar process in and around the all-time-record Hawaii rainfall event (>1000mm in 24h) earlier in 2018. It would be good to uncover what aspects of an atmospheric vertical profile can be conducive to such back-building. The HRES analysed profiles for relevant locations (relative to radar) are also shown below. There is evidence on both of some near-unidirectional lower tropospheric shear, which may be conducive, as discussed here (see "Excessive Rainfall" section near the end). However a factor also of note, for forecasting, is that the short range forecast profiles for these times do not show this characteristic as well as these analysis profiles do.
3. Predictability
...