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This effect can occur with onshore cyclonic flow of maritime air that is marginally unstable to sea surface temperatures.  At certain times SSTs can be higher close to coastlines than offshore.   In these cases the inshore sea surface temperature can be high enough for the convection scheme in IFS to trigger release of convection with high CAPE values.  Just upstream the lower sea surface temperatures offshore cannot overcome convective inhibition at low or mid-tropospheric levels.    The IFS convective scheme triggers Instantaneous shower development at each step but in Cy49 and earlier does not advect the showers down wind.  This results in repeated convective rainfall over the same inshore locations which can add up to large or implausibly large record-breaking near-coast totals (Fig9.6.2-2). Cy50r1 transfers some of the convective moisture and precipitation to the large scale precipitation calculations and this helps carry the effects of the convective precipitation inland from windward coasts.

Fig9.6.2-2:  EFI for 24h precipitation for period 00UTC 6 Nov 2023 to 00UTC 7 Nov 2023, DT 00UTC 6 Nov 2023.  Unrealistically large or extreme precipitation totals on coasts exposed to the northwest are caused by instantaneous shower development by the convection scheme over inshore waters that are slightly warmer than offshore.

(FUG Associated associated with Cy50r1)