Rapid update, high resolution, very short range precipitation forecasts are generated by blending extrapolated observations with a recent, 4km resolution forecast from the Unified Model.
Nowcasts are used by a wide range of customers as a source of detailed guidance on the location, extent and timing of imminent, high impact weather events. For example, precipitation nowcasts are utilised by the Met Office and the Environment Agency to mitigate the effects of pluvial and fluvial flooding (FFC).
Whilst significant advances in high resolution NWP prediction have been made in recent years (e.g. the implementation of 4 km and variable (1.5 km) configurations of the UM and further advances including the implementation of a RUC, storm scale (1.5 km) configuration of the UM are planned, there remains a requirement for nowcasts. Nowcasting techniques are capable of exploiting current weather observations more fully (skilfully) in the very short range (lead times < 3 hours) than is achievable with current, operational NWP models.
A suite of nowcasting algorithms is run within the UKPP system. The latter was implemented in 2007 to provide a single source of nowcasts and post-processed, short range forecasts from the 4 km and variable (1.5 km) configurations of the UM. The UKPP system incorporates a precipitation nowcasting algorithm known as STEPS.
STEPS generates control and 30 member ensemble nowcasts on 15 minute and hourly update cycles respectively. These have a range of six hours and spatial and temporal resolutions of 2 km and 15 minutes respectively. Each nowcast scale-selectively blends extrapolated observations with the most recent forecast from the 4 km UM and a time series of synthetically generated precipitation fields (noise) with space-time statistical properties inferred from weather radar. The noise component serves to account for uncertainties in the evolution of the extrapolation and NWP forecast components and also to downscale the NWP forecast.
Current R&D is focused on the use of similar techniques to generate seamless, high resolution (2km) precipitation forecast ensembles with a range of several days. These will integrate ensemble precipitation nowcasts with small ensemble NWP precipitation forecasts from the variable (1.5 km) resolution UM and the North Atlantic and European configuration of MOGREPS. The aim is to improve pluvial and fluvial flood warnings issued by the FFC.