This work investigates the link between the most extreme daily precipitation (EDP) events observed since 1900 on the west coast of Norway and the large‐scale moisture fluxes over the North Atlantic Ocean. Using station precipitation data, vertically integrated water vapor (IWV) from Special Sensors Microwave Imager/Sounder (SSMIS) satellite observations and the state of the art NOAA‐twentieth Century (NOAA‐20C) reanalysis, it is shown that 55 out of 58 EDPs are associated with narrow plumes of intense low‐level moisture defined as atmospheric rivers. Despite the high spatial correlation between IWV fields in the SSMIS and NOAA‐20C data sets, the significant positive relationship between the maximum amount of observed precipitation at all stations and the IWV content hitting the coastal terrain is only observed in the SSMIS data set. Further, the composite analyses of synoptic conditions show that the preferred circulation type consists of a mean sea level pressure (MSLP) dipole pattern where a high‐pressure system over central Europe and a series of low‐pressure systems to the east of Iceland and over the Norwegian Sea are present. The west coast of Norway is located in the exit region of the anticyclonically curved upper tropospheric polar jet stream implying that the coupling of upper troposphere and surface dynamics begins to weaken at the time of EDPs. It is also found that the primary synoptic‐scale precursors are persistent positive 500 hPa height geopotential and MSLP anomalies over central Europe up to 10 days before the occurrence of EDP events.

This series of papers (parts I and II) examines the vorticity budgets of winter North Atlantic extratropical cyclones during the period 1979–2009 using the Modern-Era Retrospective Analysis for Research and Application (MERRA). The authors use a new partitioning technique to combine the Zwack–Okossi (Z–O) equation with the omega equation. The combination provides a possibility to partition the adiabatic term in the Z–O equation into its different forcing mechanisms. Thus, both the direct effect of the dynamic and thermodynamic forcings and their indirect effect on the adiabatic term can be calculated to provide the total effect (direct plus indirect) on the 950-hPa geostrophic vorticity tendency. It is demonstrated that the total-effect diagnostic is a suitable tool to identify the dynamically consistent characteristics of cyclone development in midlatitudes because it possesses less case-to-case variability. The authors found that the vorticity advection is the major forcing process, the tendencies attributed to the ageostrophic vorticity tendency term are considerable, and the opposing effect of the friction term in moderating the deepening is significant. In general, the upper-level dynamics drive the deepening of the cyclones, except at the end of development, where a combination of midlevel latent heating, positive ageostrophic vorticity tendency, and positive indirect effect of vorticity advection contribute to the development. Additionally, the total effects of temperature advection and latent heating on the intensification of cyclones are reduced because of the inclusion of counteractive indirect effects, as are their variabilities within the cyclone composite.

In this study, the development of a warm-core and cold-core extratropical cyclone over North Atlantic is examined. The geostrophic relative vorticity tendency used to diagnose the development is calculated utilizing the so-called extended form of the Zwack-Okossi development equation. In both cases, the cyclonic vorticity advection acted to develop the system, but warm-air advection (diabatic heating) made the largest contribution to explosive development in the cold-core (warm-core) case. Further, a vertical cross section of the temperature advection in the warm-core case reveals that the largest values of this contributor are located far and ahead of the cyclone center.