Two intense cyclones over the North Atlantic during summer/autumn 2003 are investigated to determine atmospheric factors contributing to their development. One of the cases, in August 2003, led to heavy precipitation and flooding in Central Norway. The other event was a cyclone that caused heavy snowfall and wind exceeding 30m/s over East-Greenland and strong winds and snowfall over North-Iceland on 20-21 September 2003. The cyclones have been simulated with the numerical model MM5. For both cases sensitivity studies have been carried out to evaluate the significance of orography, latent heating and SST. In addition, the role of SST gradients are investigated in the September case and the role of surface albedo in the August case. This study shows that the September cyclone is strongly affected by the orography of Greenland. When orography is removed, a deep baroclinic low develops rapidly and moves eastward at 75N. Conversely, in the control run, the evolution of the primary baroclinic low is greatly suppressed by the orographic retardation of the warm air ahead of and the cold air behind the low. At the same time, a secondary low off Greenland’s east coast at 68N intensifies due to a coupling between an approaching upper level PV-anomaly and a lower level PV-anomaly generated from lee effects. The secondary cyclone is then transformed into a baroclinic low and moves eastward and causes the extreme weather conditions. In the August case the precipitation is strongly enhanced by orography due to the northely onshore winds. Removing the orography of Scandinavia leads to greatly reduced precipitation and a deformation of the cyclone. When the orography is present, a lee trough is formed over SE-Norway. A simulation with increased albedo is conducted to determine the significans of thermal effects for the trough formation. The lee trough weakened, but the impact of surface albedo was less than that of the mountain effect. In both cases, the simulations reveal that the release of latent heat had a major impact on the deepening of the cyclone in lower troposphere. In the August case, latent heating contributed to 40% of the rapid deepening during the first 30 hours of the simulation. The August cyclone occurred during a period of anomalously high sea surface temperatures(SST). Numerical tests show however that the development of the cyclone is fairly insensitive to the SST. Neighter the September cyclone was significantly influenced by the SST. In the North-Atlantic there are gradients in the sea surface temperature due to the warm Norwegian Atlantic Current and the cold East Greenland Current. The simulations show however that SSTgradients contributed relatively little to the development of the September cyclone.