Characteristics of midlatitude ionospheric disturbances during several great geomagnetic storms have been investigated using data from the European geomagnetic observatories and ionospheric stations with the aim of developing the local forecasting models, as part of the prediction and retrospective ionospheric modeling over Europe project. Based on the analysis of the geomagnetic storms of February 6, 1986, and March 13, 1989, a detailed picture of the local H component of geomagnetic field and the ionospheric critical frequency f0F2 variations is presented. The results show that f0F2 was dramatically changed above or below the monthly median level in a relatively narrow band about 15¿ of latitude and 30¿ longitude during the different phases of the storms. These results support the view that day-to-day F region ionospheric variability is essentially altered in great storms. Consequences of those effects for short-term modeling purposes are discussed. ¿ 1998 American Geophysical Union