Some derechos develop a radar signature resembling that of a hurricane in the low levels. They may have a central eye free of precipitation, with a minimum central pressure and surrounding bands of strong convection, but are really associated with an MCS developing multiple squall lines, and are not tropical in nature. These storms have a warm core, like other mesoscale convective systems. One such derecho occurred across the Midwestern U.S. on 21 July 2003. An area of convection developed across eastern Iowa near a weak stationary/warm front and ultimately matured, taking on the shape of a wavy squall line across western Ohio and southern Indiana. The system re-intensified after leaving the Ohio Valley, starting to form a large hook, with occasional hook echoes appearing along its eastern side. A surface low pressure center formed and became more impressive later in the day. Another example is the May 2009 Southern Midwest derecho. Location Derechos in North America form predominantly from April to August, peaking in frequency from May into July. During this time of year, derechos are mostly found in the Midwestern United States and the U.S. Interior Highlands most commonly from Oklahoma and across the Ohio Valley. During mid-summer when a hot and muggy air mass covers the north-central U.S., they will often develop farther north into Manitoba or Northwestern Ontario, sometimes well north of the Canada–US border. North Dakota, Minnesota, and upper Michigan are also vulnerable to derecho storms when such conditions are in place. They often occur along stationary fronts on the northern periphery of where the most intense heat and humidity bubble exists. Late-year derechos are normally confined to Texas and the Deep South, although a late-summer derecho struck upper parts of the New York State area after midnight on 7 September 1998. Warm season derechos have greater instability than their cold season counterpart, while cool season derechos have greater shear than their warm season counterpart. Although these storms most commonly occur in North America, derechos can occur elsewhere in the world, with a few areas relatively frequently. Outside North America, they sometimes are called by different names. For example, in Bangladesh and parts of Eastern India, a type of storm known as "Kalbaisakhi" or "Nor'westers" may be a progressive derecho. One such event occurred on 10 July 2002 in Germany: a serial derecho killed eight people and injured 39 near Berlin. Derechos occur in southeastern