Eta 24 hour cross-section of frontogenesis (shaded) ageostrophic circulation vectors and relative humidity, valid 1/6/05 12 UTC.

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Freezing rain events over the eastern U.S. are typically associated with low-level ageostrophic flows from the north through east, which can provide a source of cold air at low levels. This continuous re-supply of cold air is necessary for a major event, as the latent heat of freezing tends to warm the low-levels of the atmosphere, and causes temperatures to rise above freezing without a counter-balancing effect. In major events, the ageostrophic flow is typically associated with the lower branch of a strong frontogenetical circualtion. This slide shows a 24-hour forecast of frontogenesis and ageostrophic flow vectors taken from the Delmarva area, north across eastern Pennsylvania and eastern NY. The circulation associated with the frontogenesis is very evident, with a northerly ageostrophic flow indicated across eastern Pennsylvania. The location of the developing ice storm is indicated by the white arrows.