Eta 24 hour forecast 950 mb ageostrophic wind (green), wind (blue) and geostrophic wind (red) valid 1/6/05 12 UTC.

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Speaker Notes:

While frontogenesis is typically the largest contributor to the ageostrophic flow in these types of situations, another contributor can be the "isallobaric effect", where a component of the ageostrophic wind can be directed from pressure rises towards pressure falls. In this case, weak pressure rises are indicated over eastern New England, while strong pressure falls (associated with the intensifying low pressure area) are indicated over Michigan. An axis of pressure falls extends eastward from Michigan across southern Pennsylvania and off the New Jersey coast. The data on this slide shows a strong ageostrophic wind component from the north over northeast Pennsylvania and southern NY. Since the ageostrophic wind component is roughly parallel to the isalobars (lines of constant pressure change), the implication is that most of the ageostrophic wind in this case was being generated by the frontogenetical circulation, and not from any isalobaric effects (the ageostrophic wind would have a component across the isalobaric gradient, towards the strongest pressure falls, if the isalobaric effect was a significant contributor to the ageostrophic flow).