Eta 6-hr forecast vertical circulation, frontogenesis and negative EPV (shaded blue) valid August 30, 18 UTC.

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This slide shows to distinct maxima of frontogenesis, one associated with the low-level front southwest of Syracuse, and one associated with the upper-level front over southern Canada. The interaction between these two frontal zones is forcing a deep layer of ascent. Negative EPV, indicating areas of instability, covers much of the area below 600 mb.