The 10-m wind plots show in more detail both the effect of lower boundary forcing and the incipient cyclone development.
As with the near-surface temperature and moisture fields, the 10-m winds were different from the beginning of the forecast at 12 UTC December 29th. The operational Eta wind field was fairly uniform over the waters east of the Carolinas and Virginia, while in the EtaX there already was a convergence center southeast of Cape Hatteras. Note the confluence line in the EtaX oriented northeast to southwest intersecting this center, also contributing to frontogenesis.
At 12 hours (00 UTC December 30), the operational Eta showed the low already forming off the South Carolina coast and sharp convergence extending along the coast up to the Chesapeake Bay. The EtaX also showed something going on in the same area off South Carolina, perhaps related to the shortwave starting to interact with the coastal front. However, it maintained its primary circulation center well offshore of northern North Carolina with divergence apparent over eastern North Carolina and southeast Virginia, where the operational model had its coastal front convergence axis.
By 18 hours into the forecast (06 UTC December 30), the surface cyclogenesis location has become clearly defined, with strong convergence into the developing low centers over Virginia Beach (operational Eta) and offshore northeast of there (EtaX). The circulation centers are just to the left of the model convection, and the orientation of the convergence zone and windshift axis matches the orientation of the model convective precipitation maximum even though the BMJ convective scheme does not directly use convergence or vertical motion. The location of the developing low in the EtaX coincides with the location of its SST warm pool and the greatest differences between its SSTs and those in the operational Eta.
10-m winds for Eta versus EtaX: 00hr, 12hr, 18hr
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Model |
Eta |
EtaX |
Windspeed scale |
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00 hr |
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12 hr |
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18 hr |
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