Oct 5, 2009

Non - Tropical Storm Grace

Last night, a very small area of convection in the northeast Atlantic was named Tropical Storm Grace. In this visible satellite image from early this morning, it is about halfway between the Azores and the English Channel.It is not much bigger than New Jersey. Over the last several years, NHC has been increasingly naming everything in sight. This particular example goes a long way toward showing this trend. Water vapor imagery shows the original disturbance rotating around the back(west) side of a large mid/upper trough stalled out over the Azores:

It was named a tropical storm at 3Z and at 41.2 N lat, 20.3 W lon over 70 degree F (21 C) waters.Over such cool waters, deep convection can only be supported by the steep lapse rate environment provided by the mid-level trough.A moist adiabat from 21 C at sea level will only reach around -8 C at 500 mb. If that parcel were placed in the tropics, where 500 mb temps are seen to be about -5 C, this would be likley produce a positive lifted index.Another non-tropical feature is the horizontal thermal contrast - although given Grace's small size it might be argued that the storm itself doesn't even cross more than one isotherm.
In addition, the best low-level moisture is displaced well to the southeast of the low pressure center - just like almost any typical mid-latitude system.
This is especially obvious at 850:Even with systems like this being named, tropical activity in the Atlantic should be below normal this year.

No comments:

Post a Comment