Van Leer WxDiscussion

Forecast Analysis for Puerto Rico - valid 5:30 pm EST 23 Feb 2011

A rain event will pass over the island of Puerto Rico sometime between Friday night and Saturday morning as a large high pressure system moves off the East Coast and over the Atlantic.  A band of precipitation will be moving south and has the possibility to bring light precipitation over the island.  The large scale forcing for this event is quite small but never the less a quick QG exercise was performed to analyze the vorticity advection at that forecast time (23 Feb 06Z GFS HD at forecast hour 69).  A parallelogram approach was used to manually calculate the vorticity advection by the geostrophic wind at 500mb over San Juan (the northeast part of the island).  The calculated value from the parallelogram ended up to be about 4.097x10-11 s-2 just off the north-northeastern coast.  This was verified by a value of about 1x10-10 s-2 when calculated using GEMPAK.  This points to a slight amount of PVA at 500mb.  Due to lack of time this cannot be verified to be increasing PVA with height as another height was not calculated.  But due to the cold air advection in the area this vorticity advection seems to be the driving force behind the precipitation event.