05 August 2012

Ernesto and Florence both struggling with dry air

After a day of dramatic organization on satellite, the aircraft flying into Ernesto were not able to find the anticipated winds, and actually had a challenge even finding the center on some passes!  The dry air that was wrapped all around Ernesto managed to get entrained into the circulation and choke off the deep convection.  The result is a rather sickly tropical storm with an exposed low-level center:

At 15Z today (11am EDT), the intensity was 45kt and 1006mb, exactly what it was 3 days ago.  It's centered about 200 miles south of Jamaica and racing westward at 20kts.  The image below shows the TPW (total precipitable water) in the storm's vicinity from several hours ago.  TPW is the integrated amount of water in a column if all of the water were condensed out... the vast majority of the contribution comes from the lowest part of the atmosphere; the upper levels are always relatively dry.  Here, the storm is the orange blob in the middle, but the blues and greens ahead of and behind the circulation are indicative of very dry air.

So, this has spared Jamaica from a destructive encounter, and will also likely spare the Yucatan from a strong landfall.  The latest model guidance keeps the track decidedly further south, ranging from southern Belize to the central Yucatan peninsula by late on Tuesday.  The intensity on the other hand, is all over the place.  Ernesto is on a very fine line between erupting into a major hurricane and dissipating into an open wave.  Normally, a storm in this location at this time of year would be a huge concern... But this blob of dry air covering the Caribbean is atypical and a storm-killer.  The various models reflect that large uncertainty and bifurcation in possibilities.  Once again, despite it's poor satellite appearance now, it still should not be underestimated in Belize and the Yucatan... just in case.

Florence is also battling with dry Saharan air to its north and west.  In this visible satellite image, the dusty air actually shows up as a milky plume streaming off of Africa (off to the east).  It's a 50kt tropical storm, and is forecast to weaken in the coming days due to the dry air and vertical shear as it tracks WNW over open ocean.

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