Satellite imagery of the tropical wave approaching Puerto Rico shows a less impressive system than yesterday. The wave covers less area, has reduced upper level outflow, and is battling increased wind shear. However, the dry air on the wave's west side that was inhibiting thunderstorm development last night is less apparent on today's water vapor satellite imagery.
The most most impressive sign of organization of this wave is in the pressure falls observed over the Lesser Antilles islands this morning. A plot of the past week's pressures at one of our personal weather stations in Anguilla, an island in the northern Lesser Antilles about 150 miles east of Puerto Rico, shows a significant pressure fall this morning:
Keep in mind that it is a bit tricky to interpret pressure readings in the tropics. The pressure plot shows a curious regular oscillation with amplitude 1 mb, and a peak twice per day at about 11am and 11pm local time. This oscillation is due to the presence of atmospheric tides
in the atmosphere. Like tides in the ocean, atmospheric tides move as a wave around the planet with a very regular period in sync with the Earth's rotation rate. The atmospheric tides we see in the Anguilla data are primarilly due to solar heating of the atmosphere. When the sun rises and starts heating the air, the air expands in response, lowering the pressure in the vicinity, and pushing a wave alternating high and low pressure air all the way around the globe. The amplitude of this wave is highest--about 1.5 mb--where the sun's heating is greatest, near the equator. The wave's minimum amplitude of about 0.5 mb occurs near the poles. The period of the wave is 12 hours.
Mentally remove the wriggles from the pressure plot above, and you can see that the drop in pressure this morning in Anguilla was not due just to the atmospheric tidal effect, but must also have had a component due to a low pressure system moving in. Similar pressure falls are being observed on nearby islands that the tropical wave is affecting, such as St. Maarten
A hurricane hunter aircraft will investigate the area at 4pm today to see if a tropical depression is forming. If a depression does form, its potential track is highly uncertain. The GFS model maintains that it will track WNW and threaten the East Coast, while the ECMWF has the system moving into the Gulf of Mexico. The GFDL model moves the system into the Bahamas and disipates it in 36 hours. In any case, if a depression does form, it will have some significant barriers to overcome in order to intensify. If the storm moves north, there is a large upper-level trough with strong shearing winds to contend with. If the storm stays further south, interaction with the land masses of Puerto Rico and Hispanolia will hinder it.
Dr. Jeff Masters