M, N, O... P?

The tropics are really cooking now, with activity typical of what sees at the height of a busy hurricane season. We are only in the 4th inning of a 9-inning ball game, and already have our 14th named storm, with the 15th likely on the way! At this rate, the old record of 21 tropical storms set in 1933 will easily be eclipsed, and we'll have to start naming storms using the Greek Alphabet--Alpha, Beta, Gamma, etc. So let's dig in and start discussing the storms, from most important to least important.

Depression developing near Miami
The area of most serious immediate concern is the stationary tropical low centered about 75 miles east of Ft. Lauderdale, FL. This low has increased its deep convection markedly in the past six hours, and appears that a tropical depression will form here later today. The hurricane hunters are scheduled to investigate the system this afternoon.

The best to way to track the storm is on Miami radar. The radar loop shows a large circular ring of strong echoes developing. Doppler radar estimates of the wind velocity from the Miami radar (see image below) show peak winds in the 20 - 26 knot range, which is still below the threshold for a tropical depression (30 kt).


Figure 1. Doppler wind velocities from the Miami radar. The storm to the east of Miami is rotating counter-clockwise, so winds to the northeast of the city blow towards the radar, and winds to the southeast of the city are blowing away from the radar. Wind speeds over the ocean to the northeast of Miami are coded shades of green, meaning winds are blowing at 10 - 26 knots towards the radar (the radar is located in the exact center of the image, and is denoted by a hard-to-see white cross). To the southeast of the radar, the winds are colored yellow to orange, meaning winds are blowing 10 - 26 knots away from the radar. Ignore the echoes over land, most of these echoes are ground clutter. Velocities marked pink (RF) mean that the echoes are too far from the radar for velocities to be determined.

This system should move very slowly the next three days, since it is trapped under a strong high pressure ridge where steering currents are very weak. A slow northward or northwestward motion is indicated by most of the models, which would bring heavy rains to the east coast of Florida starting Wednesday. Most of the models bring the system inland over Central Florida by Thursday as a weak tropical storm and dissipate it. However, some models keep it just off the coast of Florida, and forecast that as the storm tracks further north, it will move more northeasterly away from Florida, following Nate out through a weakness in the ridge. This all depends upon how strong Nate becomes. However, none of the models forecast that the system will follow Nate all the way out to sea. A strong ridge of high pressure is forecast to build in behind Nate and force the system back towards Florida. This system seems destined to spend most of the coming week hanging around Florida. As far as intensity goes, shear levels over the system are 5 - 10 knots, which should allow for some modest strengthening. It is likely we'll have Tropical Storm Ophelia by Thursday. Shear levels are forecast to decrease even lower the next few days, so if the system remains off of the coast of Florida, it has a chance to be a hurricane by the end of the week.


Figure 2. Early forecast model tracks for the developing system east of Ft. Lauderdale.

Tropical Storm Nate
Nate formed last night on the eastern lobe of the same trough of low pressure that spawned the system near Ft. Lauderdale. Remarkably, the GFS forecast from last Thursday correct predicted that two separate storms might emerge from this trough. Nate's satellite signature has improved markedly the past six hours, is over warm waters, and has light shear overhead. We're likely to see Hurricane Nate by Thursday. Nate is moving very slowly, as it is trapped under a strong ridge of high pressure. A trough moving off the east coast Friday should create a weakness in the ridge that will allow Nate to follow Maria out to sea. The only threat from Nate to the U.S. coast will be some high surf that may develop late in the week.

Hurricane Maria
Maria peaked in intensity early this morning, and is now showing significant deterioration thanks to wind shear and cold water. She barely made it to major (Category 3) status last night, with 115 mph winds, the 4th major hurricane of the season. Maria is expected to continue zooming northeastward and turn into a huge and powerful extratropical low that will slam Iceland with high winds and heavy rain on Saturday. Maria's remnants will then weaken, but still bring Norway significant wind and rain on Tuesday.

P?
So, we've talked about M, N, and O, but not P yet. Well, I have no speculations about where the "P" storm might form. My long-range eyes can't see any evidence of other threats in the tropics. The tropical wave that just entered the eastern Caribbean has strong easterly winds at high levels that are shearing it, so that is not a threat. There is a low pressure center off the coast of Africa near 13N 33W, but convection is limited there. The ITCZ is pretty quiet, so for now we just have the M, N, and O storms to worry about.

Dr. Jeff Masters