28 July 2020

Warnings issued ahead of tropical disturbance

The tropical wave I first mentioned last Thursday has still not developed into a tropical cyclone, but it's getting close. Now centered about 550 miles (1 day) east of the Leeward Islands, this disturbance is forecast to become Tropical Storm Isaías this week. (that's pronounced ee-sah-EE-ahs)

"Potential Tropical Cyclone 9" (formerly "Invest 92L") is moving quickly toward the west; tropical storm warnings have been issued for the Leeward Islands, Puerto Rico, and Dominican Republic.  Note that a "potential tropical cyclone" is the same critter as an "invest"... both are disorganized systems that are not yet tropical depressions. However, the PTC label was developed in 2017 to facilitate the issuance of watches and warnings related to a system that had not yet formed (like this one). Prior to that invention, they had to wait until something formed to be able to warn on it.

Model guidance is pretty scattered with this, so the static-sized cone doesn't adequately capture the present level of track uncertainty, and the "S" doesn't adequately capture the intensity uncertainty. The positions and intensities on Friday, Saturday, and Sunday are big question marks, so don't take them literally. A reconnaissance flight into the area is planned for Tuesday afternoon, and that will reveal some details on the structure and the data that are collected will be assimilated into the evening's suite of model runs.

One big thing working against PTC9 is a huge mass of dry air... a Saharan Air Layer surge.  It has been battling against that since it left the African coast, and it doesn't seem like it will escape it any time soon. This map shows the center of the developing storm with a red I (Invest), a background satellite image, and then the yellow-orange-red shading indicates increasing amounts of dry, dusty air.  Forecast models keep that dry air really close for the next several days.

Furthermore, if it takes a track close to or over the big islands like Puerto Rico and/or Hispaniola, that will also disrupt intensification.  If it tracks a bit north of those islands, it could get stronger. That leads to the track scenarios. The more southern tracks that pass over Puerto Rico, Hispaniola, and even Cuba show a storm that remains very weak, or even dissipates.  A nudge northward includes some land interaction, then over the Bahamas. And further north misses most land interaction but still passes over the Bahamas.

Then there's a lot of spread regarding a track that continues into Florida, one that recurves slightly up into the southeast US, or one that recurves completely before reaching the U.S. Since we don't even have a tropical cyclone yet, it's too early to have confidence in forecasts that far out.  But for timing purposes, a south Florida encounter would be on Saturday-Sunday, while a Carolinas encounter would be Monday-Tuesday.

These track maps show output from the ECMWF and GFS global model ensembles; the tracks are colored by intensity, with Category 1 hurricanes starting at yellow. In addition to these, there are several deterministic runs that are used, but I haven't shown them here yet.

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