Will Bratton

The Difference Between a Strong Hurricane and a Weak Tropical Storm The two most recent runs of the GFS provide an excellent illustration of why many different outcomes are still on the table. Same model, six hours apart, and they do not agree on what reaches the northern Gulf. Both close off and become tropical cyclones within the next 48-60 hours. Both come north from the Bay of Campeche (BOC) toward the northern Gulf coast. The difference is how fast one develops, and whether it is still strengthening when the trough arrives. The split is already there Wednesday morning. At 1 a.m. Wednesday, the 12Z run has a tight, circular 850 mb vorticity max in the Bay of Campeche and a 999 mb surface low. The 06Z run, at the same hour, is 1006 mb and elongated. By 7 p.m. Wednesday, the 12Z is already at 983 mb with a tight wind core. In contrast, the 06Z is 1002 mb and broad, and the jet is too far north to open an outflow channel. At 7 a.m. Thursday, both are still in the BOC, and both are still in the light-shear pocket. The 12Z low has deepened to 974 mb, with a tight wind core, a closed circular 850 mb max, and the streamlines already turning out to the northeast. The 06Z low has intensified much more slowly into a 996 mb tropical storm. Same pocket, but the 06Z has no outflow channel and is 22 mb behind. The environment is not what separates them. The vortex is. From there, one strengthens all the way to the coast and the other does not. The deeper vortex extends up into the southwesterly flow ahead of the trough, so the trough can pick it up and ventilate it. It is 975 mb just off the southeast Louisiana coast at 1 a.m. Saturday, still in the light-shear pocket with the outflow channel still open, and it crosses extreme southeast Louisiana between 1 a.m. and 7 a.m. By 7 a.m., it has already made landfall near the Louisiana–Mississippi border as a strong hurricane. It got there before the shear axis did. The slower one peaks early and weakens coming in. It is a closed tropical storm on Thursday, but it remains too weak to get picked up by the approaching trough and drifts slowly northward. By 1 a.m. Saturday, it is 1004 mb and filling, still in the western Gulf, while the other run is at the coast. It reaches the western Louisiana coast on Sunday as a weak tropical storm, a day and a half later, and that late arrival is the problem. The shear axis has already come through. The same trough that ventilated the faster storm is overhead by the time this one gets there, so it gets sheared instead. Timing is the whole difference. A faster, stronger low in the short term is what lets it move north and arrive while it is still being ventilated, via the favorable trough interaction I mentioned in the previous post. A slower one does get to the coast. It just misses the window, and it arrives even weaker than it was on Thursday because the trough is no longer an outflow channel. It is destructive wind shear. Nothing else in the pattern has to change. That is why a strong #hurricane at the coast and a weak tropical storm a day and a half later are both still on the table, and why one model cycle should not be taken verbatim as a definitive forecast. #92L #Isaias