26 July 2020

As Hanna makes landfall, focus shifts to east Atlantic

Hanna made landfall on Saturday afternoon at about 5pm local time on Padre Island, Texas as an upper-end Category 1 hurricane.  It continued to intensify right up until landfall, reaching peak sustained winds of 90 mph. The Corpus Christi area was in the northern eyewall (with the onshore winds) and as a result experienced the worst of the storm surge, which was about 6 feet.

Over the past two days (Friday morning through Sunday morning), preliminary rainfall estimates show a peak of about a foot near Brownsville, with scattered rain gauge data showing 8-10 inches, and a larger swath of at least 3 inches.

These storms that rapidly intensify as they're about to make landfall are a challenge, to say the least. This chart shows the 6-hourly observed intensity in the thick black line (note that these are not necessarily the intensities at the advisories), then each forecast made by the National Hurricane Center is shown in the colored lines. I added the time of landfall as a reference point.  This storm far exceeded model guidance and human forecasts.  Fortunately, it didn't have more time to work with or it certainly would have gotten even stronger.

Hanna is now a weakening tropical storm over northeast Mexico, and continues to dump rain but will lose tropical cyclone characteristics by Monday. There are a couple long radar loops covering Hanna's landfall at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

Now on to the tropical wave I've been mentioning for a few days... it left the African coast on Friday and continues to show signs of gradual organization.  NHC is giving it a 60% probability of becoming a tropical cyclone within the next two days, and a 90% probability within the next five days. Models support this high likelihood.  It looks ragged on satellite images so far on Sunday, but regardless, it's on track to reach the Lesser Antilles on Wednesday-Thursday, likely as a tropical storm.

Water temperatures are very warm ahead of it and across the deep tropics, quite a bit warmer than average for this time of year (2-3°F). We think of ocean temperatures warmer than about 26°C as sufficient, 28°C as plenty warm, and 30°C+ as rocket fuel. 


Long-range models are trending toward keeping Invest 92L (likely future Isaias) toward the north end of the Lesser Antilles, then Puerto Rico and Hispaniola, as opposed to remaining in the deep tropical Caribbean. This latest example from the 50-member European global model ensemble illustrates the general behavior: members that remain far south tend to dissipate or remain very weak in the Caribbean, but roughly 1/3 of members pass north of Hispaniola and are able to become stronger. Among those, some turn northward more quickly and other more slowly. The "L" symbol positions on this map are valid on August 3rd. As with any long-range model product, never take it literally. The details are irrelevant, but the patterns and trends are useful.


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