Satellite of Hanna
Visible GOES-17 satellite image of Tropical Storm Hanna at 15:40Z (11:40 a.m. EDT) Friday, July 24, 2020. (Image credit: NOAA/RAMMB)

Tropical storm warnings were up for the South Texas coast at 2 p.m. EDT Friday as a strengthening Tropical Storm Hanna headed west-northwest at 9 mph, with top sustained winds of 50 mph.

Satellite loops on Friday afternoon showed Hanna as a medium-sized system, with tropical storm-force winds of 39 mph or higher extended out 60 miles from the center. Conditions were favorable for intensification, with sea surface temperatures (SSTs) near 29.5 – 30 degrees Celsius (85 – 86°F) and low to moderate wind shear of 5 – 15 knots. However, there was dry air to the west of Hanna, and this dry air was interfering with development.

Figure 1
Figure 1. Radar image of Hanna at 15:54Z (11:54 a.m. EDT) Friday, July 24, 2020. (Image credit: National Weather Service via Mark Nissenbaum)

Hanna is the earliest eighth (“H”) storm on record in the Atlantic. The prior record holder, Tropical Storm Harvey, was named on August 3 during the hyperactive year of 2005, when a record 28 named storms formed (including one unnamed storm designated after the season was over.) Hanna follows the tropical storms Cristobal, Edouard, Fay, and Gonzalo, which were the earliest third, fifth, sixth, and seventh named storms for any Atlantic season in records going back to 1851.

Figure 2
Figure 2. Predicted surface wind (colors) and pressure (black lines) at 15Z (11 a.m. EDT) Saturday, July 25, 2020, from the 6Z Friday, July 24, 2020 run of the HWRF model. The model predicted that Hanna would make landfall near Corpus Christi, Texas, as a strong tropical storm with peak winds of 62 knots (71 mph, dark red colors), and a central pressure of 977 mb. (Image credit: Tropical Tidbits)

Forecast for Hanna

The intensity forecast models and official National Hurricane Center (NHC) forecast on Friday morning all called for steady strengthening of Hanna until landfall, which is expected to occur on Saturday afternoon. Hanna will likely be a strong tropical storm with top sustained winds in the 60 – 70 mph range at landfall, though a category 1 hurricane with 75 – 80 mph winds cannot be ruled out.

Regardless, heavy rains will be the main threat from Hanna, with widespread rains of 4 – 8 inches in South Texas and a few areas of up to 12 inches. NHC did not issue a storm surge warning for Hanna, and the expected one- to three-foot storm surge will not lead to mass evacuations.

Fibure 3
Figure 3. Visible GOES-17 satellite image of Tropical Storm Gonzalo at 15:40Z (11:40 a.m. EDT) Friday, July 24, 2020. (Image credit: NOAA/RAMMB)

Gonzalo struggling with dry air as it approaches the Lesser Antilles

Tropical storm warnings were up for the southern Windward Islands of the southeast Caribbean at 11 a.m. EDT Friday, as Tropical Storm Gonzalo steamed west at 18 mph, with top sustained winds of 50 mph. Satellite loops on Friday afternoon showed Gonzalo to be a small system, with tropical storm-force winds of 39 mph or higher extending out just 25 miles from the center.

Gonzalo was struggling to maintain its heavy thunderstorm activity because of dry air, with the 8 a.m. EDT Friday run of the SHIPS model diagnosing a mid-level relative humidity of 50 – 55%, which is very dry for a tropical cyclone. However, favorable conditions for development included SSTs near 28.5 degrees Celsius (83°F) and low wind shear of 5 – 10 knots.

Figure 4
Figure 4. Predicted surface wind (colors) and pressure (black lines) at 6Z (2 a.m. EDT) Sunday, July 26, 2020, from the 6Z Friday. July 24, 2020, run of the HWRF model. The model predicted that Gonzalo would pass through the Windward Islands as a tropical storm with peak winds of 53 knots (61 mph, orange colors) and a central pressure of 996 mb. (Image credit: Tropical Tidbits)

Gonzalo’s compact size adds to forecast uncertainty

Small tropical cyclones affect smaller areas, but they are also famously more difficult to predict. Most of the top intensity models, along with the official NHC forecast, call for Gonzalo to struggle with dry air and remain at tropical storm strength through early next week, when the storm will likely dissipate in the eastern Caribbean. However, Gonzalo may find a favorable pocket for growth within the larger-scale dry air surrounding the system, which could allow the storm to reach the Windward Islands as a category 1 hurricane.

Even if Gonzalo becomes a hurricane, it likely won’t cause a lot of wind damage in the islands, since it will be a small storm. Gonzalo’s primary threat will be heavy rain, with rain totals of up to eight inches potentially causing flash floods and mudslides.

The track forecast for Gonzalo is relatively straightforward, with all of the top track models predicting a westerly motion that would take the storm through the Windward Islands on Saturday night.

Figure 5
Figure 5. MODIS satellite image from Friday morning, July 24, 2020, of a tropical wave that emerged from the coast of Africa on Thursday. (Image credit: NASA Worldview)

Tropical wave off coast of Africa is a threat to develop and affect the Caribbean

A strong tropical wave emerged off the coast of Africa on Thursday, positioned about 300 miles south-southeast of the Cape Verde Islands at 2 p.m. EDT Friday. This system had not been given a designation by NHC by Friday morning, but will likely be labeled 92L by Saturday.

The wave had conditions marginal for development on Friday afternoon, with SSTs near 27 degrees Celsius (81°F) and moderate wind shear of 10 – 20 knots. The system was embedded in a moist atmosphere, but it will have to contend with dry air from the Saharan Air Layer, located to its northwest, over the next few days. This dry air, moderate wind shear, and marginal SSTs will likely prevent development into a tropical depression until Monday at the earliest. Satellite images on Friday showed that the wave was poorly organized, and had a modest amount of spin.

Port Arthur floodingUnusually warm sea temperatures fueled Harvey’s devastating rains

By Tuesday, July 28, when the system will be approaching the Lesser Antilles Islands, it will encounter warmer SSTs near 28 – 29 degrees Celsius (82 – 84°F), but it may still have to deal with dry air and moderate to high wind shear. Of the three best models for predicting tropical cyclone genesis – the European, GFS, and UKMET models – two of them, the European and UKMET – supported intensification of the wave into a tropical depression by Wednesday. Steering currents will carry the system to the west or west-northwest, and on Wednesday, it will likely pass through the Lesser Antilles Islands and enter the eastern Caribbean. In their 2 p.m. EDT Friday Tropical Weather Outlook, NHC gave this African tropical wave two-day and five-day odds of formation of 0% and 40%, respectively. I give the wave higher five-day odds of development: 60%, and it is something all residents and visitors in the Caribbean islands should pay attention to.

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Topics: Weather Extremes