Ex-Harvey May Regenerate; 92L Still Disorganized

August 20, 2017, 3:08 PM EDT

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The Eastern Caribbean is a notorious graveyard for aspiring tropical cyclones, and it claimed another victim on Saturday afternoon, when Tropical Storm Harvey met its demise. On Friday, Harvey buffeted the Windward Islands as minimal tropical storm with 40 mph winds, dumping up to 8 inches of rain on Barbados, where some flood damage was reported. But high wind shear near 20 mph, combined with the accelerating east-to-west blowing trade winds that Harvey was embedded in, were sufficient to rip the storm apart on Saturday in the Eastern Caribbean.

However, ex-Harvey is likely to regain tropical storm status at some point in the week as it heads west to west-northwest through the Western Caribbean, across the Yucatan Peninsula, then into the southern Gulf of Mexico’s Bay of Campeche. Satellite images on Sunday morning showed that Harvey was beginning to reorganize, with a decent-sized area of heavy thunderstorms that were increasing in intensity and organization. Wind shear was moderate, 10 – 15 knots, the atmosphere had a not-terrible-for-development mid-level relative humidity of 55 – 60%, and the ocean was warm, with sea surface temperatures (SSTs) of 28.5°C (83°F). An Air Force hurricane hunter aircraft will investigate the system on Sunday afternoon to see if a closed surface circulation is forming.

The atmosphere will grow moister and the SSTs will grow warmer as ex-Harvey progresses to the west. None of the 0Z Sunday operational runs of our three reliable global models for predicting tropical cyclone genesis—the GFS, European and UKMET models—developed the system before it made landfall on Tuesday morning in Belize. However, the UKMET and European model predicted development of ex-Harvey back into Tropical Storm Harvey by Wednesday, when the storm is forecast to be in the southern Gulf of Mexico’s Bay of Campeche. The 0Z and 6Z Sunday runs of the GFS model predicted that ex-Harvey would stay farther to the south and not have enough time over the Bay of Campeche to do much development. About half of the European ensemble members from 0Z Sunday, and about a third of GFS ensemble members, revive Harvey to tropical storm strength over the Bay of Campeche. In its tropical weather outlook issued at 8 am EDT Sunday, the National Hurricane Center gave ex-Harvey 2-day and 5-day odds of development of 50% and 60%, respectively. Regardless of development, northern Honduras, northern Guatemala, Belize, and portions of Mexico can all expect very heavy rains in excess of four inches from ex-Harvey, which will likely cause dangerous flooding and mudslides.

92L
Figure 1. Invest 92L as seen by the GOES-16 satellite at 9:50 am EDT Sunday, August 20, 2017. 92L was struggling with dry air ingestion, and had a modest amount of heavy thunderstorm activity. Image credit: NOAA/CIRA/RAMMB. NOAA’s GOES-16 satellite has not been declared operational and its data are preliminary and undergoing testing.

92L disorganized as it heads towards The Bahamas

Tropical disturbance 92L was located about 300 miles north of Puerto Rico, near 22°N, 67°W, at 8 am EDT Sunday, and was headed west-northwest at 20 mph. Satellite images on Sunday morning showed 92L had a modest amount of heavy thunderstorm activity. No surface circulation was apparent, and 92L was poorly organized, with only limited upper-level outflow. The system continued to struggle with dry air ingestion, as seen by arc-shaped low-level cumulus clouds moving outwards from 92L’s heavy thunderstorms. When the thunderstorms of a tropical disturbance ingest a large amount of dry air at mid-levels of the atmosphere, the resulting strong downdrafts rob the storm of moisture and hit the ocean surface with a lot of momentum, kicking up arc-shaped bands of cumulus clouds as the downdraft spreads out along the ocean surface.

Conditions were marginal for development on Sunday morning, thanks to moderate wind shear of 10 knots, combined with a very dry surrounding air mass with a mid-level relative humidity of 45%. 92L did have warm SSTs of 29°C (84°F) to work with, though. The disturbance will bring a few heavy rain showers to Puerto Rico, the Virgin Islands, and northern portions of the Dominican Republic on Sunday afternoon. A continued west-northwest motion for 92L is expected through Wednesday, with heavy rains from the system spreading into eastern Cuba and the central Bahamas on Monday and Tuesday, and into the northwestern Bahamas and Florida on Tuesday and Wednesday.

None of the 0Z Sunday operational runs of our three reliable global models for predicting tropical cyclone genesis—the GFS, European and UKMET models—developed the system through Tuesday afternoon. However, conditions for development will steadily improve this week. 92L’s environment will get moister, and wind shear is expected to be mostly in the moderate range, 10 – 15 knots, according to the 12Z Sunday run of the SHIPS model. By Tuesday night, when the mid-level relative humidity is predicted to rise to 55%, we may see some development of 92L, and the 0Z Sunday run of the UKMET model predicted that 92L would become a tropical depression over the northwestern Bahamas, a few hundred miles east of Florida, on Tuesday night. In its tropical weather outlook issued at 8 am EDT Sunday, the National Hurricane Center gave 92L 2-day and 5-day odds of development of 10% and 20%, respectively. I think the 5-day odds of development should be higher, at 30%. Late in the week, 92L will likely get entangled with a trough of low pressure moving off the U.S. East Coast, and move to the northeast, out to sea.

Large tropical wave in the mid-Atlantic no threat to land

Another tropical wave with a marginal potential to develop into a tropical depression was located about 1000 miles east of the northern Lesser Antilles Islands on Sunday morning, and was moving northwest at about 15 mph. Satellite images on Sunday morning showed the large wave had plenty of spin, but heavy thunderstorm activity was thin and poorly organized. Wind shear was high, over 20 knots, and the wave was embedded in the dry air of the Saharan Air Layer, which was interfering with development.

The 0Z Saturday operational runs of our three reliable models for predicting tropical cyclone genesis—the GFS, European and UKMET models—did not show development of this system over the next five days. The wave will pass well to the northeast of the Lesser Antilles Islands, and is likely not a threat to any land areas. In its tropical weather outlook issued at 8 am EDT Sunday, the National Hurricane Center gave this system 2-day and 5-day odds of development of 10%.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Dr. Jeff Masters

Dr. Jeff Masters co-founded Weather Underground in 1995 while working on his Ph.D. in air pollution meteorology at the University of Michigan. He worked for the NOAA Hurricane Hunters from 1986-1990 as a flight meteorologist.

emailweatherman.masters@gmail.com

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