A tropical disturbance in the western Gulf of Mexico designated Potential Tropical Cyclone Four (PTC 4) by the National Hurricane Center, NHC, remained disorganized as of midday Saturday, but with the potential to organize into a short-lived tropical storm. before moving inland just south of the Texas/Mexico border by Saturday evening.
As of 11 a.m. EDT Saturday, the disturbance was heading northwest at around 13 mph. The system had somewhat favorable conditions for development, with sea surface temperatures near 30 degrees Celsius (86°F), moderate wind shear of 15 to 20 knots, and a moderately humid atmosphere (an average relative humidity of 65%). However, satellite images Saturday afternoon showed dry air on the north side of PTC 4 being drawn into the system’s core, resulting in telltale arc-shaped surface clouds that rushed outward, a sign that PTC 4 thunderstorms were collapsing due to dry air intrusions, robbing the system of moisture and energy. A hurricane-hunting mission late Saturday morning found a broad area of calm winds near the center of PTC 4, but no sign of an organized surface circulation.
Steering currents favor a northwesterly track, which will take PTC 4 inland over northeastern Mexico by Saturday evening. In its advisory Saturday at 11 a.m. EDT, the NHC gave PTC 4 a 60% chance of forming over two days and five days.
The next name on the Atlantic Storms list is Danielle, one of the most recycled names on the rotating Storms list, with seven previous incarnations of Danielle, starting in 1980.
The main hazard of PTC 4 will be flooding due to heavy rains. However, the rains from the system will be more beneficial than harmful, with the heaviest rains falling over northeastern Mexico, which is experiencing moderate to severe drought. Beneficial rains from PTC 4 will also spread to southern Texas, where severe drought conditions were aided earlier this week by heavy rains from an earlier tropical disturbance, 98L.
From Drought to Flood: A Classic Transition Ahead for North Texas
PTC 4 may also contribute to even heavier rainfall in northern Texas long after the system has pushed inland. A moisture-rich mass, with precipitable water values between 2 and 2.5 inches, will be transported to Texas on the east side of PTC 4. It will join some of the still-rotating energy and moisture there. around tropical disturbance 98L, which made landfall earlier this week in southern Texas. Although 98L never developed as it crossed the western Gulf, it managed to hold together as a surprisingly consistent low on its inland journey last week, moving along the US-Mexico border from the Rio Grande Valley to the Gulf of California.
Towards the end of the weekend, some remnants of PTC 4 and 98L will join forces over northern Texas, strengthening a weak east-west frontal area near the tail of an upper level trough and concentrating moisture there. This will result in several days of very heavy rains in this drought-stricken region. The heaviest rainfall swath is likely to be centered over a broad area near the Dallas-Fort Worth area initially, gradually moving south to the Austin-San Antonio area later in the week. Details will depend on small-scale features that are difficult to predict.
The NOAA/NWS Weather Prediction Center predicts precipitation totals for the week of August 20-25 greater than five inches over a significant portion of North Texas (see Figure 1), with most falling early in the week. The wettest three-day spells in Dallas-Fort Worth include a record high of 10.73″ from April 24-26, 1922. More recently, 8.34″ fell from September 20-22, 2018, fueled by the remnants of the tropical depression 19-E of the eastern Pacific. .
More than one gully washer will be needed to clear the huge accumulated rainfall deficits across Texas. More than 60% of the state was covered by extreme to exceptional drought as of Tuesday, August 16, according to the US Drought Watch. More than 15 inches of precipitation falling over the next month is needed to lift most of Texas out of long-term drought conditions (Figure 2), and parts of the state requires more than 20 inches of precipitation to break the drought. The rains over the coming week will certainly help, but localized flash flooding is possible.
It’s time to watch the Eastern Atlantic
August 20 is the typical start of the peak portion of the Atlantic hurricane season, when tropical waves rolling off the African coast near the Cape Verde Islands should be watched for development. One of these waves is expected to leave African shores by Saturday evening. The wave will move west-northwest to northwest at 15-20 mph over the next five days, and into the central tropical Atlantic, where dry air is likely to be a problem for it. Waves here generally curve out to sea without affecting the Caribbean or North America. In a tropical weather forecast for Saturday at 8 a.m. EDT, the NHC gave this newly emerging wave two- and five-day formation chances of 0% and 20%, respectively.
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