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Tropical Roundup: the Latest on Hilary, Emily, Franklin, Gert, and a Texas-Mexico Threat

To coin the understatement of the century, it’s not often that a tropical storm from the Eastern Pacific moves through Southern California on the same weekend that three tropical storms erupt in the Atlantic. That’s where things stood on Sunday, Aug. 20, 2023 – a landmark day in the annals of hurricane history and a very busy day for the National Hurricane Center, which is responsible for tracking tropical cyclones in both of these areas.

The Atlantic continued busy on Monday: two landfalling storms may arrive by midweek, one near the Texas-Mexico border and another in Hispaniola.

Hilary’s havoc: Record summer rains, widespread floods

Former category 4 Hurricane Hilary made landfall around 2 p.m. EDT Sunday as a 65-mph tropical storm on the coast of Baja California, Mexico, about 215 miles south-southeast of San Diego. More than 300,000 customers in Mexico lost power as Hilary ripped through the area, and one death was reported in Santa Rosalía, hard hit by flash flooding.

Hilary’s circulation quickly decayed as it moved northward and inland, but according to the National Hurricane Center, or NHC, it remained a tropical storm until just after reaching the southern San Joaquin Valley early Monday morning EDT. This was the first time in warning practices going back to the mid-20th century that the NHC had issued a Tropical Storm Watch and Tropical Storm Warning for Southern California.

As rainbands intensified Sunday night in the Los Angeles area, it appears the increasingly diffuse center of Hilary briefly reconsolidated over or near the coast before shifting inland. In fact, Hilary’s center as of 11 p.m. EDT Sunday was placed by NHC directly over south-central Los Angeles (latitude 33.9 north, longitude 118.2 west).

The Los Angeles area experienced some of the region’s heaviest rainfall totals from Hilary, especially at higher terrain, with amounts comparable to an intense winter storm but unprecedented for summertime. Downtown Los Angeles, where weather observations began in 1877, notched 2.48 inches on Sunday alone and 2.99 inches for the entire storm through 10 a.m. EDT Monday. Both readings topped the all-time June-July-August records of 2.06 inches and 2.26 inches set by the remnants of Hurricane Doreen in mid-August 1977. The storm total of 8.56 inches on Mount Wilson in Los Angeles County was almost certainly a summertime record as well. San Diego, where weather observations began in 1850, also recorded its wettest single summer day on record (June through August) with 1.82 inches on Sunday, beating 1.8 inches on Aug. 12, 1873.

High winds from Hilary buffeted the Los Angeles and San Diego areas, especially at higher elevations where the circulation of Hilary was slower to fragment. A gust of 87 mph was recorded at Magic Mountain (elevation 4,482 feet) at 6 a.m. EDT Monday, and there were widespread gusts of 30-50 mph along the coast and in higher terrain.

Widespread flash flooding extended eastward from the Los Angeles and San Diego areas into the Inland Empire region, along and near Interstate 10, and onward to the Palm Springs area, which was virtually cut off by the flooded I-10 on Monday morning. Palm Springs racked up 3.18 inches of rain on Sunday, making for its wettest summer day on Sunday in 101 years of recordkeeping (ahead of 2.80 inches on July 23, 1948). The city’s average annual rainfall is 4.61 inches.

No deaths had been reported from Hilary across California or Nevada as of Monday morning, and damage in the Los Angeles area was mostly minor, according to Mayor Karen Bass.

It’s a high heat: Midpoint of the atmosphere soars to record heights above the torrid central U.S.

The remnants of Hilary were dropping heavy rain on parts of eastern Oregon and western Idaho on Monday morning. Exceptionally high amounts of atmospheric moisture will continue flowing northward and eventually eastward across southern Alberta and Saskatchewan, rounding the north side of a record-strong dome of high pressure over the central United States.

It’s possible that sinking air on the eastern fringes of Hilary’s circulation added a slight bit of oomph to the strength of this heat dome. At 8 a.m. Monday, weather balloon data confirmed that the height of the atmosphere’s midpoint (the 500 millibar level) was at the highest level in 70-plus years of data for several cities in the central U.S.; these high heights signify an exceptionally warm airmass from 500 millibars down to the surface.

Many record highs are tumbling as a result of this air mass (more on that in a future post), but there’s been a smaller number of all-time highs than one might expect in this record-hot air mass, as some of the heat near the surface is going into evaporating moisture over dense agricultural lands rather than heating the ground. The result is a dangerous blend of intense heat and humidity that will threaten millions of people across the central United States this week.

Two main threats in the Atlantic this week: Potential Tropical Cyclone 9 and Franklin

As noted by Michael Lowry, only twice before in Atlantic records going back to 1851 have three tropical cyclones developed within 24 hours, as was the case on Sunday with Emily, Franklin, and Tropical Depression 6, which became Gert by late Sunday evening. Fortunately, Emily and Gert – both located well east of the Lesser Antilles in the open tropical Atlantic – will pose no threats. Emily was declared post-tropical at 11 a.m. EDT Monday, and Gert was barely hanging on, likely to dissipate by late Monday.

The most immediate threat for landfall is from Potential Tropical Cyclone (PTC) 9, spinning in the eastern Gulf of Mexico. The PTC category is used for systems that are not yet classified as tropical cyclones but that could threaten land within 48 hours.

It’s good that PTC 9 did not organize any further on Sunday night while over the near-record-warm waters of the eastern Gulf of Mexico, where sea surface temperatures are between 86 and 90 degrees Fahrenheit (30-32 degrees Celsius). Although it still lacked a closed low-level center as of midday Monday, PTC 9 was hauling a large field of moisture westward on its north side, where low-level winds are strongest, toward the coasts of South Texas and Northeast Mexico. A Hurricane Hunter flight was in progress on Monday afternoon.

The National Hurricane Center predicts that PTC 9 will become a tropical storm on Monday night before making landfall near the U.S.-Mexico border on Tuesday morning. The system will have no time to strengthen into a hurricane, and whether it arrives as a tropical depression, weak tropical storm, or tropical wave, the impacts from PTC 9 will be the same: a corridor of two- to four-inch rains across far South Texas and extreme northeast Mexico, with totals as high as four to six inches in South Texas and up to 10 inches as the system hits the rugged terrain further west in Mexico. Apart from the risk of localized flash flooding, the rains from PTC 9 may be largely beneficial to these drought-stricken areas.

The other likely landfall early this week will be from Tropical Storm Franklin, now intensifying as it drifts northwest in the eastern Caribbean with top sustained winds of 50 mph as of 11 a.m. EDT Monday. Moderate to strong wind shear (10-20 knots) will keep Franklin from intensifying rapidly on Monday, but Franklin will be passing over very warm waters around 86°F (30°C), about 1°F warmer than average, as well as high oceanic heat content extending well below the surface. Franklin is also embedded in a fairly moist atmosphere (midlevel relative humidity around 60%). Franklin’s large but somewhat disorganized mass of showers and thunderstorms, or convection, may consolidate during the normal nighttime convective peak on Monday night. A Hurricane Hunter flight was in progress on Monday afternoon.

Franklin is predicted to approach Hispaniola (Haiti and the Dominican Republic) as a strong tropical storm — perhaps even a minimal hurricane — on Tuesday night. Franklin’s main threat to Hispaniola and nearby Puerto Rico will be torrential rains over a prolonged period through Wednesday. Rainfall totals could reach six inches in Puerto Rico and 10-15 inches over Hispaniola, and substantial flooding is quite possible. After Franklin moves well north of the Caribbean, it could reintensify later this week, with Franklin’s course beyond that point highly uncertain.

Elsewhere in the Atlantic, a tropical wave near the Cabo Verde islands designated Invest 92L is likely to develop into a depression or tropical storm by later this week as it heads west-northwest. The next two names in the Atlantic list are Harold and Idalia.

Source : Yale Climate Connections