Tropical Depression Ida was moving through the Tennessee Valley on Tuesday afternoon and is a threat to bring dangerous heavy rains in excess of six inches along its path over the next two days. At 11 a.m. EDT Tuesday, Ida was located about 100 miles southwest of Nashville, Tennessee, headed northeast at 15 mph, with a central pressure of 996 mb.

Ida to douse much of Eastern U.S. into Thursday; flooding also a concern from Nora’s remnants in Southwest U.S.

The remnants of two hurricanes will drench separate parts of the United States from Tuesday into Wednesday, with significant flood risk in both cases. The main concern is with Tropical Depression Ida, as the storm migrates northeast and gradually merges with a pre-existing front from the Ohio Valley across southern New England, becoming post-tropical by Thursday morning. Moist tropical flow from the south toward Ida will help strengthen the front, while the location of an upper-level jet will favor rising motion near the frontal zone. Upslope flow into the higher terrain of the Appalachians will further accentuate the rainmaking potential. Moreover, soils are unusually wet across much of Tennessee and eastern Kentucky, and from eastern Pennsylvania and New Jersey across southern New England, in the wake of two earlier tropical cyclones, Fred and Henri.

NOAA’s Weather Prediction Center (WPC) has tagged a belt from near Atlanta to near Pittsburgh with a Day 1 moderate risk (the second-highest of four risk levels) of excessive rain leading to flooding, through Tuesday night. There is also a moderate risk for parts of southern Arizona, where moisture streaming in from former Hurricane Nora may cause heavy showers and thunderstorms. Nora hit Mexico near Puerto Vallarta on Sunday as a category 1 hurricane, causing heavy damage.

For Wednesday, the Day 2 moderate risk area will engulf much of West Virginia, most of Pennsylvania, and the highly urbanized belt from northern Virginia to western Connecticut, including the Washington and New York metro areas. Parts of the Day 2 area may be upgraded to high risk once the locations of heaviest rain become more clear-cut. Update (4 p.m. EDT Tuesday): There is now a high risk of flood-producing rain on Wednesday from southern Pennsylvania to western Connecticut, including the New York City area. Based on output from the High Resolution Ensemble Forecast System, or HREF (see image at top), WPC warned that there was a more-than-95% chance at some locations of rains equal to an average recurrence interval of more than 100 years.

From a climatological standpoint, the most exceptional rains will likely be in south-central Pennsylvania into northern New Jersey, where model ensembles suggest that a few locations will see two- or three-day total rainfall amounts that would be expected on average only every 50 to 100 years. Flash flooding is a distinct possibility with the heaviest downpours as they slam hilly, flood-prone terrain. The highly paved I-95 corridor will also be vulnerable. See the excellent explanatory thread posted Tuesday by Tomer Burg for more analysis of the factors at play:

A noteworthy tornado threat may set up on Wednesday over the mid-Atlantic in the warm sector south of Ida’s remnants and the frontal zone. As moist, warm tropical air streams toward Ida across eastern parts of North Carolina, Virginia, and Maryland, as well as the D.C. and Delmarva areas, it will encounter vertical wind shear, and atmospheric instability will be on the increase. Typically, the tornadoes associated with hurricane remnants are short-lived and difficult to warn for, but they can still cause significant damage. One of the mid-Atlantic’s largest tornado outbreaks on record occurred with the remnants of Hurricane Ivan on September 17, 2004: 58 tornadoes were reported, including 38 in Virginia. The National Weather Service Storm Prediction Center has much of the mid-Atlantic in an enhanced risk for severe weather, including tornadoes, on Wednesday.

One cautionary tale of a Gulf Coast hurricane’s subsequent impact is 1969’s Hurricane Camille, one of the nation’s four landfalling Category 5 storms on record. Camille’s 175-mph sustained winds and 24-foot storm surge caused immense damage and took around 100 lives as the hurricane made landfall in Mississippi. Yet the death toll was even higher as Camille’s remnants moved into Virginia and interacted with a frontal zone, dumping widespread rains of eight inches and localized amounts of as high as 27 inches. The ensuing flash floods and landslides killed some 153 people. In Pennsylvania, the most ominous analog to ex-Ida is 2004’s Ivan, which led to record rains and severe flooding across much of the state, including Pittsburgh’s all-time 24-hour rain record of 5.95″.

Ida’s toll in Louisiana still unclear

Four deaths were being blamed on Hurricane Ida as of 1 p.m. EDT Tuesday: two in Louisiana, and two in Mississippi. However, Ida’s full impact was still unclear a full two days after its catastrophic landfall as a category 4 hurricane with 150 mph winds at Port Fourchon, Louisiana. However, the first photos from a government aerial survey were posted Tuesday morning and showed widespread heavy damage.

Ida dumped 10-15 inches of rain in Louisiana near its landfall location and brought a storm surge between 5-12 feet to multiple tide gauges. On Tuesday afternoon, water levels in Lake Pontchartrain, along the northern edge of New Orleans, were still 3-5 feet above normal.

Ida made landfall at the key oil industry hub of Port Fourchon, Louisiana, at 11:55 a.m. CDT August 29, with 150 mph winds and a central pressure of 930 mb. Wind and storm surge damage to the port was severe, with much of the 22-mile road to the port heavily damaged (see tweet below).

Ida’s toxic threat

Louisiana is home to a vast network of active and former heavy industry sites with the potential to release toxic material into the environment. As detailed in an article at nola.com, almost 600 sites with toxic chemicals were in Hurricane Ida’s path, and there is a high risk that Ida will cause multiple releases of toxic chemicals and oil into the environment.

A fly-over of Port Fourchon on Monday by Congressman Garret Graves (Figure 1) showed heavy damage at this critical hub for the oil industry. There was no evidence that any of the oil storage tanks had been smashed or moved, but multiple sheens of oil were visible on the water.

In 2019, the Government Accountability Office compiled a list of 945 Environmental Protection Agency Superfund sites that are potentially vulnerable to climate change-heightened extreme weather risks. One of these, the Delta Shipyard Superfund Site in Houma, Louisiana, took a direct hit from Hurricane Ida when the storm was at category 4 strength. The site was a cleaning and repair facility for small cargo boats, fishing boats, and oil barges. Oily waste from the cleaning process was stored in several unlined earthen pits used as evaporation ponds. Large volumes of waste remain on site, and hazardous substances have been found in ground water, surface water and soil. Wetlands surrounding the site are contaminated with metals and polyaromatic hydrocarbons, and Ida could well have caused additional releases of toxic substances to ground water, surface water, and soil.

Next up in the Atlantic: potential for a long-lived Hurricane Larry

A vigorous tropical disturbance named 90L that rolled off Africa late Monday appears destined to become our next named storm — and most likely our next hurricane. Model ensembles are emphatic that 90L will intensify into a named storm in the next several days, and that it will likely become a powerful hurricane by this weekend as it moves west and eventually northwest into the central Atlantic. In its Tropical Weather Outlook at 2 pm. EDT Tuesday, the National Hurricane Center gave 90L a 90% chance of developing into at least a tropical depression by Thursday. The next name on the Atlantic list is Larry.

Wind shear will remain on the light to moderate side for the next few days, mainly around 10 knots. 90L will also benefit from a very moist atmosphere (mid-level relative humidity of 75-80%) and warm sea surface temperatures (27-28 degrees Celsius, or 81-82°F). Two of our top short-range dynamical intensity models, the HWRF and HMON, steadily strengthen 90L into a hurricane over the next several days: The 12Z Tuesday run of the SHIPS statistical model is also bullish on the prospects for 90L. The SHIPS Rapid Intensification Index gives a 23% chance that 90L will be a 65-mph tropical storm by Wednesday, and a 25% chance it will be a top-end category 2 storm with 110-mph winds by Friday. The newer DTOPS RI product shows a 38% chance of a Cat-2-by-Friday outcome.

90L is expected to move mainly westward until around Thursday, when it will begin feeling the influence of a weakness left by Kate in the Bermuda-Azores High and start heading northwest. This track may continue well into next week, allowing 90L to gain latitude and size as it prowls the open central Atlantic, far to the east of North America.

The National Hurricane Center is also monitoring the potential for development in the southwest Caribbean, where a low pressure area is expected to take shape later this week and move west-northwest to northwest. There is only modest model support for any development in this area, and any such low could move inland over Central America before any serious strengthening. In its 2 p.m. EDT Tuesday Tropical Weather Outlook, the National Hurricane Center gave the system 2-day and 5-day odds of development of 10% and 20%, respectively.

Tropical Storm Kate downgraded to a tropical depression

The eleventh tropical storm of the hyperactive 2021 Atlantic season, Tropical Storm Kate, weakened to a tropical depression at 11 a.m. EDT Tuesday over the central Atlantic. High wind shear and dry air are expected to combine to keep Kate weak, despite relatively favorable sea surface temperatures of 28 degrees Celsius (82°F), and Kate is not expected to regain tropical storm status. During the rest of the week, Kate is predicted to head northward over the central Atlantic, far from any land areas.

How to help hurricane recovery efforts

The Partnership for Inclusive Disaster Strategies (formerly portlight.org), co-founded by members of the Weather Underground community, is responding to the Hurricane Ida disaster. Also, the Weather Channel has put together a list of many excellent charities that will be active in Hurricane Ida recovery.

Also see: Safety tips: How to clean up after a hurricane

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