By Bob Henson and Dr. Jeff Masters
A hurricane warning was in effect from Grand Isle, Louisiana, to the Alabama/Florida border on Friday evening as Tropical Storm Nate, with sustained winds of 70 mph as of 11 p.m. EDT, sped through the narrow Yucatan Channel between Mexico and Cuba and into the Gulf of Mexico.
Update: Nate was upgraded to hurricane strength by the NOAA/NWS National Hurricane Center at 11:30 p.m. EDT Friday, with top sustained winds of 75 mph based on Hurricane Hunter reported.
Very intense thunderstorms were erupting on Friday night near Nate's center, located about 90 miles northeast of Cozumel as of 8 p.m. EDT Friday. Nate was in the process of closing off an eyewall, and it is likely to be a Category 1 hurricane on Saturday night when it makes landfall on the U.S. Gulf Coast between Southeast Louisiana and the Florida Panhandle.
Nate's strongest winds and heaviest rains missed the northeast tip of the Yucatan Peninsula, which was located on the left (weak) side of the storm. As of 9 p.m. EDT Friday, the top winds in Cozumel, Mexico and Cancun, Mexico had not exceeded 20 mph, and only intermittent light rain had been reported. The Hurricane Hunters found Nate's strongest winds were the southeast of the center, over the Yucatan Channel, where Buoy 42056, 120 nm ESE of Cozumel, reported sustained winds of 56 mph, gusting to 69 mph, just after 4 p.m. EDT Friday.
Above: Microwave satellite view of Nate taken at 7:45 p.m. EDT Oct. 6, showing a calm area at Nate's center surrounded by precipitation. The wavelength at which this image was collected, 37 GHz, senses precipitating clouds but does not highlight deep convection (intense showers and thunderstorms). Imagery at the 85 GHz wavelength, which does distinguish deep convection, showed that Nate lacked a complete eyewall at this point. Naval Research Laboratory
Dangerous heavy rains from Nate have affected large parts of Central America. As of Friday evening, Nate had led to a total of 25 deaths in Central America; hardest hit were Nicaragua with 12 deaths, and Costa Rica with nine. Satellite rainfall estimates show that Nate has dumped 8+" of rain on the Pacific side of Costa Rica, Nicaragua and Panama, and also along the northern coast of Honduras and the eastern coast of the Yucatan Peninsula, in both Mexico and Belize.
Figure 1. Infrared GOES-16 satellite image of Tropical Storm Nate at 9:02 p.m. EDT Oct. 6. The white areas indicate very strong thunderstorms surrounding Nate's center. Image credit: NASA/MSFC Earth Science Branch. GOES-16 data are considered preliminary and non-operational.
Forecast for Nate Through Landfall
Satellite images early Friday evening showed that Nate continued to struggle to consolidate; the storm lacked symmetry and had an odd, clumpy appearance to its heavy thunderstorms. However, by late evening, Nate had finally developed a well-formed central dense overcast (CDO) over its center—the large, thick area of high cirrus clouds that normally appears when a storm becomes well-organized and nears hurricane strength. Microwave imagery showed that Nate appeared to be wrapping an eyewall around its east side.
Conditions will be quite supportive of intensification through Saturday. Wind shear will be light to moderate, around 10 knots, and the surrounding atmosphere will remain quite moist, with mid-level relative humidity in the 70 – 80 percent range. Sea-surface temperatures along Nate's path will remain near or above 29°C (84°F)—about 1°C above average for early October—until a few hours before landfall. The eastern part of Nate's circulation will be passing over a warm eddy associated with the Loop Current, which will help keep Nate from churning up cooler water. Nate's rapid forward speed will have a similar effect.
Figure 2. Ocean Heat Content (OHC) for Oct. 6. Forecast positions for Nate from the 8 a.m. EDT Friday NHC forecast are also shown. OHC values in excess of 80 kilojoules per square centimeter (yellow-green colors) are often associated with rapid intensification of hurricanes. On Saturday, the eastern part of Nate's circulation will be passing over the northern portion of the Loop Current, where a warm eddy appears to be attempting to break off. University of Miami Rosenstiel School of Marine and Atmospheric Science
NHC defines rapid intensification as an increase in sustained winds of at least 30 knots (35 mph) in 24 hours. Given the factors above, this seems possible, though it's far from guaranteed. Nate remains an asymmetric storm, embedded in a complex larger circulation, so it is unlikely to crank up as dramatically as some other rapid intensifiers we've seen this year. The 0Z Saturday run of the SHIPS statistical model gave Nate a 31 percent chance of gaining at least 30 knots of intensity in 24 hours, and a 47 percent chance of gaining at least 25 knots. However, Nate's less-than-ideal structure is not reflected in the SHIPS output. Our top dynamical models, including those tuned for intensity prediction such as the HWRF and HMON, were surprisingly low-key on the odds of Nate strengthening. The 18Z Friday runs of the GFS, HWRF, and HMON dynamical models all bring Nate to the U.S. Gulf Coast as a tropical storm. Nate's upgrade to hurricane strength late Friday night makes these model projections quite suspect, though.
Taking all these mixed signals into account, the prudent approach is to get ready for Nate to arrive on the Gulf Coast as a higher-end Category 1 storm—with a chance of pushing into the Cat 2 range—and keep our fingers crossed that Nate will fail to maximize its potential, as the dynamical models insist.
Impacts From Nate
The track forecast for Nate is quite straightforward. As Nate enters a region of strong upper-level steering, it will continue briskly toward the north-northwest on Saturday and will be approaching the U.S. Gulf Coast on Saturday night, most likely somewhere between New Orleans and Pensacola. Nate is projected to angle toward the north-northeast around this time, and the exact location of this turn will be crucial to the landfall location. If the turn is delayed, landfall could be in far southeast Louisiana, whereas a faster turn will bring the center closer to the coast of Alabama or the far western Florida Panhandle.
Winds: Peak winds in Nate will hinge on how quickly the storm intensifies on Saturday. The hurricane-force wind field at landfall is not expected to be broad—perhaps just 20 or 30 miles wide. Tropical-storm-strength winds could affect a much broader region, up to 200 miles wide, as Nate pushes inland. As we saw with Hurricane Irma in Florida, sustained winds below hurricane strength can still be enough to bring down trees and power lines in wet soil and produce widespread power outages.
Since Nate will make landfall as a fairly fast-moving system (around 15 – 20 mph), there will be a stronger-than-usual asymmetry to its wind field, with the bulk of the strong winds to the right (east) of Nate's center. NOAA's Atlantic Oceanographic & Meteorological Laboratory explained: "In general, the strongest winds in a hurricane are found on the right side of the storm because the motion of the hurricane also contributes to its swirling winds. A hurricane with a 90 mph [145 km/hr] winds while stationary would have winds up to 100 mph [160 km/hr] on the right side and only 80 mph [130 km/hr] on the left side if it began moving (any direction) at 10 mph [16 km/hr]. Note that forecasting center advisories already take this asymmetry into account and, in this case, would state that the highest winds were 100 mph [160 km/hr]."
Figure 3. Diagram showing the additive and subtractive effect of storm motion on peak winds at either side of a tropical cyclone's center. Chris Landsea (NOAA / NHC), courtesy NOAA / AOML
Surge: A storm surge warning is up for the Gulf Coast from Morgan City, Louisiana, to the Okaloosa/Walton county line in Florida, as well as along the northern and western shores of Lake Pontchartrain. The highest surge from Nate will arrive quickly on Saturday night, and will likely peak before dawn Sunday, so residents need to take the surge threat seriously and make final preparations as soon as possible on Saturday. As of 8 p.m. EDT Friday, the following inundations above ground level are possible with Nate, assuming the storm were to arrive during high tide:
- Morgan City, Louisiana to the mouth of the Mississippi River ... 4 to 6 ft
- Mouth of the Mississippi River to the Alabama/Florida border ... 5 to 8 ft
- Alabama/Florida border to the Okaloosa/Walton County Line ... 4 to 6 ft
- Okaloosa/Walton County Line to Indian Pass, Florida ... 2 to 4 ft
- Indian Pass to Crystal River, Florida ... 1 to 3 ft
Because the daily high tide across this region occurs during the pre-dawn hours, it is quite possible that Nate will reach the coast near high tide. Tidal range between low and high tide is 1 - 1.3' along the central Gulf Coast, so the timing of Nate's storm surge with respect to the high tide can cause an additional foot or so of flooding. High tide in Mobile, Alabama is at 1:46 a.m. local time Sunday, and it will be one of the highest high tides of the year, due to the full moon. Low tide is at 10:12 a.m. Saturday. At Shell Beach, Louisiana, on the east side of New Orleans, high tide is at 4:29 a.m. local time Sunday, and low tide is at 12:14 p.m. Saturday.
Rains: Nate's rapid motion will limit the total amount of rainfall at any one spot, and the overall accumulations should be less than those observed during slower-moving tropical cyclones. However, Nate is embedded in a very moist atmosphere, and torrential rain could still fall in short periods. Nate's quick-hitting rains could exacerbate the strain on the troubled levee system in and around New Orleans, especially if Nate tracks toward the western end of its forecast range. Localized rainfall totals of 6 - 10" are expected close to Nate's landfall location, and intense rainbands will stream onshore well east of Nate's center across the Florida Panhandle. A large area of 4 - 12" rains, perhaps including Atlanta and Nashville, may develop as Nate accelerates into the southern Appalachians on Sunday. Rains of 2 - 6" will stream across the northern Appalachians and into southern New England on Monday, as Nate races northeast as a fast-weakening tropical cyclone.
This animation of NOAA's GOES East satellite imagery of Tropical Storm Nate from Oct. 5 at 5:45 a.m. EDT (0945 UTC) to Oct. 7 ending at 6:00 a.m. EDT (1000 UTC) ends after Nate strengthened into a hurricane while moving through the Gulf of Mexico. TRT: 00:27. NASA / NOAA GOES Project
Reposted with permission from our media associate Weather Underground.
These longer periods of stratification could have "far-reaching implications" for lake ecosystems, the paper says, and can drive toxic algal blooms, fish die-offs and increased methane emissions.
The study, published in Nature Communications, finds that the average seasonal lake stratification period in the northern hemisphere could last almost two weeks longer by the end of the century, even under a low emission scenario. It finds that stratification could last over a month longer if emissions are extremely high.
If stratification periods continue to lengthen, "we can expect catastrophic changes to some lake ecosystems, which may have irreversible impacts on ecological communities," the lead author of the study tells Carbon Brief.
The study also finds that larger lakes will see more notable changes. For example, the North American Great Lakes, which house "irreplaceable biodiversity" and represent some of the world's largest freshwater ecosystems, are already experiencing "rapid changes" in their stratification periods, according to the study.
'Fatal Consequences'
As temperatures rise in the spring, many lakes begin the process of "stratification." Warm air heats the surface of the lake, heating the top layer of water, which separates out from the cooler layers of water beneath.
The stratified layers do not mix easily and the greater the temperature difference between the layers, the less mixing there is. Lakes generally stratify between spring and autumn, when hot weather maintains the temperature gradient between warm surface water and colder water deeper down.
Dr Richard Woolway from the European Space Agency is the lead author of the paper, which finds that climate change is driving stratification to begin earlier and end later. He tells Carbon Brief that the impacts of stratification are "widespread and extensive," and that longer periods of stratification could have "irreversible impacts" on ecosystems.
For example, Dr Dominic Vachon – a postdoctoral fellow from the Climate Impacts Research Centre at Umea University, who was not involved in the study – explains that stratification can create a "physical barrier" that makes it harder for dissolved gases and particles to move between the layers of water.
This can prevent the oxygen from the surface of the water from sinking deeper into the lake and can lead to "deoxygenation" in the depths of the water, where oxygen levels are lower and respiration becomes more difficult.
Oxygen depletion can have "fatal consequences for living organisms," according to Dr Bertram Boehrer, a researcher at the Helmholtz Centre for Environmental Research, who was not involved in the study.
Lead author Woolway tells Carbon Brief that the decrease in oxygen levels at deeper depths traps fish in the warmer surface waters:
"Fish often migrate to deeper waters during the summer to escape warmer conditions at the surface – for example during a lake heatwave. A decrease in oxygen at depth will mean that fish will have no thermal refuge, as they often can't survive when oxygen concentrations are too low."
This can be very harmful for lake life and can even increase "fish die-off events" the study notes.
However, the impacts of stratification are not limited to fish. The study notes that a shift to earlier stratification in spring can also encourage communities of phytoplankton – a type of algae – to grow sooner, and can put them out of sync with the species that rely on them for food. This is called a "trophic mismatch."
Prof Catherine O'Reilly, a professor of geography, geology and the environment at Illinois State University, who was not involved in the study, adds that longer stratified periods could also "increase the likelihood of harmful algae blooms."
The impact of climate change on lakes also extends beyond ecosystems. Low oxygen levels in lakes can enhance the production of methane, which is "produced in and emitted from lakes at globally significant rates," according to the study.
Woolway explains that higher levels of warming could therefore create a positive climate feedback in lakes, where rising temperatures mean larger planet-warming emissions:
"Low oxygen levels at depth also promotes methane production in lake sediments, which can then be released to the surface either via bubbles or by diffusion, resulting in a positive feedback to climate change."
Onset and Breakup
In the study, the authors determine historical changes in lake stratification periods using long-term observational data from some of the "best-monitored lakes in the world" and daily simulations from a collection of lake models.
They also run simulations of future changes in lake stratification period under three different emission scenarios, to determine how the process could change in the future. The study focuses on lakes in the northern hemisphere.
The figure below shows the average change in lake stratification days between 1900 and 2099, compared to the 1970-1999 average. The plot shows historical measurements (black), and the low emission RCP2.6 (blue), mid emissions RCP6.0 (yellow) and extremely high emissions RCP8.5 (red) scenarios.
Change in lake stratification duration compared to the 1970-1999 average, for historical measurements (black), the low emission RCP2.6 (blue) moderate emissions RCP6.0 (yellow) and extremely high emissions RCP8.5 (red). Credit: Woolway et al (2021).
The plot shows that the average lake stratification period has already lengthened. However, the study adds that some lakes are seeing more significant impacts than others.
For example, Blelham Tarn – the most well-monitored lake in the English Lake District – is now stratifying 24 days earlier and maintaining its stratification for an extra 18 days compared to its 1963-1972 averages, the study finds. Woolway tells Carbon Brief that as a result, the lake is already showing signs of oxygen depletion.
Climate change is increasing average stratification duration in lakes, the findings show, by moving the onset of stratification earlier and pushing the stratification "breakup" later. The table below shows projected changes in the onset, breakup and overall length of lake stratification under different emission scenarios, compared to a 1970-1999 baseline.
The table shows that even under the low emission scenario, the lake stratification period is expected to be 13 days longer by the end of the century. However, in the extremely high emissions scenario, it could be 33 days longer.
The table also shows that stratification onset has changed more significantly than stratification breakup. The reasons why are revealed by looking at the drivers of stratification more closely.
Warmer Weather and Weaker Winds
The timing of stratification onset and breakup in lakes is driven by two main factors – temperature and wind speed.
The impact of temperature on lake stratification is based on the fact that warm water is less dense than cool water, Woolway tells Carbon Brief:
"Warming of the water's surface by increasing air temperature causes the density of water to decrease and likewise results in distinct thermal layers within a lake to form – cooler, denser water settles to the bottom of the lake, while warmer, lighter water forms a layer on top."
This means that, as climate change causes temperatures to rise, lakes will begin to stratify earlier and remain stratified for longer. Lakes in higher altitudes are also likely to see greater changes in stratification, Woolway tells Carbon Brief, because "the prolonging of summer is very apparent in high latitude regions."
The figure below shows the expected increase in stratification duration from lakes in the northern hemisphere under the low (left), mid (center), and high (right) emission scenarios. Deeper colors indicate a larger increase in stratification period.
Expected increase in stratification duration in lakes in the northern hemisphere under the low (left), mid (centre) and high (right) emissions scenarios. Credit: Woolway et al (2021).
The figure shows that the expected impact of climate change on stratification duration becomes more pronounced at more northerly high latitudes.
The second factor is wind speed, Woolway explains:
"Wind speed also affects the timing of stratification onset and breakdown, with stronger winds acting to mix the water column, thus acting against the stratifying effect of increasing air temperature."
According to the study, wind speed is expected to decrease slightly as the planet warms. The authors note that the expected changes in near-surface wind speed are "relatively minor" compared to the likely temperature increase, but they add that it may still cause "substantial" changes in stratification.
The study finds that air temperature is the most important factor behind when a lake will begin to stratify. However, when looking at stratification breakup, it finds that wind speed is a more important driver.
Meanwhile, Vachon says that wind speeds also have implications for methane emissions from lakes. He notes that stratification prevents the methane produced on the bottom of the lake from rising and that, when the stratification period ends, methane is allowed to rise to the surface. However, according to Vachon, the speed of stratification breakup will affect how much methane is released into the atmosphere:
"My work has suggested that the amount of accumulated methane in bottom waters that will be finally emitted is related to how quickly the stratification break-up occurs. For example, a slow and progressive stratification break-up will most likely allow water oxygenation and allow the bacteria to oxidise methane into carbon dioxide. However, a stratification break-up that occurs rapidly – for example after storm events with high wind speed – will allow the accumulated methane to be emitted to the atmosphere more efficiently."
Finally, the study finds that large lakes take longer to stratify in spring and typically remain stratified for longer in the autumn – due to their higher volume of water. For example, the authors highlight the North American Great Lakes, which house "irreplaceable biodiversity" and represent some of the world's largest freshwater ecosystems.
These lakes have been stratifying 3.5 days earlier every decade since 1980, the authors find, and their stratification onset can vary by up to 48 days between some extreme years.
O'Reilly tells Carbon Brief that "it's clear that these changes will be moving lakes into uncharted territory" and adds that the paper "provides a framework for thinking about how much lakes will change under future climate scenarios."
Reposted with permission from Carbon Brief.
Currently the U.S. Supreme Court has on its docket a case between Texas, New Mexico and Colorado and another one between Mississippi and Tennessee. The court has already ruled this term on cases pitting Texas against New Mexico and Florida against Georgia.
Climate stresses are raising the stakes. Rising temperatures require farmers to use more water to grow the same amount of crops. Prolonged and severe droughts decrease available supplies. Wildfires are burning hotter and lasting longer. Fires bake the soil, reducing forests' ability to hold water, increasing evaporation from barren land and compromising water supplies.
As a longtime observer of interstate water negotiations, I see a basic problem: In some cases, more water rights exist on paper than as wet water – even before factoring in shortages caused by climate change and other stresses. In my view, states should put at least as much effort into reducing water use as they do into litigation, because there are no guaranteed winners in water lawsuits.
Alabama, pay attention to Supreme Court ruling against Florida in water war #Water #SDG6 https://t.co/wIjdoY6Ccr— Noah J. Sabich (@Noah J. Sabich)1617800452.0
Dry Times in the West
The situation is most urgent in California and the Southwest, which currently face "extreme or exceptional" drought conditions. California's reservoirs are half-empty at the end of the rainy season. The Sierra snowpack sits at 60% of normal. In March 2021, federal and state agencies that oversee California's Central Valley Project and State Water Project – regional water systems that each cover hundreds of miles – issued "remarkably bleak warnings" about cutbacks to farmers' water allocations.
The Colorado River Basin is mired in a drought that began in 2000. Experts disagree as to how long it could last. What's certain is that the "Law of the River" – the body of rules, regulations and laws governing the Colorado River – has allocated more water to the states than the river reliably provides.
The 1922 Colorado River Compact allocated 7.5 million acre-feet (one acre-foot is roughly 325,000 gallons) to California, Nevada and Arizona, and another 7.5 million acre-feet to Utah, Wyoming, Colorado and New Mexico. A treaty with Mexico secured that country 1.5 million acre-feet, for a total of 16.5 million acre-feet. However, estimates based on tree ring analysis have determined that the actual yearly flow of the river over the last 1,200 years is roughly 14.6 million acre-feet.
The inevitable train wreck has not yet happened, for two reasons. First, Lakes Mead and Powell – the two largest reservoirs on the Colorado – can hold a combined 56 million acre-feet, roughly four times the river's annual flow.
But diversions and increased evaporation due to drought are reducing water levels in the reservoirs. As of Dec. 16, 2020, both lakes were less than half full.
Second, the Upper Basin states – Utah, Wyoming, Colorado and New Mexico – have never used their full allotment. Now, however, they want to use more water. Wyoming has several new dams on the drawing board. So does Colorado, which is also planning a new diversion from the headwaters of the Colorado River to Denver and other cities on the Rocky Mountains' east slope.
Drought conditions in the continental U.S. on April 13, 2021. U.S. Drought Monitor, CC BY-ND
Utah Stakes a Claim
The most controversial proposal comes from one of the nation's fastest-growing areas: St. George, Utah, home to approximately 90,000 residents and lots of golf courses. St. George has very high water consumption rates and very low water prices. The city is proposing to augment its water supply with a 140-mile pipeline from Lake Powell, which would carry 86,000 acre-feet per year.
Truth be told, that's not a lot of water, and it would not exceed Utah's unused allocation from the Colorado River. But the six other Colorado River Basin states have protested as though St. George were asking for their firstborn child.
In a joint letter dated Sept. 8, 2020, the other states implored the Interior Department to refrain from issuing a final environmental review of the pipeline until all seven states could "reach consensus regarding legal and operational concerns." The letter explicitly threatened a high "probability of multi-year litigation."
Utah blinked. Having earlier insisted on an expedited pipeline review, the state asked federal officials on Sept. 24, 2020 to delay a decision. But Utah has not given up: In March 2021, Gov. Spencer Cox signed a bill creating a Colorado River Authority of Utah, armed with a $9 million legal defense fund, to protect Utah's share of Colorado River water. One observer predicted "huge, huge litigation."
How huge could it be? In 1930, Arizona sued California in an epic battle that did not end until 2006. Arizona prevailed by finally securing a fixed allocation from the water apportioned to California, Nevada and Arizona.
Litigation or Conservation
Before Utah takes the precipitous step of appealing to the Supreme Court under the court's original jurisdiction over disputes between states, it might explore other solutions. Water conservation and reuse make obvious sense in St. George, where per-person water consumption is among the nation's highest.
St. George could emulate its neighbor, Las Vegas, which has paid residents up to $3 per square foot to rip out lawns and replace them with native desert landscaping. In April 2021 Las Vegas went further, asking the Nevada Legislature to outlaw ornamental grass.
The Southern Nevada Water Authority estimates that the Las Vegas metropolitan area has eight square miles of "nonfunctional turf" – grass that no one ever walks on except the person who cuts it. Removing it would reduce the region's water consumption by 15%.
Water rights litigation is fraught with uncertainty. Just ask Florida, which thought it had a strong case that Georgia's water diversions from the Apalachicola-Chattahoochee-Flint River Basin were harming its oyster fishery downstream.
That case extended over 20 years before the U.S. Supreme Court ended the final chapter in April 2021. The court used a procedural rule that places the burden on plaintiffs to provide "clear and convincing evidence." Florida failed to convince the court, and walked away with nothing.
Robert Glennon is a Regents Professor and Morris K. Udall Professor of Law & Public Policy, University of Arizona.
Disclosure statement: Robert Glennon received funding from the National Science Foundation in the 1990s and 2000s.
Reposted with permission from The Conversation.