Sep. 01, 2020 – NASA’s IMERG Analyzed Heavy Rains from Hurricanes Laura and Marco
The northern Gulf Coast, specifically Louisiana, saw two tropical cyclones make landfall in the same week just days apart. NASA’s IMERG surface rainfall accumulations were calculated for the period 22 to 27 August 2020 for both Hurricanes Marco and Laura. The two systems, however, could not have been more different when they arrived.
Marco’s History and Track
Despite forming a day later, Marco was the first system to make landfall on the Gulf Coast. Marco originated from a tropical easterly wave that was moving from the central to the western Caribbean. After becoming a tropical depression (TD) on August 20, TD #14 turned northwestward the following day as it approached the coast of Central America and moved into the northwest Caribbean.
According to the National Hurricane Center (NHC), TD #14 reached tropical storm intensity on the night of the 21st and named Marco. Marco remained over warm open water as it passed through the Yucatan Straight and into the Gulf of Mexico on the afternoon of the 22nd. Marco was now a strong tropical storm with sustained winds reported at 65 mph by NHC and seemed poised to intensify.
However, the warm waters were offset by the effects of southwesterly wind shear, which were to become a major inhibiting factor as Marco moved through the Gulf of Mexico.
Although the wind shear kept Marco from really intensifying, the warm Gulf waters supported enough convection for Marco to reach hurricane intensity around midday on Sunday the 23rd. By now Marco was in the central Gulf and moving north between a ridge of high pressure to the east and an upper-level trough over the western Gulf. However, as Marco continued northward the wind shear increased, pushing the thunderstorms that were fueling the system along with most of the rain away from the center off to the northeast, causing Marco to weaken back down to a tropical storm that very same evening.
On the 24th, as it neared the northern Gulf Coast, Marco continued to weaken and slow down as high pressure to the east built westward, blocking Marco’s path and forcing the cyclone to turn westward. The center of circulation just made landfall near the mouth of the Mississippi at 6:00 pm CDT on Monday the 24th with sustained winds reported at 40 mph by NHC before being shunted westward back over open water and weakening further. By now, the center was essentially devoid of thunderstorms with the bulk of the rain falling well east of the center.
NASA Rainfall Accumulations on Marco
IMERG surface rainfall accumulations were compiled for the period 22 to 27 August 2020 for the southeastern U.S., Gulf of Mexico, the northwest Caribbean and adjacent regions. Accumulations include the effects from both Laura and Marco. However, the rainfall due to Marco is evidenced by its north-south orientation and generally extends from the west-central Caribbean up through the Yucatan Strait and the eastern Gulf of Mexico and into the Florida Panhandle.
The heaviest rainfall totals due to Marco were just offshore and along the coast of the Florida Panhandle and reached up to 225 mm (~9 inches, shown in dark red) while a good portion of the Florida Panhandle is estimated to have received at least 75 to 100 mm (~3 to 4 inches, shown in bright green and yellow). Rainfall totals near the mouth of the Mississippi where the center made landfall are much lower and reflects the storm’s asymmetric structure due to the strong wind shear where most of the thunderstorms and associated rainfall occurred well northeast of the center.
Laura’s History and Track
Laura too originated from an easterly wave but began as TD #13 well out into the central tropical Atlantic on the night of August 19. Over the next couple of days, despite passing over warm water, TD #13 was held in check by a combination of mid-level wind shear and dry air that kept the system poorly organized. However, as it neared the Leeward Isles on the 21st, it finally reached minimum tropical storm intensity and named Laura.
Laura then passed through the Leeward Islands as a weak tropical storm later that same day. As it approached Puerto Rico on the 22nd, Laura was still unorganized, and although the wind shear abated, Laura was now inhibited by its close proximity to land. After passing very near the southwest coast of Puerto Rico, Laura crossed directly over the island of Hispaniola on the night of the 22nd; however, despite being over land, Laura continued to generate deep convection and emerged off the west coast of Haiti with sustained winds reported at 50 mph by NHC on the morning of the 23rd.
Laura gained a little strength before crossing over southeastern Cuba on the night of the 23rd; again, despite being over land, Laura continued to tap into the warm waters off the south coast of Cuba and actually intensified slightly. On the morning of the 24th, Laura re-emerged over open water south of Cuba.
At this point, another key factor came into play. The ridge of high pressure north of Laura that was steering the storm expanded westward, keeping the center well over water and further away from the south coast of Cuba. Despite this, northerly wind shear, drier air and some land interaction initially weakened the storm slightly as it paralleled the south coast of Cuba. However, as Laura neared and crossed western Cuba on the evening of the 24th, it was already showing signs of becoming better organized though it remained at tropical storm intensity.
When Laura emerged out into the southeast Gulf of Mexico early on the morning of the 25th, it did so over deep, warm water in a humid, relatively low wind-shear environment, the perfect conditions for intensification. Almost immediately, strong convection fired up near Laura’s core, lowering the central pressure, and by 7:15 a.m. CDT on Tuesday August 25th, Laura was a Category 1 hurricane. Now well organized and in ideal conditions for strengthening, Laura was primed for further, rapid intensification, which is exactly what happened.
Over the next 36 hours, as Laura gradually turned northward around the western edge of a high pressure ridge across Florida and headed for the northern Gulf Coast, it underwent a rapid deepening cycle. Reports from NHC showed that maximum sustained winds increased from 75 to 150 mph over this period, taking Laura from Category 1 to a Category 4 hurricane.
By this time, Laura was very near to the coast of western Louisiana where it then made landfall near Cameron, Louisiana at 1:00 a.m. CDT at the same 150 mph intensity. A wind gust to 128 mph was reported at the Lake Charles Regional Airport. It is estimated that Laura maintained hurricane intensity for the next 10 hours as it moved northward into northern Louisiana.
NASA Rainfall Accumulations on Laura
IMERG surface rainfall accumulations from Laura on the northern Gulf Coast were actually lower than they are for Marco and generally range from over 50 to 100 mm (~2 to 4 inches, shown in green and yellow) with local pockets of higher amounts (shown in orange) extending across most of Louisiana and Arkansas. Rainfall amounts depend more on system speed than intensity; while Marco slowed down near the coast, Laura continued to push northward. However, elsewhere in the Caribbean, IMERG estimates show from 125 to over 225 mm (~5 to over 9 inches, shown in orange and darker red) of rain over the southern half of Hispaniola, where a total of 35 fatalities were reported to due to the storm, 31 of which were in Haiti. So far, 22 deaths are being blamed on Laura in the US, 14 of those in Louisiana.
About IMERG
The near-real-time rain estimate comes from the NASA’s Integrated Multi-satellitE Retrievals for GPM (IMERG) algorithm, which combines observations from a fleet of satellites, in near real time, to provide near-global estimates of precipitation every 30 minutes. This satellite-based rain estimate is somewhat coarse in resolution and can miss short-lived, intense storm-cells, but the IMERG algorithm often does captures the large-scale features of storms wherever they form in the world. While the United States is fortunate to have a network of ground radars that can provide higher-resolution precipitation estimates, in other parts of the world, notably over most of the world’s oceans, the IMERG rain estimate is an important reference point.
This near-real time rain estimate comes from the NASA’s IMERG algorithm, which combines observations from a fleet of satellites, in near-real time, to provide near-global estimates of precipitation every 30 minutes. By combining NASA precipitation estimates with other data sources, we can gain a greater understanding of major storms that affect our planet.
IMERG fills in the “blanks” between weather observation stations. IMERG satellite-based rain estimates can be compared to that from a National Weather Service ground radar. Such good detection of large rain features in real time would be impossible if the IMERG algorithm merely reported the precipitation observed by the periodic overflights of various agencies’ satellites. Instead, what the IMERG algorithm does is “morph” high-quality satellite observations along the direction of the steering winds to deliver information about rain at times and places where such satellite overflights did not occur. Information morphing is particularly important over the majority of the world’s surface that lacks ground-radar coverage.
By Steve Lang
NASA’s Goddard Space Flight Center
Aug. 25, 2020 – NASA’s Terra Satellite Catches the Demise of Post-Tropical Cyclone Marco
NASA’s Terra satellite passed over the Gulf of Mexico early on Aug. 25 and found a very small area of convection from post-tropical cyclone Marco, northeast of its center. All watches and warnings have been dropped as the storm continues to weaken toward dissipation.
Visible imagery and surface observations indicated that Marco made landfall around 7 p.m. EDT on Aug. 24 near the mouth of the Mississippi River. The center continued to move west and moved offshore and south of Louisiana by Aug. 25.
NASA’s Terra Satellite Reveals Effects of Wind Shear
NASA’s Terra satellite uses infrared light to analyze the strength of storms by providing temperature information about the system’s clouds. The strongest thunderstorms that reach high into the atmosphere have the coldest cloud top temperatures.
On Aug. 25 at 12:30 a.m. EDT (0430 UTC), the Moderate Resolution Imaging Spectroradiometer or MODIS instrument that flies aboard NASA’s Terra satellite observed Marco in infrared light and found a small area of storms where cloud top temperatures as cold as minus 50 degrees Fahrenheit (minus 45.5 Celsius) over the western Florida Panhandle and coastal Alabama. Those storms were being pushed northeast of Marco’s center from southwesterly wind shear. Satellite imagery also shows the low-level circulation center was a swirl of clouds south of Louisiana, over the Gulf of Mexico.
In the Aug. 25, Marco discussion at 5 a.m. EDT, NHC Senior Hurricane Specialist Stacy Stewart noted, “Marco has been devoid of any significant convection for at least 12 hours. [NOAA’s Advanced Scatterometer] ASCAT scatterometer surface wind data around 0239Z (10:39 p.m. EDT on Aug. 24) suggested that Marco might have degenerated in a north-to-south elongated trough (elongated area of low pressure). Based on this information, Marco has been downgraded to post-tropical remnant low [pressure area].”
About Wind Shear
The shape of a tropical cyclone provides forecasters with an idea of its organization and strength. When outside winds batter a storm, it can change the storm’s shape and push much of the associated clouds and rain to one side of it. That is what wind shear does.
In general, wind shear is a measure of how the speed and direction of winds change with altitude. Tropical cyclones are like rotating cylinders of winds. Each level needs to be stacked on top each other vertically in order for the storm to maintain strength or intensify. Wind shear occurs when winds at different levels of the atmosphere push against the rotating cylinder of winds, weakening the rotation by pushing it apart at different levels.
Marco’s Final Status
At 5 a.m. EDT (0900 UTC) on Aug. 25, NOAA’s National Hurricane Center (NHC) reported the center of Post-Tropical Cyclone Marco was located near latitude 28.8 degrees north and longitude 91.2 degrees west. That is about 60 miles (100 km) south of Morgan City, La. and 110 miles (175 km) south-southeast of Lafayette, La. The post-tropical cyclone was moving toward the west near 10 mph (17 kph), and this general motion is expected to continue for the next day or so. Maximum sustained winds were near 30 mph (45 kph) with higher gusts. The estimated minimum central pressure was 1008.
Marco Nears its End
Brisk southwesterly vertical wind shear of 30 knots is forecast to increase to near 35 knots in 24 hours, which should prevent the redevelopment of deep convection near the center. On the forecast track, Marco should continue moving westward just offshore the coast of Louisiana until the system dissipates.
NASA Researches Tropical Cyclones
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For updated forecasts. visit: www.nhc.noaa.gov
