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IMERG Rainfall Totals from Hurricanes Marco and Laura

Hurricanes Laura and Marco Bring Heavy Rains, Destruction to the Northern Gulf Coast

The northern Gulf Coast, specifically Louisiana, saw two tropical cyclones make landfall in the same week just days apart.  The two systems, however, could not have been more different when they arrived.  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 the 20th of August, 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 was given the name 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.

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IMERG surface rainfall accumulations are shown here for August 22nd to 28th, 2020, for the southeastern US, 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 are just offshore and along the coast of the Florida Panhandle and reach 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 too originated from an easterly wave but began as TD #13 well out into the central tropical Atlantic on the night of the 19th of August.  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 was 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 of 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 of 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 still 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 am 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 am 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.  

IMERG surface rainfall accumulations due to Laura on the northern Gulf Coast are 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.

Credits:

Text and image by Steve Lang (NASA GSFC)