Hurricane Marco Takes Aim At U.S. Gulf Coast

Hurricane Marco strengthened from a tropical storm this morning, though it continues to battle with moderate wind shear from the west.

Marco has beaten the record for the earliest “M” Atlantic named storm, previously held by Maria on September 2nd, 2005 and is the first August hurricane in the Gulf of Mexico since Hurricane Harvey in 2017.

This tropical cyclone is no threat to Trinidad, Tobago, and the Eastern Caribbean.

At 5:00 PM AST, the center of Hurricane Marco was located near latitude 25.8 North, longitude 87.8 West. Marco is moving toward the north-northwest near 13 MPH (20 KM/H), a turn to the northwest is expected later tonight followed by a turn to the west-northwest by Monday night. On the forecast track, Marco will be near the Louisiana coast Monday afternoon, and move near or over the coast through Tuesday.

Maximum sustained winds are near 75 MPH (120 KM/H) with higher gusts. Little change is strength is expected over the next 24 hours. Gradual weakening is expected thereafter, and Marco could become a remnant low by Tuesday night.

Hurricane-force winds extend outward up to 15 miles (30 kilometers) from the center and tropical-storm-force winds extend outward up to 105 miles (165 kilometers). The estimated minimum central pressure is 991 millibars.

Hurricane Marco Forecast Track as of 5:00 PM Sunday 23rd August 2020. (National Hurricane Center)
Hurricane Marco Forecast Track as of 5:00 PM Sunday 23rd August 2020. (National Hurricane Center)

Watches & Warnings

Hurricane Marco Watches and Warnings as of 5:00 PM Sunday 23rd August 2020. (National Hurricane Center)
Hurricane Marco Watches and Warnings as of 5:00 PM Sunday 23rd August 2020. (National Hurricane Center)

A Storm Surge Warning is in effect for:

  • Morgan City Louisiana to Ocean Springs MississippiLake Borgne

A Hurricane Warning is in effect for:

  • Morgan City Louisiana to the Mouth of the Pearl River

A Storm Surge Watch is in effect for:

  • Sabine Pass to Morgan City Louisiana
  • Ocean Springs Mississippi to the Mississippi/Alabama border
  • Lake Pontchartrain and Lake Maurepas

A Hurricane Watch is in effect for:

  • Intracoastal City to west of Morgan City
  • Lake Pontchartrain, Lake Maurepas, and Metropolitan New Orleans

A Tropical Storm Warning is in effect for:

  • Mouth of the Pearl River to the Mississippi/Alabama border
  • Lake Pontchartrain, Lake Maurepas, and Metropolitan New Orleans
  • Cameron to west of Morgan City

A Tropical Storm Watch is in effect for:

  • Mississippi/Alabama border to the Alabama/Florida border

A Storm Surge Warning means there is a danger of life-threatening inundation, from rising water moving inland from the coastline, during the next 36 hours in the indicated locations. This is a life-threatening situation. Persons located within these areas should take all necessary actions to protect life and property from rising water and the potential for other dangerous conditions. Promptly follow evacuation and other instructions from local officials.

A Hurricane Warning means that hurricane conditions are expected somewhere within the warning area. A warning is typically issued 36 hours before the anticipated first occurrence of tropical-storm-force winds, conditions that make outside preparations difficult or dangerous. Preparations to protect life and property should be rushed to completion.

A Storm Surge Watch means there is a possibility of life-threatening inundation, from rising water moving inland from the coastline, in the indicated locations during the next 48 hours.

A Hurricane Watch means that hurricane conditions are possible within the watch area.

A Tropical Storm Warning means that tropical storm conditions are expected somewhere within the warning area within 36 hours.

A Tropical Storm Watch means that tropical storm conditions are possible within the watch area, generally within 48 hours.

Hazards Affecting Land

Hurricane Marco forecast storm surge inundation. These values represent the peak height the water could reach above normally dry ground somewhere within the specified areas. (National Hurricane Center)
Hurricane Marco forecast storm surge inundation. These values represent the peak height the water could reach above normally dry ground somewhere within the specified areas. (National Hurricane Center)

STORM SURGE: The combination of a dangerous storm surge and the tide will cause normally dry areas near the coast to be flooded by rising waters moving inland from the shoreline. The water could reach the following heights above ground somewhere in the indicated areas if the peak surge occurs at the time of high tide:

  • Morgan City LA to Mouth of the Mississippi River: 4-6 ft
  • Mouth of the Mississippi River to Ocean Springs MS including Lake Borgne: 3-5 ft
  • Lake Pontchartrain and Lake Maurepas: 2-4 ft
  • Intracoastal City LA to Morgan City LA: 2-4 ft
  • Sabine Pass to Intracoastal City: 1-3 ft
  • Ocean Springs MS to the AL/FL Border including Mobile Bay: 1-3 ft

The deepest water will occur along the immediate coast in areas of onshore winds, where the surge will be accompanied by large and dangerous waves. Surge-related flooding depends on the relative timing of the surge and the tidal cycle, and can vary greatly over short distances.

Hurricane Marco probabilities for tropical-storm-force winds as of 5:00 PM Sunday 23rd August 2020, as well as the most likely time of arrival of tropical-storm-force winds. (National Hurricane Center)

WIND: Hurricane conditions are expected within the hurricane warning area by midday Monday, with tropical storm conditions possible by early Monday. Tropical storm conditions are possible within the tropical storm watch area on Monday, and hurricane conditions are possible within the hurricane watch areas late Monday.

Hurricane Marco forecast rainfall as of 5:00 PM Sunday 23rd August 2020. This graphic is created by the NWS/NCEP Weather Prediction Center (WPC) and shows rainfall potential for the United States when a tropical cyclone threatens land. The graphic is displayed as a Quantitative Precipitation Forecast (QPF), which shows rainfall totals for a specified time period, based on forecaster discretion. (National Hurricane Center)
Hurricane Marco forecast rainfall as of 5:00 PM Sunday 23rd August 2020. This graphic is created by the NWS/NCEP Weather Prediction Center (WPC) and shows rainfall potential for the United States when a tropical cyclone threatens land. The graphic is displayed as a Quantitative Precipitation Forecast (QPF), which shows rainfall totals for a specified time period, based on forecaster discretion. (National Hurricane Center)

RAINFALL: Marco is expected to produce total rainfall accumulations of 3 to 5 inches with isolated maximum amounts of 7 inches across the Central U.S. Gulf coast through Tuesday.

This rainfall may result in scattered areas of flash and urban flooding along the Central U.S. Gulf Coast.

SURF: Swells generated by Marco are likely to affect portions of the northern Gulf Coast for the next couple of days. These swells are likely to cause life-threatening surf and rip current conditions. Please consult products from your local weather office.

TORNADOES: An isolated tornado is possible early Monday morning near the southeast Louisiana coast. Isolated tornadoes are possible across southeast Louisiana, southern Mississippi, southern Alabama, and the Florida Panhandle Monday and Monday night.

This system is of no direct threat to the Windward Islands, including Trinidad and Tobago.

Hurricane Marco Forecast Discussion

Hurricane Marco In the Gulf of Mexico (Weathernerds)
Hurricane Marco In the Gulf of Mexico (Weathernerds)

Shortly after the 10 AM CDT advisory was issued, the Air Force Reserve Hurricane Hunter aircraft investigating Marco measured SFMR winds of 65 to 69 kt for several minutes. A dropsonde around the same time also measured winds equivalent to 64 kt at the surface. Based on these data, it was evident that Marco had strengthened into a hurricane. Since that time, the convective pattern has changed little, and the final leg of the aircraft mission did not reveal any notable changes to the cyclone’s intensity. Therefore, the initial intensity has been set to 65 kt, which is also in agreement with the latest Dvorak intensity estimate from SAB. Marco continues to move north-northwest at around 11 kt.

Model guidance is in good agreement on a north-northwest and then northwest motion into Monday morning as Marco nears the northern Gulf coast. However, there has been a major shift in the track guidance beyond Monday morning, and the majority of the forecast models now keep Marco offshore of the northern Gulf Coast for the next few days rather than moving it inland over Louisiana Monday afternoon. Since this shift was so abrupt, I would rather split the difference between the previous official forecast track and the latest consensus tracks until another round of model runs can confirm this new suggested track is higher confidence.

Based on this shift in track, tropical storm conditions are expected farther west along the Louisiana coast, and a Tropical Storm Warning has been issued from Morgan City to Cameron, Louisiana. It should be noted that if the trend in the models are correct, some of the impacts over portions of the north-central Gulf coast could be lessened.

The current shear analysis from UW-CIMSS shows that Marco is experiencing about 20-25 kt of southwesterly shear, yet Marco has been able to slowly strengthen in this environment today, likely due to the presence of very warm waters and plenty of atmospheric moisture. By late tonight and through Monday, the SHIPS guidance suggests this shear will increase to over 30 kt. This should limit any further intensification and could cause Marco to weaken before it nears the northern Gulf coast Monday afternoon.

By Monday night, the shear is forecast to increase to close to 40 kt, which should strip the convection away from the center of the cyclone, causing it to weaken. After 48 h, Marco is now expected to become a remnant low devoid of deep convection. The official NHC intensity forecast is similar to the previous one, but is at the high end of the guidance through 24 h.

Key Messages:

  1. Hurricane conditions, life-threatening storm surge, and heavy rainfall are expected from Marco along portions of the Gulf Coast beginning on Monday. Interests in these areas should follow any advice given by local government officials.
  2. Tropical Storm Laura could bring additional storm surge, rainfall, and wind impacts to portions of the U.S. Gulf Coast by the middle of the week. This could result in a prolonged period of hazardous weather for areas that may also be affected by Marco. Interests there should monitor the progress of Marco and Laura and updates to the forecast during the next few days.

Forecast Discussion by Forecaster Latto from the NHC.

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