1. Marco is forecast to strengthen over the northwestern Caribbean Sea through Saturday as it approaches the northeast coast of the Yucatan Peninsula. A Hurricane Watch and a Tropical Storm Warning are in effect for portions of that region.

2. The system is expected to move into the south-central Gulf ofMexico as a tropical storm on Sunday. Although some strengtheningis anticipated on Sunday, weakening is forecast as the systemapproaches the northwestern Gulf coast on Tuesday. It is still toosoon to know exactly the location and magnitude of impacts thesystem will produce along the central or northwestern Gulf Coast,and interests in that area should continue monitoring the progressof this system over the next few days.

CHANGES WITH THIS ADVISORY:

None.

SUMMARY OF WATCHES AND WARNINGS IN EFFECT:

A Hurricane Watch is in effect for...
- Punta Herrero to Cancun Mexico

A Tropical Storm Warning is in effect for...
- Punta Herrero to Dzilam Mexico

A Hurricane Watch means that hurricane conditions are possible within the watch area, in this case within 24.

A Tropical Storm Warning means that tropical storm conditions are expected somewhere within the warning area, in this case within 24 hours.

For storm information specific to your area, please monitor products issued by your national meteorological service.

WIND: Tropical storm conditions are expected to first reach the eastern Yucatan coast within the warning area by Saturday afternoon and will spread northward and westward within the warning area Saturday night, making outside preparations difficult or dangerous. Hurricane conditions are also possible within the hurricane watch area by late Saturday.

RAINFALL: Marco is expected to produce the following rainfall accumulations through Sunday:

Eastern portions of the Mexican states of Quintana Roo and Yucatan: 3 to 6 inches, isolated maximum totals of 10 inches. This rainfall may result in areas of flash flooding.

Northeast Nicaragua and the Cayman Islands: 1 to 2 inches.

At 1100 PM EDT (0300 UTC), the center of Tropical Storm Marco was located near latitude 18.7 North, longitude 84.9 West. Marco is moving toward the northnorthwest near 13 mph (20 km/h). A slightly slower northwestward motion is expected for the next day or so, followed by an increase in forward speed by early next week. On the forecast track, the center of Marco will approach the east coast of the Yucatan Peninsula of Mexico on Saturday. The center will then cross the northeastern part of the Yucatan Peninsula Saturday night and move over the central Gulf of Mexico toward the northwestern Gulf on Sunday and Monday.

Maximum sustained winds have increased to near 40 mph (65 km/h) with higher gusts. Additional strengthening is forecast during the next couple of days as the system approaches the Yucatan peninsula and Marco could be near hurricane strength when it moves over the central Gulf of Mexico on Sunday.

Tropicalstormforce winds extend outward up to 90 miles (150 km) from the center.

The estimated minimum central pressure is 1003 mb (29.62 inches) based on reports from the Hurricane Hunter plane.

An Air Force Reserve Hurricane Hunter aircraft has investigated the system over the northwest Caribbean during the past few hours. The plane reported a number of unflagged SFMR winds between 35 and 40 kt and max flight level winds of 41 kt. A blend of these data supports an intensity of 35 kt, and therefore, this system has been designated as Tropical Storm Marco. Deep convection has increased near and to the east of Marco's center during the past few hours. Although there still isn't much evidence of inner-core banding, the data from the plane does indicate that the center of Marco has become better defined since the afternoon and that the minimum pressure has dropped.

Unfortunately the intensity forecast has not become any clearer and confidence in that aspect of the forecast is quite low. Marco is embedded within an environment that could support a fast rate of strengthening. However, recent microwave data does not indicate that the system has developed an inner-core, and only gradual strengthening is likely until it does. The intensity guidance spread is quite high, with the GFS and ECMWF global models both showing little further strengthening, while the HMON regional model rapidly makes Marco a hurricane before it reaches the northeast tip of the Yucatan peninsula. That possibility can not be ruled out, but a majority of the intensity guidance favors the weaker solution of the global models. Even with the HMON outlier included, the NHC intensity forecast is above the model consensus. Once Marco moves over the central Gulf of Mexico, a rapid increase in wind shear associated with an upper-level trough should limit the potential for further strengthening, and weakening is still anticipated before Marco nears the northern Gulf Coast, as shown in the previous official forecast.

Confidence in the track forecast is also lower than normal, as the models spread remains quite high. Only small adjustments were made to the NHC forecast which heavily favors the GFS and ECMWF solutions on the left side of the track guidance. It is worth noting that the NHC track forecast is near middle of the GFS and ECMWF ensembles. Marco is currently forecast to move northwestward toward a weakness in the subtropical ridge over the central Gulf of Mexico for the next day or two, before the ridge builds and turns the tropical cyclone farther west. Near the end of the period, Marco's track and intensity could be also influenced by Tropical Storm Laura which is also forecast to be over the Gulf of Mexico, however the details of that interaction are highly uncertain at this time. Given the high uncertainty in the forecast, larger than normal changes could be required to future advisories.