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Tropical Depression Twenty-Nine Forecast Discussion


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000
WTNT44 KNHC 312050
TCDAT4

Tropical Depression Twenty-Nine Discussion Number   1
NWS National Hurricane Center Miami FL       AL292020
500 PM EDT Sat Oct 31 2020

Showers and thunderstorms have continued to become better organized 
in association with a tropical wave which has been moving westward 
across the central Caribbean Sea.  It was unclear this morning if 
the system had developed a closed low-level circulation, since 
scatterometers have avoided the system over the past 24 hours, but 
recent visible and microwave satellite images suggest that the 
system almost certainly now has a well-defined center.  For that 
reason, the system is being designated as a tropical depression with 
30-kt winds, based on Dvorak classifications of T2.0 from both TAFB 
and SAB.

A low- to mid-level ridge axis that extends from the subtropical 
Atlantic southwestward to Cuba and the Bahamas is currently steering 
the depression toward the west (270 degrees) at an estimated speed 
of 13 kt.  Model guidance is in fairly good agreement on the 
depression's future track for the first 48 hours or so.  The cyclone 
is expected to continue westward for the first 36 hours and then 
slow down and turn west-southwestward by 48 hours as it approaches 
the coasts of Nicaragua and Honduras, in response to a building 
ridge over the Gulf of Mexico.  After that time, however, there is 
significant divergence in the models.  For example, the ECWMF and 
its ensemble members continue on a faster westward motion across 
Central America, while the GFS and its ensemble members stall the 
system over the western Caribbean Sea through day 5.  Given this 
discrepancy, the NHC official track forecast shows a slow motion on 
days 3 through 5, and brings the cyclone's center slowly across 
northern Nicaragua, more or less in line with the multi-model 
consensus aids.  This forecast is of generally low confidence, 
however, and significant changes could be required in later advisory 
packages depending on model trends.

The waters over the Caribbean Sea remain very warm--around 29 
degrees Celsius--and the environment is characterized by low 
vertical shear of 10 kt or less.  Along with plenty of ambient 
moisture, these parameters suggest the system is primed for steady, 
if not significant, strengthening during the next few days.  The NHC 
official forecast generally lies between the SHIPS guidance and the 
HCCA corrected-consensus aid, which lie near the upper bound of the 
intensity guidance, and it brings the system to hurricane strength 
in 48 hours.  The intensity forecast hinges greatly on whether or 
not the cyclone's center moves inland over Central America, but 
regardless, the system is expected to be a hurricane when it 
approaches the Honduras and Nicaragua coasts in a few days.

Key Messages:

1. The depression is expected to strengthen to a hurricane early 
next week as it approaches the coast of Central America late Monday 
and Monday night, and there is a risk of storm surge, 
hurricane-force winds, and heavy rainfall for portions of Nicaragua 
and Honduras.  Hurricane Watches could be needed for portions of 
those areas later tonight. 

2. Through Thursday afternoon, heavy rainfall from the system will 
likely lead to flash flooding and river flooding across portions of 
Jamaica, the Cayman Islands, and Central America, which could result 
in landslides in areas of higher terrain.  Flooding is also possible 
near the southern coast of Hispaniola.


FORECAST POSITIONS AND MAX WINDS

INIT  31/2100Z 15.0N  73.2W   30 KT  35 MPH
 12H  01/0600Z 15.1N  75.6W   35 KT  40 MPH
 24H  01/1800Z 15.4N  78.3W   45 KT  50 MPH
 36H  02/0600Z 15.5N  80.4W   55 KT  65 MPH
 48H  02/1800Z 15.1N  82.1W   65 KT  75 MPH
 60H  03/0600Z 14.3N  83.1W   75 KT  85 MPH
 72H  03/1800Z 13.8N  83.7W   70 KT  80 MPH...INLAND
 96H  04/1800Z 13.5N  85.5W   40 KT  45 MPH...INLAND
120H  05/1800Z 14.0N  86.9W   25 KT  30 MPH...INLAND

$$
Forecaster Berg