Tropical Cyclone Probabilities

NHC's tropical cyclone strike probability forecasts give the percentage chance that the center of a tropical cyclone will pass within 65 NM (75 mi) of specific locations within 72 hours. Probabilities are based on the official forecast track and the past distribution of track forecast errors. They are issued when the 72-hour forecast position approaches the coast with updates issued until the storm makes landfall.

The maximum probabilities by forecast period are displayed in the table below and are based on NHC's average forecast errors for the various periods.

In most cases, if you wait until the probability is 50% or greater, it will be too late to take effective actions. Consequently, this table provides critical information for decision making.

Forecast Period	Maximum Probability*
72 hours	10%-15%
48 hours	20%-25%
36 hours	25%-35%
24 hours	40%-50%
12 hours	75%-85%
*These probabilities are those which would be computed if the forecast position (at the given time period) were directly over a community. A range of probabilities is given because forecast errors differ by location. The probability can be 100% if the center is already close to a location. In addition, the probabilities can exceed the maximums listed in the table if the hurricane center is actually forecast to be at the location at an earlier time period.

The example below is a portion of a strike probability issued for Hurricane Georges in 1998. The rows in the table are the locations, some of which are given in latitude and longitude and others by name. The different columns (labeled "A","B", "C", "D") are probabilities at various forecast times. The column labeled "E" is the sum of the probabilities in the four periods.

According to this example, Key West FL had the highest probability (99%) of the hurricane center passing within 65 NM (75 mi) during the period between 11 a.m. EDT on Friday, September 25 (the current time) and 8 a.m. on Saturday, September 26. There was a 2% chance that the center would pass within 65 NM of the location
GULF 29N 87W before 8 a.m. Saturday, but a 25% chance between 8 a.m. Saturday and 8 p.m. Saturday. The 2% probability is low for a 24-hour forecast, but the 25% chance at about 36 hours is about as high as is possible.

ZCZC MIASPFAT2 ALL
TTAA00 KNHC DDHHMM
HURRICANE GEORGES PROBABILITIES NUMBER 41
NATIONAL WEATHER SERVICE MIAMI FL
11 AM EDT FRI SEP 25 1998

PROBABILITIES FOR GUIDANCE IN HURRICANE PROTECTION
PLANNING BY GOVERNMENT AND DISASTER OFFICIALS

AT 11 AM EDT...1500Z...THE CENTER OF GEORGES WAS LOCATED NEAR
LATITUDE 24.3 NORTH...LONGITUDE 81.7 WEST

CHANCES OF CENTER OF THE HURRICANE PASSING WITHIN 65 NAUTICAL MILES OF LISTED LOCATIONS THROUGH 8AM EDT MON SEP 28 1998

LOCATION	A	B	C	D	E	LOCATION	A	B	C	D	E
26.7N 85.3W	54	X	X	X	54	APALACHICOLA FL	8	14	1	1	24
28.1N 86.7W	16	18	X	1	35	PANAMA CITY FL	3	17	3	1	24
29.6N 87.9W	X	15	11	1	27	PENSACOLA FL	X	11	12	2	25
MUHA 230N 824W	16	X	X	X	16	MOBILE AL	X	4	16	3	22
KEY WEST FL	99	X	X	X	99	GALVESTON TX	X	X	X	3	3
MARCO ISLAND FL	56	X	X	X	56	GULF 29N 85W	20	9	1	X	30
FT. MYERS FL	39	X	1	X	40	GULF 29N 87W	2	25	2	1	30

COLUMN DEFINITION PROBABILITIES IN PERCENT
A IS PROBABILITY FROM NOW TO 8AM SAT
FOLLOWING ARE ADDITIONAL PROBABILITIES
B FROM 8AM SAT TO 8PM SAT
C FROM 8PM SAT TO 8AM SUN
D FROM 8AM SUN TO 8AM MON
E IS TOTAL PROBABILITY FROM NOW TO 8AM MON
X MEANS LESS THAN ONE PERCENT

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