The Graphical Tropical Weather Outlooks (GTWO) are a set of products issued by the National Hurricane Center (NHC). The 2-day and 5-day GTWOs provide the probability of tropical cyclone formation during the next 2 and 5 days, respectively, in 10% increments for both the Atlantic and eastern North Pacific Ocean basins. The GTWO is intended to be a visual companion to the text Tropical Weather Outlook (TWO). The GTWOs are issued four times daily for each basin and will be available shortly after the issuance of the text TWO.
The 5-day GTWO became operational in 2015. In addition, the definitions of low, medium, and high chance of development were redefined. Low is now 0–30%, medium is 40–60%, and high is 70–100%. The original category definitions were established to best reflect forecasters' ability to assess formation potential. Unfortunately, the original definitions did not correspond with some users' expectations of what the everyday terms "low", "medium" and "high" meant. This led some users to overestimate the chance of formation when they heard that a system had entered the medium or high categories. Improvements in forecast accuracy over the past few seasons now allow for a more natural assignment of the three bins.
The 2-day GTWO depicts significant areas of disturbed weather and their potential for development during the next 48 hours. The 2-day GTWO also shows the locations of any active tropical cyclones. The location of areas of disturbed weather on the graphic are denoted by an X and are numbered, with text discussions for each disturbance given beneath the graphic. The potential for tropical cyclone formation for each disturbance within the next 48 hours is indicated by the color of the X: yellow indicates a low probability of development (0-30%), orange indicates medium likelihood (40%-60%), and red indicates a high likelihood of development (70%-100%).
The 5-day GTWO provides formation potential for individual disturbances during the next 5-day period. The areas enclosed on the graph represent the potential formation area during the forecast period. As with the 2-day GTWO, the areas depicted in the 5-day GTWO are color-coded based on the potential for tropical cyclone formation, but in this case for the 5-day period. Areas in yellow indicate a low probability of development (0-30%), orange indicates a medium probability (40-60%), and red indicates a high probability (70-100%). The locations of existing disturbances are indicated by Xs. If the formation potential of an existing disturbance does not include the area in which the disturbance is currently located, an arrow will connect the current location of the disturbance to its area of potential formation. Areas without an X indicate that no disturbance currently exists, but that one is expected to develop during the 5-day period. Active tropical cyclones will not be depicted on the 5-day GTWO.
Information on the motion and potential impacts of each disturbance is available in the text descriptions but is not displayed graphically in the 2-day or 5-day GTWO.
The 2-day and 5-day GTWOs are interactive; users can mouse over cyclones or disturbances in the graphic and pop-up windows will appear with cyclone advisory information or the TWO discussion for that disturbance. Clicking on a tropical cyclone symbol on the 2-day GTWO will take the user to a new web location that contains all advisories and products for that cyclone. Clicking on a disturbance on the 2-day GTWO will show a zoomed satellite image of that disturbance. Clicking on a disturbance on the 5-day GTWO will take the user to a graphic that shows only that disturbance.
GTWOs are issued every six hours from 1 June–30 November for the Atlantic Basin and from 15 May–30 November for the eastern North Pacific Basin at 0000, 0600, 1200, and 1800 UTC. Local issuance times are shown in the table below. The GTWO is also updated whenever a Special TWO is issued. A special TWO may be issued at any time when important changes in areas of disturbed weather over tropical or subtropical waters need to be conveyed before the next scheduled release of the TWO.
|Basin||Local Issuance Times During Daylight Savings Time||Local Issuance Times During Standard Time|
(1 June–30 November)
|2:00 am EDT
8:00 am EDT
2:00 pm EDT
8:00 pm EDT
|1:00 am EST
7:00 am EST
1:00 pm EST
7:00 pm EST
|Eastern North Pacific
(15 May–30 November)
| 5:00 am PDT
11:00 am PDT
5:00 pm PDT
11:00 pm PDT
| 4:00 am PST
10:00 am PST
4:00 pm PST
10:00 pm PST
The location of disturbances on the 2-day and 5-day graphics, and the location, size, and shape of the potential formation areas on the 5-day graphic are based on subjective forecaster interpretation of all available data. Data sources include, but are not limited to, remotely sensed data, in-situ observations, objectively analyzed meteorological fields, and global model forecasts fields.
Brown, D., Franklin, J., and J. Rhome, 2008: Verification of the National Hurricane Center's Experimental Probabilistic Tropical Cyclone Genesis Forecasts. AMS 26th Conf. on Hurricane and Tropical Met., Orlando, FL, 28 April – 2 May 2008.