Graphically Depicting Threat Assessment Information for Flood Situations in East Central Florida

The National Weather Service (NWS) office in Melbourne (MLB), Florida has initiated a local project with the intent to better convey threat assessment information regarding hazardous weather in east central Florida (ECFL). Fundamentally, this has been accomplished through the development of experimental graphical products which complement the official text versions of both the "Hazardous Weather Outlook" (HWO) and the "Hurricane Local Statement" (HLS). The HWO is a routine product issued daily at 7 AM (at the request of Florida Emergency Management) as a planning tool that describes all forms of expected hazardous weather during the next 12 to 24 hours (with intermediate updates issued as needed). The spectrum of hazardous weather in ECFL is wide-ranging and includes Florida-style "flash" flooding, river flooding, and coastal flooding. In contrast, the HLS is issued only during tropical storm/hurricane watch/warning events and is updated at least every three to six hours during such situations. The HLS speaks to the direct affects of a tropical cyclone on the local area and addresses the hazards of storm surge and flooding rain. Due to the diversity of Florida weather threats, as well as the chance for multiple coincident threats, the text version (alone) of either the HWO or HLS is often insufficient to properly express comprehensive concerns. This is especially true during dangerous flooding situations when simple quantitative precipitation forecasts may not adequately depict the actual flood potential. As a result, the text version may become overwhelmingly large in order to accommodate detail or overgeneralized to accommodate product length manageability. Both are undesirable. Adding a graphical component to the HWO and HLS greatly improves their usefulness and may change the way the NWS provides flooding information to its users.

The graphical component of the HWO and HLS uses uniquely developed "Degree of Threat" (DoT) bar charts for each flood type. The DoT is assessed through a series of locally defined threat assessment rules which are specific to ECFL. Rules for longer-fused flooding threats (such as river flooding and coastal flooding) are based on pre-existing advisory criteria or are empirically derived while shorter-fused threat rules use equation calculations. DoTs range from Threat Level 1 (threat of hazard) to Threat Level 5 (extreme threat of hazard) and are colored coded. For the HLS, the DoT levels are carefully calibrated so that tropical cyclone hazards can be compared against each other to determine if the local impacts may be more wind-related (damaging winds and/or tornadoes) or flood-related (surge and/or heavy rain). Importantly, it has provided the means to address the flooding threat from rainfall without challenging the integrity of the Saffir-Simpson scale to sufficiently describe a tropical cyclone. In tandem with the DoT, a Threat Area Map (TAM) is also provided. The TAM is used to depict those geographic areas at greater risk or to delineate the timing of a flooding hazard. The graphical component greatly enhances the HWO/HLS by offering a complete package of hazardous weather (e.g. flooding) information for decision makers.