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WT Staff

NWS: New resources for flood forecasting
Advanced Hydrologic Prediction Service expands

February 6, 2024

In January the National Weather Service announced a new on-line tool for river flood forecasting and modeling, now available to the public. The new resource is experimental, carrying a warning that the forecasts should not be used for decision making, yet.

Flood inundation mapping has been updated for approximately 10% of the US surface area to date. The pairing of the inundation mapping with the Advanced Hydrologic Prediction Service extended as far as 7 days out could provide emergency managers with a considerable advantage in preparing a flood response.

The Experimental Flood Hazard Outlook (FHO) tool offers emergency managers and response coordinators with a national view, according to the NWS, "intended to provide a coordinated inland hydrologic flood graphic for use in regional and national Impact-Based Decision Support Services (IDSS) briefings, and support broad messaging of flood threats to National Weather Service (NWS) stakeholders."

With this national flood map tool, projected flood events are color-coded and categorized first by potential impact: Catastrophic, Considerable and Limited, allowing planners to triage flood events occurring in populated areas or impacting critical infrastructure.

Flood extent is further categorized as major, moderate or minor, corresponding to flood stages established and periodically re-evaluated at each streamflow monitoring station. With the monitoring tools currently in use, measured streamflow breaching moderate or major flood stage does not immediately present the risk or impact. Combining the streamflow data measurements and flood stage parameters with the inundation maps and precipitation forecast, a clear picture emerges allowing emergency response managers to mobilize resources more efficiently and effectively.

From the National Weather Service bulletin describing the Advanced Hydrologic Prediction Service (AHPS):
AHPS is a new and essential component of our Climate, Water, and Weather Services. AHPS is a web-based suite of accurate and information-rich forecast products. They display the magnitude and uncertainty of occurrence of floods or droughts, from hours to days and months, in advance. These graphical products are useful information and planning tools for many economic and emergency managers. These new products will enable government agencies, private institutions, and individuals to make more informed decisions about risk based policies and actions to mitigate the dangers posed by floods and droughts.

A Brief Overview of the NWS Precipitation and River Forecasting and the River Forecast on the AHPS Hydrograph

The National Weather Service (NWS) uses river forecast models to estimate the amount/level of water flowing through the US Rivers. These models estimate the amount of runoff a precipitation event generates, computes how the water will move downstream, and then predicts the flow of water at a given location (AHPS forecast point) throughout the forecast period (every six hours, out 3 to 5 days in many locations).

As the precipitation event unfolds and actual reports of precipitation amounts from rain gauges and radar estimated rainfall become available, the precipitation forecast values are replaced with the observed amounts in the river model. For an ongoing precipitation event, the river model will use both observed and forecast precipitation amounts to predict the river levels. In addition to precipitation, some other factors the NWS River Models account for are: snowmelt, base flow/groundwater, reservoir operations, and routed water from upstream.

This discussion on river models provides the background information on the river forecasts provided on the AHPS hydrograph. Additionally, itís beneficial for AHPS users to understand the process of issuing river flood watches and warnings.

Every morning, hydrometeorologists at the NWS River Forecast Centers (RFCs), collect and quality control precipitation observations and precipitation forecasts. Depending on the particular weather scenario and the forecaster confidence in the precipitation amounts forecasted (timing, location and total volume), forecasters will use anywhere from 6 to 48 hours of forecast rainfall.

RFC hydrologists enter these data into their river models. Model output is examined and adjusted to produce the best forecast possible. The river forecasts at each AHPS location are sent to the NWS Weather Forecast Offices (WFOs), who in turn issue flood watches and warnings to the public.

WaterToday Media Group aggregates data collected by state and federal sources including NWS Hazardous Weather Outlooks, Weather Advisory, watches and warnings, special weather statements. USGS extreme high and low flow data points are pinned on WT licensed maps, showing the watershed areas collecting and concentrating rainfall and runoff indicating the general direction of flow. Rivers in action stage approaching flood stage are pinned to the WT state maps and flood alerts are sent to WT subscribers via SMS.

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