National Weather Service

Introduction

Quantitative precipitation nowcasting (QPN) is an important component of NWS flash flood warning services. It refers to the forecasting of rainfall out to 1-3 hours in the future based heavily on current observed data in the near-term forecast period when numerical weather prediction models currently have lesser skill. The Flash Flood Potential (FFP) algorithm is an experimental, prototype, operational QPN algorithm developed at the Hydrology Laboratory that uses current and recent-past WSR-88D radar data to estimate the future location of storms, their associated rainfall, and flash flood threat up to one hour into the future. These QPNs can provide forecasters with additional guidance in evaluating flash flooding threat and additional lead time in issuing warnings to the public.

This web page is intended to be a graphical interface to real-time products generated by this FFP algorithm for pre-deployment evaluation by the scientific developers and forecast office staff. This demonstration project is one component of the collaborative Baltimore Flash Flood Project at the NWS Hydrology Laboratory to demonstrate improved operational hydrologic services focused on the short time and small space scales typically associated with flash flooding.

Why a Web-based Demonstration Project?

This web page has been developed to provide a limited group of NWS developers and forecasters with real-time, 24 hours-a-day access to these experimental QPN products for critical field evaluation prior to the actual deployment of the algorithm within existing NWS operational computer systems. The World Wide Web represents a cheap, efficient forum to test new scientific algorithms and display their products to the field forecasters prior to full-scale and expensive deployment on computer systems at the 122 forecast offices. It allows them to use the products as needed during operational shifts, to learn the algorithm and its strengths and weaknesses, and to provide feedback to the developers regarding the added value and any necessary enhancements so that when the functionality is ultimately deployed nationwide it will better serve their needs from the start. This is typically called “alpha testing” by the NWS.

Users

The FFP algorithm is targeted for general use by hydrologic forecasters at Weather Forecast Offices (WFO) whose primary responsibility it is to monitor flash flooding potential and issue appropriate warnings to the public. In particular, since we are using Sterling, VA WSR-88D radar data for this web page, it is targeted for use and evaluation by the Baltimore/Washington WFO forecasters for which this radar covers their county warning area. It is also targeted for use and evaluation by the scientific developers of the algorithm at the Hydrology Laboratory.

Quick Description of the Algorithm

This algorithm produces rainfall nowcasts up to one hour in the future on a 4-km HRAP grid by extrapolating current storm movement (as represented by instantaneous rain rate fields) into the future and then estimating the amount of rainfall that would fall. The WSR-88D Digital Hybrid Scan Reflectivity (DHR) product is used as input currently and Digital Storm-total Precipitation (DSP) product in the near future when available. The algorithm computes storm motion using an extrapolative, pattern-matching technique and then assumes the storms will continue to move in that direction in the future. This storm motion estimation is done at a grid spacing of about 20 km, and therefore it permits a spatially-variable grid of storm motion vectors to be estimated. This is important since flash flood-producing storms often move at slower speeds and/or different directions compared to neighboring storms in the region. Options exist within the FFP to estimate and incorporate future growth or decay of the storms based on recent-past storm intensity changes. The one-hour rainfall nowcasts are updated every 5 minutes as new radar data arrives. The algorithm will reinitialize all the output products to zero when there has been a 3 hour period without any detectable rain in the radar domain.

Although not a main focus of this demonstration project, the FFP then compares both the observed and forecasted gridded rainfall with the latest gridded Flash Flood Guidance (FFG) from the River Forecast Centers to estimate flash flood threat within the next hour in a probabilistic sense in a manner similar though somewhat different from the existing AWIPS Flash Flood Monitoring and Prediction algorithm. Currently all such comparisons are done on the 4-km grid and not within hydrologic basins although the extension of the functionality to perform basin averages of flash flood threat within FFP is straightforward. 

Additional details of the FFP algorithm and processing logic can be found in Fulton and Seo (2000), referenced cited therein, and the associated 15th Hydrology Conference Corel slide show. The “algorithm enunciation language” for the Projection and Assessment subalgorithm of the ffp describes the processing logic in greater detail.

Description of the FFP Image Products

The image products are separated into Observations, Forecasts, and Verification.

Rainrate - The observed or forecasted rainrate (mm/hr) as computed using the reflectivity factor field converted to rainrate using the current Z-R relationship as defined in the WSR-88D and stored in the DHR product header.  The 1-hour rainrate verification image compares the current observed rainrate field with the 1-hour rainrate forecast from one hour ago.

1-hour Rainfall - A running rainfall accumulation (mm) for the past or future hour.  The 1-hour rainfall verification image compares the current observed 1-hour rainfall field with the 1-hour forecast from one hour ago.

Storm-total Rainfall - The observed total accumulation of rain (mm) since the event started. The start time of the rainfall is displayed on the image.

Storm Motion - Quality-controlled local storm motion vectors (with length proportional to speed) overlaid on a “reliability” field. The reliability field is a relative measure of the algorithm’s ability to identify storms in the rainrate field and thus compute their velocity. You can place greater confidence in the computed storm motion vectors where the reliability values are larger.  The current hourly-averaged storm motion vector over the whole radar umbrella is plotted in the lower left corner.

Storm-Relative Motion - Same as above except that the storm motion vectors are relative to the current mean hourly-averaged, radar-umbrella-averaged storm motion vector (as plotted in the lower left corner of the image).  Non-zero storm-relative motion vectors, particularly those that point opposite to the mean motion vector, identify potentially dangerous storms that are back-building or stationary and therefore capable of producing flash floods.

Storm Growth/Decay - The local, lagrangian rate of change of intensity of the storms (mm/hr/hr) over the last ~15 minutes.  This information is used to adjust the forecasted rainrates locally.

Observed Probability of FFG Exceedance - The probability that the observed rainfall up to the current time will exceed FFG. This is computed as the maximum value for the associated 1-hour, 3-hour, and 6-hour durations of FFG.

Forecasted Probability of FFG Exceedance - The probability that the observed+forecasted rainfall will exceed FFG. This is computed as the maximum value for the associated 1-hour, 3-hour, and 6-hour durations of FFG.

Observed Critical Rainfall Probability - The probability that the observed rainfall at some time during the rainfall event has exceeded the FFG. This is computed as the maximum value for the 1-hour, 3-hour, and 6-hour durations of FFG. It will always monitonically increase throughout a rain event for a given location.

Forecasted Critical Rainfall Probability - The probability that the observed+forecasted rainfall at some time during the rainfall event has exceeded or will exceed the Flash Flood Guidance in the next hour. This is computed as the maximum value for the 1-hour, 3-hour, and 6-hour durations of FFG. It will usually monitonically increase throughout a rain event for a given location.

Flash Flood Guidance (FFG) - The RFC model-estimated basin-averaged rainfall over either 1, 3, or 6 hours duration that would cause small streams to reach bankfull.

There are certain situations when a rainfall forecast is not attempted, for example due to nonexistent or weak radar echoes.  In these cases, "No Projection Attempted" is displayed on the forecast images.

Some Limitations

This algorithm has been developed and optimized primarily for warm season convection. It does not currently perform optimally for stratiform rainfall events where the spatial gradients of reflectivity are weak. This is because the algorithm’s ability to track storms depends on its ability to successfully perform pattern matching of consecutive, recent-past, radar rainrate fields. If it cannot identify well-defined storm features in the consecutive radar images, it will intentionally not estimate their motion and will therefore not produce a rainfall nowcast. This is a built-in quality-control feature that helps prevent the algorithm from issuing anomalous and/or questionable rainfall forecasts when the data does not adequately support it.

Future Plans

There will be on-going enhancements to the algorithm and its web presence to improve the scientific integrity of the products and the web-based user interface. There will be scientific enhancements to the algorithm itself that are briefly described below.

1) One of the current limitations of the FFP is that it is single-radar-centric. We have plans to enhance this QPN algorithm so that it produces regionally-mosaicked rainfall nowcasts at the 4-km grid scale using all available data from regional radars. In the near future, the new prototype Multisensor Precipitation Nowcaster (MPN) algorithm, currently under development, will functionally replace the existing FFP (http://www.nws.noaa.gov/oh/hrl/papers/wsr88d/qpe_enhance_hasconf.pdf). It will be an extension of the existing Multisensor Precipitation Estimator algorithm (http://www.nws.noaa.gov/oh/hrl/papers/papers.htm#wsr88d) which estimates observed rainfall up to the current time using radar rainfall estimates from the WSR-88D’s Precipitation Processing System, rain gauges, and satellite.

2) In living up to its multisensor billing, the MPN will also utilize real-time rain gauge data so that the rainfall estimates and forecasts are properly calibrated. It will also utilize satellite rainfall estimates from NESDIS and numerical weather prediction model output so that, when combined with radar and rain gauge rainfall data, it will produce optimal rainfall estimates and forecasts using as much information as available.

3) Higher resolution (e.g., 1 km) QPN products may be justified. This will be possible when using the FAA’s Terminal Doppler Weather Radar located near Baltimore-Washington International Airport. We are investigating methods to use this data.

4) Some QPN algorithms (e.g., NCAR, MDL, MIT/LL) utilize additional data besides radar reflectivity data, including satellite and numerical weather prediction model data as well as radar doppler velocity data. Incorporation of this technology into the existing algorithm may add value and probably improve performance, particularly as the forecast period increases beyond one hour. In particular, this data may aid in forecasting the growth or decay of new or existing storms in the future which is an active area of research currently.

5) Evaluate FFP performance for forecast periods beyond 1 hour. The development of the MPN as described above is one prerequisite for this because the current single-radar algorithm will be unable to forecast storms moving into radar range from upstream since it cannot see them yet. Multi-radar mosaicking will alleviate this deficiency. Also it will be necessary to utilize atmospheric model guidance by merging the current extrapolation-based forecasts with numerical model forecasts especially as lead time increases beyond an hour as is currently done at the UK Met Office.

6) Display of real-time QPN verification statistics on the web page.

7) Currently the FFP uses River Forecast Center model-generated Flash Flood Guidance (FFG) as a surrogate measure of antecedent, basin-average soil moisture for basins typically about 100 square miles in size. In the near future, in association with the Baltimore Flash Flood Project, radar-derived rainfall and QPNs will be input directly into high resolution distributed hydrologic forecast models with explicit soil moisture accounting models. This will then obviate the need for comparisons of radar rainfall with surrogate measures such as FFG with its well-known deficiencies. We will produce explicit point forecasts of streamflow and depth at ungauged locations as well as high resolution flood inundation maps.

Additional information on our future plans for improving multisensor rainfall estimation and nowcasting algorithms can be found at http://www.nws.noaa.gov/oh/hrl/papers/papers.htm#wsr88d or here.

The Product Description Document, as required by the NWS Office of Climate, Water, and Weather Services, is also available. Click Here.

Comments or Questions?

Send them to Richard.Fulton@noaa.gov.