SCAN

This lesson presents information on the System for Convection Analysis and Nowcasting (SCAN) application. While this lesson presents the basics, more detailed examples are included in WES Exercise #8 on your local WES-2 Bridge machine.

Required WES Exercise #8 (System for Convection Analysis and Nowcasting (SCAN))

About SCAN

The System for Convection Analysis and Nowcasting (SCAN) is an application suite for monitoring single radar algorithm output from the RPG and a few other related convective storm datasets. SCAN provides popup tables outside the CAVE editor in D-2D that can be used to sort storms by their attributes, generate time trends of WSR-88D algorithms like maximum expected hail size and time-height trends of Digital Mesocyclone Detection (DMD) output. There are two main SCAN tables for each radar source, the Storm Cell/Site Storm Threat table (primarily driven by the Combined Attributes Table from the Composite Reflectivity product, the Mesocyclone Detection product, and the Tornado Vortex Signature product) and the DMD Icons and Table (driven by the Digital Mesocyclone Detection product).

SCAN displays a few of its own products such as VIL density (VIL divided by Echo Tops), Quantitative Precipitation Forecast (QPF) probability estimates, and an instantaneous precipitation rate (based off the Digital Hybrid Reflectivity). SCAN also calculates a few of its own severe weather parameters such as a severe weather and heavy precipitation threat using relatively limited datasets that have limited utility over different areas of the US. SCAN has extensive configurability on table colors and alert monitoring thresholds that can be configured to alert the user for reaching thresholds, and it can highlight areas where thresholds are met and no warning has been issued for a particular county. While many of the alerting features of SCAN may look attractive to new warning forecasters, many offices have found them to be distracting in warning environments and have turned them off. In general, the default settings in SCAN are suitable for most uses.

Much of the RPG algorithm data SCAN displays can be displayed using simple D-2D radar graphics, but SCAN’s strength is in its ability to tabularize this output and provide displays of time trends. Radar algorithms in general have lots of limitations when used for making warning decisions, and expert warning forecasters tend to rely more on base data analysis, so WDTD recommends algorithm data and tools like SCAN be used carefully, only when needed, in more of a safety net capacity (with holes!). SCAN use will vary greatly from office to office and many offices choose not to use SCAN. While SCAN will be available in the RAC workshop for you to experiment with, WDTD will be focusing primarily on base data analysis and issuing effective storm-based warnings. Before you make your own decision about whether you want to use SCAN you should have a basic proficiency with loading and manipulating SCAN. You will notice the mouse clicks and table conventions in SCAN are similar to other applications in D-2D like FFMP, SNOW, SAFESEAS, and the Fog Monitor.

Starting SCAN

All SCAN tables and products are loaded from the SCAN menu in CAVE. The SCAN products are grouped together by radar input source. Loading the Storm Cell table or the DMD table will launch a separate popup table and a linked display in the CAVE editor. When loading this product, you will likely want to load a base reflectivity (or even a composite reflectivity) product into this same display to better visualize what storms SCAN is triggering on at a given time.

If your CWA has multiple dedicated radars that provide optimal coverage of certain areas, you will need to choose which radar source is best for loading SCAN (usually the closest radar). In general you don’t want to load multiple instances of SCAN because of the overhead with having multiple tables.

SCAN has a number of products available in the menus:

• QPF - probability of QPF for different thresholds and 1hr rainfall forecast
• SCAN CWA Threat Index - SCAN threat values based on the SCAN Severe Weather Probability algorithm, whether there is a mesocyclone or TVS, and also VIL and composite reflectivity.
• Hail Diagnostic Grids - VIL Density Grids
• Precipitation Rate - instantaneous precip rate in in/hr from DHR product

Using SCAN

The SCAN Storm Cell Table and the DMD Table are the primary tables loaded from the SCAN menu. While the Storm Cells table is primarily organized around storm-cell information, you can also drill down further into the MESO and TVS tables from the Tables menu.

The Storm Cell/Site Storm Threat table includes the following parameters:

• ident - storm cell id from WSR-88D SCIT algorithm
• azm - azimuth in degrees
• rng - range in nmi
• tvs - Y or NONE from TVS algorithm
• mdaSR - mesocyclone algorithm strength rank (note 3, 5, 7 rule of thumb for weak, moderate, and strong rotation)
• posh - probability of severe hail from WSR-88D hail algorithm
• poh - probability of hail
• hSize - maximum expected hail size in inches from the WSR-88D Hail Index algorithm
• VIL - cell-based VIL in kg/m2 (note different than gridded VIL image loaded from the radar menu)
• dbz - maximum reflectivity in dBZ
• dbzHt - height of maximum reflectivity in Kft
• top - storm top from SCIT
• dir - direction of motion
• spd - speed in kts
• polh - SCAN algorithm probability of “large” hail (2 cm which is ~ 0.79” and not large by today’s operational standards)
• svrwx - SCAN severe weather probability which uses VIL, freezing level, wind speed
• hvyPr - heavy precip probability
• pPos - percent positive NLDN cloud-to-ground lightning
• cgRate - cloud-to-ground NLDN lightning rate in flashes/min
• cape - CAPE from RAP13 model (can be configured)
• sreh - storm-relative environmental helicity from RAP13 (can be configured)
• county - state and county name

Some of the main functionality of the Storm Cell table includes the following:

• Left-Mouse Button on column header - rank rows by selected attribute
• Left-Mouse Button on cell ident - zoom in on cell
• Right-Mouse Button on cell ident - generate trend set time trend
• Left-Mouse Button on parameter value - generate time trend (some trends like dbzHt highlight radar tilts; note axis are colored red when very high values exist)
• File menu - save and load configuration settings
• Configurations menu- configure hexagons displayed and their characteristics based off user-defined parameter, define alarm settings, box colors, and trend set configurations
• Rank menu - identifies what table is ranked by (note ranked parameter is also colored purple)
• Attributes menu - configurable attributes in table
• Tables menu - launch MESO and TVS tables
• Link to Frame toggle - keeps table in sync with D-2D display
• CWA Filter toggle - filters contents by CWA
• Unwarned toggle - configure unwarned county based on thresholds and whether warning exists for that county
• Vert - vertical or horizontal table orientation toggle
• Tips - tooltips toggle

While the Cell Table is mostly dominated by reflectivity information, you can drill down into mesocyclone and TVS algorithm output using the Tables button in the CELL Table. The MESO and TVS Tables do not have dynamic time trend capabilities with the Left and Right-Mouse Clicks.

The MESO table includes the following parameters:

• strmID - MD storm ID
• ident - SCIT cell id
• azm - azimuth
• rng - range in nmi
• mdaSR - MDA Strength Rank
• llVr - low-level rotational velocity (outbound velocity - inbound velocity divided by two) in kts
• llgtg - low-level gate-to-gate velocity difference (velocity difference across two azimuths at constant range) in kts
• base - base of mesocyclone in Kft
• depth - depth of mesocyclone in Kft
• relDep - relative depth of mesocyclone in Kft
• maxVr - maximum rotational velocity in kts
• htMxVr - height of maximum rotations velocity in Kft
• tvs - TVS detected Y or N
• dir - direction of meso motion from
• spd - speed of meso motion
• msi - mesocyclone strength index (integrated rotation parameter)
• county - state and county

The TVS table includes the following parameters:

• strmID - SCIT storm ID
• ident - TVS id
• type - TVS type
• azm - azimuth
• rng - range in nmi
• avgDv - vertically averaged velocity difference delta V in kts
• llDV - low-level gate-to-gate velocity difference (velocity difference across two azimuths at constant range) in kts
• maxDv - maximum velocity difference in kts
• mxDvHt - height of maximum velocity difference
• base - base of TVS in Kft
• depth - depth of TVS in Kft
• top - top of TVS in Kft
• shear - max shear in per seconds
• shrHt - height of max shear in per seconds
• county - state and county