SPC Heavy Rain Analysis

Data Courtesy Storm Prediction Center

Precipitable Water

The magnitude of the vector wind difference from the effective inflow base upward to 50% of the equilibrium level height for the most unstable parcel in the lowest 300 mb. This parameter is similar to the 0-6 km bulk wind difference, though it accounts for storm depth (effective inflow base to EL) and is designed to identify both surface-based and "elevated" supercell environments. Supercells become more probable as the effective bulk wind difference increases in magnitude through the range of 25-40 kt and greater.

Precipitable Water with 850 mb Moisture Transport

The 850 mb moisture transport is the product of the wind speed (m s-1) and the mixing ratio (g g-1) at 850 mb. Values are scaled by factor of 100, such that a 40 kt (~20 m s-1) wind speed and a 12 g kg-1 mixing ratio (0.012 g g-1) results in a moisture transport of 24 m s-1 (the first pink shade in the color fill). High values of moisture transport have been related to heavy rainfall potential with convective systems.

SFC-6 km Vertical Shear Vector (kts)

The surface through 6-km above ground level shear vector denotes the change in wind throughout this height. Thunderstorms tend to become more organized and persistent as vertical shear increases. Supercells are commonly associated with vertical shear values of 35-40 knots and greater through this depth.

850mb Moisture Transport

0-500 m Storm Relative Helicity (m2 s-2)

SRH (Storm Relative Helicity) in the lowest 500 m AGL has been found by Coffer et al. (2019), October issue of Weather and Forecasting, to be a better discriminator than effective SRH between significant tornadoes and nontornadic supercells. This calculation of 0-500 m SRH is limited to within the effective inflow layer, as long as the inflow base is at the ground.

925 mb Moisture Transport

The 925 mb moisture transport is the product of the wind speed (m s-1) and the mixing ratio (g g-1) at 925 mb. Values are scaled by factor of 100, such that a 40 kt (~20 m s-1) wind speed and a 12 g kg-1 mixing ratio (0.012 g g-1) results in a moisture transport of 24 m s-1 (the first pink shade in the color fill). High values of moisture transport have been related to heavy rainfall potential with convective systems.

Upwind Propagation Vector

The upwind "vector approach" is a method developed by Corfidi et al. (1996) to forecast MCS (or more specifically mesoscale beta element - MBE) movement. It is the vector sum of the mean flow through the cloud-bearing layer and the propagation component. The magnitude and direction of the propagation component is assumed to be equal and opposite to that of the low-level jet (850 mb).

Precipitation Potential Placement

Precipitation Potential Placement is a derived parameter combining precipitable water and low-level mean RH to help better place where rainfall will occur. Research has been published in the National Weather Association Digest in 2003 and stems from research and operational use of this product originally developed at NESDIS and Rod Scofield for satellite rainfall estimates dating back to 1981.

Rainfall is usually maximized where the best low level convergence and instability overlay with the highest values for this parameter.

The risk for heavy rainfall increases as values go up. Additionally, thresholds for precipitation also change based on temperatures. Onset of rainfall ranges from around 0.3 inches with temperatures below 30 to 1.0 inches above 80. Values above 1.0-1.4 inches with temperatures below 60 usually increase the risk for heavy rainfall while values above 1.6-2.0 inches increase the risk for heavy rainfall events with temperatures above 60.