Significant Hail Parameter

The Sig. Hail Parameter (SHIP) was developed using a large database of surface-modified, observed severe hail proximity soundings. It is based on 5 parameters, and is meant to delineate between SIG (>=2" diameter) and NON-SIG (<2" diameter) hail environments.

SHIP = [(MUCAPE j/kg) * (Mixing Ratio of MU PARCEL g/kg) * (700-500mb LAPSE RATE c/km) * (-500mb TEMP C) * (0-6km Shear m/s) ] / 42,000,000

0-6 km shear is confined to a range of 7-27 m s-1, mixing ratio is confined to a range of 11-13.6 g kg-1, and the 500 mb temperature is set to -5.5 C for any warmer values.

Once the initial version of SHIP is calculated, the values are modified in the following scenarios:

1) If MUCAPE < 1300 J kg-1, SHIP = SHIP * (MUCAPE/1300); 2) if 700-500 mb lapse rate < 5.8 C km-1, SHIP = SHIP * (lr75/5.8); 3) if freezing level < 2400 m AGL, SHIP = SHIP * (fzl/2400)

It is important to note that SHIP is NOT a forecast hail size.

Since SHIP is based on the RAP depiction of MUCAPE - unrepresentative MUCAPE "bullseyes" may cause a similar increase in SHIP values. This typically occurs when bad surface observations get into the RAP model.

Developed in the same vein as the STP and SCP parameters, values of SHIP greater than 1.00 indicate a favorable environment for SIG hail. Values greater than 4 are considered very high. In practice, maximum contour values of 1.5-2.0 or higher will typically be present when SIG hail is going to be reported.

Figure 1 - Distribution of SHIP for two data sets - Ping-Pong or smaller hail (left), and Tennis Ball and larger (right). All 1.75"-2.00" reports are filtered out in order to magnify separation based on SHIP. The 10th, 25th, 75th, and 90th percentiles are shown for each subset.