Large hail is not a killer, but does considerable damage

Hail

Falling from 20,000 feet or higher, large hailstones pack quite a punch!

Large hail does significant damage to cars.

Average annual number of days with hail

This is the Aurora hailstone, the largest one at the time. It fell at Aurora, NE on June 22, 2003.

The Aurora hailstone was 7 inches in diameter. Imagine that falling

on your head from 20,000 feet up!

This is the previous record-holder, the Coffeyville hailstone with some props (egg, hand) for comparison.

The Coffeyville hailstone cut into sections. Notice the layers

This stone fell on Coffeyville, KS on Sept 3, 1970.

The rings are caused by different growth regimes. Clear ice is deposited slowly and contains few air bubbles. Cloudy ice occurs

when water freezes quickly, trapping the air bubbles.

The current record-holder is the Vivian (SD) hailstone, July 23, 2010.(http://www.noaa.gov/features/02_monitoring/hailstone.html)

Hail is often categorized by its size in relation to everyday objects

Sizes of hail and the everyday objects used as adjectives (e.g., baseball-sized hail is 2.75” in diameter)

Marble

Oneonta hail, June 15, 2009. This is marble-sized

Those are small pine cones for comparison.

A schematic supercell radar echo with a hook echo. Tornadic thunderstorms often also have large hail.

The hailfall area is close to the tornado.

Ingredients for Hail Growth

In a basic sense, 3 Main “ingredients”3 Main “ingredients” are required:

1. Adequate updraftAdequate updraft to keep hailstone aloft for an appropriate amount of time, -10 to -30C

2. Sufficient supercooled waterSufficient supercooled water near the hailstone to enable growth during transit through an updraft

3. A piece of ice or snow (embryoembryo) for it to grow upon

Vertical cross section of a supercell hailstorm.

Forecasting Hail

Deep, Moist Convection (DMC) has 3 Deep, Moist Convection (DMC) has 3 Ingredients:Ingredients:

1. Sufficiently deep low level moisture

2. Steep lapse rates (related to instability)

3. Sufficient lifting from LCL to LFC

Once DMC identified as possibility, look for hail signal

Hail Ingredients

1.1. Strong Strong updrafts updrafts necessary but necessary but not sufficient not sufficient

High CAPE in High CAPE in hail growth hail growth zoneszones

Hail Ingredients (continued)

2.2. Storm scale Storm scale winds: winds: Speed/Direct-ional Speed/Direct-ional shear is important. shear is important. Directional shear helps Directional shear helps create a rotating create a rotating updraft. Rotation can updraft. Rotation can enhance updraft enhance updraft strength by a factor of strength by a factor of 2 or more, above that 2 or more, above that of an updraft created of an updraft created by buoyancy alone.by buoyancy alone.

12 hours earlier

Wet-bulb Zero (WBZ): This is the level where evaporative cooling reduces a parcel temperature to 0C. It correlates well with large hail when the altitude of the WBZ is between 2200 meters and 2800 meters.

If the WBZ is higher than 2800 meters, hailstones must fall through a large layer which is above 0C and usually do not reach the ground in frozen form.

If the WBZ is lower than 2200 meters, the lower atmosphere is relatively cold and stable so the large updrafts needed for hail formation don’t exist.

The exception to the above guidelines is when the ground level is significantly above sea level. That usually reduces the thickness of the low-level warm layer and hail becomes more common. This is the reason for the hail maximum in the High Plains and especially east of the Colorado Rockies.

Source: A World of Weather by Lee Grenci and Jon Nese (2001)

The concept of the Wet-bulb Zero

Example of Skew-T Log P determination of WBZ

WBZ 729 mb 2788m

Tennessee Colony

SHIP - Significant Hail ParameterStrengths Weaknesses

• Based on large number of soundings and is simple to calculate

•High POD of SIG HAIL (>90%)

•Statistically separates SIG from NON SIG hail

• Widely available: (SPC Mesoanalysis page, SFCOA, NMAP, NSHARP)

• Only forecasts SIG Category, not actual hail size(values > 1 favorable for SIG)

•Conditional on >0.75” hail

“going to occur”

•Based on MUCAPE, thus may exhibit noise at times

• Again, dependent upon accurate thermodynamic profile forecast

• Will instability be realized? Forcing is not parameterized

SHIP = MUCAPE * MUMR * 7-5LR * 500T* 0-6 SHR / ConstantMUMR = Mixing Ratio of Most Unstable Parcel

Next: The most common type of severe weather: Downbursts