Looking For Direction

Ever take a long walk in a thunderstorm and notice that your umbrella can't possibly keep you dry? You tuck it just above your head, your hair catching in the spokes. Then you rotate it 90 degrees to the right. Oops. Now it has to go straight in front of you—en guard. When the thing inverts and turns into a rain vase, you resign yourself to getting soaked and chuck it in the nearest bin.

Think of your personal battle with the elements next time you assess hail damage. It won't be enough just to know the storm's path, especially if the date of loss is unknown. If a date of loss is unknown, it is often assumed that hail has fallen in the same direction as the storm path. However, decades of weather research show that winds associated with storms are not uniform in direction and speed. As a result, hail damage can occur on surfaces of a building other than those facing the storm's origination point. That's why an insured might file for damage from a storm claiming that wind stripped siding from one side and hail pock marked the other—all one claim, all one occurrence.

Thunderstorms are very complex systems, with wind speed and direction often changing abruptly in both vertical and horizontal directions. The supercell is a unique storm variety since, by definition, it has a rotating updraft and an elevated propensity to produce large hail, tornadoes and damaging winds.

Winds flowing into and out of supercell storms are from different directions, and an observer's location relative to a position within a storm will experience different wind direction and speed. Figure 1 represents a conceptual model of a supercell thunderstorm and illustrates the winds that a structure would likely experience as the storm passes over its location. Initially, winds would likely be from the southwest coincident with the direction the thunderstorm is moving when a structure is located at position A. Hail fall would be likely as the most intense portion of the thunderstorm reaches the structure at house position B where winds may be from a northerly direction. Furthermore, the structure would likely be impacted by continued strong winds and rain from a northerly direction followed by winds from a northeasterly direction as the storm moves away from the structure depicted relative to house position C.

These storms can leave a wide swath of destruction or be very isolated in their effects. They can howl from one corner of the earth on their approach then suddenly burst downward or horizontally, driving hail into the west side of a house then causing a tree to fall toward the north. Often, adjusters will use the services of a meteorological consultant to confirm a weather event in a certain area, especially if the damage isn't widespread or the property is isolated. While that is an important procedure, general event information won't go much further than confirming locally clocked wind speeds, lightning strikes, temperatures, timing and types of precipitation. To verify the likelihood of specific damage, much more detail is needed on wind patterns. Some of that is available through Doppler radar analysis.

One documented hailstorm that illustrates that hail may fall in a direction other than the storm's direction was observed on August 8, 1963, near Weldon, Ill. This event was captured by nearby weather radars maintained and operated by the Illinois State Water Survey staff. Staff members conducted a site survey of the impacted area to determine the characteristics of hail fall and the magnitude of crop damage. The investigation also included interviews of local citizens. The observed hailstorm moved in an uncharacteristic direction toward the south-southwest at approximately 25 mph. An assessment of the crop damage showed that the horizontal component of the hail fall was from the east-southeasterly direction, which was at a right angle to the hailstorm's direction of motion. It was also noted that damaged trees were blown toward the south-southwest. Similar hail fall behavior was noted in another Illinois hailstorm the previous year in which the horizontal component of the observed falling hail was skewed relative to the storm's direction of motion.

Another case in point occurred during the late afternoon of April 21, 2004, in Oklahoma City, Okla. (Figure 2).

Local Doppler radar observations of this event showed that the storm moved toward the east at approximately 20 mph. Hail up to three inches in diameter was reported with this storm, and some observers reported hail falling for a 20-minute duration as the storm moved through Oklahoma City. A subsequent hail damage assessment exercise composed of professional engineers and claims adjusters sponsored by the Roofing Industry Committee on Weather Issues noted damage to roof faces in all directions.

Analysis of the available radar data captured during this event revealed that the storm had a supercell structure characterized by a strong horizontal rotation (counterclockwise) associated with the storm's updraft, a well defined hook-echo, and other characteristics which are attributable to supercell storms. Depending on the location of an observer at the ground relative to the storm's movement, surface winds would likely have begun from the west and then become northerly and northwesterly near the storm's rotating updraft as the storm moved eastward.

Relatively little attention by the weather research community has been given to studying the direction of hail fall as it relates to structural damage. Most meteorological research regarding hailstorms has focused on identifying and estimating the size of hail using Doppler radar. Therefore, much of the information about the direction of hail fall near the ground is anecdotal and comes as a side note when referenced in hail formation and detection studies.

There is, of course, the possibility that previous damage will be claimed under a new event. That is something that an adjuster has to build into the evaluation, but denying a claim based on hail direction implied by the direction of a storm path can be an iffy prospect. If a hail claim is going to be rejected based on directional characteristics, a thorough application of wind measurement technology is recommended.

E. DeWayne Mitchell is a senior meteorologist, Matt Gaffner is a forensic meteorologist, and Lynne Lawry is director of forensic product sales at Weather Decision Technologies, Inc. www.weatherforensics.com.