During the afternoon of April
28, 1999, numerous thunderstorms affected central
Kentucky and south-central Indiana. Although the atmosphere was
not conducive for a major severe weather outbreak nor tornadoes,
warm, moist air at the surface combined with cold, relatively
dry air aloft made large hail producing storms likely. Numerous cases of dime (3/4 inch diameter) and
larger size hail were reported to the National Weather Service
(NWS) in Louisville. One of these storms occurred at the NWS office, just south of downtown Louisville.
The hail coated the ground with continuous hail for around 10
minutes. Initially, the hail was pea to marble size, then increased
in diameter to dime to quarter (1 inch) size. Near the end of
the storm, numerous larger, odd-shaped pieces of hail fell, with
the length of some pieces around 2.5 inches and a width around
1 inch. The storm moved north across downtown Louisville and gradually
weakened thereafter. The following images show the
storm at its maximum intensity, as well as examples of hail that
fell from the storm.
This is a low-level base reflectivity
image of the severe thunderstorm over the NWS office in south-central
Jefferson county, Kentucky, just south of Louisville. Very heavy
rain and large hail were occurring within the dark red colors
on the southwestern flank of the storm. Rain, but no hail, was
occurring in the yellow and green areas to the north and east
of the severe portion of the storm.
This reflectivity image shows the
same storm at the same time as above, but higher up in the storm,
i.e., in the middle-levels. Note that very high reflectivity
values (65-70 dBZ; pink and small purple colors) are evident
in the southwestern part of the convective cell. This indicates
that very large hail was being suspended aloft in the storm by
the updraft. Velocity data, not shown, revealed weak rotation
in the updraft aloft, which enhances its potential to suspend
large particles, but no rotation near the ground. As this elevated
hail core descended, reported hail size increased from dime size
to well over one inch, as shown below.
This reflectivity vertical cross-section
shows a separate severe storm that produced around one inch diameter
hail just southeast of the Jefferson county storm above. The
storm shown at left exhibits vertical tilt and a high reflectivity
core (65-70 dBZ; pink color) suspended aloft. The storm top is
not particularly high (about 30,000 ft), but this can be misleading
in assessing severe storm potential. On this day, the atmosphere
precluded tall storms, but supported severe pulse storms, as
exhibited in this cross-section.
This picture, taken outside the
NWS Louisville office, shows a hail pile accumulating during
the storm as hail fell from off the roof and congregated at this
location on the ground. Note the falling hail in the upper left
portion of the image. This image was taken before pieces of very
large hail began falling.
Marble and dime size hail began
to coat the sidewalk, gravel area, and grass outside the NWS
office during the storm, before larger hail pieces began to fall.
Some other storms across central Kentucky and southern Indiana
on this day also caused hail to coat or cover the ground.
These are examples of some of the
odd-shaped pieces of large hail that fell near the NWS Louisville
office during the height of the storm. Initially, the smaller
hail that fell was circular in shape. However, note that none
of these very large pieces were circular shaped. Hail shape and
size are a function of atmospheric conditions, and internal dynamics
and physics within a thunderstorm. It appears that pieces of
smaller hail may have froze together into larger pieces within
this storm, thereby producing the clumps of hail shown here.
This large hail piece measured up
as one of the longest that fell during the storm. Note that it
has a length of nearly 2.5 inches, i.e., just smaller than baseball
size (2.75 inch diameter). However, the width of the piece shown
here is only about one inch.
Another measurement shows 2 of the
larger hail pieces that fell. The piece on the left is the same
as in the picture above. The hail piece on the right has a length
of at least 2.75 inches, although again the width is smaller.
Fortunately, there was little hail damage from the storm at the