1996 Lake Huron Cyclone - Storm History

Storm History

On September 11, 1996, a weak low-pressure area was situated close to Lake Superior. Near the surface, its central pressure was 1,012.0 hectopascals (29.88 inHg) over Lake Michigan. There was also a shortwave at the 500 hPa level (18,000 ft above mean sea level) over Ontario. The northwest tilt of the low with height indicated that the surface center was in a stage of baroclinic development. In addition to this, analysis concluded that the cyclone had a cold core in the mid-troposphere. Analysis at the 700 (around 10,000 ft above mean sea level) and 850 millibar (around 5,000 ft above mean sea level) pressure levels indicated that the cyclone also had a cold core within the lower troposphere.

The cyclone's overall strength increased, with surface winds building from from 11 mph (18 km/h) to 67 mph (108 km/h). This occurred while positive vorticity and thermal advection were causing moderate rain showers over Ontario, with a broken area of thunderstorms present just ahead of an advancing cold front. By September 12, the cyclone moved southward to a position over Lake Huron, with upper portions centered to the west over Michigan. During this 24-hour period, the cyclone's central pressure fell from 1,012.0 hectopascals (29.88 inHg) to 1,006.0 hectopascals (29.71 inHg). After 1200 UTC September 12, the low moved southeastward and became vertically stacked from the surface up to the 500 millibar pressure surface (located 18,000 ft above mean sea level).

For a brief time, the portions of the cyclone at the 300 and 200 mbar pressure levels moved southeastward over Lake Huron. The greatest intensification occurred below the 500 mbar level. As the cyclone's tilt was neutral, or stacked with height, the storm was in the occlusion (or warm seclusion) stage of cyclone development. As the cyclone proceeded through this stage, the cold front, which was connected to the surface low, became an occluded front as it caught up with the surface warm front. The occluded front extended from Lake Huron to Pennsylvania on September 13. A 155 mile (250 km) swath of showers and thunderstorms was positioned across the frontal zone. Also at this time, a third area of showers was centered near the occluded low.

Between 1200 UTC September 13 and 0000 UTC September 14, a shortwave rotated throughout the area of the occluded low. This caused the mid-level portion of the cyclone to move eastward, centering itself just east of Lake Huron. During the same period, the surface low moved slowly over Lake Huron and fell five more millibars to 999.0 hectopascals (29.50 inHg). In response to this change, the maximum sustained winds increased as well. After 0000 UTC September 14, the lower and mid-tropospheric portions of the cyclone moved westward, becoming stacked atop one another again, but this time up through the 200 mbar pressure level. In contrast to the earlier developments, the baroclinicity of the system diminished rapidly. However, the lower tropospheric area continued to intensify, dropping to 993.0 hectopascals (29.32 inHg), with major height falls seen aloft. The lowered baroclicity determined that the heat fluxes may have spawned the intensification.

Between 1200 UTC September 14 and 0000 UTC September 15, visible satellite imagery of the vortex revealed a resemblance to a tropical cyclone, with an eye-like feature about 19 miles (30 km) wide. In addition to the eye, convective clouds had also formed, creating the eyewall of a tropical cyclone. Furthermore, feeder bands of convective showers continued extending westward to about 310 miles (500 km). Soon after the cyclone passed buoy 45008, winds turned to the east-northeast and rapidly increased. In contrast to earlier in the storm's life, which had more energy in the high levels (like a cold-core low), now it was stronger near the surface (like a warm-core low). Surface analysis indicated that a weaker cyclonic circulation perisisted over the eastern shore of Lake Huron with another center north of Lake Ontario. During this 12-hour period, the cyclone diminished rapidly, mainly in the lower troposphere. Twelve-hour height rises also occurred ranging from 66 – 197 feet (20 – 60 m). On September 15, the cyclone decayed and left Lake Huron.