|Snowfall accumulation (in inches) for the February 16, 1993 snowstorm. Heavy snow fell over the northwestern third of Kentucky and all of southern Indiana, with a few reports of 8 to 10 inches in southern Indiana. Farther south across Kentucky, snow changed to rain, thereby limiting snowfall accumulation during the precipitation event.|
TOP ROW: Composite synoptic charts at 0000 UTC (top left) and 1200 UTC 2/16/93 (top right). On these charts, surface fronts are shown, including a cold front in solid blue color and a warm front in red. In addition, a surface low is indicated by a red L and labeled with minimum pressure (e.g., 06 at 1200 UTC indicates minimum surface pressure of 1006 mb). Also shown are 1) 850 mb temperatures every 5 deg C from +10 to -10 deg C (dashed red lines with 0 deg isotherm in sold red), 2) the axis of the low-level (850 mb) jet (bold black line with arrowhead) with wind speeds in knots indicated along the jet, 3) 500 mb heights in meters every 120 m (thin black lines), 4) 300 mb isotachs (lines of equal wind speed) in knots (green lines), and 5) 300 mb jet core (bold green line with arrowhead).
BOTTOM ROW: Contours of frontogenesis at 850 mb (blue) and 700 mb (red) at 0000 UTC (bottom left) and 1200 UTC 2/16/93 (bottom right). Values are in deg Kelvin per 100 km per 3 hours (K/100 km x 3 hr). Strongest frontogenetical forcing for lift usually occurs along and just south/east of the axis of strongest frontogenesis, with frontogenetical forcing tilted with height toward cold air. Also shown is 850 mb positive equivalent potential temperature (theta-e) advection (dashed green) with values in 10 to the minus 1 power deg Kelvin per hour (10 e-1 K/hr).
Summary of Event
Surface: A low center was located over New Mexico at 1200 UTC 2/15 with a warm front extending east through central Texas. The low and warm front moved east-northeastward across the southern Plains, Arkansas, then into central Kentucky by 1200 UTC 2/16. The minimum pressure of the low at 1200 UTC 2/16 was 1006.5 mb. Cold east-northeast surface flow persisted in the snowfall zone north of the warm front.
850 mb: At 1200 UTC 2/15, a trough was located across New Mexico, with a thermal boundary extending east across the Texas Panhandle to southern Tennessee. A 40-50 kt low-level jet existed south of the boundary across eastern Texas. As an 850 mb low moved to Oklahoma by 0000 UTC 2/16, the warm frontal boundary lifted northward into southern Kentucky, while the low-level jet intensified to around 60 kts over Mississippi and Alabama. Strong theta-e advection was observed across the lower Ohio and Tennessee Valleys at 0000 UTC 2/16. The 850 mb low continued to move to the east-northeast between 0000 and 1200 UTC 2/16 as the warm boundary became nearly stationary, and as the theta-e maximum propagated across the Ohio Valley. The strong low-level jet and low-level convergence resulted in a compaction of the isotherms (0 deg C isotherm over southern Kentucky at 0000 UTC), which promoted strong warm air and theta-e advection, isentropic lift, and frontogenetical forcing across the Ohio Valley. These factors contributed significantly to heavy snow amounts across southern Indiana and northern Kentucky.
700 mb: The 700 mb trough axis moved east during the period resulting in a tightening thermal gradient, convergence, and frontogenetical forcing over Kentucky and Indiana.
500 mb: Mid-level synoptic flow was characterized by broad southwest, positively-tilted flow over the Ohio Valley ahead of an active southern stream shortwave trough and associated height falls across the southern Rockies and southern Plains. The shortwave shifted quickly east-northeastward to the lower Ohio Valley by 1200 UTC 02/16.
300 mb: An anticyclonically-curved jet streak across the Great Lakes and a cyclonically-curved streak over the southern Plains and Tennessee Valley became better-defined and increased in strength to 130 and 90 kts, respectively. The entrance region of the northern jet exhibited significant along-stream variation. The Ohio Valley was positioned to the south of the northern jet core within the right entrance region and within the left exit region of the southern jet. The presence of the two jets, particularly the pronounced anticyclonically-curved jet entrance region, resulted in substantial upper-level divergence which increased with time. At low levels, the northerly ageostrophic flow, which is part of the direct thermal circulation in the entrance region, promoted increased low-level cold air advection and convergence which enhanced frontogenetical forcing across the Ohio Valley. The southerly low-level environmental flow (low-level jet) to the south responded to the forcing and upper divergence maximum by increasing with time (to 60 kts at 0000 UTC 2/16) and lifting adiabatically along steeply sloped isentropic surfaces. Strong adiabatic cooling (due to strong ascent) and the entrance region ageostrophic circulation played a significant role in keeping enough cold air in place across the Great Lakes and Ohio Valley during the snowstorm, despite pronounced low-to-mid-level warm advection. The importance of a well-defined anticyclonically-curved northern stream jet streak and its associated entrance region also was noted during the January 17, 1994 major snowstorm over Kentucky.
Isentropic/Frontogenesis: Isentropic data (e.g., 290 K surface) during the period of heavy snowfall showed a tight pressure gradient across the Ohio and Tennessee Valleys with strong southerly flow normal to the isobars. Also, significant northward moisture transport from the Gulf was occurring. As a result, strong upward vertical motion of moist air was present (with low condensation pressure deficits) across Tennessee, Kentucky, and southern Indiana. The maximum lift on this surface was positioned within the left exit region of the cyclonically-curved southern jet streak and the right entrance region of the anticyclonically-curved northern jet. Substantial low-to-mid-level Q vector convergence and frontogenetical forcing was present across the lower Ohio Valley and western Tennessee at 0000 UTC 2/16 which moved northeastward across the Ohio Valley and area of heavy snow between 0000 and 1200 UTC. The frontogenetical forcing maximum was located within the northern stream jet entrance region.