|Snowfall accumulation (in inches) for the March 20, 1996 snowstorm. Heavy snow fell across much of Indiana (only southern Indiana amounts are analyzed) and portions of central Kentucky. Highest reported amounts were locally from 12 to 15 inches, especially across southern Indiana. Considerable mesoscale (small scale) banding was noted in the snow accumulation pattern across Indiana.|
TOP ROW: Composite synoptic charts at 1200 UTC 3/19/96 (top left) and 0000 UTC 3/20/96 (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., 988 mb at 1200 UTC 3/19/96). 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) and 500 mb low centers (black L), 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 1200 UTC 3/19/96 (bottom left) and 0000 UTC 3/20/96 (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 within 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: This A 1003 mb surface low across eastern Texas at 1200 UTC 3/18 deepened rapidly to 988 mb as it moved northeastward to eastern Tennessee by 1200 UTC 3/19. A surface trough extended north from the low through eastern Ohio with easterly flow off the Atlantic Ocean east of the trough and northeast to northerly flow west of the trough across Kentucky and southern Indiana. The low moved slowly northeastward to eastern Ohio (986 mb) by 1200 UTC 3/20. The heaviest snow fell roughly from 1800 UTC 3/19 to 0600 UTC 3/20 with lighter snow continuing on the 20th.
850 mb/700 mb: The 850 mb low across the lower Mississippi and Tennessee Valleys at 1200 UTC 3/19 deepened as it moved to West Virginia by 0000 UTC 3/20. Pronounced east to northeast flow (40-55 kts) off the Atlantic Ocean caused the thermal ridge axis to wrap westward around the north and west side of the strong low. This resulted in warm air advection, theta-e advection, and moisture convergence across Ohio, Indiana, and northern Kentucky. By 1200 UTC 3/20, the 850 mb low slowly moved to eastern Ohio with still some "wrap-around" warm air and theta-e advection from the north, but weaker than at 1200 UTC 3/19 and 0000 UTC 3/20. At 700 mb, a strong, closed low also existed during the period with pronounced easterly flow and warm advection extending westward across the Ohio Valley. This case is much different than the heavy snow pattern in the 2/16/93 and 1/17/94 cases. In this case, strong 850 and 700 mb lows produced strong easterly flow and warm advection along a north-south oriented thermal gradient (i.e., isotherms were aligned north-south with warmest air to the east and coldest air to the west). In contrast, strong southerly flow along a more west-east oriented thermal gradient existed in the 2/16/93 and 1/17/94 events.
500 mb: A strong southern stream shortwave (height values about 5550 m) across Texas at 1200 UTC 3/18 closed off and intensified rapidly as it reached middle Tennessee by 1200 UTC 3/19 (5330 m). Strongly diffluent flow was noted across the Ohio Valley. At the same time, a northern stream closed low sagged southward across the upper Mississippi Valley. The two low centers experienced a "pinwheeling" effect as the southern low moved slowly northeastward into the middle Atlantic states while the northern low weakened as it dropped southward into Missouri and Arkansas. The strong, dynamic closed low 500 mb pattern in this case also was vastly different from that associated with the 2/16/93 and 1/17/94 snowstorms. In those cases, positively-tilted southwest flow existed with embedded shortwaves moving northeastward within the flow.
300 mb: An active cyclonically-curved southern stream jet was present throughout the storm event from the southern Plains to the Gulf states to the Carolinas. Observed jet winds reached 140 kts. North of the jet, an elongated closed height trough and shear axis extended from the middle Mississippi Valley through Kentucky at 0000 UTC 3/20 with easterly flow across the Ohio Valley. The trough axis pivoted eastward by 1200 UTC 3/20. Significant upper-level divergence existed across much of the Ohio Valley at 1200 UTC 3/19 and 0000 UTC 3/20 within the left exit region of the southern jet.Isentropic/Frontogenesis: Cross-isobaric flow was noted on multiple surfaces across Ohio, Indiana, and northern Kentucky as an east-west trajectory of warm, moist air lifted isentropically across the region. This lifting was rather prolonged as the low and mid-level closed low centers only progressed slowly northeastward between 1200 UTC 3/19 and 1200 UTC 3/20. At 0000 UTC 3/20, warm advection and isentropic lift were occurring from northeast-southwest along the low-level flow. Modest Q vector convergence was noted in the 850-700 mb layer, suggesting frontogenetical forcing as well. Finally, total totals index values above 40 were present across the southeastern U.S. and middle Atlantic states with a ridge axis extending westward toward the Ohio Valley, suggesting elevated weak convective instability or perhaps CSI may have been present. The snowfall pattern showed considerable mesoscale banding, especially across Indiana.