MARCH 20, 1996 HEAVY SNOW EVENT
Snowfall Map
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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. |
Composite Charts
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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.
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