|Snowfall accumulation (in inches) for the January 17-18, 1994 major snowstorm. Heavy snow fell over most of Kentucky and southern Indiana, after changing from rain and freezing rain early in the event. The heaviest snow fell in a narrow northeast-southwest oriented band across the northern third of Kentucky. Reported snowfall reached 16 inches in Louisville and up to 22 inches east of the city. Mesoscale (small scale) banding is common in winter storms, making accurate prediction of precipitation amounts and locations a forecast challenge.|
TOP ROW: Composite synoptic charts at 0000 UTC (top left) and 1200 UTC 1/17/94 (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., 11 at 1200 UTC indicates minimum surface pressure of 1011 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), 5) 300 mb jet core (bold green line with arrowhead), and 6) total totals index (measure of instability) at 0000 UTC in (small green numbers) along low-level jet.
BOTTOM ROW: Contours of frontogenesis at 850 mb (blue) and 700 mb (red) at 0000 UTC (bottom left) and 1200 UTC 1/17/94 (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 weak low center moved east across the lower Mississippi Valley during the storm event, while an arctic cold front extending north from the low moved across the middle Mississippi Valley into the Ohio Valley. Ahead of the front, southerly flow existed from the Gulf Coast to the Great Lakes within a tight pressure gradient between the front and high pressure along the East Coast. Surface maps did not suggest a pattern normally associated with heavy snow events.
850 mb: At 0000 UTC 1/17, no organized 850 mb low centers were present. Instead, an open trough extended from the western Great Lakes, associated with a northern stream shortwave, to western Texas, associated with a southern system. Ahead of the trough, a strong, moist low-level jet (50-70 kts) extended from Arkansas to southern Ohio. Strong warm air and theta-e advection were occurring across western Kentucky and southern Indiana as 850 mb temperatures ranged from +5 deg C at Paducah to -8 deg C in southern Ohio. By 1200 UTC 1/17, the southerly low-level jet (40-50 kts across Kentucky and 50-65 kts across the southeastern U.S.) and warm advection extended from the Gulf states to the Ohio Valley and central Appalachians. Temperatures at 850 mb cooled significantly across Kentucky between 0000 and 1200 UTC (during the period of heavy precipitation), despite strong warm advection.
500 mb: At 500 mb, broad southwest flow existed across the Ohio and Tennessee Valleys throughout the heavy snow episode, ahead of a height trough from the northern Plains to the lower Mississippi Valley. Within the trough, however, two distinct shortwaves were present, a northern stream system associated with an arctic surge, and an active southern stream system that progressed eastward during the period. No deep, closed 500 mb low existed. The 500 mb pattern was similar to the February 15/16, 1993 snow event. An active southern stream shortwave appears to be a key in developing a pronounced moist, low-level jet, warm advection, and isentropic lift across the Ohio and Tennessee Valleys.
300 mb: A well-defined anticyclonically-curved jet streak was present across the Great Lakes with a secondary jet streak across Texas into the Southeast at 0000 UTC 1/17. Kentucky was positioned between the two jets, with the entrance region of the northern jet exhibiting substantial along-stream variation in the wind field. As a result, the ageostrophic wind response to the geostrophic deformation was significant, resulting in strong upper-level divergence. The divergence maximum within the entrance region coupled with the existence of the southern upper jet enhanced the southerly low-level jet across the lower Mississippi and Ohio Valleys (e.g., up to 70 kts at 0000 UTC at Little Rock), resulting in very pronounced isentropic lift across Kentucky toward the upper divergence area. By 1200 UTC 1/17, wind speeds within the northern jet increased substantially and appeared to backbuild across the Ohio and middle Mississippi Valleys, due in part to a tightening height gradient between an advancing height trough to the northwest and convective latent heat release to the south. As in the February 15/16, 1993 event, the existence and location of a distinct northern jet streak entrance region with substantial along-stream variation facilitated cold northerly ageostrophic low-level flow, increased frontogenesis, and very strong lift and adiabatic cooling which kept cold air in place across the Ohio Valley, despite strong environmental warm advection.
Isentropic/Frontogenesis: Strong lift and upward moisture transport along steeply sloped isentropic surfaces was present across Arkansas, western Kentucky, and western Tennessee at 0000 UTC 1/17 (e.g., on the 290 and 295 K surfaces). The average isentropic mixing ratio on the 292 K surface at 0000 UTC was 4-5 g kg-1 over western Kentucky. Strong lift continued through 1200 UTC across Kentucky. Q vector convergence and frontogenetical forcing was evident across the lower Ohio Valley to Arkansas at 850 and 700 mb at 0000 UTC 1/17, coincident with an area of enhanced precipitation. Pronounced frontogenetical forcing continued over Kentucky in the 850-700 mb layer through the morning hours on 1/17 (i.e., during the period of heavy snowfall) within the entrance region of the northern anticyclonically-curved jet streak. Finally, unstable air was located across the lower Mississippi Valley at 0000 UTC 1/17 which supported convective development in that area. Strong isentropic lift resulted in elevated convective instability across the Tennessee Valley into Kentucky (occasional thunder and lightning accompanied part of the snowstorm).