Freezing Rain Event 19-21 February 2018 (81)

Kris Sanders, NOAA/National Weather Service Topeka Kansas, Topeka, KS

Chad Gravelle, NOAA/National Weather Service Operations Proving Ground, CIMMS – University of Oklahoma

John Gagan, NOAA/National Weather Service Sullivan Wisconsin

Brian Barjenbruch, NOAA/National Weather Service Valley Nebraska

 

Abstract:

Abstract

During the period of 19-21 February 2018, a strong arctic air mass progressed southward across the Plains and Midwest, undercutting warm temperatures aloft, and creating an environment conducive for rain, freezing rain and graupel. Multiple convective complexes formed above sub-freezing surface air, with freezing rain and graupel accumulation extending from portions of central Texas northward into southern Minnesota. The impacts associated with this event varied considerably. Some areas experienced sufficient freezing rain accumulation to cause damage to trees and power lines. Meanwhile, other locations experienced higher amounts of graupel, limiting ice accumulations and infrastructure impacts. The spatial variability of precipitation types, the performance of numerical weather prediction (NWP) depicting the low-level thermodynamic profile, and the presence of convection resulted in substantial challenges to the forecast and associated messaging.

Overall, the synoptic and mesoscale patterns associated with this event were conducive to producing substantial freezing rain accumulations somewhere in the central United States (Sanders 2013). However, NWP struggled to accurately depict the evolution of the shallow arctic air mass as sub-freezing surface temperatures were observed much farther south and much quicker than predicted. Significant model guidance errors of surface temperatures occurred even in the hours leading up to this event, and freezing rain was observed farther southeast than first anticipated. The presence of steep midlevel lapse rates supported elevated instability and resulted in convective elements that altered precipitation types and coverage. Late on the evening of the 19th, elevated thunderstorms developed within the strong instability and vertical wind shear above the sub-freezing surface air. Some of these storms moved as fast as 70 mph while exhibiting mid-level rotation and producing hail up to the size of nickels, freezing rain and graupel.

This presentation will explore the forecast challenges associated with this event. Temperature variations of only a few degrees were the difference between substantial rain and significant icing, and specific attention will be paid to how various NWP models handled the shallow arctic air mass. The conceptual understanding of Midwestern ice storms, model biases, forecast precision, and situational awareness will be discussed to better understand, predict, and message potential impacts with future events.

Sanders, K. J., C. M. Gravelle, J. P. Gagan, and C. E. Graves, 2013: Characteristics of major ice storms in the central United States. J. Oper. Meteor., 1 (10), 100-113,