National Weather Service, Duluth, MN
A precipitation shadow, as defined in the AMS Glossary of Meteorology under the term "rain shadow", is a region of sharply reduced precipitation on the lee side of an orographic barrier, as compared with the upwind side of the barrier. Enhanced precipitation occurring on the upwind side of a hill or mountain leaves less moisture available for precipitation on the downwind side. Air descending on the downwind side warms and dries to further erode the precipitation. The ridge of higher elevation running northeast to southwest along the North Shore of Lake Superior rises abruptly from the lake. The drop in elevation to the northwest is much less and more gradual. Although less pronounced as with a true mountain barrier, enhanced precipitation along the ridge due to orographic lift from onshore winds is still significant. The extra release of moisture results in less precipitation to the northwest. The radar loop shows what appears to be a shadow in the snow on radar during a snow event on November 4, 2018. Be advised that other atmospheric processes may also be contributing to the reduction in snowfall.
The topographic map shows the drop off in elevation farther inland from Duluth with the narrow jagged line of the Mesabi Iron Range to the north in the central part of Saint Louis County.
Surface winds at the Duluth International Airport during the time covered by radar were from slightly south of due east. Winds were not directly into the hill but still a strong component. The following surface observations from the NOAA Aviation Weather Center database show winds from 70 to 80 degrees around 15 knots with gusts up to 25 knots.
KDLH 042155Z 08016G23KT 1/2SM R09/5000VP6000FT -SN BR OVC008 01/00 A2980 RMK AO2 PK WND 07026/2111 SFC VIS 3/4 SLP110 P0005 T00110000Also important, the upper air maps retrieved from the Storm Prediction Center archive for 00Z November 5, 2018 (6 PM CST, November 4, 2018) showed near unidirectional wind flow up through the 700 mb pressure level. Extra moisture is being condensed out of the air as the entire layer is lifted and cooled, leaving much less moisture for areas farther northwest.
