Cold Air + Warm Water = Lake-Effect Snow

February 4th, 2007 |

MODIS true color image of Great Lakes region

The coldest air of the 2006-2007 winter season (surface temperatures on 04 February 2007 were as cold as -42º F / -41º C at Embarrass, Minnesota, -31º F / -35º C at Tomahawk, Wisconsin, and -21º F / -29º C at Randville, Michigan) was flowing over the still-warm waters of the Great Lakes (water temperatures were generally +35º to +41º F / +2º to +5º C), producing widespread bands of lake-effect snow (LES) which were very apparent on MODIS true color imagery (above). Note the transition from a multiple-LES-band regime over Lakes Superior and Michigan to more of a single-LES-band regime over Lakes Erie and Ontario (below). If you look closely, you can see that ice was beginning to form along portions of the northern shore of Lake Superior, as well as along the western shore of Lake Michigan and Lake Huron. The cold temperatures were limiting snow crystal growth (creating a fine, powdery snow), but snowfall accumulations still managed to reach amounts of 17 inches / 43 cm at Grandville in lower Michigan and 12 inches / 30 cm at Rainbow Lodge in the Upper Peninsula of Michigan; farther to the east, 18 inches / 46 cm fell at Boston in western New York. The strong winds were also producing wind chill temperatures in the -30º to -40º F / -34º to -40º C range across parts of Minnesota, Iowa, and Wisconsin.

MODIS true color image of Lake Superior

MODIS true color image of Lake Michigan

MODIS true color image of Lakes Erie and Ontario

Fatal tornado outbreak in Florida

February 2nd, 2007 |

GOES-12 10.7µm IR image
GOES-12 10.7µm InfraRed (IR) imagery (above; Java animation) shows the development of multiple lines of convection that produced numerous reports of damaging winds and tornadoes across northern and central Florida during the pre-dawn hours  on 02 February 2007. The counties where damaging winds and/or tornadoes were reported are highlighted in blue around the times of the SPC storm reports. At least 3 of these tornadoes have been rated as producing EF-3 damage, and 20 deaths have been reported so far as a result of these storms. While fairly cold IR cloud top temperatures were noted at times (-60º to -66º C, red to dark red enhancement), there were no “enhanced-V” or other typical severe storm IR signatures associated with this convection.

.
These supercells formed within an environment of high vertical wind shear, which enabled the thunderstorm updrafts to develop strong rotation; in addition, a strong upper tropospheric jet stream was located over the southeastern US, producing high wind speeds aloft and supporting large-scale lift over Florida. The lines of severe convection were forming within a zone of pre-frontal lower-tropospheric convergence; while it was too cloudy to sample the immediate pre-convective environment, GOES-12 sounder derived product imagery (DPI) at 00 UTC and at 12:00 UTC (below) did show that the air mass ahead of the approaching frontal boundary was relatively moist (PW values of 40-50 mm) with modest winter-season instability (LI values of 0º to -6º C, CAPE values of 0 to 1000 J/kg).
AWIPS images of GOES sounder derived products