January 29th, 2008
A very cold arctic air mass was in place across parts of the Canadian Prairies, with many sites in Alberta reporting surface temperatures colder than -40º F (-40º C) on the morning of 29 January 2008; the leading edge of this arctic air was surging rapidly southward across the Great Plains of the US as a strong cold frontal boundary. AWIPS images of the GOES-12 6.5µm “water vapor channel” (above) actually showed a signature of a gravity wave along the leading edge of the cold front as it moved southward across Nebraska and Kansas into Texas and Oklahoma. This lee-side cold frontal gravity wave feature could be detected on the water vapor channel imagery due to the fact that the airmass ahead of the front was quite dry, which shifted the altitude of the GOES-12 water vapor channel weighting function to much lower altitudes compared to what would be seen in a more typical airmass.
Strong pre-frontal and post-frontal winds were responsible for creating a large area of blowing dust across Texas during the afternoon hours, as seen on consecutive Terra (18:15 UTC, or 12:15 pm local time) and Aqua (19:50 UTC, or 1:50 pm local time) MODIS true color images from the SSEC MODIS Today site (below). This blowing dust reduced surface visibility to as low as 2 miles at San Angelo, Texas.
The windy conditions (several wind gusts in Texas were in excess of 60 mph) and very dry air were also creating an environment favorable for small wildfires; note that a number of fire “hot spots” (black pixels) could be seen on an AWIPS image of the MODIS 3.7µm shortwave IR channel (below).
The SSEC MODIS Today true color image from the Aqua satellite displayed using Google Earth (below) showed exactly which counties and highways in Texas were being impacted by the large cloud of blowing dust. In addition, a smoke plume from small wildfire that was burning just southeast of Abilene, Texas could be seen streaming south/southeastward.
January 24th, 2008
AWIPS images of the 4-km resolution GOES-12 10.7µm IR channel (above) showed that the land surfaces across the Upper Midwest region exhibited very cold brightness temperature values (darker blue enhancement) during the pre-dawn hours on 24 January 2008 — GOES IR brightness temperatures were as cold as -36ºC in northern Wisconsin, and -38ºC in northern Minnesota. Most of the area seen in the images above was cloud-free (except for the lake-effect cloud bands downwind of Lake Superior and Lake Michigan); the cloud-free surfaces could then be seen warming very quickly after sunrise. Much of the northcentral US had a deep snow cover of at least 5-10 inches, with snow depths of 15-30 inches common in northern Minnesota, Wisconsin, and Michigan; this deep snow cover (along with cloud-free skies and light winds) allowed for very strong radiational cooling of the air near the surface.
A higher-resolution (1-km) view using the MODIS 11.0µm IR channel and the MODIS fog/stratus product (below) revealed an amazing amount of structure in the surface brightness temperature field over the region (much of which was driven by terrain, with cold air drainage into low-lying areas such as river valleys); the coldest MODIS IR brightness temperatures sampled on the AWIPS images were -38ºC in northern Wisconsin and -42ºC in northern Minnesota. The MODIS fog/stratus product (created by computing the 11.0µm – 3.7µm brightness temperature difference) confirmed that there were no areas of fog or stratus cloud contributing to the interesting IR temperature structure seen across much of the Upper Midwest region around 08:20 UTC (2:20 AM local time). Note the appearance of “urban heat islands” (warmer IR temperatures, cyan enhancement) around cities such as Minneapolis, Minnesota and Sioux Falls, South Dakota.
NWS cooperative observer overnight minimum temperatures were as cold as -31ºF (-35ºC) at Sparta, Wisconsin and -39ºF (-39ºC) at Embarrass, Minnesota. There was also a reported minimum temperature of -51ºF (-46ºC) at a Minnesota Department of Transportation site northwest of Duluth (below), but the temperature data from that particular site appeared to be suspect.
Such cold temperatures aided in the formation of ice along the near-shore waters of western Lake Superior, as seen the following afternoon in a 250m-resolution MODIS true color image from the MODIS Today site (below). Lake Superior is the largest and deepest of the Great Lakes, and is usually the last to experience significant ice formation during the winter months.
January 20th, 2008
A Terra MODIS true color image from the SSEC MODIS Today site (above) revealed a swath of fresh snow cover across parts of Mississippi, Alabama, and Georgia on 20 January 2008. Snowfall amounts in the state of Alabama (which fell on the previous day) were as high as 5.8 inches near Verbena, 5.0 inches at Orrville, and 4.0 inches at Toxley (located on the MODIS imagery using Google Earth, below); snowfall up to 3.0 inches was reported in Mississippi, with 1.5 inches falling in Georgia.
AWIPS images of the MODIS visible channel, the 1.6µm “snow/ice channel”, and the Land Surface Temperature product (below) confirmed that this feature was indeed snow cover (snow is a strong absorber at the 1.6µm wavelength, and appears darker on the “snow/ice channel” image); in addition, the land surface temperatures within the area of snow cover were generally several degrees F colder (upper 20s to low 30s F, darker green enhancement) compared to the surrounding areas with bare ground (where land surface temperatures were generally in the mid 30s to around 40 F).