AWIPS images of the MODIS visible channel, 1.6 µm near-IR “snow/ice” channel, and Land Surface Temperature (LST) product (above) depicted a broad swath of heavy snow on the ground across parts of eastern South Dakota and western Minnesota on 26 April 2008. Snowfall amounts included 19.0 inches at Watertown, South Dakota (which set an all-time record for 24-hour snowfall accumulation there) and 15.5 inches at Brandon, Minnesota. On the MODIS visible image, the unfrozen lakes stand out as dark features against the bright white snow-covered ground; the snow cover appears very dark on the MODIS snow/ice image (since snow is a very strong absorber at the 1.6 µm wavelength), in contrast to supercooled water droplet clouds which appear as varying shades of white. Note how the MODIS Land Surface Temperature values within the snow swath were in the 30-40º F range (green colors), compared to much warmer LST values of 50-60º F (yellow to orange colors) over the bare ground regions on either side of the deep snow cover. The MODIS LST product gives an indication of the temperature of the actual land surface (or “skin temperature”), which can be several degrees different than the air temperatures measured in instrument shelters located about 5 feet above ground level.

A MODIS true color image from the SSEC MODIS Today site (below, viewed using Google Earth) further demonstrated the large contrast between the significant snow cover and the surrounding bare ground. According to the National Weather Service forecast office at Sioux Falls, a Trough of Warm Air Aloft (TROWAL) contributed to the heavy snow event (which forced Interstate 29 to be closed from Brookings – about 50 miles north of Sioux Falls – all the way to the North Dakota border).

View only this post Read Less

Clusters of severe thunderstorms developed along the Texas / New Mexico border region during the late morning and afternoon hours on 23 April 2008. GOES-12 visible images (above) showed multiple low-level convective outflow boundaries that moved westward — and these outflow boundaries then played a role in subsequent convective development later in the day. The GOES-12 satellite had been placed into Rapid Scan Operations (RSO) mode, allowing images to be available as frequently as every 5-10 minutes during the daylight hours.

A comparison of 1-km resolution MODIS and 4-km resolution GOES-12 IR images (below) demonstrated the superior storm top temperature detection capability of the MODIS data — the largest storm over northwest Texas (located in the center of the image, between Abilene KABI and Lubbock KLBB) exhibited a well-defined “enhanced-v” signature on the MODIS imagery, in addition to a “warm trench” signature surrounding an area of overshooting tops just to the west of the enhanced-v. Note that the features on the GOES-12 IR image appear to be displaced a bit to the north/northwest compared to the MODIS IR image — this is a result of “parallax error” due to the large viewing angle of the GOES-12 satellite (which is positioned over the Equator at 75º W longitude).

These thunderstorms produced heavy rainfall, and were also responsible for a number of reports of tornadoes, large hail, and damaging winds (SPC storm reports). Isolated locations reported rainfall of 2 inches or more, creating several large swaths of wet ground which were very evident on MODIS IR and Land Surface temperature (LST) images (below) from the next day (24 April 2008). While the swaths of wet ground remained relatively cool during the following day (yellow to orange to light red colors), the surrounding dry ground areas were able to heat up much more quickly, exhibiting significantly warmer IR brightness temperature and LST values (darker red to black colors). In addition, a burn scar from a large fire that burned back in February 2008 could still be seen, with slightly warmer IR temperatures and LST showing up in the area of the burn scar (located just northwest of San Angelo, KSJT).

View only this post Read Less

An AWIPS image of the MODIS Sea Surface Temperature (SST) product (above) indicated that the surface water temperatures over much of Lake Michigan were still in the 38-39º F or 3-4º C range (darker blue colors) on 20 April 2008. However, as a southerly flow of warm air moved across the region on the following day (21 April 2008),  a large area of warm air advection fog formed over parts of Lake Michigan (due to the warm air flowing over the cold water and being cooled to temperatures at or near the dew point). 

An animation of GOES-12 visible images (below) showed the movement of the fog features as they moved northward across the lake during the day. Note that after 21:00 UTC (4 PM local time), narrow fingers of fog were seen moving westward past the small islands north of Door County, Wisconsin and drifting into the northern parts of Green Bay. In addition to the lake fog features, a well-defined lake breeze could also be observed moving inland from both the western and eastern shorelines of Lake Michigan. In eastern Wisconsin, the temperatures at 19:00 UTC (2 PM local time) near the Lake Michigan shore were only 52º F (11º C) at Manitowoc (KMTW) and 55º F (13º C) at Racine (KRAC), while temperatures a short distance inland included 70º F (21º C) at Green Bay (KGRB) and 73º F (23º C) at Burlington (KBUU). Across the state of Wisconsin on 21 April, the daytime maximum temperatures ranged from 81º F (27º C) well inland at Devils Lake (station identifier KDLL) to only 49º F (9º C) at Port Washington (located east of station KETB), where to persistent fog and onshore winds during the day kept temperatures quite cool.

The GOES-12 visible images also suggested that significant snow cover still remained across much of the northern portion of the Upper Peninsula of Michigan on 21 April — this snow cover is more obvious on 250-meter resolution MODIS true color imagery from the SSEC MODIS Today site (below, viewed using Google Earth). A snow depth of 22 inches (56 cm) was reported that morning at Phoenix Farms, while 13 inches (33 cm) was reported on the ground at Munising. However, with daytime high temperatures later that day reaching 63º F (17º C) at Phoenix Farms and 72º F (22º C) at Munising, the snow depths on the following morning at those two sites had diminished to 16 inches (41 cm) and 8 inches (20 cm), respectively.

View only this post Read Less

A sequence of 3 daily AVHRR false-color images from the 16-18 April 2008 period (above; viewed using Google Earth) showed large amounts of smoke (the hazy-looking, lighter gray areas on the images) from fires that were burning in parts of Argentina. One of the largest fires was located between Rosario and Buenos Aires — according to media reports, thick smoke covered the capitol city of Buenos Aires for several days, causing hazardous air quality, aircraft flight delays, highway closures, and traffic accidents that claimed several lives. As many as 292 fires burned a total of 173,000 acres (70,000 hectares); these fires were apparently set by ranchers in an attempt to clear land to create fresh pasture for cattle.

View only this post Read Less