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.

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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.

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AWIPS images of the MODIS sea surface temperature (SST) product on 16 April 2008 (above) displayed some interesting warm water eddy structures (green to yellow colors) over the western Atlantic Ocean, not far off the coast of the Mid-Atlantic states. A comparison with the SST analysis field from the High Resolution Real Time Global (RTG_SST_HR) model less than 6 hours later (at 00:00 UTC on 17 April) demonstrated the advantage of the 1-km resolution MODIS data for resolving the small-scale detail in the structure of warm water eddies; even the magnitude and placement of the largest warm eddy (in the middle 60s F or 18-20º C, light green colors) was incorrectly depicted by the RGT_SST_HR model. On the other hand, the model did do a fairly good job of portraying the warm plume of the Gulf Stream (denoted by the yellow to orange colors in the bottom center of the image).

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An animation of GOES-12 (GOES-East) visible channel imagery (above) showed the development of an elongated plume of blowing dust over the south-central US on 10 April 2008. The dust plume originated in eastern New Mexico and western Texas, and was being drawn northeastward into the circulation of a large and powerful mid-latitude cyclone that was centered over the central Plains states.

Also note the haziness that was seen moving northward across the western Gulf of Mexico — this was due to smoke from widespread fires that had been burning in parts of southern Mexico and Central America (NOAA Hazard Mapping System fire and smoke product).

AWIPS images of the MODIS visible and “cirrus detection” channels (above) showed the early stages of formation of the dust plume over the Texas / New Mexico border region at 17:20 UTC (11:20 AM local time) on 10 April. At that particular time, the blowing dust was not yet readily identifiable on 1-km resolution MODIS visible imagery (nor on 250-meter resolution MODIS true color imagery); however, a subtle light gray “dust signal” was evident on the 1.4 µm near-IR “cirrus detection” image (since that channel is very sensitive to the presence of particles that are efficient scatterers of light).

The conventional method used to detect airborne dust or sand is to calculate the difference in brightness temperatures between the 11.0 and 12.0 µm IR channels; higher concentrations of dust exhibit a brighter yellow signal (a more negative temperature difference) on such a MODIS IR difference product (above). The dust feature was obvious over New Mexico and Texas on the first image (17:25 UTC on 10 April), with the second image (08:42 UTC on 11 April) showing the dust reaching from northeastern Texas to far southern Wisconsin. Note that a “false positive” dust signal (lighter yellow colors) appears over parts of the western US on the IR difference product, due to the high emissivity of the soils found over certain regions.

The GOES-West (GOES-11) imager has the 11.0 µm and 12.0 µm IR channels used to calculate this type of IR difference product, but the GOES-East (GOES-12 ) imager lacks the 12.0 µm channel — so using this IR brightness temperature difference technique to detect airborne dust (or volcanic ash) over the eastern US is only possible using data from either the GOES sounder or the polar-orbiting NOAA AVHRR and Terra/Aqua MODIS instruments.

The strong winds associated with the large storm system in the central US transported the dust rapidly northeastward, helping it to reach northern Illinois and southern Wisconsin. Backward air mass trajectories calculated using the NOAA Air Resources Laboratoty HYSPLIT model (above) verified the rapid transport of dust from New Mexico and Texas all the way to southern Wisconsin during the 24-hour period from 12 UTC on 10 April to 12 UTC on 11 April. In fact, there were public reports of dust-laden moisture seen on cars in southern Wisconsin (in the La Crosse, Madison, and Racine areas) on the morning of 11 April — some of the dust aloft was scavenged by falling precipitation overnight and brought down to the surface.

A similar event of blowing dust from Texas reaching southern Wisconsin occurred back in December of 2003 (CIMSS GOES Gallery | National Weather Service Milwaukee/Sullivan).

UPDATE: The National Weather Service forecast office at Green Bay, Wisconsin received photos of “dirty snow” (below) that fell near Lac du Flambeau in far northern Wisconsin on 11-12 April. In addition, the NWS forecast office at Marquette, Michigan passed along a report of “brown snow” that fell on 11 April at Marenisco in the Upper Peninsula of Michigan.

(Photos courtesy of Linda Albers, NWS Cooperative Observer at Lac du Flambeau, Wisconsin)

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