Yes, we have been posting a lot of fog/stratus product images lately, but this comparison of AWIPS images of the 4-km resolution GOES-12 and the 1-km resolution MODIS fog/stratus product from around 07:15 UTC (3:15am local time) on 21 September 2007 was deemed to be highly Blog_Worthyâ„¢. Widespread fog (yellow to orange to red enhancement) can be seen forming in valleys from Ohio and West Virginia to Vermont and New Hampshire during the overnight hours. Note that the patches of high-altitude cirrus cloud (black enhancement) appear to be located farther to the north/northwest on the GOES fog/stratus product image — this “parallax shift” is due to the large viewing angle of the geostationary satellite, which is positioned over the equator. The MODIS instrument flys aboard the polar orbiting Terra and Aqua satellites (which pass directly overhead), so the imagery does not exhibit significant parallax error for high-altitude cloud features.

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As pointed out by the NWS forecast office at Milwaukee/Sullivan WI, a narrow ribbon of cooler water temperatures was seen along parts of the western nearshore waters of Lake Michigan on 19 September 2007. Strong westerly winds in the wake of a cold frontal passage aided in the upwelling of cooler water from below the lake surface, which was evident in a comparison of AWIPS images of MODIS Sea Surface Temperature (SST) from 18 and 19 September (above).

A comparison of the 19 September MODIS SST image with the MODIS visible channel image on the following day (20 September, below) shows that patchy low-level fog and stratus cloud had formed over much of the western portion of Lake Michigan (where cooler SST values were seen on the previous day).

An animation of GOES-12 visible imagery (below) indicated that the offshore fog and stratus persisted for much of the day on 20 September, and even began to move inland along parts of the lakeshore counties in southeastern Wisconsin shore as an easterly component of the surface winds increased during the afternoon hours. The cooler nearshore waters seen on the MODIS SST images above likely played a role in maintaining the fog/stratus along the west coast of Lake Michigan.

Also evident in the early portion of the GOES-12 visible animation are fingers of river valley fog across southwestern Wisconsin, northeastern Iowa, and southwestern Minnesota — this fog (which formed during the nighttime hours) quickly burned off with daytime heating. A comparison of the MODIS and GOES-12 fog/stratus products (below) around 08 UTC (3am local time) shows the more detailed river valley fog structure that was apparent on the 1-km resolution MODIS imagery (compared to the 4-km resolution data from GOES-12).

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AWIPS images of the GOES-12 6.5µm “water vapor” channel (above) show an upper-level low that was intensifying over Florida during the day on 18 September 2007. As warm/dry middle to upper tropospheric air (yellow enhancement) wrapped around and into the southwestern quadrant of the cyclone, convection was seen to develop along the southern Gulf Coast of Florida. This dry air was associated with a tropopause anomaly, where the “dynamic tropopause” (taken to be the 1.5 Potential Vorticity Unit contour) descended to around the 380 hPa pressure level according to the GFS40 model (below).

A cross section of GFS40 model fields (oriented from east on the left to west on the right) across Florida (below) reveals the lowering tropopause (color shaded potential vorticity values) and descending dry air (dashed green specific humidity contours) associated with the tropopause anomaly. Surface pressures were beginning to fall across that region in response to the tropopause anomaly, raising speculation that a subtropical or a tropical cyclone could eventually develop from this system as it moves slowly westward into the Gulf of Mexico.

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AWIPS images of the MODIS fog/stratus product (above) and the corresponding GOES-12 fog/stratus product (below) revealed areas of fog that were forming in the valleys of Pennsylvania and New York around 08 UTC (4am local time) on 17 September 2007. The 1-km resolution MODIS imagery offered a more detailed view of the fog structure compared to the 4-km resolution GOES-12 product. Also note the “false signal” of fog that appeared on the GOES-12 image along certain land/water boundaries (the Finger Lakes region of New York, portions of Lakes Erie and Ontario, and the New Jersey coast of the Delaware Bay). With clear skies and light wind conditions associated with high pressure in place across much of the northeastern US, fog formation (due to strong radiational cooling during the night-time hours) was prevalent over a large area.

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