A weak tornado occurred Thursday 27 September in Racine County of southeast Wisconsin. (North Cape Wisconsin, the site of the brief EF-0 touchdown, is in north-central Racine County). The tornado occurred about 15 minutes before the radar image below; the cell that produced the tornado is moving offshore into Lake Michigan at the time of the radar image.

The weather in Wisconsin on Thursday, 27 September was initially dominated by descent in the wake of a departing cold front. Generally clear skies were the rule for most of the morning. However, solar heating combined with cooling at 500 mb and an approaching mid-level shortwave helped cause the development of isolated thunderstorms.

The loop above shows the quick eastward progression of the cold front that started the day (at 1200 GMT) in eastern Wisconsin — the front propagates rapidly through lower Michigan. At the same time, 500-mb temperatures (as depicted by the RUC forecast from 1200 GMT, above) steadily fall as a short wave (depicted by colder brightness temperatures in the water vapor imagery) moves eastward across Wisconsin.

The cooling temperatures and approaching shortwave were discussed in the convective outlook issued by the Storm Prediction Center in Norman, OK at 1200 GMT on Thursday 27 September:

…MN/WI…
WATER VAPOR IMAGERY SHOWS STRONG SHORTWAVE TROUGH DIGGING
SOUTHEASTWARD ACROSS ND. THIS FEATURE WILL MOVE INTO WESTERN MN BY
THIS EVENING…AND LIKELY RESULT IN ONE OR MORE CLUSTERS OF
THUNDERSTORMS TRACKING EASTWARD INTO WI. COLD TEMPERATURES ALOFT
/500MB TEMPS AOB -20C/ AND STEEP MID LEVEL LAPSE RATES ARE FORECAST
OVER THIS AREA…RESULTING IN MARGINAL INSTABILITY DESPITE LIMITED
LOW LEVEL MOISTURE. THERMODYNAMIC PARAMETERS APPEAR FAVORABLE FOR A
RISK OF HAIL IN STRONGER CELLS THIS AFTERNOON AND EVENING.

The visible loop (below) shows the development of vigorous convection in the destabilizing atmosphere over the course of the afternoon. In fact, structures in the convection at the end of the loop suggest convective towers overshooting the tropopause. The coldest brightness temperatures associated with the deep convection were around 234 K; the 00 UTC sounding from Green Bay Wisconsin suggests those temperatures occurred around 350 mb. Lightning in this tornadic storm was uncommon; although other parts of the convective line showed more electrical activity. Dewpoint temperatures before the thunderstorms moved through were in the low 50s, adequate to support tornadoes, but not ideal.

One additional note: Sounder DPI products nicely captured the development of the instability over Wisconsin. This image, from 2046UTC, shows lifted indices in the -4 to -8 range.

Finally, the image below is from GOES-West, and it shows the updraft of the tornadic storm merging with a spreading cirrus shield aloft. The outlines of Racine County are included on the satellite image.

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A NOAA-15 1-km resolution Advanced Very High Resolution Radiometer (AVHRR) 10.8µm IR image centered over Madison, Wisconsin at 11:15 UTC or 6:15 AM local time on 26 September 2007 (above) shows that several of the smaller inland lakes across southern Wisconsin were still relatively warm (IR brightness temperatures as warm as +17ºC, red enhancement) — the surrounding land areas exhibited significantly cooler IR brightness temperatures around +8ºC (yellow enhancement), in agreement with most of the 11 UTC surface temperatures at METAR sites. The cyan-colored features in the northwestern and northcentral portions of the image were patches of stratus cloud, which exhibited cloud top IR brightness temperatures around +3ºC.

The corresponding GOES-12 10.7µm IR image (below) showed similar warm signatures for a few of the larger inland lakes (but only as warm as +10 to +12ºC); however, many of the smaller lakes could not be resolved by the 4-km resolution GOES-12 IR data. Also note the patch of cold cirrus cloud over far southeastern Wisconsin (dark blue to violet enhancement): a “transverse banding” structure was evident with this cirrus feature on the AVHRR IR image (a signature of potential high-altitude turbulence), which was not as apparent on the GOES-12 IR image.

About 25 minutes after the time of the IR images above, an interesting orinthological meteorological radar signature of bird “roost rings” was observed. Large numbers of birds dispersed from their nocturnal roost sites during the early morning hours to begin feeding, and as the flock crossed the radar beam, ring-like signatures were seen on the Milwaukee/Sullivan radar composite reflectivity product (below; QuickTime animation) as the birds spread out across the area. Similar radar signatures are also occasionally seen with bat swarms.

At least two of the roost rings appear to have originated from the general vicinity of some of the larger (warmer) lakes noted on the MODIS and GOES IR imagery above: Beaver Dam Lake (located northeast of Madison, KMSN), and Lake Koshkonong (located southeast of Madison). As close as one of the larger roost rings came to Dane County Regional Airport in Madison, one also has to wonder if the high density of birds may have potentially been an aviation hazard for a brief period of time?

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Strong winds in the middle troposphere (50-70 knots at 500 hPa) were seen around the base of a large trough of low pressure that was centered over the Great Basin region of the western US (above) on 24 September 2007.

A sequence of three AWIPS images of the MODIS 6.7µm “water vapor channel” from that day (below) revealed widespread areas of mountain wave signatures indicating that the strong winds were interacting with the rugged terrain across the southern and central Rocky Mountains. These types of mountain waves are known to be a satellite signature of lee turbulence; the pilot reports of turbulence were only isolated around the times of the 3 MODIS images (05:13, 09:22, and 20:30 UTC), but it could be that no aircraft were flying in the exact areas (or altitudes) where the lee wave signatures (and any associated turbulence) might have been present.

The corresponding sequence of three GOES-12 water vapor images (below) did indicate a few of the more well-defined areas of mountain waves signatures (especially over central Colorado), but the presentation of these features using 4-km resolution GOES data was not as clear as it was using the 1-km resolution MODIS data.

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A sequence of three Terra MODIS true color images from 02 August, 30 August, and 22 September 2007 centered over Lake Superior (above) reveals the onset of the brilliant orange Autumn season tree colors across parts of northeastern Minnesota, the Upper Peninsula of Michigan, and northern Wisconsin (ground truth: photo 1 | photo 2). Much of that area has experienced moderate to severe drought conditions, which could be causing the trees to begin to change color a bit earlier than normal due to stress.

Farther to the south, a similar sequence of three MODIS images centered over the state of Wisconsin (below) shows another (unrelated) change of colors — note the change from green to light brown colors across southern parts of the region where trees are sparse and corn cultivation is prevalent (southeastern Minnesota, northeastern Iowa, and northern Illinois). Once the majority of the corn fields begin to mature and dry down leading up to harvest time, those areas begin to take on a light brown hue on the MODIS true color imagery.

In addition, note the hazy appearance across a good deal of the southern half of the first MODIS image below (on 02 August) — this was due to thick smoke from widespread wildfires that had been burning in Idaho and Montana (IDEA MODIS Aerosol Optical Depth). This thick smoke had been lingering over the northern Rocky Mountain region during the last few days of July 2007, and was then transported eastward and concentrated along a cold frontal boundary that moved across the Great Lakes region on 02 August.

The image below shows a zoomed-in view centered on Lake Gogebic (located in the western portion of the Upper Peninsula of Michigan) on 23 September, using the AWIPS MODIS True Color Imagery Viewer developed by Jordan Gerth at CIMSS. This tool allows National Weather Service forecasters to view MODIS true color imagery within the AWIPS environment.

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