Clearing skies in the wake of a cold frontal passage on 08 October 2007 allowed AWIPS 4-panel images of the MODIS visible, snow/ice, 3.7µm, and 11.0µm IR window channels (above) to reveal extensive swaths of slightly darker (and slightly cooler) wet ground which were oriented southwest-to-northeast across parts of Texas, Oklahoma, Kansas, Nebraska, and Iowa. One-day precipitation amounts within these swaths were as high as 1.0-2.5 inches in Kansas, with similar amounts falling across western Nebraska a day earlier.

A closer view of western and central Kansas using the MODIS 3.7µm shortwave IR channel (above) shows that the IR brightness temperatures were several degrees cooler within the rain swaths (23-26º C or 73-79ºF, yellow enhancement), versus 28-33ºC or 82-91ºF (red enhancement) outside of the rain swaths — surface METAR data also depicted early afternoon (2:00 PM local time) surface temperatures a few degrees F cooler in the swaths where significant rain had fallen. The dew point temperatures were also generally a few degrees F higher at stations located within the rain swaths.

The GOES sounder Skin Temperature derived product image (below) showed a similar temperature difference between the areas that received heavy rain (skin temperature values of 25-27º C, or 77-81º F, green enhancement) and those areas that received little to no rain (skin temperature values of 31-37º C, or 88-99º F, yellow to orange enhancement).

UPDATE: From an operational forecasting standpoint, there was some concern that the areas in Kansas having higher soil moisture might be more prone to fog formation during the upcoming nighttime hours. An AWIPS image of the 1-km resolution MODIS fog/stratus prodcut (below) did not exhibit a fog signal (yellow enhancement) in the Kansas rain swaths at 08:39 UTC (03:39 AM local time) on 09 October. However, it was interesting to note that a few of the stations located within the primary rain swaths — most notably, Hayes (KHYS) and Dodge City (KDDC) in western Kansas, as well as Salina (KSLN) and Pratt (KPTT) in central Kansas — apparently remained a few degrees warmer than adjacent sites just outside of the rain swaths, thereby not cooling close enough to their dew points for widespread radiation fog to form.

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AWIPS composite images of the 6.7µm GOES-11 and 6.5µm GOES-12 “water vapor” channels (above) show the eastward progression of synoptic-scale frontal systems across the US and Canada — as well as the westward motion of what could evolve into a subtropical disturbance in the eastern Gulf of Mexico — on 02 October 2007. Regions of greater middle-tropospheric moisture (blue to white enhancement) and clouds (white to green enhancement) are evident along and ahead of many of these fronts, while areas of subsidence and dry air (yellow enhancement) are seen in the wake of some of the frontal boundaries.

The CIMSS Regional Assimilation System (CRAS) numerical weather prediction model has the ability to output synthetic satellite imagery — a “forecast” of what GOES imagery might look like in the near future. AWIPS images of CRAS predictions of the GOES water vapor channel imagery (below) show the forecast covering the 02-05 October 2007 period. The CRAS imagery indicated that the pattern of alternating moist and dry features (associated with subsequent frontal passages) would continue for the next 2-3 days, while the moisture associated with the Gulf of Mexico disturbance would eventually move inland across the western and central Gulf Coast states during the forecast period.

CIMSS has been generating CRAS water vapor and IR forecast imagery since the mid-1990s, and this forecast imagery has been made available to AWIPS (via LDM subscription) since August 2006. The CRAS model forecast imagery in AWIPS has been well-received so far, being mentioned in several Area Forecast Discussions issued by the National Weather Service office at Milwaukee/Sullivan, Wisconsin. For more information, see the CRAS Forecast Imagery in AWIPS VISIT lesson.

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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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