An occluded storm that affected the upper midwest early in the morning of 16 October displayed cloud characteristics that can be interpreted as the development of a TROWAL (A TROugh of Warm air ALoft). The highest, coldest clouds tops that at the start of the loop are in north-central Illinois can be interpreted as the edge of the warm conveyor belt in this occluded system. As that edge translates northeastward across Lake Michigan and into lower Michigan, cold cloud tops blossom over southeast and east-central Wisconsin and then move westward or northwestward. These cold clouds can be interpreted as the development of precipitation in the TROWAL airstream, which originates in the warm sector and then turns cyclonically into the cold air on the cyclonic shear side of the warm conveyor belt.

TROWALs are characterized by warm air aloft, and RUC model output for this day shows warmth aloft in a region that overlays a region of precipitation. Here is the temperature on the 320 K theta-e surface at 0900 UTC — the midpoint of the IR loop above. A tongue of warm air — which would be a depression on this isentropic surface — extends west-northwestward from eastern Wisconsin towards northwest Wisconsin. Note how well the canyon correlates spatially with radar echoes shown here. Note also that the warm tongue at 850 mb, shown here, does not overlay as well with the radar echoes. (A loop of the three images is here). TROWALs move through isobaric surfaces and lie along isentropic surfaces. Care must therefore be taken when interpreting thermodynamic parameters associated with a TROWAL.

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The National Hurricane Center began issuing advisories on Tropical Depression 15 (located in the central Atlantic, east of Bermuda) on 11 October 2007. A GOES-12 visible image with ASCAT wind data sourced from the CIMSS Tropical Cyclones website (above) suggested that maximum winds were only around 30 knots.

There was strong southwesterly flow aloft over the developing cyclone with increasing deep layer wind shear, so the bulk of the deep convection was confined to the northeastern quadrant of TD #15 — an animation of GOES-12 visible images (below) shows that the low-level center was close to becoming exposed in that strong wind shear environment.

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“Lee waves” are sometimes seen just downwind of significant topography (such as high mountain ranges) when strong winds are perpendicular to the ridge lines — but even relatively subtle terrain features can produce lee wave clouds that are evident on satellite imagery (one such example of lee wave clouds in a region of subtle terrain was seen over the Bay of Fundy in Canada in August 2007). Brisk northwesterly winds were prevalent across much of southern Wisconsin during the day on 10 October 2007, and an animation of GOES-12 visible channel imagery (above) revealed a packet of quasi-stationary lee waves immediately downwind of the Baraboo Bluffs.

An AWIPS high resolution topography image (below) shows that the narrow Baraboo Bluffs terrain feature (located just to the south-southeast of Baraboo/Wisconsin Dells airport, station identifier KDLL) only has a maximum altitude of about 1503 feet above sea level (brown enhancement), but that was high enough to act as a barrier to the boundary layer winds and induce the lower tropospheric wave cloud features. Aircraft flying in the vicinity of such lee waves can encounter turbulence at times — however, in this case there was only one isolated pilot report of turbulence at 3500 feet above ground level over far southwestern Wisconsin (well to the west of the lee wave cloud features).

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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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