Hot and dry Santa Ana conditions across southern California (with wind gusts as high as 91 mph in Los Angeles county and 87 mph in Ventura county) caused widespread blowing dust/sand (reducing surface visibility to 3 miles at Ontario, station identifier KONT) and created an environment that allowed several large wildfires to burn out of control on 21 October 2007. Note that the dew point temperature at Ontario dropped from 50ºF (+10º C) down to +2ºF (-17º C) in a 3-hour period once the strong northeasterly Santa Ana winds developed. AWIPS images of the GOES-11 3.9 µm IR channel (above) showed very hot pixels (black to yellow to red enhancement) associated with the larger fires (including one fire that caused multiple injuries and at least one fatality near San Deigo, and another fire farther to the north near Malibu – just northwest of Los Angeles KLAX – which shut down portions of the Pacific Coast Highway).

The IDEA MODIS Aerosol Optical Depth product (below) and GOES-11 visible imagery (QuickTime animation) revealed several large plumes of smoke (which could have also contained some blowing dust and/or blowing sand) drifting westward over the adjacent offshore waters of the Pacific Ocean. The airborne particulate matter was causing air quality problems at several sites across southern California on that day.

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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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