A deepening mid-latitude cyclone (located over southeastern Colorado / southwestern Kansas at 12 UTC) began to transition from the mature stage to the occluded stage during the morning hours on 21 September. The tell-tale occluding cyclone signature (consisting of a wrapping “dry slot swirl”) was depicted on the 8-km resolution GOES-10 6.7 micrometer “water vapor channel” imagery (above) — a 200-image QuickTime animation shows the evolution of the occlusion process at 1-minute intervals, since the GOES-10 satellite was still in Super Rapid Scan Operations (SRSO) mode. By 15 UTC, HPC analyzed the feature as an occluded low; later that afternoon several tornadoes were reported in northcentral Kansas as the low moved over that region.

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Additional structure is revealed on the 1-km resolution MODIS water vapor imagery at 17 UTC. Also, strong winds were causing a plume of blowing dust along the advancing cold frontal boundary — this dust plume exhibits a signal (yellow to orange enhancement) on the MODIS 11-12 micrometer IR difference product (below). Note the observation of blowing dust at Lubbock, Texas (station identifier LBB in the lower left corner of the image), where westerly surface winds were 38 mph (gusting to 47 mph); the blowing dust was reducing visibility to 3 miles at that particular time.

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Cold air advection in the wake of a secondary cold front helped to generate some well-defined lake effect rain bands over Lake Huron on 20 September (QuickTime animation of GOES-12 visible images). The cold air mass allowed daily minimum temperatures at the surface to fall into the 30s to 40s F across the Great Lakes region (getting as cold as 26 F at Hayward WI and Embarrass MN). The air temperatures aloft (850 hPa, near 5000 feet) were 0 to -3 C; as this cold air flowed over the relatively warm waters of Lake Huron (water temperatures of 15-18 C), the release of the thermal instability helped elongated cloud bands to form (generally parallel to the boundary layer wind direction. The AWIPS MODIS cloud phase product indicated that most of the lake-effect clouds were water phase clouds (blue enhancement), although it did flag portions of the longest cloud band as “mixed phase” (darker gray enhancement) where the cloud top temperatures were as cold as -12 C. With daytime surface air temperatures warming into the 50s F, the precipitation falling from these cloud bands was in the liquid form; similar cloud band features produce significant lake-effect snowfall during the colder winter months.

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The large (74,000-acre) “Day Wildfire” was still burning in the Los Padre National Forest just northwest of Los Angeles on 17 September — the MODIS true-color image (above) shows the thick plumes of smoke drifting westward. Strong offshore winds helped the fire (which had been burning since 04 September) to grow quickly and spread to the west on this particular day. A QuickTime animation of GOES-11 visible images the following day (18 September) shows that the smoke began drifting in a more northwesterly to northerly direction as the surface winds gradually shifted direction.

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The AWIPS MODIS 6.7 micrometer “water vapor channel” image (above) depicts an excellent example of a mountain wave signature over parts of the southern and central Rocky Mountains region, as strong winds associated with a jet stream axis were interacting with the rugged terrain. Note that the corresponding MODIS visible image reveals that very few clouds were present across much of the area where the mountain waves were evident (southern Colorado and northern New Mexico). Aircraft occasionally experience moderate to severe turbulence when they encounter mountain waves — in this case, there were numerous pilot reports of turbulence (below), which included this report in the Denver area at 19,000 feet:

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DEN UUA /OV DVV 224064/TM 1931/FL190/TP E120/TB CONT MOD OCNL SEV/RM SEV MTN WV-15 SECONDS-PILOT SAID LIKE HITTING A WALL-ZDV

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