AWIPS images of the GOES-12 6.5µm “water vapor” channel (above) show an upper-level low that was intensifying over Florida during the day on 18 September 2007. As warm/dry middle to upper tropospheric air (yellow enhancement) wrapped around and into the southwestern quadrant of the cyclone, convection was seen to develop along the southern Gulf Coast of Florida. This dry air was associated with a tropopause anomaly, where the “dynamic tropopause” (taken to be the 1.5 Potential Vorticity Unit contour) descended to around the 380 hPa pressure level according to the GFS40 model (below).

A cross section of GFS40 model fields (oriented from east on the left to west on the right) across Florida (below) reveals the lowering tropopause (color shaded potential vorticity values) and descending dry air (dashed green specific humidity contours) associated with the tropopause anomaly. Surface pressures were beginning to fall across that region in response to the tropopause anomaly, raising speculation that a subtropical or a tropical cyclone could eventually develop from this system as it moves slowly westward into the Gulf of Mexico.

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AWIPS images of the MODIS fog/stratus product (above) and the corresponding GOES-12 fog/stratus product (below) revealed areas of fog that were forming in the valleys of Pennsylvania and New York around 08 UTC (4am local time) on 17 September 2007. The 1-km resolution MODIS imagery offered a more detailed view of the fog structure compared to the 4-km resolution GOES-12 product. Also note the “false signal” of fog that appeared on the GOES-12 image along certain land/water boundaries (the Finger Lakes region of New York, portions of Lakes Erie and Ontario, and the New Jersey coast of the Delaware Bay). With clear skies and light wind conditions associated with high pressure in place across much of the northeastern US, fog formation (due to strong radiational cooling during the night-time hours) was prevalent over a large area.

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GOES-11 visible channel imagery (above) showed a nice example of lee wave “lenticular clouds” immediately downwind of the Spring Mountains (whose highest peak is Mt. Charleston at 11,918 ft or 3362 m) just to the northwest of Las Vegas, Nevada (Google maps) on 14 September 2007. The vertical motions associated with lee waves can cause moderate to severe turbulence which is a hazard to aviation, but on this particular day there were no pilot reports of turbulence noted in the immediate vicinity of the lenticular cloud formations (McCarran International Airport is located about 20 miles or 50 km southeast of the Spring Mountain range, and no lenticular clouds were seen on the satellite imagery directly over the Las Vegas metropolitan area or the airport itself).

Such lee waves are generated when strong atmospheric flow encounters a barrier to the flow — in this case, the axis of a strong southwesterly jet stream was located over the region, as indicated by an AWIPS image of RUC80 model 250 hPa wind fields overlaid on GOES-11 water vapor channel imagery (below). The lenticular clouds associated with this lee wave were seen to dissipate later in the day as the strongest jet stream winds propagated northeastward away from the region.

In order for these types of stationary lee wave clouds to form and be maintained, there often needs to be a stable layer located at altitudes above the top of the terrain obstruction. In this case, the 12 UTC rawinsonde data from Desert Rock, Nevada (below) did indeed indicate the presence of a shallow stable layer between about 600 and 650 hPa (with a 50 knot wind speed maximum at the 650 hPa level).

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The slow movement of Tropical Storm Humberto over the warm waters of the Gulf of Mexico (above) allowed the system to intensify to become Hurricane Humberto (a Category 1 storm) during the pre-dawn hours on 13 September 2007. Humberto strengthened from a tropical depression (with 35 mph winds) to a hurricane (with wind gusts to 84 mph) in just 18 hours, which is the fastest rate of intensification near landfall ever observed. Humberto also became the first hurricane to make landfall in the US since Hurricane Rita back in September 2005.

GOES-12 IR imagery from the CIMSS Tropical Cyclones site (above) shows the compact cluster of cold cloud top brightness temperatures moving inland across parts of Texas and Louisiana. A hint of a partial eyewall structure was seen in the MIMIC microwave imagery (below). Hurricane Humberto produced heavy rainfall across parts of the southeastern US, with amounts as high as 14.13 inches in Texas.

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