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.

View only this post Read Less

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).

View only this post Read Less

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.

View only this post Read Less

Tropical Depression #9 formed early in the day on 12 September 2007, and quickly intensified in the warm waters of the western Gulf of Mexico to become Tropical Storm Humberto (just off the coast of Texas). AWIPS images of the MODIS IR and visible channels (above; upper 2 panels) shows the early stages of a spiral band that began wrapping around the core of the cyclone during the afternoon hours. DMSP SSM/I imagery (above; lower 2 panels) depicted rainfall rates during the morning that as high as 30 mm per hour, and total precipitable water values of 55-65 mm in the near-storm environment.

GOES-12 IR imagery and derived winds products from the CIMSS Tropical Cyclones site (below) indicated that Humberto developed in an environment that was characterized by very low deep layer wind shear (5-10 knots within the 850-200 hPa layer), which was a factor that aided in the intensification from tropical depression to tropical storm.

View only this post Read Less