Several inches of snow fell in parts of Texas, Oklahoma, Arkansas, and Missouri during the evening of 03 March 2008 into the morning of 04 March 2008 — snowfall amounts included 8.5 inches at Springfield, Missouri, 8.0 inches at Broken Arrow, Oklahoma, 6.0 inches at Bella Vista, Arkansas, and 5.0 inches at Pottsville, Texas. Snow was even reported briefly at Shreveport in far northwestern Louisiana, and the 1.0 inch of snow at Dallas-Ft. Worth in Texas set a new daily snowfall record for 03 March. GOES-12 6.5 µm water vapor images (above) show that many of the reports of snow (along with some rain and freezing rain) were occurring along the periphery of a well-defined dry slot that was advancing northeastward from Texas into Arkansas. In addition, a deformation zone just northwest of the dry slot was also acting to help organize the southwest-to-northeast oriented banded structure of the resulting precipitation.

AWIPS images of the GOES sounder Total Column Ozone product (below) depicted a lobe of high values of ozone (greater than 400 Dobson units, red colors) associated with the sharp trough aloft that was swinging eastward across the southern and central Plains. Such elevated ozone levels are often seen in regions where the tropopause altitude is quite low — in fact, the NAM fields indicated that the “dynamic tropopause” (taken to be the pressure level of the 1.5 Potential Vorticity Unit surface) was as low as the 700 hPa pressure level at 12:00 UTC.

GOES-12 visible images from the following morning (below) revealed several large patches of snow-covered ground as the clouds began to retreat eastward. Note the aforementioned southwest-to-northeast orientation of the bands of snow cover in Oklahoma, Arkansas, and Missouri. The snow on the ground melted quickly under the warmth of the March sun, eroding quickly from the edges inward on the visible satellite imagery. Also, if you look closely, you can see a small puff of smoke move northward from a fire that was burning briefly in west-central Oklahoma (toward the end of the animation).

If we compare AWIPS images of the MODIS visible channel and the 2.1 µm near-IR “snow/ice channel” at 16:57 UTC 0r 11:57 am local time (below), we see that both clouds and patches of snow cover appear as brighter white features on the visible channel image; however, on the snow/ice image the snow cover features appear much darker (in contrast to the much lighter appearance of supercooled water droplet clouds). Cloud features in Missouri and Arkansas that were composed of ice crystals also appear darker on the snow/ice channel image.

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Minimal cloud cover (MODIS true color image) off the US East Coast and over the western North Atlantic Ocean on 03 March 2008 allowed a nice view of the ribbon of warm water temperatures associated with the Gulf Stream, as shown on an AWIPS image of the MODIS Sea Surface Temperature (SST) product at 17:56 UTC (above). MODIS SST values along the Gulf Stream were generally in the 72-79º F (22-26º C) range (yellow to orange colors); clouds over water are depicted with as black on the MODIS SST product. A comparison with the SST analysis field from the High Resolution Real Time Global (RTG_SST_HR) model 18 hours earlier (at 00:00 UTC) demonstrated the advantage of the 1-km resolution MODIS data for resolving the small-scale detail in the structure of warm and cold water eddies that existed on either side of the Gulf Stream axis.

In particular, note Buoy 41002, located south of Cape Hatteras, North Carolina (or about 250 nautical miles east of Charleston, South Carolina): the National Data Buoy Center water temperature data for 41002 (below) revealed that the SST had increased 7-8º F since 29 February 2008 as an eddy of warm water drifted across the buoy location, and the water temperature was in the 74-75º F range during the daytime hours on 03 March. The 17:56 UTC MODIS SST value of 74.2º F in the vicinity of Buoy 41002 was in much better agreement with the NDBC water temperature data (compared to the SST value of less than 69º F at Buoy 41002 indicated by the RTG_SST_HR analysis at 00:00 UTC).

At its greatest, the Gulf Stream is about 100 miles wide and several hundred feet deep — much larger than any river on Earth. It also moves much faster than any river, moving as much as about 140 miles in a single day; in terms of volume per second, the Gulf Stream moves about 1000 times faster than the Mississippi River.

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Several large wildfires began to burn across parts of western Texas (and also southeastern New Mexico) late in the day on 25 February 2008, as seen by the large number of fire “hot spots” (black to red pixels, with red being the hottest temperatures) on GOES-12 3.9 µm “shortwave IR” images (above). Strong northerly winds — gusting as high as 67 mph (30 m s-1) at Guadalupe Pass, 51 mph (23 m s-1) at Midland, and 45 mph (20 m s-1) at San Angelo — helped the grass fires quickly grow out of control and spread rapidly southward; this extreme fire growth was evident by the fast rate of increase in areal coverage of “hot” black-to-red pixels on the shortwave IR imagery. Very warm surface air temperatures were also noted across Texas on that day, with a high of 93º F (34º C) at San Angelo and a high of 99ºF (37º C) at Del Rio (which tied the all-time record high for the month of February at that location).

A comparison of the 1-km resolution MODIS 3.7 µm IR image with the corresponding 4-km resolution GOES-12 3.9 µm IR image (above) as the fire intensity and coverage was beginning to diminish around 05:00 UTC or Midnight local time (near the end of the GOES-12 shortwave IR animation shown at the beginning of this post) demonstrates the superior fire detection capability offered by the improvement in spatial resolution on the MODIS instrument. The largest of the active fires was located to the west/northwest of San Angelo (station identifier KSJT), and hot pixels associated with this large fire (black to red enhancement) could be seen across parts of several different counties.

An extensive burn scar resulting from the largest fire (the “Glass Fire”, which was located between San Angelo and Midland) was apparent on MODIS true color imagery (viewed using Google Earth, above) two days later (on 27 February 2008). In fact, one small fire was still burning along the southeastern periphery of the burn scar, with a subtle smoke plume seen drifting northeastward toward Highway 87. The Glass Fire burned over 219,000 acres in Sterling county alone (InciWeb).

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Some interesting cloud features were seen in the western North Atlantic Ocean on 26 February 2008, about 300 miles southeast of New England (and about 300 miles south of Nova Scotia, Canada): a narrow line of cumulus clouds (oriented approximately north-south), and a patch of stratus cloud (or possibly fog?) that developed just east of the cumulus line. These cloud features were apparently related to very sharp gradients in the Sea Surface Temperature (SST) over that particular area.

AWIPS images of the GOES-12 10.7µm IR channel (above) showed that there were large cold eddies (darker green enhancement) located over the western North Atlantic, along the north wall of the Gulf Stream (RTG_SST analysis). These cold water eddies remained stationary as patches of colder clouds (cyan to blue enhancement) drifted eastward across the region.

An 8-day composite of the MODIS Sea Surface Temperature product (above) confirmed the presence of a very sharp SST gradient in that area (near 40º N latitude, 65º W longitude), with a patch of water having SST values in the 40s F (cyan to blue enhancement) located just to the east of water having SST values in the 60s F (green enhancements) — black features on the SST image are clouds. Low-level baroclinicity enhanced by the sharp SST gradient was aiding the development and maintenance of the cumulus line; as warm air moving eastward encountered the colder pocket of water, stratus cloud quickly formed as the marine boundary layer air was rapidly cooled to its dew point.

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