Large MCS over Texas

March 12th, 2007 |

AWIPS GOES 10.7µm IR image
A very large and long-lived Mesoscale Convective System (MCS) developed over Texas during the pre-dawn hours on 12 March 2007, which produced reports of tornadoes, hail up to 2.5 inches in diameter, and damaging winds up to 95 mph (SPC storm reports). AWIPS images of the GOES-12 10.7µm InfraRed (IR) channel (above; QuickTime animation) shows the areal extent of cold cloud top temperatures associated with the developing MCS — IR cloud top temperatures were as cold as -72º C / -97º F (black enhancement) at 03:30 and 11:15 UTC. The supercell convection that first developed across the Big Bend region and over central Texas did exhibit enhanced-v signatures from about 23:00 UTC on 11 March to 03:00 UTC on 12 March.

The MCS eventually developed Line Echo Wave Pattern (LEWP) structures, which were evident in the cloud to ground (CG) lightning strike data (below; QuickTime animation) — in fact, this severe convection produced over 1000 CG strikes every 15 minutes during the 09-10 UTC period. Later in the day, new convective development produced heavy rainfall (including reports of 2.00 inches in a 1 hour period, and a total of 6.94 inches) in the Brownsville, Texas area — GOES sounder derived product imagery displayed Total Precipitable Water (TPW) values of 46 mm (1.8 inch) and Convective Available Potential Energy (CAPE) values of 2480 J/kg over that region several hours earlier.
AWIPS GOES 10.7µm IR image with lightning data

Stratus cloud deck over snow pack

March 9th, 2007 |

AWIPS MODIS near-IR, visible, and IR images

An extensive deck of stratus cloud was evident across much of northern Minnesota and adjacent portions of the Dakotas and Wisconsin during the daytime hours on 08 March 2007. An AWIPS 4-panel display of MODIS imagery (above) shows how different this cloud feature appeared in the various channels. The stratus appeared much darker than the surrounding clear (but snow-covered) ground on the 3.7 µm shortwave IR (Band 20) image (above; upper left panel), due to that channel’s sensitivity to solar radiation reflected off the top of the supercooled droplet cloud layer. On the other hand, this supercooled water droplet cloud feature appeared much brighter than the dark surrounding snow-covered ground on the 2.1 µm near-IR (Band 7) image (above; upper right panel), since snow and ice are strong absorbers at that wavelength. Cloud top temperatures were fairly uniform (around -8 to -10º C) on the 11.0 µm IR Window (Band 31) image (above; lower right panel). However, it is interesting to note that this stratiform cloud deck was semi-transparent on the 0.65 µm visible (Band 1) image (above; lower left panel), allowing several of the larger frozen lakes in northern Minnesota to be seen through the cloud — the frozen lakes (and other surface features such as river valleys and ice in Lake Superior) could also be seen through the stratus cloud deck on GOES-12 visible channel imagery (Java animation).

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The transparency of the stratus deck was somewhat surprising given the opaque appearance on the other near-IR and IR MODIS channels, and the fact the cloud layer was apparently fairly thick: surface reports across that region indicated cloud bases between 300-800 ft above ground level, and the GOES sounder Effective Cloud Amount product (below; lower left panel) and the Cloud Top Height product (below; lower right panel) indicated overcast (100%) coverage (yellow enhancement) with cloud tops in the 11,000-13,000 ft range (green enhancement). The rawinsonde profile at Minneapolis, Minnesota at 00 UTC on 09 March (a few hours after the MODIS images) indicated that a very strong and deep temperature inversion existed within the 800-900 hPa layer, which was probably where the bulk of the stratiform cloud layer actually existed. The air was not particularly moist above the temperature inversion, but another cloud deck could also have existed near the top of the 625-800 hPa layer — such a higher cloud layer may have been responsible for the cloud top height values of 11,000-13,000 ft seen on the GOES sounder product below.

MODIS and GOES sounder images

Southeast US Tornado Outbreak, Northcentral US Blizzard

March 2nd, 2007 |

AWIPS water vapor image

A very large and very intense mid-latitude cyclone was responsible for a major tornado outbreak in the southeastern US, as well as widespread blizzard conditions across the northcentral US on 01 March 2007. AWIPS images of the GOES water vapor channel (above; 48-image QuickTime animation) and the GOES 10.7 µm InfraRed (IR) channel (below; 48-image QuickTime animation) show the unusually large size of the storm during the 01-02 March period. Widespread reports of tornadoes, hail, and damaging winds were noted over much of the southeastern quarter of the US, including several fatal tornadoes (Caulfield, Missouri; Enterprise, Alabama; and Americus, Georgia — all rated EF3). A total of 31 tornadoes were reported, with 20 fatalities; hail was as large as 2.5 inches in diameter in parts of Missouri and Arkansas. The Storm Prediction Center issued a rare “High Risk” Convective Outlook early in the day, and eventually issued 3 Severe Thunderstorm Watches and 13 Tornado Watches during the prolonged severe weather event. Meanwhile, in the northcentral US, blizzard conditions were common across parts of North and South Dakota, Nebraska, Minnesota, and Iowa (US watches, warnings, and advisories) — several locations in that region reported snowfall totals of 15-21 inches, and many schools and highways across the region were closed due to the blowing and drifting snow.
AWIPS IR image

A closer look at the GOES-12 10.7 µm IR imagery (below; 100-image QuickTime animation) shows the cloud top temperature structure from 12:15 UTC on 01 March to 03:40 UTC on 02 March — GOES-12 was in Rapid Scan Operations, so images were available at 5-10 minute intervals during much of the period. There were no well-defined “enhanced-v” signatures evident on the IR images, but cold cloud top temperatures in the -60º to -70º C range (orange to red enhancement) were common; the coldest cloud top temperature noted on the GOES-12 IR imagery was -76 C at 19:15 UTC (green enhancement), which was around the time of the Enterprise, Alabama tornado (which caused 8 fatalities when it struck a high school). Radar images of the Enterprise tornado can be seen on the NWS Tallahasse site and the  Weather Underground WunderBlog. Vertical wind shear was enhanced over the southeastern US by strong winds in the middle to upper troposphere associated with an approaching jet streak — this helped to create an environment favorable for the development of rotating updrafts and supercell thunderstorms.
GOES-12 10.7µm IR image

More blowing dust in Texas

March 1st, 2007 |

GOES-11 IR difference image / GOES-12 visible image

Another case of blowing dust was noted across the southern Texas panhandle region on 01 March 2007. Unlike the stronger blowing dust event in that same area 5 days earlier (on 24 February), there was no distinct dust cloud feature evident on the GOES-12  visible channel imagery (above, right) — however, an IR difference product using the 10.7 µm and 12.0 µm channels available on GOES-11  did reveal a subtle “dark blowing dust signature” (above, left; Java animation). No surface stations in the area reported blowing dust during the time period shown; visibilities did drop to 2-4 miles at Big Spring TX (station identifier KBPG), but remained at 9-10 miles at San Angelo TX (station identifier KSJT).

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A color-enhanced version of the GOES-11 IR difference product or “split window difference” (below) shows that this “dust signature” occurs when the difference between the 10.7 µm and 12.0 µm brightness temperatures is in the -1º K to -2º K range (yellow enhancement). Note that there is also a “false dust signal” farther to the west over regions of higher albedo soil — but the “dust signature” would propagate in the direction of the boundary layer wind flow (in this case, toward the southeast) on an image animation, while the “false signal” would remain stationary. The 12.0 µm IR channel is not available on GOES-12, having been replaced by a 13.3 µm IR channel that is used for better cloud top height assignment; however, the 12.0 µm IR channel is available on the polar-orbiting MODIS and AVHRR instruments, which is useful for the detection of airborne dust and volcanic ash.
GOES-11 IR difference image