AWIPS images of radar reflectivity and GOES-12 visible and InfraRed (IR) imagery (above) showed a well-defined mesoscale convective eddy that was moving southward across Lake Michigan during the day on 30 January 2007. For a few hours, the core of this mesoscale vortex even exhibited an eye-like structure on the radar (Java animation) and visible channel satellite imagery (Java animation). To the south of this lake vortex, the clouds covering the southern half of Lake Michigan exhibited fairly cold brightness temperatures (-20º to -30º C, cyan to blue enhancement) on the 10.7µm IR imagery (Java animation), yet at the same time appeared significantly warmer (+10º to +20º C, darker gray enhancement) on the 3.9µm IR channel imagery (Java animation). This suggests that much of the cloud cover over southern Lake Michigan was composed of supercooled water droplets, rather than ice crystals (water droplets are efficient reflectors of solar radiation, making water-based clouds appear “warmer” than ice-based clouds on the shortwave IR imagery, due to that channel’s sensitivity to solar reflectance). The MODIS Cloud Phase product (below, upper left panel) supported this idea, indicating predominantly “water phase” cloud (blue enhancement) across much of the southern half of Lake Mighigan. It is interesting to note that the precipitation type at Muskegon, Michigan (station identifier KMKG) fluctuated between “snow” and “snow with freezing fog” as the clouds over southern Lake Mighigan were moving inland (there was some hint of “Mixed” or “Uncertain” cloud phase features moving over that station, which could have seeded the supercooled cloud deck below with enough ice to produce mostly snow at times) — however, the changing precipitation types could be due more to inherent ASOS uncertainties. As this lake eddy moved inland across southwestern lower Michigan later in the day, up to 15 inches of snow was reported at Allegan.

On a side note, during the morning hours over Madison, Wisconsin (station identifier KMSN) it appeared to be snowing very lightly, even though the skies overhead were practically cloud-free — actually, there were ice crystals growing within the boundary layer, rather than snowflakes falling from a cloud. The BUFKIT model sounding analysis (below) indicated a fairly moist “snow growth region” (yellow line) within the lowest 1.5 km of the atmosphere, along with some negative omega in that layer indicating upward vertical motions.

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GOES-12 visible channel images (above) revealed numerous aircraft dissipation trails — otherwise known as “distrails” or “hole punch clouds” — during the day over eastern Texas, northern Louisiana, southern Arkansas, and Mississippi on 29 January 2007. Corresponding GOES-12 10.7 µm InfraRed (IR) imagery showed that cloud-top infrared brightness temperatures over that region were generally between -20º and -35º C; as aircraft (likely air traffic to/from Dallas-Fort Worth airport KDFW) penetrated that cloud layer, they caused the supercooled cloud droplets to glaciate and begin to fall out of the cloud (causing the “holes” and “streaks” that were evident on the visible imagery).

Higher-resolution views of these cloud features were available from Terra MODIS (sourced from the NASA Rapidfire site) and Aqua MODIS True Color Red-Green-Blue (RGB) images (below).

12 UTC rawinsonde data from Fort Worth, Texas (below) indicated that the likely elevation of the supercooled cloud deck was around 25,000 feet.

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Around an inch of snow accumulated across parts of southeastern Virginia and northeastern North Carolina during the overnight hours on 28-29 January 2007. This patch of snow cover was evident the following morning on GOES-12 visible channel imagery (above). A Java animation of visible images shows that the northern portion of the snow cover (in Virginia) began to melt rather quickly during the late morning hours. The darker region within the area of snow cover is the Great Dismal Swamp National Wildlife Refuge; snow was not able to accumulate over most of that marshy surface, so it does not appear as white as the surrounding snow-covered land (MODIS true color image). In addition, we can see that the snow cover extended as far to the southeast as coastal sections of the Albemarle and Pamlico Sounds in North Carolina.

By examining AWIPS imagery of some of the MODIS channels, we can confirm that this particular image feature is indeed snow on the ground: snow is a very strong absorber at the 2.1 µm wavelength, and therefore shows up as a dark feature on the Snow/Ice (Band 7) image (below, upper right panel). Note that the MODIS IR brightness temperatures (below, bottom left panel) are several degrees colder in the region of snow cover (-5º to -8º C, darker blue enhancement) than over the surrounding bare ground (0º to -3º C, cyan enhancement).

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Because of relatively mild and snow free conditions in the first half of January, many stories appeared in the media about the lack of winter and mild temperatures. In a classic case of Beware What you Wish For, cold winter temperatures have struck back with a vengeance. Dangerously cold weather covered eastern North America this morning, spilling out over the Atlantic Ocean. Under clear skies overnight, temperatures in eastern Canada plunged below -30 (F) at many locations. The infrared cloud image from 12:45 UTC this morning bears testament to the cold air, with brightness temperatures colder than 240 K (-33 C, or -27 F) in the clear air. These cold temperatures match those of high clouds associated with a storm moving towards the north Atlantic at the eastern edge of the image.

A common feature observed in images recorded when Arctic air spills out over the ocean is the development of a band of stratocumulus clouds. The edge of the cloudbank roughly parallels the coast, and the distance between the coast and the cloud edge tells you something about the air-sea temperature difference and the initial moisture content of the air. As dry, cold air leaves the continent, heat and moisture are added to the boundary layer from the ocean surface. The warming and moistening will reduce the stability of the lowest part of the atmosphere, allowing the development of shallow convection. At some point from shore, sufficient moisture is added that the convection becomes visible. That is, clouds develop. Note in the image how air flow down Delaware Bay and across Chesapeake Bay is likely adding moisture to the offshore flow. Clouds develop downstream of these features much closer to shore than over adjacent waters where the airflow emerges unmoistened from the Continent.

The color-enhanced version of the grey-scale image highlights several features. Note that the coldest air (dark blue) over central New Hampshire and southern Maine is just upstream of a region where clouds develop very close to shore. This is a small region of mid-level cloudiness, with a surface echo in marginally higher dewpoints (-12 F at Portland vs. -15 to -17 at points north and south alont the coast) In adjacent, drier, areas, cloud development occurs farther offshore. Clouds extending to Cape Cod in this image were producing road-slickening snow showers. The enchancement also highlights the relative warmth of rivers, such as the ice-free Susquehanna (and its North Branch) in Pennsylvania and the Hudson in New York, and the ridge and valley geography of south-Central Pennsylvania.

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