Lake Michigan “pneumonia front”

May 27th, 2008

MODIS Sea Surface Temperature product

A strong cold front moved southward across the Upper Midwest and Great Lakes states on 26 May27 May 2008. During the preceding month, temperatures had been colder than normal at many sites across the region (mean air temperature departures for May 2008 included 3.0º F below normal at Milwaukee and 3.1º F below normal at Madison, Wisconsin, 3.2º F below normal at Chicago, Illinois, and 5.1º F below normal at Marquette, Michigan) — as a result, an AWIPS image of the MODIS Sea Surface Temperature (SST) product (above) showed that the water temperatures across much of central Lake Michigan were still in the rather cold 39-45º F range (blue colors). This cold lake water contributed to the formation of a well-defined “pneumonia front(defined by Behnke (2005) as a lake-modified synoptic scale cold front that results in a rapid air temperature drop of at least 16º F in a 1 hour period) that moved inland across eastern Wisconsin during the evening and night-time hours.

Examining surface temperature data from Milwaukee and Madison in Wisconsin, Behnke (2005) documented 25 cases of pneumonia fronts during the period 1948-2003. It was found that reduced roughness over the north-south oriented Lake Michigan allowed for stronger impact of the pneumonia fronts over the southern portions of the lake and shoreline, and colder lake temperatures in the north helped to increase the nearshore temperature gradient and frontal propagation. Topography appeared to have little influence on pneumonia front generation and maintenance — the associated temperature falls were strongly dependent on diurnal landmass heating and Lake Michigan surface water temperatures.

Milwaukee (KMKX) base reflectivity (Animated GIF)

The inland progression of the “pneumonia front” boundary was clearly evident on Milwaukee (KMKX) base reflectivity radar data (above; QuickTime animation). Note that a sequence of 3 distinct boundaries could be seen: Boundaries 1 and 2 were likely the primary cold front (moving south-southeastward across south-central Wisconsin), and an initial “weak pulse” of lake-cooled air moving southward (roughly perpendicular to the lakeshore), respectively; then, Boundary 3 (the true “pneumonia front”) raced rapidly inland from east to west, with surface air temperatures quickly dropping from the 70s F to the 40s F across far eastern Wisconsin. At Milwaukee (KMKE, where the daytime maximum temperature was 81º F), the air temperature later dropped from 78º F to 49º F in one hour, with northerly winds gusting to 38 mph; even as far inland as Madison (KMSN, where the daytime maximum temperature was 82º F), the temperature later dropped from 64º F to 55º F in one hour, with northeasterly winds gusting to 24 mph as the pneumonia front passed.

GOES-12 3.9µm IR images (Animated GIF)

An extensive area of post-frontal stratus clouds could be seen moving southward and southwestward on 4-km resolution GOES-12 3.9µm IR images (above), but the leading edge of the stratus clouds became masked by a veil of high cirrus clouds streaming eastward from convection across Iowa and Missouri. The 1-km resolution MODIS fog/stratus product (below) was more helpful in accurately locating the leading (southern) edge of the stratus clouds around 04:32 UTC (11:32 PM local time).

MODIS fog/stratus product

Additional reference:

Synoptic and Local Controls of the Lake Michigan Pneumonia Front, C. Behnke and P. Roebber, 16th Annual U.S./Canada Great Lakes Operation Meteorology Workshop, Milwaukee, WI, Sep. 5-7, 2007.

Severe thunderstorms over the Front Range and the central Plains

May 22nd, 2008

GOES IR images (Animated GIF)

AWIPS images of the GOES 10.7 µm IR channel (above) showed the development of severe thunderstorms across parts of Colorado, Wyoming, Nebraska, and Kansas on 22 May 2008 (see also: CIRA RAMMB Satellite Case Study). According to the SPC storm reports, a number of tornadoes were produced by these storms (one tornado-related fatality was reported near Greely, Colorado), along with large hail (up to 2.75 inches in diameter near Windsor, Colorado and 3.00 inches in diameter near Grinnell, Kansas).

MODIS IR image

A closer view using the 1-km resolution MODIS 11.0 µm IR channel (above) from around 17:57 UTC (11:57 am local time) revealed a rather diffuse “enhanced-v” signature just east/northeast of Fort Collins, Colorado (station identifier KFNL). This storm went on to produce tornadoes and large hail in southeastern Wyoming, with one tornado doing extensive damage in the Laramie area.

Another MODIS IR image from around 19:34 UTC (1:34 pm local time) displayed impressive detail in the overshooting top structure of the storms developing over western Kansas and far southwestern Nebraska (below), with the coldest cloud top brightness temperatures in the -70º to -80º C range (black to while enhancement).

MODIS IR image

The GOES sounder derived Convective Available Potential Energy (CAPE) product (below) indicated that the pre-convective environment across northeastern Colorado during the morning hours was becoming unstable, with CAPE values in the 2000-4000 J kg-1 range (yellow to red enhancement) at 16:00 UTC (10 am local time). The air mass south of the warm front and east of the dryline (from southern Kansas into Texas) was also very unstable, with CAPE values in some areas over 4000 J kg-1 (purple enhancement).

GOES sounder CAPE product

Vegetation across Southern California

May 21st, 2008

MODIS true color images (Animated GIF)

A comparison of Aqua MODIS true color images from 05 March 2008 and 17 May 2008 (above) showed the “browning” of vegetation across Southern California after the Spring season rains (90-day total rainfall | 90-day percent of normal rainfall) had ended. A significant amount of snowmelt was also evident during that period over the southern Sierra Nevada mountains (as well as over the higher elevations of smaller ranges such as the San Gabriel, San Bernardino, and San Jacinto Mountains). The images (courtesy of Liam Gumley, CIMSS) were created in Google Earth using publicly available data from the SSEC MODIS Today website.

Even though the MODIS true color images above suggested an overall dry-down of the vegetation across the region, a comparison of AWIPS images of the MODIS Normalized Difference Vegetation Index (NDVI) and visible channel (below) indicated that many areas of southern California still exhibited a NDVI value as high as 0.5-0.6 on 20 May 2008. A National Public Radio story pointed out that the rapid growth of non-native plant species across parts of southern California could actually be a warning sign of an increased potential for future wildfire activity (once that vegetation continued to dry out over the upcoming Summer and Fall months).

MODIS NDVI + visible image (Animated GIF)

Occluding cyclone over the North Atlantic Ocean

May 20th, 2008

Northern hemisphere water vapor composite imagery (Animated GIF)

AWIPS images of the water vapor  satellite composite (above) showed the evolution of a wide variety of synoptic-scale features across the Northern Hemisphere at 3-hour intervals during the 19 May – 20 May 2008 period. Of particular interest was the water vapor signatures associated with an occluding cyclone over the North Atlantic Ocean (located south of Iceland and west of the British Isles). A closer view using Meteosat-9 images of the 6.2 µm water vapor channel at 15-minute intervals (below) revealed a great deal of interesting mesoscale structure as bands of moist and dry air wrapped into the occluding cyclone as it began to fill .

Meteosat-9 water vapor images (Animated GIF)