The long nights of November allow ample cooling in clear airmasses, and fog is a frequent occurrence over rivers that are still relatively warm compared to the surrounding land. In these near-water locations, cooling to the dewpoint, and resultant saturation, allows fog to form in and along River valleys as shown in the above visible image from GOES-12. The Ohio River and its many tributaries in Pennsylvania, West Virginia and Kentucky are plainly visible.

Detection of fog at night occurs by comparing observed brightness temperatures at about 11 microns and about 3.9 microns. Small water droplets in fog are not effective emitters of radiation at 3.7 or 3.9 microns — that is, they do not emit like blackbodies — but the water droplets are effective emitters of radiation at 11 microns. Thus, the temperature inferred by the satellite (that assumes all bodies are emitting like blackbodies) is cooler for the 3.9-channel than for the 11-micron channel. A difference between the two temperatures, then, can be used to highlight fog.

The MODIS Fog product image, above, from AWIPS, shows the channel difference at 0736 UTC on 2 November, and it suggests ongoing fog in river valleys from New York State southwestward to Kentucky. The nearly simultaneous GOES image (from 0730 UTC) is below. The degraded resolution of the GOES infrared sensor over the Ohio Valley (about 5-km pixels, vs. 1-km pixels for MODIS) means that developing fog, by its nature at very small horizontal scales, is not initially detected. The finer-resolution detector on the MODIS instrument provides earlier warning of developing fog.

Fog in the river valleys is more obvious in the GOES Fog Product image from 1030 UTC, three hours later. However, the pixel footprint of GOES IR sensor means a less defined horizontal mapping of the true fog locations. The higher resolution MODIS instrument gives earlier detection and better horizontal delineation of fog events.

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The Halloween Blizzard of 1991 was an early-season storm that moved north from the Gulf of Mexico to the upper Great Lakes. Unseasonably cold air allowed the rich moisture-laden airmass to deposit a long band of snow from the Panhandle of Texas northeastward to western Lake Superior. Many early-season snow total records were broken, and single-storm records fell at Minneapolis (28.4″) and Duluth (36.9″) Typically storms from the Gulf of Mexico do not move due north; however, eastward motion of this system was blocked by a large nor’easter off the coast of New England (the so-called “Perfect Storm”).

In the visible loop above, notice the rapid melting of snow deposited by the system in the Texas Panhandle, despite record cold (30 and 31 October 1991 are the only October days in Amarillo history when the surface temperature stayed below 30 F all day). Snowcover in South Dakota (the Missouri River stands out) also speaks to the chill in the airmass on the cold side of the storm. A larger-scale visible animation is available here.

Update

The 1991 “Halloween” storm is the “single storm record for the metropolitan (Twin Cities)” area. A comparison of a GOES-7 Infrared and visible image on November 1, 1991 at 21 UTC.

H/T

These NOAA GOES-7 data was accessed via the University of Wisconsin-Madison SSEC Data Services, using the McIDAS-X software.

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AWIPS images of the MODIS visible and 2.1 µm near-IR “snow/ice” channels (above) showed areas of snow cover across parts of eastern Colorado, far northwestern Kansas, and Nebraska on 23 October 2009. Snow is a strong absorber at the 2.1 µm wavelength, so it appears very dark on the snow/ice channel image.

The corresponding MODIS Land Surface Temperature product (below) revealed significantly colder LST values in the middle to upper 30s F (darker green colors) where the snow cover was deeper. There was a lack of surface reports in the exact areas of deeper snow cover, except for Limon in eastern Colorado (station identifier KLIC), which was reporting a surface air temperature of 39º F at the time.

Snowfall amounts from this particular storm (which moved through the region on 22 October) included 15 inches at Elizabeth, Colorado, 12 inches at Brady, Nebraska, and 4 inches at Saint Francis, Kansas. These locations are marked on a MODIS Red/Green/Blue (RGB) true color image from the SSEC MODIS Today site (below, displayed using Google Earth).

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Eastern Pacific Hurricane Rick, shown above near peak intensity at sunset on 17 October 2009, is the second strongest hurricanes on record in the eastern Pacific — weaker only than 1997’s Linda. Sustained winds at this time were estimated to be 180 miles per hour, and the central sea level pressure was estimated to be 906 mb. Note in the visible imagery the presence of gravity waves in the cirrus shield that makes up the central dense overcast (CDO). In addition, as noted in the Tropical Prediction Center discussion issued near this time, the stadium effect in the Hurricane eye is readily apparent.

Rick formed out of a tropical disturbance southwest of the Gulf of Tehuantepec (a loop of 3-hourly water vapor imagery here, and a loop of 6-hourly 11-micron imagery here show an interesting flare-up of convection in the Gulf of Tehuantepec in the days before Rick formed. It is worth pondering how that convection influenced Rick’s early and rapid growth). The evolution from strong tropical depression (here, at 2100 UTC on 15 October) to minimal hurricane (here, at 1500 UTC on 16 October) to category 4 hurricane (here, at 1500 UTC on 17 October to category 5 hurricane, above, was rapid indeed and speaks to the ideal environment through which the disturbance traveled. Consider the image below from the CIMSS Tropical Weather Website.

The image shows that the theoretical minimum to the central pressure in the region through which the system traveled was below 880 mb! (This value is a function of sea surface temperature, and of atmospheric thermodynamic profiles as described here. Note that Rick was moving across ocean waters with surface temperatures close to 30 C as it intensified rapidly. Wind shear as the storm rapidly intensified time was also very low (as diagnosed by Satellite winds). Very warm ocean waters and low vertical wind shear are key ingredients in allowing the strengthening of tropical systems.

The ideal environment resulted in a category 5 storm with a very tall circular ring of convection around the eye. The GOES-11 10.7-micron image, below, shows temperatures of nearly -80 C (the purple pixels within the grey) in the tallest convection around the eye.

(Added: Note in the water vapor and infrared imagery loops, above, the presence of what looks to be a binocular-shaped eye. This is an artifact of the interpolation used to blend GOES-12 and GOES-11 imagery to combine one cohesive picture. In individual images from either satellite, only a single eye is present).

Polar orbiting satellites, such as NOAA-19, give high-resolution images of the storm. The 10.8-micron example above, from 2020 UTC on 17 October, as the storm neared its peak intensity, shows pixels northwest of the storm center (this NOAA-19 pass is ascending, so north is towards the bottom of the image) with brightness temperatures of -84 C. Note also the more circular aspect ratio that comes from the polar-orbiter’s more top-down view, versus the Geostationary satellite’s oblique view. Visible imagery, below, at 0.65 and 0.86 microns, from the NOAA-19 AVHRR instrument, show better storm structure as well.

MODIS imagery from the Terra and Aqua satellites can also be used to investigate the storm. Unfortunately for this storm, the Aqua overpass granule split was right across the storm eye (granules are created so that the vast amount of data created by the satellite are more easily transportable). Gluing the two images together does not re-capture all the missed points, but it does give a good representation of the storm intensity here. A later MODIS image from TERRA, below, from 1755 UTC on 18 October (that is, about a day after the image from Aqua), below, shows a somewhat cloudier, but still quite distinct, eye. At this point, Rick has passed its peak in intensity.

(added: Jesse at Accu-Weather has other imagery of Rick here).

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