Clear skies over the Great Lakes (except for Lake Superior) allowed for VIIRS data to record temperatures over the Great Lakes in the early morning of 5 August 2021. In situ data from moored buoys closely match ACSPO (Advanced Clear-Sky Processor for Oceans) values. Southwestern Lake Erie is warmest, with temperatures as warm as 78ºF (pink/white enhancement). Green Bay and Saginaw Bay are as warm as 75ºF. The analysis below (from this link at the Great Lakes Environmental Research Lab) also shows values largely in agreement with the above analysis.

AWIPS-ready files of these fields are available from the CIMSS LDM feed.

There plots of basin-wide average temperatures suggest that the Great Lakes (except for Lake Superior) are a bit cooler in early August of this year than they have been at this time in the recent past.

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The animation above shows NUCAPS sounding availability plots from sequential NOAA-20 overpasses on 5 August 2021. Stations that are north of about 39 N will occasionally see profiles from two sequential overpasses, as was the case above at Buffalo NY, and also Pittsburgh PA. A benefit of NUCAPS soundings is that they show up halfway between 0000 and 1200 UTC upper-air soundings. The animations below show 0000 UTC and 1200 UTC soundings from Buffalo (top) and Pittsburgh (bottom) with two soundings from NUCAPS from the closest point to the upper air launch.

When sequential overpasses happen, a forecaster can use information to tell how things are changing on 90-minute timescales. Even with only one profile, however, one can view changes between the synoptic upper-air soundings. It is worth mentioning a couple obvious differences between the two data sources: Radiosondes are sampling a line that moves up in the atmosphere over a period of time (up to two hours!); that line can extend over a significant horizontal distance; in contrast, NUCAPS observations are volumetric and instantaneous. NUCAPS profiles are very smooth compared to radiosondes, and extreme values, or very thin layers are unusual in NUCAPS profiles.

NUCAPS profiles are available at this OSPO site. Gridded NUCAPS fields are here.

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True-color imagery from the VIIRS Today website, below (click here for a direct link to the image below at the VIIRS Today site), shows an unusual sunglint pattern over the eastern Gulf of Mexico, to the southwest of the Florida peninsula. Typically, sunglint features are fairly wide in VIIRS imagery, as evidenced from this Suomi-NPP image, also from 2 August, but to the east of Florida. However, the winds over the eastern Gulf were very light on 2 August, so surface wave action was reduced. When the ocean approaches glassy calm, solar reflection becomes more unidirectional, and a brighter spot becomes visible in True-Color imagery. When seas are choppier (as was the case to the east of Florida), solar reflection off the ocean is diffuse, and a less concentrated region of brightness results. An good analogy might be reflection off flat aluminum foil (representing a flat sea state) or very crinkled aluminum foil (representing an agitated sea state).

What evidence is present of light winds? Consider the Metop-A Scatterometry image, below, from this site. Both the ascending pass (about 0145 UTC) and descending pass, below (about 1410 UTC; orbit imagery available here, from this site) show very weak winds over the Gulf to the southwest of Florida.

The effect of the sun glint occurs in both visible and shortwave infrared channels. The imagery below, downloaded from the CIMSS Direct Broadcast ftp site (https://ftp.ssec.wisc.edu/pub/eosdb/j01/viirs/, from this ephemeral site — https://ftp.ssec.wisc.edu/pub/eosdb/j01/viirs/2021_08_02_214_1837/images/ — in particular), shows the five Image bands from VIIRS, a prominent signal is apparent in all but the longest wavelength.

Does the sunglint affect products? The Cloud Mask product is tricked by the large reflectance into believing a cloud is present, as highlighted by the box below. The image also includes Cloud Type and Cloud Phase. Because liquid water clouds are believed present, the ACSPO Sea-surface temperature algorithm (at bottom) produces no values in that region. Careful inspection of the image, however, shows marginally warmer waters (a lighter pink color is apparent) adjacent to the sunglint where the lack of wind means surface mixing of waters is suppressed and a thin layer of very warm water can develop on the top of the ocean.

This animation steps through all the VIIRS M-bands. The sunglint is apparent in nearly all of them, except for wavelengths longer than 4 µm. The sunglint is also missing from the 1.38 µm (M09) imagery because of strong absorption of energy at that wavelength by water vapor.

Other examples of diagnosing areas of light winds over water (via a lack of sunglint or moonglint) can be found here, here, here and here.

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Smoke from fires (unless it is extraordinarily thick) is very challenging to detect at night in infrared imagery. When there is sufficent lunar illumination, however, as occurred on 29 July 2021 over Canada (the image above shows northwestern Ontario and southern Manitoba — Lakes Winnepeg, Winnepegosis and Manitoba are apparent, and the city of Winnipeg’s lights are apparent along the southern border of the image), the pall of smoke can be detected just as during the daytime using Day Night band visible imagery. In the image above, bright signals are showing light emitted by active fires, and streams of smoked from the fires are apparent. The toggle below of the Day Night band image and the VIIRS I04 3.74 µm imagery underscores that the bright spots are also very warm spots; that is: fires. Note that a smoke signal is not present at all in the infrared.

A zoomed-out Day Night band image shows the horizontal extent of the smoke pall that extends southward into Minnesota. This kind of night-time imagery can be useful to compare observations to model estimates of smoke coverage as created by the HRRR Smoke model. Compare the image above, for example, to the two-hour forecast of vertically integrated smoke valid at 1300 UTC on 29 July, shown below. There is good agreement in the coverage over northern Minnesota. What does that kind of smoke look like from the ground? Here’s a image from a webcam at Lake Bemidji (from this source). GOES Imagery, at bottom, from just before Noon Central time (and from the CSPP Geosphere site) also shows the very thick smoke over Minnesota.

Use the Day Night band image (as available here, for example) to identify smoke plumes when lunar illumination is present.

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