Monthly Archives: September 2017

Dense Fog over Missouri and over Alabama

GOES-R IFR Probability Fields, Hourly from 0215-1315 UTC on 19 September 2017 (Click to enlarge)

GOES-R IFR Probabilities are computed using Legacy GOES (GOES-13 and GOES-15) and Rapid Refresh model information; GOES-16 data will be incorporated into the IFR Probability algorithm in late 2017

Dense fog developed over Missouri on Tuesday 19 September and Dense Fog Advisories were issued. The animation above shows the hourly development of GOES-R IFR Probability fields; values increased from northern Missouri to southern Missouri as dense fog developed, first north of I-70, then south into the rest of the state. The morning of 19 September was mostly devoid of mid-level and high-level clouds over Missouri (exception: west-central Missouri starting after 0900 UTC), and that kind of night means that traditional methods of fog detection work well. The brightness temperature difference field between the shortwave Infrared and the Longwave Infrared (3.9 µm and 10.3 µm on GOES-16, 3.9 µm and 10.7 µm on GOES-13) shows the fog development.

Note that the IFR Probability field, above, does not show fog dissipating around sunrise. That’s in contrast to the Brightness Temperature Difference field below. As the sun rises, the amount of solar radiation at 3.9 µm that is reflected off the clouds increases; this changes the brightness temperature difference from positive (cyan in the color enhancement shown) to negative (grey or black in the enhancement shown).

GOES-16 has better spatial resolution than GOES-13; thus, the small valley fogs that can develop in the rugged (ish) terrain of southern Missouri are resolved in GOES-16, but not in GOES-13. When GOES-R IFR Probability is created using GOES-16 data (slated to begin in late 2017), the resolution improvements in GOES-16 will migrate to IFR Probability fields.

GOES-16 data posted on this page are preliminary, non-operational and are undergoing testing.

GOES-16 Brightness Temperature Difference (10.3 µm – 3.9 µm), hourly from 0412 to 1112 UTC on 19 September (Click to enlarge)


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On 20 September, Dense Fog developed over Tennessee and Alabama, leading to the issuance of Dense Fog Advisories. The GOES-R IFR Probability field, below, shows good agreement between high probabilities and reduced ceilings/visibilities.

GOES-R IFR Probability Fields, Hourly from 0415-1315 UTC on 20 September 2017 (Click to enlarge)

As on the 19th over Missouri, top, this was a night with relatively few middle- and upper-level cloud decks. On such nights, the GOES-16 Brightness Temperature Difference field can capably identify regions of stratus (it’s up to a human to decide if the stratus deck extends to the surface; on this night, much of the stratus did). The 2-hour animation of Brightness Temperature Difference, below, highlights two particular strengths of GOES-16: Better spatial resolution that allows small valleys to be sampled correctly, and good temporal resolution (every 5 minutes vs. every 15 minutes for GOES-13) that allows superior monitoring of the cloud evolution with time. Note that the rising sun is eroding the GOES-16 Brightness Temperature Difference signal by the end of the animation below.

GOES-16 Brightness Temperature Difference (10.3 µm – 3.9 µm), hourly from 1002 to 1202 UTC on 20 September (Click to enlarge)

The Brightness Temperature Difference field (10.3 µm – 3.9 µm) is a key component to the Nighttime Microphysics Red/Green/Blue Composite. As the toggle below shows, the Brightness Temperature Difference field overwhelmingly controls the region identified by the RGB as one with a potential for fog.

Fog over southwest Lower Michigan

GOES-R IFR Probability Fields, 0330-1430 UTC on 14 September 2017 (Click to animate)

GOES-R IFR Probabilities are computed using Legacy GOES (GOES-13 and GOES-15) and Rapid Refresh model information; GOES-16 data will be incorporated into the IFR Probability algorithm in late 2017

GOES-R IFR Probability fields (From this site, but also available via LDM feed in AWIPS), above, show the development of dense for over Lower Michigan, leading to the issuance of advisories. IFR Probabilities on this morning remained fairly low over the relatively warm late Summer waters of Lake Michigan. There is also a noteworthy gap in Fog of unknown origin from Grand Rapids Michigan southwestward to Lake Michigan as fog forms on either side of that line. This is especially evident around 0845 UTC; the gap subsequently fills in as fog becomes more widespread.

GOES-16 data posted on this page are preliminary, non-operational and are undergoing testing

GOES-16 also viewed the fog field, from the top of the cloud deck, as shown below in an animation (courtesy of Nathan Jeruzal the National Weather Service Grand Rapids Office) of the Brightess Temperature Difference field (10.3  µm – 3.9  µm) for two hours before sunrise on 14 September 2017. A still image at 1117 UTC suggests that fog over Grand Rapids’ city limits was not widespread, perhaps due to slight warming in the city due to an Urban Heat Island that would reduce the relative humidity.

At the end of the animation, there is a consistent change in signal over eastern Michigan, from cyan indicating low clouds/stratus to grey, indicating no cloud, because of increasing amounts of reflected solar radiation at 3.9 as the Sun rises. The GOES-R IFR Probability field animation at top suggests that the fog persists through sunrise.

GOES-16 Brightness Temperature Difference fields (10.3 µm – 3.9 µm), 0947-1142 UTC on 14 September 2017;  City outlines are denoted in Yellow (Click to enlarge)