Dense fog formed over south Georgia early in the morning of 9 October 2015 (National Weather Service Homepage in Peachtree City). The imagery above, from 1115 UTC, demonstrates a strength of the GOES-R IFR Probability field: it maintains a signal where dense fog is ongoing even as high clouds overspread the region. At 0800 UTC on 9 October (below), high clouds had not yet overspread south Georgia, and the image below shows that both IFR Probabilities and Brightness Temperatuere Difference fields outline the region of developing fog. When high clouds appear (as in the image above), the Brightness Temperature Difference signal that can be used in clear skies loses its utility in fog detection.
URGENT – WEATHER MESSAGE
NATIONAL WEATHER SERVICE GRAND FORKS ND
303 AM CDT FRI OCT 9 2015
INCLUDING THE CITIES OF…CANDO…LANGDON…FORT TOTTEN…
MADDOCK…LEEDS…MINNEWAUKAN…DEVILS LAKE…NEW ROCKFORD…
303 AM CDT FRI OCT 9 2015
…DENSE FOG ADVISORY IN EFFECT UNTIL 10 AM CDT THIS MORNING…
…FROST ADVISORY REMAINS IN EFFECT UNTIL 9 AM CDT THIS MORNING…
THE NATIONAL WEATHER SERVICE IN GRAND FORKS HAS ISSUED A DENSE
FOG ADVISORY…WHICH IS IN EFFECT UNTIL 10 AM CDT THIS MORNING.
* TEMPERATURES…IN THE LOWER 30S.
* VISIBILITIES…AREAS OF VISIBILITY UNDER ONE QUARTER MILE.
VISIBILITIES WILL BE HIGHLY VARIABLE HOWEVER.
* TIMING…FOG WILL BURN OFF MID MORNING.
A FROST ADVISORY MEANS THAT WIDESPREAD FROST IS EXPECTED.
SENSITIVE OUTDOOR PLANTS MAY BE KILLED IF LEFT UNCOVERED.
A DENSE FOG ADVISORY MEANS VISIBILITIES WILL FREQUENTLY BE
REDUCED TO LESS THAN ONE QUARTER MILE. IF DRIVING…SLOW DOWN…
USE YOUR HEADLIGHTS…AND LEAVE PLENTY OF DISTANCE AHEAD OF YOU.
The GOES-R IFR Probability fields increased in this region as the fog developed (above). Notably in this case, IFR probabilities were lower over Minnesota and the Red River Valley where mid-level stratus was occurring. The toggle below compares IFR Probabilities and Brightness Temperature Differences at four times during the night. The Brightness Temperature Difference fields (animation from 0400-1100 UTC is here) greatly overpredict the region of dense fog. Because the IFR Probabilities incorporate information about the surface, however, regions of mid-level stratus can be screened out. These are regions where the Rapid Refresh model does not show saturation in the lowest layers.
Note that Cirrus is overspreading the region from the west at the end of the animations. High Clouds change the signal in the brightness temperature difference field. Harvey ND at 1100 UTC reports Zero visibility and an obscured ceiling (with freezing drizzle). The station is under cirrus, however, so a brightness temperature difference signal associated with fog is not present. There is a modest IFR Probability value there because data from the Rapid Refresh suggests low-level saturation is occurring.