Toggle between Brightness Temperature Difference (3.9µm – 10.7µm) and GOES-R IFR Probability fields, 2300 UTC on 11 December 2016 (Click to enlarge)

Dense Fog developed over south Texas during the early morning of 12 December 2016 (IFR Sigmet from this website shown here ; Advisories from the weather.gov website shown here). The toggle above shows in the brightness temperature difference field a signature of high clouds — and where those high clouds exist, IFR Probability fields rely on Rapid Refresh Model data to diagnose where IFR conditions might be occurring, or where IFR conditions might develop. The animation of Brightness Temperature Difference fields from 0215 through 1115 UTC, below, shows that the high clouds over south Texas diminished with time: by 0815 UTC only low stratus is present over south Texas.  But is that stratus also hugging the ground — that is, is it fog?  From the satellite’s perspective, the top of a stratus deck and the top of a fog bank can look very similar.

GOES-13 Brightness Temperature Difference (3.9µm – 10.7µm), 0215 through 1115 UTC on 12 December (Click to enlarge)

GOES-R IFR Probability fields give a more complete estimate about the presence of fog/low stratus because Rapid Refresh data and satellite data are used to diagnose the probability of IFR conditions. If the Rapid Refresh model shows low-level saturation, then the presence of stratus clouds also likely indicates the presence of fog; conversely, if the Rapid Refresh Model does not show low-level saturation, then the presence of stratus cloud need not indicate the presence of fog. IFR Probability fields below, from 0215 through 1115 UTC, start off regions with uniform values where only Rapid Refresh data are used in the algorithm — where high clouds block the satellite view of low clouds/fog. As the high clouds dissipate, the field acquires larger values because there is higher confidence of the presence of clouds (in part because satellite data can be used to observe them). In addition, these larger values have pixel-sized variability because of variability in the satellite observations.

IFR conditions are observed latest over far south Texas — this is also where IFR Probabilities are slowest to reach large values.

GOES-13 Brightness Temperature Difference (3.9 µm – 10.7 µm, left) and GOES-R IFR Probability Fields (Right), hourly from 0215 through 1415 UTC on 2 December 2016 (Click to enlarge)

Near-IFR and and IFR Conditions developed over the High Plains of Texas on 2 December 2016, and a SIGMET for IFR conditions was issued as shown below.

The animation above shows plentiful cirrus (in the brightness temperature difference enhancement used in the imagery on the left, above, cirrus clouds are dark) over south Texas, with occasional breaks.  This makes continual monitoring via satellite of the developing stratus/fog field problematic:  the satellite cannot monitor what it is blocked from being observed by intervening cloud layers — in this case cirrus.  (Click here for a brightness temperature difference only animation;  Click here for an IFR Probability only animation)  Because IFR Probability fields include model-based data about saturation in the lower troposphere, in the form of Rapid Refresh model output, a useful and coherent signal can be generated underneath cirrus clouds.  The GOES-R IFR Probability signal can be better used for situational awareness and anticipation of the development of the IFR conditions shown below.

In the animation above, note the change between 1315 and 1415 UTC fields — in the Brightness Temperature Difference fields (1315 UTC ; 1415 UTC), this change arises because of increasing amounts of reflected solar 3.9 µm radiation:  this causes a sign change in the brightness temperature difference.  For IFR Probability fields (1315 UTC ; 1415 UTC), the change occurs because the Predictors used at night (1315 UTC) and during the day (1415 UTC) are different.

1605 UTC screen capture from Aviation Weather Center. Note IFR Sigmet over west Texas (Click to enlarge)

GOES-13 Brightness Temperature Difference (10.7µm – 3.9µm) Fields, 0100-1400 UTC on 25 March 2015, along with surface observations of ceiling and visibility (Click to enlarge)

GOES-13 Brightness Temperature Difference Fields, above, demonstrate how high and middle clouds can complicate or prevent the detection of low clouds from satellite. The beginning of the animation shows signatures of cirrus streaking across the northern part of the domain. There are also signals of low clouds/fog over south Texas that diminish by ~0300 UTC before reappearing. No fog is reported during the early part of the animation. In the latter half of the animation, cirrus moves in to the southern part of the domain from the southwest (cirrus appears black in the enhancement used), interfering with the detection of low clouds and fog over south Texas. At 1400 UTC, the end of the animation, reflected 3.9µm radiation originating from the rising sun has flipped the sign of the brightness temperature difference field.

GOES-13-based IFR Probability Fields, 0100-1400 UTC on 25 March 2015, along with surface observations of ceiling and visibility (Click to enlarge)

The GOES-R IFR Probability fields, above, show little signal until after 0500 UTC when the satellite and model data both start to suggest the presence of fog/low stratus.  When the cirrus impinges on the southern part of the domain, becoming noticeable at 1000 UTC, IFR Probability values drop because satellite predictors cannot be used in the algorithm — only model data is driving the field over south Texas.  The model data strongly suggests fog is present however (and IFR and near-IFR conditions are reported).  Using the fused product allows a forecaster to have a consistent signal as the high clouds move in.  IFR conditions persist — and IFR Probabilities remain high — after sunrise over portions of south Texas.