Dense Fog Advisories were issued before sunrise on 5 July 2016 by the National Weather Service Offices in St Louis Missouri and Lincoln Illinois (link). This post compares GOES-R IFR Probability fields with Brightness Temperature Difference Fields for that event. At 0500 UTC, the Brightness Temperature Difference Field had a very strong signal over central/northern Illinois. This region of mid-level stratus was de-emphasized by the GOES-R IFR Probability fields because of a lack of low-level saturation in the Rapid Refresh Model Fields. At 0500 UTC, IFR Probability Fields show an increase in values over the lower Ohio River Valley.
At 0700 UTC, below, an axis of higher probabilities has developed northwest to southeast across central Missouri; in addition, IFR Probabilities are increasing slowly over central Illinois to the south of the mid-level stratus that persists over east-central parts of Illinois. IFR Conditions are present at stations over central Illinois (Springfield — KSPI — and Litchfield — K3LF, for example)
By 1000 UTC, below, when Dense Fog Advisories have been issued, a strong brightness temperature difference signal is present over much of northern Missouri, but mid-level clouds over southeast Missouri prevent a strong signal from occurring there where fog is occurring. IFR Probability Fields’ use of Rapid Refresh information mitigates this lack of satellite observations. IFR Probability fields at 1000 UTC show a signal over much of southern Illinois where IFR Conditions are widespread.
Use IFR Probability fields as a tool to become situationally aware to the development of lowered ceilings and reduced visibilities. There are many times when Brightness Temperature Difference fields cannot tell the entire story — when multiple cloud layers exist, for example, or when mid-level stratus is present. A slow increase in GOES-R IFR Probability will often suggest lowering ceilings/reduced visibilities before Brightness Temperature Difference fields do.