Halliday & Matthiessen (2014: 172-3):
But interpersonal judgements, or assessments, extend beyond the ‘core’ grammatical system of modality to include assessments of temporality and intensity realised like modality through mood Adjuncts (e.g. It is/It already is/It almost is), and also other types of assessments beyond the mood itself that relate either to the proposition being exchanged (e.g. Fortunately it is: ‘it is, which is fortunate’) or to the act of exchanging it (e.g. Frankly it is: ‘I’m telling you frankly it is’).