Forecaster summary - Issued 2021-05-20T00:19:21

The high energy proton flux (greater than 10 MeV) is expected to be at background levels. The high energy electron (greater than 2MeV) flux is expected to remain at background levels on Day 1, before potentially rising above the Alert threshold at times on Days 2 or 3 at the diurnal maxima, though confidence is lower than usual due to the timing of coronal hole 59 and it's expected high speed solar wind stream. The associated 24-hour fluence may also rise accordingly later in the period, and may reach the Active (1e8 integrated pfu) threshold on Days 3 and 4 (22nd and 23rd).

The REFM 27-day recurrence shows a declining trend from above the Active threshold, although on the previous occasion the driving solar winds were much faster than on this rotation. The 3-day REFM forecast has a low trend, which is broadly supported. The recurrence-persistence model has a peak percentage of 22% on Day 2 (22nd), then a decline, but this is simply an artefact of the statistics, and not considered real for this occasion. If Earth receives a couple days of solar winds near 500 km/s, then a rise to the Active threshold is possible. There is a chance (~30%) of a rise above the threshold by Day 3 due to the incoming high speed stream.