Moderate geomagnetic storm conditions (NOAA Kp = 6 + ) were observed between 21:00 UTC on May 15 and 00:00 UTC on May 16. Locally, several intervals of minor storm conditions were observed, with K BEL reaching 5. This activity is associated with the arrival of the high-speed solar wind stream from the large, negative polarity transequatorial coronal hole (returning SIDC Coronal Hole 147), which started to cross the central meridian on May 13. Solar wind conditions showed a significant enhancement, with the solar wind speed increasing up to approximately 810 km/s. The interplanetary magnetic field reached up to about 17 nT, while the Bz component reached values down to -11 nT. Active to minor storm conditions may persist over the next 24 hours due to the continuous influence of the high-speed stream and depending on the evolution of the interplanetary magnetic field, particularly the Bz orientation.

Solar flaring activity was moderate over the past 24 hours, with four M-class flares. The strongest flares were two M1.9 flares: SIDC Flare 7718, peaking at 16:12 UTC on May 16, which was associated with the returning SIDC Sunspot Group 825 (NOAA Active Region 4436, magnetic type beta), and SIDC Flare 7714, peaking at 17:42 UTC on May 16, which was associated with SIDC Sunspot Group 844 (NOAA Active Region 4435). There are currently seven numbered active regions on the solar disk, with all regions classified as magnetic type beta or alpha. SIDC Sunspot Groups 871 and 872, both magnetic type beta, emerged in the southeast quadrant, while SIDC Sunspot Group 870 (NOAA Active Region 4441, magnetic type beta) emerged in the northeast quadrant. SIDC Sunspot Group 844 (NOAA Active Region 4435) has rotated across the west limb. The solar flaring activity is expected to be low over the next 24 hours, with C-class flares very likely and a chance for M-class flares. A wide and fast Coronal Mass Ejection (SIDC CME 656) was observed in LASCO/C2 and C3 coronagraph imagery starting from around 16:36 UTC on May 16, directed mainly toward the northwest. The CME is associated with an M1.9 flare peaking at 16:12 UTC on May 16 from SIDC Sunspot Group 825 (NOAA Active Region 4436), with associated dimming and field line movement observed in SDO/AIA data. Type IV radio emission was detected from 16:05 UTC on May 16, associated with this event. Preliminary analysis suggests that the CME has a fitted speed around 1200 km/s. Although the bulk of the ejecta appears to be directed mainly toward the northwest, preliminary modelling indicates that a glancing blow is expected at Earth around May 19. No other Earth-directed CMEs were observed in the available coronagraph imagery. The greater than 10 MeV proton flux was below the 10 pfu threshold level over the past 24 hours. It is expected to remain below the threshold level over the next 24 hours. The greater than 2 MeV electron flux measured by GOES 18 and GOES 19 was below the 1000 pfu threshold until around 16:00 UTC on May 16 and exceeded the threshold afterwards. The greater than 2 MeV electron flux is expected to remain around or above the 1000 pfu threshold levels in the next 24 hours. The 24-hour electron fluence was initially at normal levels and reached moderate levels for the rest of the period. It is expected to remain at moderate levels over the next 24 hours.

Geomagnetic conditions globally reached minor storm levels between 15:00 UTC and 18:00 UTC on May 16 (NOAA Kp 5- ), before returning to mostly quiet levels towards the end of the period. Locally, geomagnetic conditions reached active levels during several intervals at the beginning of the period (K BEL 4), before returning to mostly quiet levels towards the end of the period. Mostly quiet geomagnetic conditions are expected over the next 24 hours. Over the past 24 hours, the solar wind parameters (ACE) reflected enhanced solar wind conditions under the waning influence of the high-speed stream from the large, negative polarity transequatorial coronal hole (returning SIDC Coronal Hole 147). Speed values ranged between 620 km/s and 730 km/s. The interplanetary magnetic field values ranged between 2 nT and 5 nT. The Bz component varied between -5 nT and 4 nT. Solar wind conditions are expected to gradually return towards a slow solar wind regime over the next 24 hours.