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Based on our tool for the early diagnostics of solar eruptiongeoeffectiveness (EDSEG tool; Chertok et al., 2013, 2015, 2017), we haveanalyzed space weather disturbances that occurred in early September 2017. Twoflares, SOL2017-09-04T20:33 (M5.5) and SOL2017-09-06T12:02 (X9.3), accompaniedby Earth-directed halo coronal mass ejections (CMEs) were found to begeoeffective. We extracted the associated EUV dimmings and arcades andcalculated their total unsigned magnetic flux. This calculation allowed us toestimate the possible scales of the Forbush decreases (FDs) and geomagneticstorms (GMSs) in the range from moderate to strong, and they are close to theobserved scales. More precisely, after the first eruption, an FD approximatelyequal to 2% and almost no GMS occurred because the Bz magnetic field componentin front of the corresponding interplanetary CME (ICME) was northern. Thestronger second eruption produced somewhat larger composite disturbances (FD ~9.3% and GMS with indexes Dst ~ -144 nT, Ap ~ 235) than expected (FD ~ 4.4%,Dst ~ -135 nT, Ap ~ 125) because the second ICME overtook the trailing part ofthe first ICME near Earth, and the resulting Bz component was more intense andsouthern. Both ICMEs arrived at Earth earlier than expected because theypropagated in the high-speed solar wind emanated from an extended coronal holeadjacent to AR12673 along their entire path. Overall, the presented resultsprovide further evidence that the EDSEG tool can be used for the earliestdiagnostics of actual solar eruptions to forecast the scale of thecorresponding geospace disturbances.

Solar Eruptions, Forbush Decreases, and Geomagnetic Disturbances From Outstanding Active Region 12673