####018002302#### FXXX10 KWNP 181231 DAYTDF :Product: 3-Day Forecast :Issued: 2026 Mar 18 1230 UTC # Prepared by the US Dept. of Commerce, NOAA, Space Weather Prediction Center # Product description and SWPC contact on the Web # http://www.swpc.noaa.gov/wwire.html # # 3-Day Forecast # A. NOAA Geomagnetic Activity Observation and Forecast The greatest observed 3 hr Kp over the past 24 hours was 2 (below NOAA Scale levels). The greatest expected 3 hr Kp for Mar 18-Mar 20 2026 is 6.33 (NOAA Scale G2). NOAA Kp index breakdown Mar 18-Mar 20 2026 Mar 18 Mar 19 Mar 20 00-03UT 1.00 1.67 5.00 (G1) 03-06UT 1.00 5.33 (G1) 4.33 06-09UT 1.67 6.33 (G2) 3.67 09-12UT 1.33 5.67 (G2) 3.00 12-15UT 1.67 4.33 2.67 15-18UT 2.00 3.33 3.67 18-21UT 2.33 3.67 4.33 21-00UT 2.67 4.00 4.67 (G1) Rationale: G1-G2 (Minor-Moderate) geomagnetic storms are expected over 19-20 Mar due to a CME arrival from 16 Mar with a potential solar sector boundary crossing. B. NOAA Solar Radiation Activity Observation and Forecast Solar radiation, as observed by NOAA GOES-18 over the past 24 hours, was below S-scale storm level thresholds. Solar Radiation Storm Forecast for Mar 18-Mar 20 2026 Mar 18 Mar 19 Mar 20 S1 or greater 5% 10% 10% Rationale: There is a slight chance for S1 (Minor) or greater solar radiation storms on 19-20 Mar due to the potential for accelerated particles ahead of the CME shock. C. NOAA Radio Blackout Activity and Forecast Radio blackouts reaching the R1 levels were observed over the past 24 hours. The largest was at Mar 18 2026 0839 UTC. Radio Blackout Forecast for Mar 18-Mar 20 2026 Mar 18 Mar 19 Mar 20 R1-R2 35% 35% 35% R3 or greater 5% 5% 5% Rationale: There is a chance for R1-R2 (Minor-Moderate) radio blackouts, with a slight chance for R3 (Strong) events, through 20 Mar, due predominantly to the flaring potential of Region 4392. ####018003791#### FXXX12 KWNP 181231 DAYDIS :Product: Forecast Discussion :Issued: 2026 Mar 18 1230 UTC # Prepared by the US Dept. of Commerce, NOAA, Space Weather Prediction Center # Product description and SWPC contact on the Web # http://www.swpc.noaa.gov/wwire.html # # Forecast Discussion # Solar Activity .24 hr Summary... Solar activity reached to moderate levels, with an M2.7 at 18/0842 UTC from Region 4392 (S16W13, Cai/beta-gamma). The region had been variable with periods of growth and decay the past two days; however, today it has primarily undergone decay while it retained a mildly mixed magnetic configuration. Region 4391 (N07W61, Bxo/beta) reemerged, but is exhibiting signs of decay. The remaining regions were mainly stable and changed little. New Region 4397 (N19E67, Bxo/beta) came around the E limb and was numbered, though its classification is tentative to due foreshortening effects. The M2.7 was accompanied by a tenflare and Type II radio burst with an estimated velocity of 740-860 km/s. An EUV wave is visible in GOES SUVI, along with faint field line movement in the NW quadrant. The associated CME became visible in STEREO A COR2 at 18/0923 UTC, during a SOHO LASCO data gap. Modeling is currently ongoing. An EUV wave also accompanied a C3.9 flare at 18/0123 UTC, also from Region 4392, visible in various GOES SUVI channels. However, no CME signatures were observed in available imagery, potentially due to data gaps, earthshine interference, and the disk center source location. .Forecast... Solar activity is expected to be low, with a chance for M-class (R1-R2; Minor-Moderate) level events 18-20 Mar due primarily to the minor instability and variability of Region 4392. Energetic Particle .24 hr Summary... The greater than 2 MeV electron flux reached high levels again, with a peak of 2,230 pfu at 17/1715 UTC. The greater than 10 MeV proton flux continued at background levels. .Forecast... The greater than 2 MeV electron flux is likely to reach high levels yet again on 18 Mar. A brief return to normal-moderate levels on 19-20 Mar is expected due to a likely coronal mass ejection (CME) arrival. The greater than 10 MeV proton flux is expected to continue at background values, with a slight chance for an S1 (Minor) level event 19-20 Mar due to possible CME shock enhancement influences. Solar Wind .24 hr Summary... Solar wind parameters were indicative of a gradual return to a slow regime. Total IMF strength was at normal levels of mainly 4-5 nT, and the Bz component was predominantly northward. Solar wind speed decreased from peaks around 500 km/s to around 400 km/s. The phi angle was predominantly positive. .Forecast... The solar wind environment is anticipated to transition to a near normal regime through 18 Mar. An anticipated CME arrival near midday on 19 Mar is expected to enhance solar wind speed and total IMF strength. CME passage is likely to continue into 20 Mar with the additional factors of two other nearby CMEs passing by close to Earth, as well as a possible solar sector boundary crossing (SSBC). All this is likely to lead to a complicated and amplified solar wind regime 19-20 Mar and beyond. Geospace .24 hr Summary... The geomagnetic field was quiet. .Forecast... Quiet to unsettled conditions are expected on 18 Mar as positive polarity CH HSS influences end. The geomagnetic field is likely to become disturbed on 19 Mar, with G2 (Moderate) storm levels likely upon and after the anticipated CME arrival. Active to G1 (Minor) storm conditions are likely on 20 Mar due to continuing but weakening CME passage influences.