  Synoptic magnetograms provide us with knowledge about the evolution of
magnetic fields on the solar surface and present important information for
forecasting future solar activity. In this work, poloidal and toroidal magnetic
field components derived from synoptic magnetograms are assimilated, using the
Ensemble Kalman Filter method, into a mean-field dynamo model based on Parker's
migratory dynamo theory complemented by magnetic helicity conservation. It was
found that the predicted toroidal field is in good agreement with observations
for almost the entire following solar cycle. However, poloidal field
predictions agree with observations only for the first 2 - 3 years of the
predicted cycle. The results indicate that the upcoming Solar Maximum of Cycle
25 (SC25) is expected to be weaker than the current Cycle 24. The model results
show that a deep extended solar activity minimum is expected during 2019 -
2021, and that the next solar maximum will occur in 2024 - 2025. The sunspot
number at the maximum will be about 50 with an error estimate of 15 - 30 %. The
maximum will likely have a double peak or show extended periods (for 2 - 2.5
years) of high activity. According to the hemispheric prediction results, SC25
will start in 2020 in the Southern hemisphere, and will have a maximum in 2024
with a sunspot number of about 28. In the Northern hemisphere the cycle will be
delayed for about 1 year (with an error of $\pm 0.5$ year), and reach a maximum
in 2025 with a sunspot number of about 23.
