A non-exhaustive list of online resources to monitor weather and climate.
Global climate
NOAA PSL Weather/Climate Plotting and Analysis Tools
National Snow and Ice Data Center (NSIDC)
Climate variability & teleconnections
CPC Teleconnections (NAO, PNA, etc.)
UK climate
Met Office UK climate maps and data
Met Office surface analysis archive
Tornado and Storm Research Organisation (TORRO)
Stratosphere
MERRA-2 stratosphere time series (NASA GSFC)
NASA GEOS stratospheric seasonal evolution
Weather observations
NOAA National Data Buoy Center
UK & Ireland radar (Netweather)
Starlings Roost Weather (Dan Harris)
University of Wyoming Upper Air (radiosonde) archive