  We propose an extension of the LSST survey to cover the northern sky to DEC <
+30 (accessible at airmass <1.8). This survey will increase the LSST sky
coverage by ~9,600 square degrees from 18,900 to 28,500 square degrees (a 50%
increase) but use only 0.6-2.5% of the time depending on the synergies with
other surveys. This increased area addresses a wide range of science cases that
enhance all of the primary LSST science goals by significant amounts. The
science enabled includes: increasing the area of the sky accessible for
follow-up of multi-messenger transients including gravitational waves, mapping
the milky way halo and halo dwarfs including discovery of RR Lyrae stars in the
outer galactic halo, discovery of z>7 quasars in combination Euclid, enabling a
second generation DESI and other spectroscopic surveys, and enhancing all areas
of science by improving synergies with Euclid, WFIRST, and unique northern
survey facilities. This white paper is the result of the Tri-Agency Working
Group (TAG) appointed to develop synergies between missions and presents a
unified plan for northern coverage. The range of time estimates reflects
synergies with other surveys. If the modified DESC WFD survey, the ecliptic
plane mini survey, and the north galactic spur mini survey are executed this
plan would only need 0.6% of the LSST time, however if none of these are
included the overall request is 2.5% of the 10 year survey life. In other
words, the majority of these observations are already suggested as part of
these other surveys and the intent of this white paper is to propose a unified
baseline plan to carry out a broad range of objectives to facilitate a
combination of multiple science objectives. A companion white paper gives
Euclid specific science goals, and we support the white papers for southern
extensions of the LSST survey.
