I claim the ornamental design for a digital outdoor thermometer, as shown and described.
FIG. 1 is a front view of the digital outdoor thermometer.
FIG. 2 is a back view of the claimed design.
FIG. 3 is a side view of the claimed design.
FIG. 4 is a top view of the claimed design.
FIG. 5 is a bottom view of the claimed design; and,
FIG. 6 is a perspective view of the claimed design.
The broken line showing of structural features is included for the purpose of illustrating non-claimed subject matter and forms no part of the claimed design.