I made an inquiry to Thermoworks about their stated operating range of 32-122F and their FAQ about winter operation. Here is their response:
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Hello,
Thank you for your email. While the ideal range is 32 to 122°F, because we know that our customers need to use it in the winter, we have done extensive tests on Signals and have validated that it works well in colder temperatures. We also took care to increase its waterproof rating to IP66. We do not, however, recommend storing Signals on a permanent basis outside. If you have any other questions or concerns please let us know.
Best regards,
Sarah Burton
Customer Care Team