wx radar on 737NG question~

[citizensun]

the following words is quote from my company fcom:
"The weather radar automatically begins scanning for windshear when:
• thrust levers set for takeoff, even if engine is off or IRS not aligned, or
• in flight below 2,300 feet RA (predictive windshear alerts are issued below 1,200 feet RA).
737-800
Alerts are available approximately 12 seconds after the weather radar begins scanning for windshear. Predictive windshear alerts can be enabled prior to takeoff by pushing the EFIS control panel WXR switch."
my question are:
a. thrust set for takeoff means the TLA >53° or the thrust is set at predetermined N1?
b. alerts are available only 12 seconds after the weather radar begins scanning for windshear, does that mean without the wx radar turned on during takeoff roll we only have windshear alerts till that 12` passed? and within that 12` we may already in the air!!!
someone know about this?
thanks~