C177 pilot reported a loss of aircraft control due to a windshear event during takeoff. The event resulted in a runway excursion with no injuries or aircraft damage.

Narrative

Aggressive wind shift causing windshear immediately after liftoff requiring an abort. Aircraft 200+ lbs. below MTOW; near the center of CG envelope. 1 pax; 1 dog and half full fuel load aboard. ATIS wind 280/07; Departure Runway XXL; 3300 feet available. Receiving takeoff clearance; I noted wind patterns in the grass and a distant flagpole showing wind slightly more from the north. Requested spot wind from Tower and received a report within 20* of the ATIS report; but at 15 kts. Being well within limits; I accepted the takeoff clearance. Planned rotation was at about the 1000 ft point; anticipating some extra roll for a crosswind. During the takeoff roll I could feel what seemed to be a greater than reported crosswind; but easily manageable so was not surprised when rotation was about 200 ft further down the runway. Within seconds of liftoff the required crab angle to hold centerline drastically increased; airspeed stagnated. The aircraft would not climb. I immediately elected to abort; set the aircraft back down on the runway and announced the abort to ATC. It was instantly obvious that my groundspeed had become much higher than my airspeed and that most of the need for crosswind correction had disappeared. Using maximum braking while trying not to blow a tire I was able to maintain centerline but could not stop the aircraft until we had entered the mowed grass overrun by about 150 ft. I brought the aircraft to a stop within 15 ft of a ditch. I shutdown the engine near the end of rollout; we evacuated the aircraft; then returned to inspect the airframe and radio the Tower. There were no injuries; no damage to the aircraft; and with local assistance I was able to pivot the aircraft and have it towed to a local shop. It was inspected with no damage found. Speaking with the ATC Specialist on duty; he acknowledged my spot wind request just prior to roll. He also said he noted the wind as I was rolling; referred to it as a 'nasty wind shift' and reported it as 030-060* at 15-25 kts. I provided all requested information to him for his report. I believe the cause of this event was a very rapid wind shift; creating a windshear; and unfortunately occurring just at or immediately after I rotated the aircraft. I accept I had some startle factor delay when presented with an aircraft that lifted off normally; then suddenly refused to perform.Lessons learned. My instincts were correct. The wind was changing; and had I waited 1 minute more; the shift would have become visible; a runway change required; and the event would probably not have occurred. Secondary; I had just returned from recurrent training on a transport category aircraft. Windshear is part of that training. This experience would have been an event beyond V1 during training; so an escape maneuver would normally be flown. Not in this aircraft though; and working through that process cost me a few seconds; perhaps just enough to create this overrun. As an operator of transport aircraft with significant performance margins and also an operator of light aircraft with very little margins; another lesson for me is that I need to forcibly recalibrate the decision process used during critical flight phases. Another is changing my predeparture performance planning. Rather than just identifying my takeoff abort point on the runway; by using planning software; I can determine that point; and my stopping distance after an abort; even from 50ft. In a limited way for big picture planning only; it creates a balanced field scenario for a single engine aircraft. The total required distance is eye opening and really changes the departure decision process considering there is no V2 climb option in a light single engine aircraft. Knowing what's beyond the end of the runway is always critical; but more so in a single engine aircraft. Using this software; I can calculate normal ground roll & 50 ft numbers; then calculate the same for landing over 50 ft. Adding the two could really give me usefulinformation in the critical takeoff rotation phase. I've always used runway lights to measure distance; they're about 200 ft apart. I now brief my wife or any other passenger to count lights and announce them in the headset to support my departure decision process. It's not perfect but it drives the thought process during the predeparture briefing. The better answer however would be to practice pausing when something is changing in the departure (or arrival) phase and reconfirm the situation before proceeding. Regardless of planning; that is a key part of every informed decision.