B737-700 CREW TOOK OFF NEAR A SQUALL LINE AND ENCOUNTERED MODERATE TURB AND SEVERE WINDSHEAR.

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CAPT FLYING. WE JUST FINISHED A ROUND TRIP (CITY PAIRS) AND WERE WELL AWARE OF THE SQUALL LINE MOVING THROUGH FROM THE N. AFTER GATE ARR IN BNA; WE HAD 30-40 MINS BEFORE DEP WITH A MINIMAL PAX LOAD. THE OPS AGENT ASKED IF WE WANTED TO GO EARLY BECAUSE HE HAD JUST LOOKED AT HIS WX SCREEN. I TOLD HIM THE STORM WAS STILL 'A WAYS' OFF AND THAT WE WOULD BE GONE BEFORE IT WAS A FACTOR. I THEN LOOKED AT HIS SCREEN AND SAW THAT IT WAS 50 MI AWAY; BUT MOVING FAST. WE LANDED RWY 20L; BUT WERE INITIALLY HEADED FOR RWY 2R FOR DEP THEN SENT TO RWY 2L WHICH TOOK A FEW EXTRA MINS TO TAXI TO. JUST PRIOR TO DEP; RADAR SHOWED ECHOES 20-25 MI AWAY WITH NORMAL CONDITIONS AT THE FIELD 5-10 KTS. DURING THE TKOF ROLL; TWR STARTED RPTING GUSTS 20-30 KTS; THEN 40+ KTS AS WE GOT AIRBORNE. I KEPT THE NOSE FLAT AND NOTICED THE GUST FRONT KICKING THINGS UP AROUND THE BUSINESS AREAS. WE WERE IN MODERATE TURB IMMEDIATELY; GOT A QUICK TURN TO THE S AND ANTICIPATED CLBING THROUGH AND AWAY FROM IT QUICKLY. AFTER ROLLING OUT AND CLBING THROUGH 3500-4000 FT; WE GOT WHACKED AS IF THE GUST FRONT JUST OVERRAN US. I WAS GLAD IT HIT WHEN WE WERE WINGS LEVEL BECAUSE IT SHOOK US SO HARD TWICE; IT WAS ALL I COULD DO TO DISCERN WHAT THE VERT GYRO WAS TELLING ME. CONSIDERING TIME COMPRESSION; THERE WAS A SPLIT SECOND THAT THE ENTIRE GAUGE CLUSTER WAS A BLUR. I ESTIMATED THE EVENT AT 3-5 SECONDS; WITH 2 SEPARATE ONSETS OF SEVERE TURB. THE HIGHEST GUST RPT I REMEMBER WAS 46 KTS AND THAT WAS JUST AFTER GETTING AIRBORNE. GETTING RWY 2L TOOK MORE TIME AND PUT US A LITTLE CLOSER TO THE APCHING ECHOES; BUT IF WE HAD SEEN THE GUST FRONT; RECEIVED THE WIND RPTS OR HAD KNOWN IT WAS OVERRUNNING THE FIELD; WE COULD HAVE RE-EVALUATED THE TKOF DECISION.