#!/usr/bin/env perl

use strict;
use warnings;

my ($in,@array);
$in=0;

while(<>){
        if (/^OBS ASH CLOUD/) {
                $in=1;
        }
        elsif (/^\n/) {
                $in=0;
        }
        chomp;
        if($in) {
                @array=split(/\s+/);
                print "@array\n";
        }
}


Output:
OBS ASH CLOUD: SFC/FL120 N1705 W06244 - N1702
W06227 - N1643 W06210 - N1641 W06211 - N1633
W06220 - N1648 W06241 - N1705 W06244 MOVING NW
5-10 KNOTS.