Update: See this post for a faster version.

Thank you for sharing batch scripting on Windows. Do not fear Perl. The following book is helpful for beginners.

Perl by Example 4th Edition.

Below, am sharing a parallel version using MCE. However, am using MCE mainly for its chunking abilities. Each record is handled individually. The record separator is anchored at the start of the line with "\nINTERPOLATED HYDROGRAPH". MCE detects this and handles automatically.

use strict;
use warnings;

use MCE::Loop;
use MCE::Candy;

my $input_file  = shift || 'input.txt';
my $output_file = shift || 'output.txt';

my $match_string = "INTERPOLATED HYDROGRAPH";

open my $ofh, ">", $output_file
   or die "cannot open '$output_file' for writing: $!\n";

MCE::Loop::init {
   use_slurpio => 1, chunk_size => 1, max_workers => 4,
   gather => MCE::Candy::out_iter_fh($ofh),
   RS => "\n${match_string}",
};

## Below, each worker receives one record at a time
## Output order is preserved via MCE::Candy::out_iter_fh

## line  1 CAC40 #  INTERPOLATED HYDROGRAPH AT CAC40
## line  2       #  blank line here
## line  3       #  PEAK FLOW TIME MAXIMUM AVERAGE FLOW
## line  4       #  6-HR 24-HR 72-HR 166.58-HR
## line  5       #  + (CFS) (HR)
## line  6       #  (CFS)
## line  7 1223. #  + 1223. 12.67 890. 588. 245. 106.
## line  8       #  (INCHES) .154 .408 .509 .509
## line  9 1456. #  (AC-FT) 441. 1166. 1456. 1456.
## line 10       #  CUMULATIVE AREA = 53.67 SQ MI

mce_loop_f {
   my ( $mce, $chunk_ref, $chunk_id ) = @_;

   ## Skip initial record containing header lines including *** ***

   if ( $chunk_id == 1 && $$chunk_ref !~ /^${match_string}/ ) {
      ## Gathering here is necessary when preserving output order,
      ## to let the manager process know chunk_id 1 has completed.
      MCE->gather( $chunk_id, "" );
      MCE->next;
   }

   ## Each record begins with INTERPOLATED HYDROGRAPH.

   my ( $k1, $k2, $k3 ) = ( "", "", "" );

   open my $ifh, "<", $chunk_ref;

   while ( <$ifh> ) {
      $k1 = $1 and next if $. == 1 && /(\S+)\s*$/;
      $k2 = $1 and next if $. == 7 && /^\S+\s+(\S+)/;
      $k3 = $1 and last if $. == 9 && /(\S+)\s*$/;
   }

   close $ifh;

   ## Gather values.

   MCE->gather( $chunk_id, "$k1 $k2 $k3\r\n" );

} $input_file;
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Kind regards, Mario.