You should be able to comment out the River Flood Warnings in nws-alerts.php I believe.
Around line 760 in mine
array('N'=>'Radiological Hazard Warning', 'C'=>'#D00', 'S'=>'37', 'I'=>'WSW.gif'),
array('N'=>'Red Flag Warning', 'C'=>'#D00', 'S'=>'38', 'I'=>'FWW.gif'),
array('N'=>'River Flood Warning', 'C'=>'#D00', 'S'=>'39', 'I'=>'FLW.gif'),
array('N'=>'Shelter In Place Warning', 'C'=>'#D00', 'S'=>'40', 'I'=>'WSW.gif'),
array('N'=>'Sleet Warning', 'C'=>'#D00', 'S'=>'41', 'I'=>'IPW.gif'),