I need to return a value from json. The div#maxRecord works okay but I also need to use the variable topOfNormalRange for other conditionals. How do I do that?

<div id="maxRecord"></div>
<script>

var topOfNormalRange = getStationData('L1307')

if (topOfNormalRange > 3.0) {
do_stuff()
}
function getStationData(stRef) {

$.getJSON("https://environment.data.gov.uk/flood-monitoring/id/stations/"+stRef+"/stageScale", function(json){

// collect the data
var topOfNormalRange = json["items"]["typicalRangeHigh"];
var maxRecord = json["items"]["maxOnRecord"]["value"];

$('#maxRecord').html(maxRecord+'m');
return (topOfNormalRange);
 });
}
</script>

returns undefined