How to setup alert for x% decrease in count by market?

hi All, We are receiving web traffic to one index from multiple markets like below query. Now we have been asked to setup an alert if there is any decrease in 50% of volume in any market over a time period like an hour or in 30 mins. Can some one help me how to achieve this? Charting the Traffic by Market wise: index=webtraffic sourcetype=mobile_traffic marketName=* eventType="ProductAdded" |timechart count by marketName useother=f usenull=f Thanks!