$scomHeaders = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
$scomHeaders.Add('Content-Type','application/json; charset=utf-8')
$bodyraw = "AuthenticationMode:<DOMAIN>\<USERNAME>:<PASSWORD>"
$Bytes = [System.Text.Encoding]::UTF8.GetBytes($bodyraw)
$EncodedText =[Convert]::ToBase64String($Bytes)
$jsonbody = $EncodedText | ConvertTo-Json

$uriBase = 'http://<SCOM_SERVER>/OperationsManager/authenticate'
$auth = Invoke-RestMethod -Method POST -Uri $uriBase -Headers $scomheaders -body $jsonbody -UseDefaultCredentials -SessionVariable websession

$query = @{
"classId" = $null;
"objectIds" = $null;
"criteria"= "(((Severity = '1') or (Severity = '2')) AND (ResolutionState = '0'))" ;
"displayColumns" = "severity","monitoringobjectdisplayname","name","age","repeatcount","lastmodified","resolutionstate","ticketid"
} 

$jsonquery = $query | ConvertTo-Json
$Response = Invoke-WebRequest -Uri "http://<SCOM_SERVER>/OperationsManager/data/alert" -Method Post -Body $jsonquery -ContentType "application/json" -UseDefaultCredentials -WebSession $websession
$alerts = ConvertFrom-Json -InputObject $Response.Content
$alerts.rows | select id,monitoringobjectdisplayname,name,severity,resolutionstate,lastmodified,age,ticketid

0 - Information
1 - Warning
2 - Error

0 - Low
1 - Medium
2 - High

Acknowledged            = 249
Assigned to Engineering = 248
Awaiting Evidence       = 247
Closed                  = 255
New                     = 0
Resolved                = 254
Scheduled               = 250