From the US National Weather Service
We received some questions about wireless emergency alerts yesterday with the tornado warning that included downtown Pittsburgh, so wanted to share this graphic to help explain. Since warnings are polygon based and not county based, just because you’re in Allegheny County doesn’t mean you should have gotten the tornado warning yesterday. Your phone will only alert if you’re within the warning polygon and in a location that’s at imminent threat of danger. That means family or friends who are as close as a mile or two away may have received a wireless emergency alert while you did not, or vice versa. That is the system working exactly as it should.
Part of the process we go through every time we draw up a warning is to keep the polygon confined only to the locations that are at imminent threat of a tornado. We, of course in a matter of seconds, ensure that we make deliberate decisions on which areas need to be included and which don’t, and pay especially close attention to large points of interest like major cities and interstates. If we can safely keep the city of Pittsburgh out of a warning and not set off a lot of phones, we will, but with the specific case from yesterday, Pittsburgh was within the downstream track of the rotation signature on radar that warranted the tornado warning, and thus was included in the warning. Luckily, the rotation signature weakened before it reached Pittsburgh, and we were able to cancel the warning early. If you were impacted by that storm and received damage, please let us know, but at this time, we have not received any damage reports that warrant a survey.
