Thursday, July 27, 2006

Help - someone explain NWS POPs to me

Could someone please explain what exactly the POPs in this morning's NWS forecast mean?

Driving in to the university, in steady light to moderate rain, I heard the following forecast on NPR for the Tucson area.

Today: Mostly cloudy. Numerous showers and thunderstorms early in the morning...then scattered showers and thunderstorms. Highs 87 to 92. West wind 5 to 15 mph. Chance of precipitation 60 percent.

This is still the forecast at 9:30 am for the Tucson metro area, even though it has been raining across the metro area since 1200 UTC. The NPR forecast was followed by news about the roads that are currently closed due to flooding washes. The observations at TUS (verification point for the forecast) since 1200 UTC have been:

Day Time VSBY WX 1hr precip

27 Jul 8:55 am 10.00 -RA 0.02

27 Jul 7:55 am 10.00 -RA 0.04

27 Jul 7:30 am 3.00 RA

27 Jul 6:55 am 10.00 -RA T

27 Jul 5:55 am 10.00 -RA 0.01

27 Jul 5:10 am 10.00 -RA

27 Jul 4:55 am 10.00 -RA 0.07

Perhaps the POPs mean that when the current rain stops, then there will be a 60% chance of more rain later today?

Or, perhaps I just don't understand how the NWS defines and uses POPs anymore.

1 comment:

  1. Anonymous7:17 AM

    Perhaps the pops are an artifact in the forecast as the NWS does more important things than update the 60 % to 100%. Any idiot can see its raining.

    Effort spent updating the forecast, possibly taking away resources from some other, more important task in the NWS office would be foolish.

    Furthermore, even if the NWS did update the forecast, many media outlets would continue to read the old forecast. They don't check for updates everytime it is read.

    You are worrying about things that simply don't matter when there is actual life threatening flooding occurring. I fail to see how your commentary on this matter is at all productive?