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Discrepancies in 24/48/72 hour probabilities of QPF?

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TC
Timothy Cermak, modified 1 Year ago.

Discrepancies in 24/48/72 hour probabilities of QPF?

Youngling Posts: 1 Join Date: 8/27/14 Recent Posts

We recently had a rain event and noticed that the 24, 48, and 72-hr probabilities of the same QPF did not match, despite all three windows of time encompassing the same rain event. The event primarily fell within the 24-hour period from 12Z Friday, Nov 17 to 12Z Saturday, Nov 18. You can find images of the issues we experienced here.

 

The 24-hr probability of 1"+ (valid 12Z 11/17 - 12Z 11/18) was generally 40-60% over the northwestern half of our CWA.

The 48-hr probability (valid 12Z 11/17 - 12Z 11/19) drops to 20-40% in that same area.

The 72-hr probability (valid 12Z 11/17 - 12Z 11/20) drops further to 10-20%.

 

If all three windows of time encompass the same rain event, why are the probabilities for the same amount of rain different? If this is not a bug, can you please explain why it is correct?

 

Thanks!

probqpf24 probqpf48 probqpf72
DS
Dana Strom, modified 1 Year ago.

RE: Discrepancies in 24/48/72 hour probabilities of QPF?

Youngling Posts: 87 Join Date: 12/8/14 Recent Posts
Hi Timothy,

I see that Jeff Craven posted this to the NBM forums as well. I can confirm that WSUP is displaying the data as supplied to us. We'll coordinate with the NBM team to ensure things are running properly.

Thank you!
Dana

On Mon, Nov 20, 2023 at 10:15 AM Timothy Cermak <VLab.Notifications@noaa.gov> wrote:

We recently had a rain event and noticed that the 24, 48, and 72-hr probabilities of the same QPF did not match, despite all three windows of time encompassing the same rain event. The event primarily fell within the 24-hour period from 12Z Friday, Nov 17 to 12Z Saturday, Nov 18. You can find images of the issues we experienced here.

 

The 24-hr probability of 1"+ (valid 12Z 11/17 - 12Z 11/18) was generally 40-60% over the northwestern half of our CWA.

The 48-hr probability (valid 12Z 11/17 - 12Z 11/19) drops to 20-40% in that same area.

The 72-hr probability (valid 12Z 11/17 - 12Z 11/20) drops further to 10-20%.

 

If all three windows of time encompass the same rain event, why are the probabilities for the same amount of rain different? If this is not a bug, can you please explain why it is correct?

 

Thanks!


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Timothy Cermak Whole Story Uncertainty & Probabilities Viewer Virtual Lab Forum https://vlab.noaa.gov/web/wsup/forums/-/message_boards/view_message/33490356VLab.Notifications@noaa.gov


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Dana Strom
Visualization Task Lead
Digital Forecast Services Division
Meteorological Development Lab
NOAA/National Weather Service

301-427-9451