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RE: Editing PoT Grids to adjust my snow or ice Amount Grids

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Eleanor Dhuyvetter, modified 5 Months ago.

Editing PoT Grids to adjust my snow or ice Amount Grids

Youngling Posts: 3 Join Date: 7/28/18 Recent Posts

Hello ! 

A few of us at APX were talking through steps for editing PoT grids to change accumulations grids (such as snow or ice). Per the most recent training and documentation; to modify your snow or ice amount grids one will go back and edit the Pot, QPF, POP, T, Td, or winds grids. Below is a certain scenario we thought of for needing to make these edits (with our overall question being how to go about editing the Pot grids): 

 

For one hourly grid, at one pixel the values of POP, Pot R, and Pot S are 30 with T/Td near freezing. For how we understood things, the X amount of QPF would be split 50/50 between rain and snow. Say you are doing the forecast and want to increase the amount of snow.

To increase the amount of snow, would one simply increase the Pot S grid? It seems like you can't increase it too much, or else WxType R will be assigned to 15% if its too high and maybe you don't want to change the wording in the grids. The increase ice nuclei tool looks like it only changes FZ to SN, so that won't help us. So how would someone go about editing the Pot grid to increase the amount of QPF % that goes into snow. 

 

Overall, making these edits to an edit area of pixels over a set amount of hours initially seems to be a lot of work. Is there a simpler way to go about this? 

We appreciate all thoughts ! Thanks 

KL
Krystal Lynum, modified 5 Months ago.

RE: Editing PoT Grids to adjust my snow or ice Amount Grids

Youngling Posts: 1 Join Date: 8/17/23 Recent Posts

Hi Eleanor, 

 

The best way to edit how much QPF is split between snow/rain is to mess with the PotR and PotS grids, as you were doing. The way it works is that your PotR/PotS grids added together will be 100% of your QPF (in your case, 30 from PotR and 30 from PotS = 60, so your QPF gets split 50/50). I suggest lowering your PotR to 20 or 25 if you'd like the majority of the QPF to go to snow. For example, if PotR is 20 and PotS is 30 (total of 50), 40% of the QPF will go to rain and 60% will go to snow.  It is a little more difficult in this case since your PoP is so low, but if it matches what is expected, you take what you can get, I know haha.

 

Hopefully this helps and makes sense. Good luck!

Krystal

KH
Kevin Huyck, modified 5 Months ago.

RE: Editing PoT Grids to adjust my snow or ice Amount Grids

Youngling Posts: 5 Join Date: 7/6/14 Recent Posts
Hi, Eleanor.

To add to Krystal's comment, the T/Td plays a big role, too. "Near freezing" isn't quite descriptive. ForecastBuilder aggressively limits snow accumulation for T and/or Tw (wet bulb temp) between 32 and 36 degrees and doesn't permit snow above 36 degrees. In the "PType Step" there is an "Edit" tool called "T/Tw Tool" which will let the user adjust those thresholds for that iteration of the PType step. Adjusting those top and bottom thresholds can also help raise snow amounts.

ForecastBuilder is a very sophisticated tool and there is more than one way to get a preferred solution out of the tool. Some are less labor intensive than others.

Respectfully,

Kevin Huyck (hike, hear my name) - KB9WOB
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On Wed, Nov 13, 2024 at 1:49 PM Krystal Lynum <VLab.Notifications@noaa.gov> wrote:

Hi Eleanor, 

 

The best way to edit how much QPF is split between snow/rain is to mess with the PotR and PotS grids, as you were doing. The way it works is that your PotR/PotS grids added together will be 100% of your QPF (in your case, 30 from PotR and 30 from PotS = 60, so your QPF gets split 50/50). I suggest lowering your PotR to 20 or 25 if you'd like the majority of the QPF to go to snow. For example, if PotR is 20 and PotS is 30 (total of 50), 40% of the QPF will go to rain and 60% will go to snow.  It is a little more difficult in this case since your PoP is so low, but if it matches what is expected, you take what you can get, I know haha.

 

Hopefully this helps and makes sense. Good luck!

Krystal

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ED
Eleanor Dhuyvetter, modified 5 Months ago.

RE: Editing PoT Grids to adjust my snow or ice Amount Grids

Youngling Posts: 3 Join Date: 7/28/18 Recent Posts

Thank y'all for the great insights! Its very appreciated ! :D 

 

We are trying to build a flow chart for local use and have incorporated both of these. I was really excited to work with the T/Tw tool as it seems easier to get the grids I wanted, however larger scale and magnitude changes didn't seem ideal in this step due to not modifying the actual temp or Td grids - so it was easy to flag consistency when changing things more dramatically here. This was my observation in the few tests I did in practice mode. So, we have in our flow chart the T/Tw tool being suggested for slight changes/fixes down the road after one gets their base grids to what they want.