Winter Weather with warm T Grids?

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Chris Rohrbach, modified 4 Years ago.

Winter Weather with warm T Grids?

Youngling Posts: 4 Join Date: 5/1/15 Recent Posts

Running Forecast Builder a few days ago gave me a result I wasn't expecting and I was wondering if someone could shed some light on the issue.

A strong front is projected to come through our CWA on Christmas Eve with cold air following it. I thought the NBM was too quick to bring in the cooler air and opted to use different model output when creating the T grids. We kept temps well above freezing during the time we have PoPs in the grids, then bring in the cold air and remove PoPs. I would have thought that keeping the temperatures high would prevent forecast builder from putting in winter weather, but it did not. From the attached screenshots you can see our T grid at 03Z on 12/25 has values ranging from 46 to 54°. After running all Forecast Builder steps the potSnow values were 26% at 03Z and consequently a slight chance of snow was added to the Wx Grid. Is there nothing checking the T grids during the generation of the potSnow or Wx grids? Any advice you could give would be helpful.

Andy Just, modified 4 Years ago.

RE: Winter Weather with warm T Grids?

Padawan Posts: 99 Join Date: 6/2/15 Recent Posts

Chris,

A few thoughts:

1. What precipitation type is employed? If the precipitation type technique is set to NBM, then there is no adjustment to the precipitation types based on surface temperature.

2. Out of curiosity what are the surface dewpoints, as wet-bulb does play some role (especially if using SnowLevel as the precipitation type approach).

Thanks!

Chris Rohrbach, modified 4 Years ago.

RE: Winter Weather with warm T Grids?

Youngling Posts: 4 Join Date: 5/1/15 Recent Posts

Thanks for reaching out Andy, we appreciate the quick response!

For the gridpoint at our office (KCAE) we had a fcst T of 53° so in order to get a Tw near freezing we'd need a dew point of something like -15°F. I can check but I'm pretty certain our forecast dew points were nowhere near that.

That makes me think it was the 1st point you mentioned. Looking under the baselineVarDict method in ForecastBuilderUtility.py it does look like we have varDict["Default PType Method"] = "NBM". Is this what you mean by which precip type method is employed? So if we have this set to NBM it won't check to make sure our surface temps would reasonably accommodate winter weather?

Thanks for the help and happy holidays,

Chris

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