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Precip Type

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RM
Ronald Miller, modified 7 Years ago.

Precip Type

Youngling Posts: 6 Join Date: 9/24/12 Recent Posts

I'm still learning ForecastBuilder, so forgive me if I'm covering old ground.  I'm using the TopDown method with no ProbIcePresent.  At a given point, I have the following numbers:

T: 33F

Td: 30F

MaxTwAloft: 0.6C

I ran it with the default temperature range (34 - 37) for precip type on Step 4, and I get the following Pot grids:

PotRain: 100

PotSnow: 94

PotFreezingRain: 26

PotSleet: 4

Wx: Def RmSm SChc ZRm

 

I confused.  If I look at the graph of Precip Type vs MaxTwAloft, it clearly shows a 0% chance of Rain until the MaxTwAloft reaches +2.3C.  So why is there a non-zero PotRain and PotFreezingRain?  If I'm reading the graph right, for a MaxTwAloft of +0.6C, I should have a near 100% PotSnow (which I have) and a low but non-zero PotSleet (which I also have). 

What am I missing?

 

Ron 

DB
David Barjenbruch, modified 7 Years ago.

RE: Precip Type

Youngling Posts: 16 Join Date: 9/2/14 Recent Posts

Hi Ron,

Thanks for your question.  I'm not too familiar with this one and the output does seem strange that there's no rain, but PoTFreezingRain for this combination of parameters.  Maybe Andy could shed more light on this situation.

Dave

RM
Ronald Miller, modified 7 Years ago.

RE: Precip Type

Youngling Posts: 6 Join Date: 9/24/12 Recent Posts

I have a guess as to what happened, now that I've learned a bit more about FB.  But this is just a guess.

Since my surface Temperature was above freezing, the purple PotSleet graph on the PoWT image essentially becomes the PotRain graph (PotSleet is only valid if Surface T <= 32F).  Not sure why I got a PotRain of 100 instead of 4 (which was my PotSleet value).  But then, since my surface Temp was 33F, it was still cold enough for FB to put in a chance of Freezing Rain.  Still seems strange to me.

 

Ron

SK
Stephen Keighton, modified 7 Years ago.

RE: Precip Type

Youngling Posts: 2 Join Date: 9/24/12 Recent Posts

I wanted to add to this thread that we are having a lot of problems with high ProbFreezingRain values when MaxTwAloft and ProbIcePresent support snow or sleet. If we know it's going to be a snow vs. rain scenario, we will usually have to edit the MaxTwAloft from values that are sometimes just a little above freezing (+0.5 - 1.0) and just take it down below zero. It gets harder when we have a mix of snow/sleet/freezing rain since we if we do that we'll just get snow and we'd like to be able to show the transitions.  We also find that ProbIcePresent values are often lower than we would expect looking at forecast soundings and so sometimes we just bump those up to 100% unless we are confident we will have a shallow layer of moisture well below the ice crystal growth zone.

I really think these algorithms need to be adjusted for the next version.  Thanks.

Steve @RNK