Understanding Precipitation Forecasts on Model Graphics

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wxman57
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Understanding Precipitation Forecasts on Model Graphics

#1 Postby wxman57 » Wed Nov 17, 2010 12:25 pm

This is an issue that I've noticed time and time again here on the forum. Someone will take a look at the latest GFS run and point out what he thinks is an indication of freezing/frozen precipitation based on the location of the freezing line and the area of precip. A good example can be seen on last night's GFS run on the 240hr panel below:

Image

To the casual viewer, it looks like the GFS is forecasting precipitation across northern Georgia and Alabama while the temperature is below freezing (blue "0" line). But what is often not understood is that the precipitation depicted on the graphic is not falling at the same time as the valid time of the map (in this case, 240hrs). The precipitation on the map above is actually the projected precipitation that falls between the 228hr time frame and 240 hrs.

To see if there will still be any precip falling in northern AL/GA starting at 240 hrs, one needs to look at the NEXT map (252hrs) to see where the precip area is. Here's the 252 hr map:

Image

I've identified where the trailing edge of the precip would be starting at 240 hrs as a dashed black line on the 252 hr image above. Note that starting at 240 hrs, the edge of the precipitation is well to the south of the sub-freezing air. Actually, the model is indicating that the precip ends right behind the cold front.

What we're actually seeing forecast here is a line of pre-frontal showers/thunderstorms and not post-frontal freezing/frozen precipitation. Here's another version of that same 240 hr map but this time with the actual rear edge of the precipitation shield identified as the dashed line. That dashed line represents the northern edge of the projected precip area valid at the same time as the 240 hr map:

Image

Note that the back edge of the precip is located well to the south of the sub-freezing line. From this we can conclude that the GFS is NOT forecasting frozen precipitation across northern AL/GA.

OK, we know that the precipitation had long-since ended across northern AL/GA by 240 hrs according to the graphics above. But there WAS significant precipitation across that area between 228hrs and 240hrs. The final thing to check is the GFS panel prior to 240 hrs to verify that the freezing line was not south of northern AL/GA.

Image

From the image above, we can see that the freezing line at 228hrs was well north of northern AL/GA. We can now conclude that all of the precipitation projected to fall across northern AL/GA is associated with the frontal passage and no precipitation is forecast to fall once the temperature approaches freezing.

Keep this post in mind this coming winter when posting model graphics that may appear to indicate freezing/frozen precip across a region. It's always good to check the GFS (or any model) panels after a time period and also prior to a time period to verify the location of the projected precipitation at the time in question.
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#2 Postby Aquawind » Wed Nov 17, 2010 12:42 pm

Well done! 8-)
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Re: Understanding Precipitation Forecasts on Model Graphics

#3 Postby amawea » Wed Nov 17, 2010 9:34 pm

That is excellent stuff wxman57! Thank you for that explanation of what the models predict. So we are looking at a 12 hour difference in the precipitation and the model forecast of the freezing line?

No wonder I think the models suck. :?:

I want to make myself clear that I am not blaming the models as much as my ignorance of interpreting them. I think they are a good thing but most people believe that weather forecast were better at the two day forecast in 1965 than they are today. I agree with that.
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Re: Understanding Precipitation Forecasts on Model Graphics

#4 Postby wxman57 » Thu Nov 18, 2010 8:30 am

amawea wrote:That is excellent stuff wxman57! Thank you for that explanation of what the models predict. So we are looking at a 12 hour difference in the precipitation and the model forecast of the freezing line?

No wonder I think the models suck. :?:

I want to make myself clear that I am not blaming the models as much as my ignorance of interpreting them. I think they are a good thing but most people believe that weather forecasts were better at the two day forecast in 1965 than they are today. I agree with that.


No, not necessarily. Only the extended range GFS (beyond 192 hrs) indicates precip for the PREVIOUS 12 hours. The shorter-range GFS (0-192 hrs) indicates precip over the previous 6 hours. And if you look at the high-res GFS, the interval is cut to 3 hours prior to the map valid time.

Think about it, the graphic cannot show instantaneous precip accumulations (precip falling at the map valid time). It has to show cumulative precip that falls over a specified interval. And that interval is the time period leading up to the map valid time.
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Re: Understanding Precipitation Forecasts on Model Graphics

#5 Postby Stephanie » Fri Nov 19, 2010 7:30 pm

Great point! Thank you! :D
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