Accumulation addition

[winter_rules]

In another thread recently I saw a reference that successive forecasted accumulations can't be added together. Can someone explain the reasoning to me? I understand that it's not a smart thing to do because of the obvious margin of error issues, but how else is the public supposed to look at it?

[Burghblizz]

I think a lot of times it has to do with timing or the fact the forecast is for a broad area..

For example, if you have back to back 3" to 6" forecasts.....but only a 6-10" total....the second 3 to 6" is contingent upon what happens in the first period . If you get 5", you may have gone through most of the total precip and only 2" or 3" more is on the way in the second period. You see this with NWS CWA'S because the eastern end of a county may have different timing than the west. But ultimately they have the same forecast.

[Analog96]

I think a lot of times it has to do with timing or the fact the forecast is for a broad area..

For example, if you have back to back 3" to 6" forecasts.....but only a 6-10" total....the second 3 to 6" is contingent upon what happens in the first period . If you get 5", you may have gone through most of the total precip and only 2" or 3" more is on the way in the second period. You see this with NWS CWA'S because the eastern end of a county may have different timing than the west. But ultimately they have the same forecast.

It shouldn't be anymore when you have point and click forecasts that are based on exact points from NDFD, and not text county zone forecasts.

[Tornado Girl]

The main reason for following the guideline is to ensure everyone is measuring the snow in the same way. It will ensure that one's snowfall measurements are useful not only for themselves, but also for the NWS and other weather forecasters who may use them as well. No one benefits from artificially inflated totals just to be the person with the most snow when they report. While seeing one's reports in the news and elsewhere is a thrill, its worthless if it is not accurate.

The easiest way to measure snow is with a snowboard. Essentially this is a piece of wood about 16" by 16" that is painted white. You can measure snowfall every hour, but make sure that the board is cleared only ONCE every 6 hours. Therefore, it's easier to measure once every 6 hours and clear the board after your measurement. You should measure to the nearest tenth of an inch. If there has been a lot of drifting snow, you may want to take several measurements across your snowboard and average them together.

Overall snow depth is also important, Snow depth is important, especially after major snowstorms. Lingering snow pack causes concern when it comes to black ice, flooding, and even temperature fluctuations. Snow depth is usually measured once a day and to the nearest inch. Once again, if there has been drifting, you may want to take several measurements across your board and average them together.

[Burghblizz]

The main reason for following the guideline is to ensure everyone is measuring the snow in the same way. It will ensure that one's snowfall measurements are useful not only for themselves, but also for the NWS and other weather forecasters who may use them as well. No one benefits from artificially inflated totals just to be the person with the most snow when they report. While seeing one's reports in the news and elsewhere is a thrill, its worthless if it is not accurate.

The easiest way to measure snow is with a snowboard. Essentially this is a piece of wood about 16" by 16" that is painted white. You can measure snowfall every hour, but make sure that the board is cleared only ONCE every 6 hours. Therefore, it's easier to measure once every 6 hours and clear the board after your measurement. You should measure to the nearest tenth of an inch. If there has been a lot of drifting snow, you may want to take several measurements across your snowboard and average them together.

Overall snow depth is also important, Snow depth is important, especially after major snowstorms. Lingering snow pack causes concern when it comes to black ice, flooding, and even temperature fluctuations. Snow depth is usually measured once a day and to the nearest inch. Once again, if there has been drifting, you may want to take several measurements across your board and average them together.

Good info...wrong subject

[Analog96]

Good info...wrong subject

Lol.

[Tornado Girl]

Good info...wrong subject

Sorry, I misread the question

[Burghblizz]

It shouldn't be anymore when you have point and click forecasts that are based on exact points from NDFD, and not text county zone forecasts.

Many times the point and click seems to exceed the statement

Here is a time sensitive example....

http://forecast.weather.gov/MapClick.php?map.x=193&map.y=143&site=mpx&zmx=1&zmy=1

The statement caps at a 16" max, whereas the point and click adds up to 11" to 19" at the moment.. But I think it comes back to what I was thinking before about timing, and if you get more in the first period, you can probably expect less in the second. Most weenies will come back and say they are "calling for 19", which causes others to step in and say that adding them up may not be the greatest idea.

[thundersnow68]

Ok, here's the deal with the "point-n-click" forecast. As many of you know, the NWS forecast is done these days within a graphical editor and then the forecast data is pushed to several servers for anyone to do anything with. The NWS USES it's own data to make the forecast web pages, and other text/graphic products, etc.. When you click on a point on the map, or type in a zip code, you get a forecast "on-the-fly" for that grid point. What you see is all done by web scripts. The data underneath is NWS forecast, but on the surface for web presentation is code interpretation of the data. So, built into this code is a routine that takes the value of the snowfall forecast within the grid and adds or subtracts 1-2" from the forecast value to get a range. I believe any amount 5 inches and above gets +/- 2", so in the higher accumulation totals, you may see a period of 6-10, followed by 3-5, or whatever. I can assure you that it is not the intention of the forecaster to be forecasting 15" as a max in this case. The top end forecast would be a foot in this case since the grid point would have 8" one period and 4" the next. THIS is why one needs to be cautious to not add up the top end of the snow forecast range off the point and click, as it is a fabrication of the computer code. It would be better, IMO, to code the snowfall accumulation to say "Snowfall accumulations around (the value extracted from the grid point)".