<HTML><BODY style="word-wrap: break-word; -khtml-nbsp-mode: space; -khtml-line-break: after-white-space; ">Dear Patrick - WEASD in GFS represents water equivalent snow depth accumulated to date. The change in accumulated snow between two times is snowfall minus snowmelt, not simply snowfall. If you consider only positive values of the 6-hourly difference of WEASD, you will get a pretty good approximation of snowfall since it is unlikely (but not impossible) to have snowmelt occur in the same 6-hr period as snowfall. For example: <DIV>sum(maskout(weasd-weasd(t-1),weasd-weasd(t-1)),t=2,t=13)</DIV><DIV>should give you a good approximation of the total snowfall in the first 72 hours a forecast, whereas, </DIV><DIV>weasd(t=13)-weasd(t=1)</DIV><DIV>will not, because it may include periods of both snowfall and snowmelt. <DIV><BR class="khtml-block-placeholder"></DIV><DIV>The WEASD unit is the mass of all the snow accumulated in a square meter, which is the same mass if all the snow were melted to liquid (water equivalent). If WEASD = 1, that is a 1 kg/m^2 mass of water equivalent, which represents a depth of 1 mm (liquid). That is 1 cm of snow only if you assume a 10:1 ratio of snow volume to liquid water volume. This ratio varies from 4:1 to 100:1 in nature, but 10:1 works well on average. Hope that helps. -- Jim<DIV><BR><DIV> <SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><BR class="Apple-interchange-newline"></SPAN></SPAN> </DIV><BR><DIV><DIV>On 27Jan2007, at 3:28 PM, Patrick Reuter wrote:</DIV><BR class="Apple-interchange-newline"><BLOCKQUOTE type="cite"><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Hi Folks using Grads !</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><BR></DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Can someone tell me how to calculate the 6h accumulated snowfall frm the</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">GFS grib entry ? I saw the WEASD entry in kg/m*m, but I dont know how to</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">interpret this data. It seems to be some sort of accumulation, but from</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">when ? To get the snowfall for one 6h interval, do I have to take the</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">difference between two entries ?</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><BR></DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Example : GFS Data calculated at 00Z the 20070127. I want the snowfall</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">from 6h to 12h. Do I have to take the difference between the two values</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">(WEASD+12 - WEASD+6) ?</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><BR></DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Hope this is clear ... Any help is greatly appreciated!!</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><BR></DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">BTW : 1 kg/m*m is about 1cm snow ?</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><BR></DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><SPAN class="Apple-converted-space"> </SPAN>Patrick</DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><BR></DIV> </BLOCKQUOTE></DIV><BR></DIV></DIV></DIV></BODY></HTML>