UPDATE: Blizzard of 2011: the numbers and how they came about
Thu, 03 Feb 2011 23:53:38 GMT —
UPDATED: February 7 at 6:30 p.m.
Snowfall measuring requires patience, diligence, and attention to detail. Even with these conditions satisfied, when you have an epic storm like we had last week, you will undoubtedly find varying snowfall figures with various observers. I had a conversation with one of my former meteorology professors at Western Illinois University earlier this evening. Dr. Tom Williams and I discussed the methods of measuring snowfall and how challenging some of these methods become when you have blizzard conditions with 50-mile-per hour winds blowing for hours on end.
Snowfall measurement provides a best-estimate for the amount of snow that falls. This number, particularly on heavy snowfall situations, is different from how much snow accumulation is actually on the ground at the end of a big storm. Snow drifts and compacts. Snow is supposed to be measured in an undisturbed spot, but such locations were very difficult to come by last week. Even in a decent measuring area, with high winds, some snow still blows into, but also out of, the area being used to measure. This is why several measurements should be taken and then averaged.
Periodic measurements allow you to see a good approximation of the amount of snow that fell " before high winds have a chance to work on the snow for a longer period of time. A 2-inch snow will fall and likely be observed as a 2-inch snow, but a 20-inch snow that falls may not be recorded as a 20-inch snow. With that much snow, the snow compacts at a fairly decent rate, so for major winter storms, I like to report both the amount it snowed and the apparent amount of snow that is on the ground after the storm has finished.
Original Story: Thursday, February 3 at 6 p.m.
My official snowfall total for the Blizzard of 2011 from Tuesday, February 1, 2011 through Wednesday, February 2, 2011 is 28.8 inches. My current, compacted total of snow on the ground is a range from 16 to 22 inches for this storm.
Here is why there are differences in snowfall numbers that are reported... The challenges for measuring this snowfall included blowing and drifting snow and the compaction of snow as it accumulated both from the sheer weight of the accumulated snow and from drifting compaction. Water (snow) is very heavy! An average shovel full alone weighs between 30 and 60 pounds! Snow, especially for a massive storm like this, needs to be measured at least each hour on a solid, flat surface that TMs relatively shrouded from (at least) north winds in order to come up with an accurate amount when all is said and done.
When a big winter storm begins, you take several measurements once each hour, calculate an average of those measurements, and then record that average as an hourly total. You clean all the snow from the surface and start the process over each hour until the snow ends. When the storm does end, you can add all your hourly amounts to come up with an accurate total of how much it actually snowed. This number will be different, perhaps drastically different from the measured amount you would get if you tried to measure the snow only once at the end of the storm. Meteorologists in Lake-Effect-snow-favored regions will often report a set of snowfall numbers. They may say that 65 inches of snow has compacted down to about 37 to 42 inches. The 65-inch figure is the one that officially, meteorologically (should) get(s) recorded in the record books. The other number is actually misleading as to how much snow actually fell. The longer snow sits on the ground " the more these two numbers will vary. Even a few hours can make a huge difference on a significant snowfall like the one we just received " especially when winds of 50 miles per hour initially were working hard to compact the snow! Imagine what the first couple inches worth of snow we received would look like right now " crushed like an accordion! The tiny ice crystals of snow are pressed much closer together.Measuring snowfall, checking the computer models, working the radar, and making some calculations made for a very busy week in the weather lab! Thanks so much for joining KHQA on this historical journey. We TMll all have many memories to share later!