Technical Post: Epic battle-to-the-death of snow amount forecast between guidance, just hours before next round of precipitation
Just hours before the next round of precipitation comes through New England, and there is an epic, awesome battle to the death of guidance, and it's all about resolution. 06Z NAM (12 kilometer resolution) continues to put out heavy snow, with over 10" of accumulation in parts of Northern Plymouth County MA this morning!
12 km resolution NMM forecast, run at 1 AM:
Increase the resolution of the guidance, and you get a very different scenario, however - here's the same NAM model, run on a 4 kilometer resolution, instead of 12 kilometers - now we're down to 7-8" in highest amounts, but all of Eastern MA gets plastered:
Change the equations a bit to enhance performance in "mesoscale" banding - the setup of narrow, intense bands of precipitation like we're seeing developing over Cape Cod and the waters south of New England right now on radar, and you get a solution like the NMM, below, run on a 5 kilometer resolution at 7 PM last evening - no, your eyes do not deceive you, that is a forecast for no snow at all in Eastern MA:
Upgrades to the European (ECMWF) model have increased its resolution, even though it is a globally run model, and parameterization (calculations) have attuned it well to the mesoscale, and the result is similar from the 7 PM run - an area of light accumulations of a fraction of an inch in Southeastern MA, but nothing epic:
Finally, the near-term, high-resolution HRRR also performs well with mesoscale features and focuses only on the next 15 hours, and also shows relatively low snowfall totals, amounting to only .5" to 1" for most, but perhaps a very localized higher amount to two inches - this guidance is run every hour, and you're looking at the 3 AM run:
Incredible, fascinating stuff - I have seen this kind of battle 24 hours out but can't recall one to pronounced, so epic, so close in.
Today, I think less is more, not only because banding processes have begun and that implies mesoscale will reign supreme in this particular instance, but also because, in most areas, the temperature has been increasing over the last 4-5 hours. I'm actually a bit surprised by that, but the average temperature in Central and Southern New England is up 3-4 degrees. Heavy precipition, of course, can cool the column but warming is a strange trend, in this case perhaps related to the mesoscale, as well, as subsidence establishes west of the deformation band of heavy precipitation that is preparing to set up. Of course, it is windier inland now, too, so some of it may be a mixing of a warmer layer that was just above the undercutting cold.
As for me, I'd say there are two things to remember - 1) These snowfall total forecasts are often the least reliable from guidance, and should never be the basis of a snow forecast, but they do make for really interesting moments like these, and 2) I've learned that each event is different, and have seen some epic failures of the higher resolution, mesoscale modeling, but for today I'd put my money on those guidance products and bet on lower-end snow totals for many, but enough that many folks do at least get to see some flakes Sunday. If I'm wrong, what's a 10" blown forecast among friends? Incredible!
Regardless, we will know by midday, as the forecast burst of snow...or no snow...or very little snow, if it goes as I expect, should be happening from 7 AM through late morning. One take-away point here is that there can still, in 2014, when we like to think our app can do it all, be 10" differences 3 hours before go-time. Meteorology never has been, and never will be, an automated and application-friendly science...at least not in New England.
Going to be an interesting winter if this keeps up.