Just why is this snow storm so hard to forecast? Think weather is easy? Read this highly technical discussion below from Craig Allen: Alert! Take dramamine before reading this; itll make your head spin with all the twists and turns and ideas that have to go into this forecast. Based on morning (12z) and midday (18z) runs, one thing is certain- start time. Renegade flurries are possible early but the real stuff gets here 10pm to 2am from south to north...and it will start as snow (though Im concerned we havent dropped any lower with current s/sw winds). Right after that, it already gets complicated. Significant rise in thickness ahead and along the heavier precip, but the 850mb temp (5,000- critical level for snow) does not get above 0 until 10-14z Sat morning from southeast to northwest. In otherwords, the precip can still be snow up until that time. An exception may be along the NJ shore and many parts of L.I. where warmer air is intruding at 950-900. That could change the precip to pelting sleet. So the question now becomes, how heavy is the precip before the mix/change. Usually, it gets quite heavy for a time as tremendous lift/ vert vel pushes north ahead of the warming. However, the 540 thickness is already north of us. You usually need that in conjunction with the northbound 850 warming to get your heaviest thumps of snow before the mix and change. So with these two parameters so far apart, Im not sure how it will impact the snow rate. My first guess is it would not be as heavy as it could be but snowfall rates of 1 per hour are not out of the question prior to sun up. This is how I came up with 1-3 for the coast and 3-6 north & west of I-95 from NJ Tpk -GWB-NET-CT Tpk. Then, the coast, followed by the City eventually change to sleet to rain...and a wind-driven rain at that. All parameters aloft support liquid and fortunately all maps indicate sfcT to be aoa 32. However, west and north of that approximate I-95 line, the 32° isotherm snakes its way thru interior NJ to the Bronx to BDR. So it could mean freezing rain where it stays near 30. But a competing factor is this is not the same set up as a week ago when we had a skating rink. The ground had been at 18-22° when the rain started and instantly froze. These last few days have been above freezing by day. Sanding and salting and pre emptive brine are already being put on roads so hopefully that negates a situation like last Sunday. So now we have this wintry beginning, an awful day with wind and all kinds of precip. Are we done yet? Not a chance. Most models have indicated the back end of this storm could thump some areas with a change back to snow. This feature was lost last night by a few of the models but its pretty obvious again today. Most models show this occurring between 4-7pm. It could be a quick hitting coating to an inch or two, (the GEM actually is most robust with 2-4) the kind that plasters everything since its heavy at wet and wind blown as the surface temperature falls back below 32° All precip is over, off eastern L.I. between 7-9pm. The low deepens dramatically, nearly 20 mb in under 12 hours depending on what map you want to believe but all show it rocketing away so this is not going to be any more prolonged than it has to be. We are now well within range of the usually accurate HRRR and RAP short range models. Both are quite similar in start time, warm up with thickness rises and 850 (5-8am on average), qpf and storm track which is well within the 70/40 benchmark. One note about all the colorful snowfall accumulation maps. They are cumulative, so if you see your area shaded in colors that denote 6, 8, 10 or more, it may be a combination from both ends of the storm and does not necessarily represent what will be on the ground. Ex- lets say you get 4 in the morning and 3 in the wrap around. That first 4 may be washed away by rain in between, or hammered down by ice pellets....but in the end, theres about 3 on the ground when the storm departs once and for all. Some use a better algorithm. 2 maps posted show the same output for the euro snowfall accumulations. One just an assumed 10:1 ratio, the other takes into account real factors such as bondary and sfc temps and eliminates sleet from the equation. Amazing, huh? For now, my fingers are tired and my brain hurts. --Craig Allen / Craig Allen On-Air Inc
Posted on: Sat, 24 Jan 2015 01:20:17 +0000
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