Thursday, January 5, 2017

Fast moving snow system to hit central OK tonight

Our first accumulating snow event of the season is looking more likely tonight.  I've been monitoring the forecasts over the past several days and only by two days ago did the forecasts have enough confidence that snow would indeed happen.  The impacts will be quite clear.  Confidence is high that a period of accumulating snow will occur between midnight and 6 am, with the heaviest rates occurring within a couple hours of 3 am.  The timing and accumulations are pretty well represented by the 09 UTC short range ensemble forecast (SREF) of snow depth, to which the NWS forecast office agrees.  However, the SREF forecasts a large uncertainty in snow fall, anywhere from 0.5 to 6"!  If you're the cautious type, you may want to consider planning for a 4+ inch event with much of it coming down in a hurry just before the morning rush hour begins tomorrow.

But you may want to ask how much of that snow will stick to the roads.  Considering the possibility that no road crews go out tonight to pre-treat the roads with salt, an apt possibility for neighborhoods and minor roads, then all of the falling snow will remain frozen after landing.  We've had nearly 36 continuous hours of subfreezing air, broken only by a few hours of temperatures a little above freezing yesterday afternoon.  The Oklahoma mesonet soil temperatures 2-5 cm below ground in east OKC responded to yesterday's warm temperatures by rising to 40 deg F but since have fallen below 35 deg F this morning.  Other central OK mesonet soil temperature sites have responded similarly.  With plenty of arctic air feeding in from the north, all forecast 2 meter temperatures remain well below freezing, with a falling trend, through tonight and into tomorrow morning.  The ground temperatures should easily fall below freezing before the first snow falls.  Certainly all elevated roads will be even colder.

The impact means all untreated roads will be snow covered by 5 am tomorrow.  These include neighborhood roads and minor streets, or in other words, where most people will attempt to depart.  On the other hand, I would expect main snow routes, highways and bridges to be treated in advance of the snow.  Once they're treated the chemicals should easily melt a 2" snow cover within a couple hours of the snow ending which should be in time for the majority of a morning commute as a majority of forecast guidance anticipates snow tapering off by 6 am.  I would expect an hour or two delay should be sufficient to allow any remaining snow on treated routes to melt.

Now to be cautious, there are a few possibilities that may cause further delays for you.  The temperatures tomorrow will remain well below freezing and so all unplowed snow in shaded roads will remain and there's a good chance enough clouds will remain that even normally sunny road surfaces will stay below freezing.  Another issue could be if the DOT is delayed treating roads until the morning.  You'll know quickly about that if you come upon a normally treated road and it's white.  Or there is a third possibility of snow covered roads, even in treated areas, if the snow fall exceeds forecasts or if the heaviest falling snow gets delayed into the morning commute.

What is the possibility of getting more snow or the heaviest snow occurring after 6 am?  I expect the possibility of both to be there.  To understand why, consider the plots below.  Our event will come from the intersection of an upper level storm system, marked by the 'L' in northern California this morning, and the elevated frontal boundary marking the top of the arctic air dome shown by the strong temperature gradient at the 700 mb level seen below.  This type of upper-level storm provides forcing for rising motion well above the ground usually in the shape of a wave  and the front provides the assist a little lower in the form of bands parallel to the temperature contours.  The relative humidity pattern at the level of the frontal boundary aloft, also at 700 mb, indicates the air is already saturated and likely lifting along bands.   When the upper-level system begins to arrive late tonight one or more bands along the front may intensify and drop heavy snow, heavier than forecast.  The higher resolution models show some indications of bands, however the southern one doesn't last long appears to be more a reflection of the upper-level system.  Yet a stationary band across the metro area is in a realm of possibilties and it could last into the morning commute resulting in plowable snow accumulations and more snow covered main routes.

One final interesting thing to consider with this snow event is whether or not we'll get the beautiful six-sided dendrite crystals.  Consider that to get this kind of snow, we want to see strong rising motion occurring in the -12 to -18 deg C temperature layer aloft.  Consider a forecast sounding below from the morning NAM model.  The cold air at low levels is forecast to become cold enough to generate dendrites but there's not much in the way of rising motion.  However thanks to warm air above the arctic air layer, we extend the optimal temperatures for dendrites into the layer where strong rising motion is forecast to occur.  So with the cold surface temperatures, we should wake up to lots of dendrites.  That's in theory.  In reality, lots of factors may interrupt the processes to create spectacular dendrites but at least the possibility is there.

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