Thursday, January 12, 2017

What will roads be like Friday-Saturday central OK?

The next winter storm is upon us one week after the cold snow we experienced.  This time it's ice that's in the forecast and one big question is what the roads will be like.  After a nearly record warm day on Wednesday with temperatures near 80 deg F, the ground temperatures are at least 10 deg F warmer than right before the snow storm and with temperatures expected to drop to just a couple degrees below freezing, it'll be tough to cool the ground surface to below freezing.
But we're talking about freezing rain, right?  It makes all the difference in the world and this time the trend will be to keep ground temperatures warm.  As opposed to already frozen precipitation where upon landing and melting, extracts heat from the ground, rain deposits energy into the ground upon freezing.  If the ground, or the road surface, were to freeze, the energy will have to be extracted by another mechanism.  A continually fresh and deepening source of arctic air could accomplish this task.  However forecasts from all numerical guidance and the NWS paint a scenario where the near surface air barely remains below the melting point throughout Friday and into Saturday morning, early.  This is hardly the needed reservoir of cold required to cool the ground below the melting point in the face of all the latent heat to be added as the rain attempts to freeze.

Consider also that the rain will be falling from a layer nearly at 60 deg F a few thousand feet above ground and you are asking a lot of barely subfreezing air to cool the rain drops while also extracting heat from the ground and successfully fighting off the latent heat added by any attempts at freezing.

All this points to road surfaces remaining wet in central OK throughout the duration of the freezing rain event.

Now the exposure of elevated roads paint a different story.  The reservoir of heat will be eroded from multiple sides, allowing the surface to potentially reach a little below the melting point and allowing the potential for falling rain to freeze.  Bridges and overpasses could become slick if untreated.  But this event is well-forecast and hopefully the OKDOT attacks elevated surfaces before precipitation starts.  Since the NWS forecasts the potential for hazards to occur, they can't depend on knowing for sure what our efforts of mitigation may entail and thus pay heed to these graphics below.

The bottom line is that elevated surfaces may become slick, if untreated in central OK while colder air could be sufficient for all untreated roads in NW OK.

Ice will accumulate on all trees, power lines in central and NW OK.  However NW OK is most likely to bear the brunt of the heaviest rainfall.     Three quarter to one inch accumulation of ice will cause power outages there, and into Kansas and adjacent Missouri.  While this storm will not likely be equivalent to the catastrophic ice events of the past 16 years, it'll be bad enough.

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