Sunday, January 22, 2017

Rare and dangerous high risk of tornadoes in GA and FL

I haven't seen tornado outbreak environments like this in some years.  The latest Storm Prediction Center (SPC) outlook still has a high risk for severe storms including long-track significant tornadoes for portions of south Georgia into north Florida.  The last time that a high risk was issued by the SPC was almost three years ago according to Skip Talbot's Facebook post, and possibly no high risks have been forecast into the Florida peninsula.  Now storms are starting to form along and ahead of a cold front in the western FL panhandle and north along the GA, AL border.  Newer storms are firing up along the cold front south into the Gulf.  These should be of interest to anyone concerned about their safety which should include especially the high risk zone.

Later, more isolated storms will fire to the south and threaten the Florida peninsula.  While they may be more isolated, the environment will also support the potential for strong tornadoes.  The risk may not be high for Tampa, Orlando and Melbourne, but if you're unlucky enough to be in the path of a potentially tornadic storm, assume it'll produce significant tornadoes putting you at risk.

Areas north of the high risk may not see an obvious environment supportive of tornadoes because of the widespread rain in southern Georgia.  However this system is unusually far to the south, and our collective experience, limited.  Thus I suspect that even western to central Georgia may see a tornadic threat as the surface low deepens dramatically to something rarely seen in central GA - up to five standard deviations below normal for this time of year.  Outside of hurricanes, the sea level pressures will be very low down into FL as well.  As a result low-level winds will be strong and that means that if you're experiencing a cloudy, cool rainy atmosphere now, that may change quickly to one favorable for severe weather very quickly.   Residents in the Huntsville, AL area on the super tornado outbreak day of 2011 can relate to that.  Temperatures were in the 50's all afternoon and then in the last hour, jumped to near 70 deg F quickly followed by a mile-wide long-tracked tornado.

Furthermore, the probability that any one supercell will produce a significant tornado currently stands in the 15% range according to research by Smith and Thompson and Marsh of SPC in the last few years.  Get used to those numbers being extremely high.  As cases are gathered and return intervals calculated, you may see them as rather unusually high.  More importantly is that these numbers will go up from here as the day progresses.  The key thing to consider is that area hodographs feature large storm-relative helicity, very humid atmosphere (in an absolute and relative sense) and lots of buoyancy for thunderstorms to grow uninhibited, as seen from this sounding from the HRRR in the FL Panhandle ahead of the storms.

Bottom line, if your sheltering location is dusty, or cluttered, clear it now!

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.

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.