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Posted at 02:22 PM ET, 03/05/2013

Why Wednesday may feature intense snowfall rates and thundersnow


We’re forecasting some fairly hefty snowfall amounts Wednesday, especially in our western suburbs where 6-12 inches are possible.

It’s not going to be particularly cold, with temperatures near or slightly above freezing. But our “Snowquester” storm snowfall forecast is based - in part - on the assumption that very heavy snow will fall at times, overcoming the lack of bitter cold air.

Why do we expect this storm will produce intense precipitation rates and possibly even generate thunder and lightning? History and theory provide the clues.

This storm has a lot in common with the Commutageddon storm of January 26, 2011 which produced ripping thundersnow.

Like that crippling storm, this storm has a very strong and strengthening closed upper level low pressure center passing just to our south. In short, the storm looks like it may have the ingredients needed to produce instability and an impressive banded-type precipitation structure that often leads to snowfall rates of 1 to 2 inches per hour.

Researchers Nicosia and Grumm studied several Northeast snowstorms and developed a conceptual model (below) for when intense banded precipitation might occur. Let’s compare the ingredients present in their conceptual model to the NAM model forecast from last night for Snowquester.


(Nicosia and Grumm, 1999.)

The model highlights two areas where heavy banded precipitation and/or thundersnow could occur.

Note there is a zone of reduced stability shaded in pink (above) that sometimes results in instability and convection (fancy word for thunderstorms). This happens just north of the surface low pressure system(see the red “L” above) where a tongue of dry air (green arrow) on the storm’s back side overrides easterly winds associated with the cold conveyor belt (blue arrow) feeding the storm.

There also is an area where mid-level frontogenesis is taking place (the green shading). Frontogenesis is just a fancy way of saying that the temperature lines on the map are packing closer together with time and that the storm’s front is getting stronger. Where frontogenesis is happening, you tend to develop bands of precipitation that lead to enhanced precipitation rates.

Let’s now look to see whether the Snowquester event might have either of these features conducive for thundersnow.

As for the presence of instability, the NAM model valid at 1 p.m. Wednesday for Reagan National Airport shows just such an unstable layer.


On the right hand side of the forecast sounding above you can see the winds and the strong 50 knot (55-60 mph) easterly winds associated with the cold conveyor belt. Just above the cold conveyor belt, note how the green and red lines bend bank to the left as you go to higher elevations and how that slope is slanted back towards the curved dashed line on the sounding, indicating a dry layer. The unstable layer is evident just below where the dry air is found. The sounding essentially is what you’d expect in the pink area on our original figure. It suggests the potential for thundersnow or at least heavy banded precipitation.

The second way to possibly get thundersnow is through the frontogenesis process described earlier. Let’s look at what is happening to the temperature field at around 5000 feet (850 mb) around 1 a.m. and 7 a.m. Wednesday morning in Snowquester simulations (see below).


Left: temperatures and winds at 5,000 feet (850 mb) at 1 a.m. and 7 a.m. (right) Wednesday morning

In the figure above, note how the temperature lines are squeezing together across our area even as we have strong easterly winds at that same altitude. Strong frontogenesis is taking place favoring the development of an intense band or bands of precipitation.


Finally, let’s look at the 700 mb around (10,000 ft) forecast of relative humidity and vertical motion (to the right). Note how close the dry slot (white area under the green shades) gets to D.C. while still staying just to our south. And see how close the dry slot gets to the easterly flow on the 7 a.m. 850 mb forecast (above). The NAM is predicting enhanced upward motion and the development of an intense precipitation band. Its placement is very consistent with where Nicosia and Grumm observed intense precipitation bands from this second possible mechanism for heavy snow rates and/or thundersnow.

In summary, this storm has potential to be among the rare cases where the strong atmospheric dynamics lead to ripping snow and possible thunder.

By  |  02:22 PM ET, 03/05/2013

 
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