Wednesday, December 26, 2012

December 26, 2012: Anatomy of Winter Storm Euclid

This storm is receiving lots of attention from me...because winter storm systems rarely get as complicated as this, around here.   Today we have experienced a mix of sleet, freezing rain, snow, moderate to heavy rain...and tomorrow, strong winds...all from the same mid-latitude cyclone.

My earlier posts have illustrated the complex thermal structure of this storm, tied to our region's geography, and the influence of that thermal structure on evolving precipitation types through the day.

In this post, we step back and look at the whole enchilada of this classic, textbook winter storm.

First, let's look at the synoptic surface map from this afternoon.  Winter Storm Euclid is a complex, sprawling and intense storm consisting of two low pressure centers.  The primary low is located over Kentucky.  Through the day, a secondary or coastal low has begun to develop over the Carolinas.  Eventually, Euclid will transfer its energy, and its moisture, to "Son of Euclid" as the new center tracks up the East Coast.

Euclid's primary and secondary low pressure centers, and associated precipitation pattern.  Intellicast
The precipitation shield surrounding Euclid's counterclockwise whirl is immense, wrapping from the southern tip of Florida, northward along the East Coast, then back west into the Great Lakes.   Green colors show rain and embedded thunderstorms (Euclid's "warm side") and blue colors indicate ice and snow (the storm's "cold side").

Cyclones such as Euclid have strong temperature contrasts; they bring air masses together from different source regions.  The juxtaposition of air masses creates weather fronts within the storm system.  The weather map below, showing the thermal pattern at 5,000 feet, illustrates the classic "ying yang" pattern whereby warm, tropical-source air (yellow) wraps into the system from the south and east, and cold, Canadian-source air enters (blue-purple) from the north and west:

Winter Storm Euclid's temperature contrasts.  Unisys Corp.
These air mass contrasts are what power large cyclonic storms in the mid-latitudes.

The next graphic shows Euclid from the vantage of weather satellite, using the moisture channel.  Dry areas in the atmosphere appear black, while moist areas show up in varying chromatic shades - from blue to red, dark red indicating the greatest degree of saturation.

Adapted from WeatherTap
I have added a few annotations that reveal the complex 3D anatomy of this storm.  First, note the primary low center over Kentucky, and the secondary center beginning to form near Tidewater, VA.  Next, the massive precipitation shield is this storm's distinguishing feature.  Euclid has an impressive feed of moisture from the Gulf and Atlantic.  The moisture influx assumes the form of a narrow, high-speed conduit or "river" of air entering the system from the south, and is called the Moist Conveyor (solid green arrow).  All along this corridor of high water vapor, clouds condense.  Along the East Coast, severe thunderstorms have erupted in the warm, unstable air.  Over the Mid Atlantic, moderate snow-sleet-rain has fallen from the Conveyor for over 12 hours.  And back through the Ohio Valley, heavy snow has fallen along the Moist Conveyor where air temperature has remained below freezing.  The Moist Conveyor is the one element that connects all the disparate forms of heavy and severe weather in this single storm system.

But along the back edge of the storm, very dry air has descended from high levels over the upper Midwest.   Along the track of this descending current, called the Dry Conveyor, clouds and precipitation have evaporated.   Dry air is wrapping deep inside the inner core of Euclid.  This is the hallmark of a cyclone entering its most mature and intense phase.  Eventually, the dry air will sequester the core completely, and the primary low will dissipate.   Meanwhile, the new coastal low will deepen as it moves toward New England, effectively robbing the old core of its Moist Conveyor.

Finally, and most importantly, it is important to understand where these intense cyclonic storms come from, and what sustains them.  The answer is the polar jet stream, a river of intense air blowing from west to east between 30,000-40,000 feet.  Below is the jet stream chart from this morning, which I have annotated:

Jet Stream level winds and their relationship to Euclid.  Adapted from Unisys Corp.
You can see the core of high winds zipping along from California to the Gulf Coast.  However, there are numerous undulations along the way.  Most prominent of these is a deep trough of low pressure over the Great Lakes (labeled).  The airflow around a trough has counterclockwise curvature, and the trough contains a core of very cold air.   As the air streams along the base of the trough, it speeds up as it exits and moves over the Mid Atlantic (think of a trough as a tight curve on the Beltway - you have to slow down through the curvy part, but once you are free, you accelerate).   I have shown a hypothetical red box in the exit region of the trough.  Imagine air exits the north edge of the box, faster than it arrives from the south.  This creates a "divergence" of air flow within the box.  Because the same amount of air must always remain in the box, air must be drawn up from below, from the surface, to help fill the void.   When we remove air from the surface, we create low pressure there.   Air rising up to fill the imaginary box creates widespread cloud and precipitation.  Thus, areas of low pressure and storminess - such as Euclid - develop near the "exit" region of large troughs in the jet stream. 

There is something else helping to sustain low pressure at the surface.  Note that in addition to the flow speeding up through box, it also spreads outward, away, in a fan-shaped pattern.   This is a consequence of very intense troughs such as the one shown here, which take on a certain tilt from NW to SE.  (In meteorological parlance, the spreading apart of airflow at high levels is called "diffluence").   Nevertheless, as the air fans away, more mass is trying to leave the box from the north side, than can enter from the south side.   Once again, extra air must rise up from below to fill the box.

Finally,  note a very intense pocket of high wind in the core of the jet stream, at the base of the Great Lakes trough.  This is called a jet streak.  When jet streaks zip around the base of a trough, they enhance both the divergence and diffluence of air in the trough's exit region.   This helps surface low pressure regions deepen even further. 

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