Tuesday, February 21, 2012

February 19: A “Nostorm” for Baltimore

That’s right, we had a nostorm, not a snowstorm.   For days, the computer models went back and forth on this one.  Early on, it looked like DC and Baltimore would be ground zero for a significant Mid Atlantic snowfall.  But then the models began tracking the storm further to our south, and moving it quickly out to sea.   And the bulls eye for heavy snow turned out to be southwestern and central Virginia.
Let’s look at the factors that contributed to a “near miss” for Baltimore.
Not An Ideal “Snowmaker” Setup
To get heavy snow in Baltimore, we need three key ingredients:  (1) A rapidly intensifying coastal cyclone (nor’easter) moving slowly up the coast (tracking from south to north); (2) a deep layer of cold air, and (3) abundant oceanic moisture.
As the surface synoptic chart below shows ) (7 pm, February 19), a cyclone did develop along the Gulf of Mexico and track across the southeastern U.S.  However, the jet stream – which has been stuck in a zonal or “west to east” configuration most of this winter - never developed a deep loop or “trough” along the east coast.   These troughs tend to draw coastal cyclones northward along the coast.   The February 19 storm instead tracked nearly due east, emerging into the Atlantic south of the Outer Banks.   It moved quickly, meaning fewer hours for heavy snow to accumulate, nor did not undergo the rapid deepening that characterizes truly heavy snowfalls.

One of the key reasons why this low failed to rapidly deepen hinged on the configuration of the jet stream (see chart below).   This chart shows the winds at the 30,000 foot level.   The jet had split into two separate streams:  A weak northern stream draped across New England, and a vigorous southern branch with a core of 150 mph winds across Georgia.   This “split flow” pattern is often a precursor of big East Coast snowstorms.   However, the trough in the northern branch failed to link up or “phase” with the trough in the southern branch.   Had this occurred, a much deeper trough would have set up over the East Coast.  This would have promoted a rapidly intensifying storm that moved north along to the coast (the classic “snowmaker” type of nor’easter for the Mid Atlantic).

Cold Enough For Snow
The second main ingredient, a fairly deep layer of subfreezing air from surface through at least 5,000 feet, was present in this case.   In fact, one of the key elements is called cold air damming, in which a cold, dense air mass gets wedged up against the eastern slopes of the Blue Ridge.  The figure below shows the characteristic surface pressure ridge east of the mountains when damming is in place.   Note how the isobars (black solid lines) sag to the south east of the mountains, indicating dense, cold air flowing southward.  Superimposed on the isobars is the distribution of precipitation as revealed by regional radar.  While the precipitation fell as rain over North Carolina, the heavy batch over Kentucky, southwest Virginia and streaming east across Central Virginia is snow, forming within a deep, cold air mass.

The figure below shows the actual air temperatures at 5,000 feet, the critical snow-forming layer in the clouds.   The closed vortex of the coastal low is centered over North Carolina’s Outer Banks.  To its west, a tongue of very cold air is being drawn southward (blue and purple colors), over the Appalachians and into the cold damming region east of the mountains.   Temperatures at 5,000 feet were -4 to -8 °C, plenty cold for vigorous snow formation.

On A Knife Edge:  Insufficient Moisture Over Baltimore
The southern track of this storm put the Baltimore region on the extreme northern fringe of the storm system, where clouds were being undercut by very dry air in the lowest several thousand feet.   This resulted in a sharply delineated border or northern edge to the heavy precipitation, which remained mainly south of DC.   The sharp cutoff in snow is shown on the regional radar image below.   Superimposed on the radar image is the amount of moisture contained in the air (solid lines and green shaded regions) – a quantity called “precipitable water”.   You will note over an 1” of liquid available to the storm along the Outer Banks, but the precip water drops off rapidly moving north toward Baltimore.  Because the air was so dry over us, any snowflakes that managed to develop completely sublimated before reaching the surface. 

Major Impacts Over Appalachia
The snow that did fall north and west of the low’s center was heavy and wet, thanks to large precipitable water content close to the storm’s center.  Snow totals ranging from 6” to 10” were fairly widespread across southern WV and southwest VA, with locally higher amounts at the highest elevations.    The snow accumulation map below reveals a fairly compact region of heavy snow:

Over 1000 traffic accidents were reported across the snow region, and as predicted, the blanket of heavy snow brought down tree limbs onto electric utilities.  More than 60,000 power outages led to an extended period of power restoration.  The map below shows “ground zero” for these outages on the day after the storm:

Lucky This Time…
In a nutshell, Baltimore dodged a bullet, thanks to a weaker storm that moved rapidly and too far to our south.   But I find it instructive to do a careful post-mortem of these “near misses” because there are always important meteorological lessons to be learned!

Baltimore’s Only Brush With Winter: February 11, 2012 Arctic Front and Snowfall

This has been an unusual, but not unheard of, mild and snow-free winter.   Heavy snow in Baltimore can best be described as “episodic” in nature, a feast-or-famine in terms of total seasonal snowfall.   Recall the record-setting 2009-2010 winter in Baltimore, with a season total snowfall of 77 inches.   This season, cold air masses have sequestered well to the north of the Mid Atlantic and the jet stream has been largely zonal, running flat in a west-to-east flow.   Without big, cold meanders in the jet building down the east coast (these are called jet stream troughs), storminess and winter air outbreaks have been minimal.    Whether we can blame this behavior on La Nina, and/or the North Atlantic Oscillation, remains to be carefully assessed once the season ends.
Baltimore endured a brief winter blast on February 11-12, when a chunk of that cold air mass over Canada pushed south into the Plains and Great Lakes.   The anticyclone that propelled this cold air south and east was intense, with a surface pressure of 1046 mb (fewer than 5% of all anticyclones over CONUS ever reach this intensity).   The leading edge of this cold surge invaded the Mid Atlantic in the form of an arctic front – essentially a cold front on steroids.   The synoptic weather map at 7 AM on February 11 (below) shows the front (blue line running through West Virginia) poised to blast through Baltimore:

Ahead of the front, there was sufficient moisture and an unstable (overturning) air mass.   As this air was lifted along the front, a narrow band of convective snow showers erupted east of the Blue Ridge Mountains.   Around 2 pm, this sudden squall slammed through the area.   Visibility plummeted as a heavy fall of snowflakes and graupel (tiny white icy spheres) fell in a volley, accompanied by gusty winds, rapidly falling temperatures and even lightning.   Many areas picked up a quick ½”-1” of accumulation in a matter of 20 minutes.   The regional weather chart (below) shows this squall as detected by weather radar, along with snow streamers blowing off the Great Lakes and a patch of upslope snow along the Allegheny Plateau.

In this image, isobars (lines of constant pressure) are drawn at 2 mb pressure intervals.  The very compact squeezing of isobars implies an intense pressure gradient, which drives the wind.   Winds from the northwest rapidly increased trough the late afternoon as the heart of the cold air mass began to filter in.  The arrival of cold air from the northwest was very swift.  The diagram below shows this cold air advection at about 5,000 feet above the surface.  Cold air advection is a calculation of how rapidly the temperature changes at a location, due to the influx of cold air by the wind.  Dark blue shades in this diagram show where the chilling effect was most pronounced.   Note that the air temperature at 5,000 feet above Baltimore was 18° F!  

But really cold air was only part of the story that night.   Wind speed steadily increased through the evening, as surface pressure surged in Baltimore.    The winds were also very gusty, peaking at 49 mph.  The diagram below, called a time series, plots hourly observations of wind gust and air temperature at Dulles airport.  Note the arrival of the arctic front at 3 pm on February 11, followed by the precipitous drop in air temperature and surging winds.

Intermittent snow squalls continued through the evening, exemplified by the patch of moderate snow captured by weather radar just before midnight on the 11th:

The air temperature never climbed above 32° F the next day!
What’s interesting is that none of this extreme weather – arctic chill, thundersnow, high winds – were actually produced by an area of low pressure, but rather by an exceptionally strong anticyclone.    We normally think of low pressure regions or cyclones as bringing disturbed and stormy conditions, while anticyclones or regions of high pressure herald fair weather.  But in the case, the tables were turned!