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!

1 comment:

  1. Thanks for the great explanation! Glad we missed it. :)