Five ways we get snow in the Mid-Atlantic

Here in the Mid-Atlantic, winters can be quite capricious.  Snowfalls come in different varieties.  Sometimes we receive a dusting, other times a Nor’easter wallops our city with a foot or more of heavy, wet snow.   Often it’s snowing over our western mountains, while here in D.C. it remains cold and sunny.   And you may have wondered – what exactly is Lake Effect snow?

In this article, we look at five common types of weather systems that usher in the much-loved – and deeply despised – white stuff.  Read on to learn all about the science of snow!

Snowstorm type 1:   A snowy clipper sails through

Many times, our forecast calls for a dusting to light accumulation of snow (1”-2”) courtesy of a quick-hitting weather system called an Alberta Clipper.

Alberta Clipper Snowstorm (NWS)

Alberta Clipper Snowstorm (NWS)

A Clipper is an area of low pressure embedded in strong jet stream flow, accompanied by either a continental polar or arctic air mass.    As the figure shows, Clippers commonly arrive from the northwest.  They can deposit a continuous swath of light snow from the northern Plains to the East Coast.

But these systems are inherently moisture-starved.   This, combined with their brisk movement, limit the accumulation.  Light to moderate snow accumulates for a few hours, then the system exits.   The low moisture content and cold air mass yield snow:liquid ratios that are very high, in the 15:1 to 20:1 range – depositing a “dry dusting” to an inch or two of fine powder.

One caveat:  Infrequently, Clippers can become bad actors.  During January 1996, a Clipper-type system dumped 5”-6” in the Washington region, to the surprise of forecasters.   The low pressure can also strengthen off the coast, as it encounters warm Gulf Stream water;  if the Clipper morphs into a Nor’easter, additional snow will fall across our region.

Snowstorm type 2:  Frontal snowburst

During the warm months, an approaching cold front is often associated with a squall line of heavy showers and thunderstorms.   During winter, the atmosphere is much less unstable, and available moisture is reduced.    Vigorous cold fronts still sweep through, however.

These fronts converge and lift moist air, forming a line of convective clouds, even in winter.   Convective updrafts aren’t vigorous enough to generate lightning, but if the  temperature below cloud base remains below freezing, a sudden squall of brief, heavy snow develops.

These snowbursts, oriented parallel to the cold front, arrive with a bit of fanfare.  The wind kicks up, gusting in the 30-40 mph range.   A burst of heavy snow reduces visibility to near zero.   Driving becomes extremely hazardous.   Sometimes instead of snow, or mixed with snow, are tiny ice spherules called graupel.    The formation of graupel suggests convective vigor.   The narrowness of the convective line and its fast movement ensure that snow accumulation remains on the light side.

As quickly as the snow squall arrives, it ends.   The clouds break, the wind shifts to west-northwesterly, and cold air behind the cold front begins filtering in.  Game over!

Snowburst-Producing Squall Line (NWS)

Snowburst-Producing Squall Line (NWS)

Snowstorm type 3:  The lake effect snow machine

I mention this type for the sake of completeness, rather then its impacts over Washington.   But these infamously heavy and localized snowstorms sometimes make national news. Very rarely, a Lake Effect snowband will sneak into our region’s backyard, mainly near the Mason Dixon Line, or across western Maryland.

Lake Effect Snowstorm (NWS)

Lake Effect Snowstorm (NWS)

Lake Effect snows are not associated with low pressure systems;  they are created by a high pressure cell that sweeps cold air, from the west-northwest, across warm water.   These cold, windy air masses filter in behind a cold front.   Cold air in contact with lake water is warmed and moistened. The energy transfer is maximized by a long fetch, or wind duration, across the water.   As moist, unstable air ascends, it forms shallow, convective snow squalls along the lee shores of the lakes.

Lake Effect snow bands often form narrow plumes that extend 100 or miles downwind of the lakes.    These bands may only be a few miles wide – but repeatedly track over the same locations.    Over a day or two, their passage can lead to awe-inspiring snow accumulations, at times approaching several feet.

Lake Effect snowfall is very sensitive to wind direction.  Fetch is optimized when the cold wind parallels the long axis of a lake.  A shift in wind direction can quickly shut down a snowband.   Also, in the heart of winter, many of the Great Lakes freeze over, eliminating the transfer of heat and moisture.  So these heavy snows are most common during the late fall and early winter months.

Snowstorm type 4:  Upslope flow snow

After a wintertime cold front sweeps through, you may see winter weather advisories, perhaps even a winter storm warning, posted for Maryland’s western mountains and the West Virginia panhandle.  This occurs while skies are clearing over Washington.  What gives?

Cold air, arriving from the west-northwest, is forced to ascend steep terrain along the western slopes of the Appalachians.  This air picks up moisture as it flows across the Great Lakes.   Often an upper-level disturbance (a wave of low pressure, or a cutoff low) enhances the uplift of air, leading to a more widespread and heavier snowfall along the mountain slopes.

Upslope Flow Snowstorm (Dan Robinson, stormhighway.com)

Upslope Flow Snowstorm (Dan Robinson, stormhighway.com)

It’s not uncommon for these events to pile up 4”-6” or more of snow along the highest mountain slopes.   Upslope snows often occur in tandem with a Lake Effect snowstorm.    The radar snapshot below, from February 2012, illustrates upslope snowbands occurring in conjunction with Lake Effect snow streamers.  There’s even a snow squall over Washington, thrown in for good measure!

 Upslope Snow, Lake Effect and a Snowburst Along A Cold Front.  Solid black lines are isobars (modified from NWS radar image)

Upslope Snow, Lake Effect and a Snowburst Along A Cold Front. Solid black lines are isobars (modified from NWS radar image)

But why is Washington immune to these local winter storms?   Air that ascends the western slopes must then descend eastern slopes, before fanning out across the flatlands of Washington.   The descending air warms and dries (this follows from Meteorology 101).   Any snow clouds sublimate and evaporate.   So while a mini-blizzard rages 100-125 miles to the west of D.C., on our side of the mountain, the weather is fair, albeit chilly and breezy.

Snowstorm type 5:  The snowmaker a.k.a. “The Big One!”

I’ve saved the very best (or worst, depending on your viewpoint) for last.   When the surface weather charts take on the configuration shown below, beat the snow drum – loud and hard.   Here is what it takes to create the blockbuster snowstorms that cripple our region every few years.

Nor’easter, Miller Type A Pattern (NWS)

Nor’easter, Miller Type A Pattern (NWS)

The main player is a Nor’easter – a powerful coastal low, created by strong thermal (air mass) contrasts and interaction with a trough in the jet stream.   These are very dynamic systems, drawing in copious amounts of Atlantic moisture on their warm, eastern side.  They may develop either along the U.S. Gulf Coast, or off the North Carolina Outer Banks.  The warm Gulf Stream provides a key additional energy source.

The diagram above illustrates the first of two Snowmaker flavors. High pressure over New England pumps cold air into the Nor’easter at low levels.  As the coastal storm tracks northward along the East Coast, it lays down a widespread swath of moderate to heavy snow along its cold, western side.

The location of heavy snow depends primarily on the storm’s track.   When the low remains offshore, subfreezing air intrudes over Washington-Baltimore and a heavy snow swath lines up with the cities.  If the storm is too far offshore, Washington may gets clipped with flurries or a dusting.  If the low takes an inland track, with the center passing over Washington, mild ocean air invades – resulting in rain, or perhaps a wintry mix.

When the Gulf Stream is particularly warm and the jet stream exceptionally energetic, the central pressure of a Nor’easter can “bomb out” i.e.  decrease at a very rapid rate.  The heaviest snow typically falls while Nor’easters are rapidly deepening.

If the high pressure north of the Nor’easter is intense and firmly anchored, it impedes the northward movement of the storm.   Such “blocking” situations lead to prolonged periods of heavy snow, sometimes to the tune of 2 to 3 feet.    This was the case, repeatedly, during the pernicious winter of 2009-2010.

Based on air temperature and moisture content of the clouds, snow in a Nor’easter can either be heavy and wet or dry and fluffy.   When a plume of tropical moisture feeds a Nor’easter from the south – termed an atmospheric river – extremely wet snow develops.  Early and late season Nor’easters are more likely to work with milder air masses, and wet snow develops when temperatures hover near freezing.

Nor’easter, Miller Type B Pattern (NWS)

Nor’easter, Miller Type B Pattern (NWS)

 

This figure shows a variation on the Nor’easter pattern.  In this scenario, a vigorous storm approaches our region from the west, traversing the Ohio Valley.   As the system crosses the Appalachians, it transfers its energy to the coast.   The resulting “center jump” creates a secondary low, creating the Nor’easter.   Heavy snow begins falling on the cold backside of the Nor’easter as it deepens and begins moving up the coast.

This scenario does not always guarantee a big Washington snowstorm.   It all depends on the timing and nature of the energy transfer, the location of the new storm center, and a host of other factors.  But some of our region’s historical snowstorms were created in this manner.

When it comes to Nor’easter snows, the devil lurks in the details.   This is where the mesoscale (local-scale) forecast models offer useful guidance.   The heavy snow swath sets up about 150-200 miles to the west and north of the storm center.   It often contains intense, small-scale bands that remain nearly stationary for several hours.  These develop in a region of stretched airflow called the “deformation zone”.

The storm system may also possess a certain type of instability, leading to a type of vigorous, slantwise motion in the clouds.   Where pockets of instability develop, there is often thundersnow.

Within heavy snow bands or thundersnows, accumulation rates approach 2”-3” per hour.   These localized, dynamic features create significant gradients of snow accumulation.  This is why one side of the Washington metro region may receive two or three times more snow than adjacent counties.

No matter what the scenario, a strong anticyclone (high pressure cell) located to our north is crucial for heavy snow.   This ensures an over-land trajectory of deep, subfreezing air.   If the high repositions along the coast, airflow arrives from the northeast or east, creating a warmer marine flow – the “kiss of death” for a big snowfall.   But if high pressure digs in and builds down the eastern slopes of the Appalachians, a cold air dam or “cold wedge” develops.  This really anchors the deep, subfreezing air, a classic setup for a Washington, D.C. heavy snow.

It’s not just how much snow…

Washingtonians are obsessed with snow accumulation numbers.  Mention a coming snowstorm and the first question, without thinking, becomes “how much are we gonna get?”.  If only this were the whole story!

Why do meteorologists predict ranges of accumulation i.e. 3”-6”, 4”-8”, etc.?  Predicting snow totals to the exact inch requires that forecasters make a liquid equivalent forecast accurate to +/- 0.1”.   This is based on the common snow:liquid ratio of 10:1.   But this level of precision is exceedingly difficult, even using today’s relatively sophisticated computer models.

The snow’s liquid equivalent can vary from about 7:1, for a sopping, heavy snow…to 20:1, the type of dry powder that a leaf blower can remove. (To predict Clipper-type, dry snow to the inch, forecasters need to nail the liquid equivalent to a precision of 0.05”).

But the snow:liquid ratio is exceedingly important not just for predicting total accumulation, but the impact of the snowfall.   Whether our region receives 8” of 15:1 vs. 8:1 snow means the difference between lights and heat, and a widespread, multi-day power outage.   The snow ratio can even change during a storm, with snow often starting off wet and heavy, then ending as a dry powder.

The snow accumulation rate is another important measure of impacts.   A rapid accumulation can overwhelm snow removal efforts and severely restrict road visibility.   A 15-minute heavy burst of snow can create a multi-vehicle pileup – not from snow amount but the sudden onset of near-zero visibility.   Additionally, when road temperatures are a few degrees above freezing, rapid accumulation hastens the  cooling process, allowing snow to begin sticking earlier in the storm.

Finally, gusty winds exacerbate the impact of snow.   High winds are often generated by an intense Nor’easter that “bombs out” offshore, or by a strong cold front.  Wind gusts whip snow into whiteout situations, cause snow to drift, and create dangerous wind chill.

Share what you know!

This is just a brief primer on the types of snowstorms experienced around Washington.   The science of snow is a rich and fascinating subtopic of our region’s meteorology. Each storm is unique.   Impacts are quite variable.  Forecasting is still very problematic. At times it dominates the lexicon of our daily discourse.  So now I turn the floor over to you, CWG readers!

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