A lot of factors need to come together in a coordinated fashion in order for the Mid-Atlantic states to experience a major snow storm of the magnitude of this “President’s Day Weekend” storm. All of these weather phenomena occur individually on a fairly regular basis. It is the combination of these individual phenomena coming together in a certain way that results in a major snow storm. What follows is a brief discussion of some of the main “ingredients”, and how their juxtaposition leads to a major mid-Atlantic snow storm. For a much more detailed discussion of this subject see the "Kocin and Uccellini" book listed on the Meteorology Links Page.
A. Upper Level Troughs
An upper level trough is always present before and during a major winter storm. A trough is a dip in the upper level winds. See Figure 7 from Chapter 2 to see the 500 mb trough west of the Mid-Atlantic states at 12Z (0700 EST) Sunday February 16. A trough generates flow patterns that support positive vorticity advection (PVA), upper level divergence, and positive (upward) vertical velocities, especially between the trough and the downstream ridge. All of these factors, plus a large temperature contrast between the cold air over the Eastern US and the relatively warm air over the Atlantic just off the coast, promote the formation and intensification of a surface low pressure area.
An intensifying storm system will generally be accompanied by an increase in amplitude and decrease in wavelength of the upper level trough. Frequently the upper level trough actually forms a closed low during a major storm. This “cutting off” aids in slowing the forward motion of the surface low and keeping it close enough to the coast to cause heavy snow in the mid-Atlantic.
B. Cold Surface Anticyclones (High Pressure Areas)
High pressure areas at the earth’s surface are generally associated with fair skies and good weather, but a strong surface high is almost always a major player in a mid-Atlantic snow storm. The key is the location of the surface high. The high's role is to provide and reinforce the cold air near the surface so that precipitation can remain as snow for all or at least most of the event, and the best location from which to do this is in eastern Canada. The high responsible for this PDII snowstorm was a monster. Notice the high pressure area in Figure 8, which is a surface map for 9:33 am Sunday the 16th. The high is centered north of Lake Ontario and is pumping cold air down the East coast all the way to the Carolinas. During the intense snowfall on Sunday morning the temperature in N DE was pretty much steady at around 12 deg F, which is unusually cold for a heavy snow event in this area.
Also note the “bulges” in the isobars east of the Appalachians. This is a signature of cold air intrusion and illustrates a phenomenon known as “cold air damming”, which is cold surface air trapped between the Appalachians and the coast. Cold air damming is almost always found with a major East Coast snow storm. This does not mean that strong coastal storms cannot form without cold air damming, but it does mean that without it, precipitation will most likely be rain or sleet on the coastal plain for much of the event. The reason for this is that without the strong high to keep pumping in the cold air, and thereby forcing the warmer moist air to flow over top of it, warmer (i.e. above freezing) air will take over and change the precipitation to rain. Shallow cold air damming with a warm (above freezing) layer above can result in significant ice storms.
Notice on Figure 8 that a low pressure area has formed off the coast of SC. This is probably a result of the steep temperature contrast between the cold arctic air inland along the coast and the (relatively) warm air over the Atlantic Ocean, with the upper level trough to the west supplying the upper air dynamic support. These regions of strong horizontal temperature contrast are called baroclinic zones.
C. Latent Heat Release
When water condenses it gives up its heat of condensation, which is considerable. Heat of condensation (or vaporization) is called latent heat and it is released or absorbed when water changes phase from vapor to liquid (condensation) or liquid to vapor (evaporation). When water vapor condenses to form precipitation, the latent heat released helps to increase the vertical velocity by making the air slightly warmer and more buoyant. This process is believed to be an important factor in the rapid intensification of some coastal storms.
D. Putting It All Together
Most, if not all, of the above factors are necessary for a heavy snow along the East coast. There are other factors (for example jetstreaks) usually involved as well. Many coastal lows form and intensify, and many have most of the features described above. But if the precipitation falls as rain along the coastal plain (very generally E of I-95), or the precipitation is light in nature, it’s probably because one of the major factors listed above is missing or weak.
The development of a major coastal storm can be generally described as follows. First there will be an upper-level trough (sometimes called a short wave) approach from the west, accompanied by the development of a surface low. The warm air and moisture intrusion (from the Gulf and/or Atlantic) and latent heat of condensation all act to warm the air to the north and east of the surface low. Warm is used in the relative sense here; for heavy snow, the air will be at or below freezing at all levels. This warming slows the eastward progression of the downstream ridge ahead of the trough and builds its amplitude, while the trough continues to move east and amplifies (amplifies means the trough grows in the N-S direction). Meteorologists call this a “digging” trough, which basically means intensification. The decrease in wavelength between the trough axis and the upstream ridge axis, and the corresponding increase in maximum upper level wind speeds, strengthen divergence in the mid and upper level flows above the surface low. The increased upper level divergence further deepens the surface low.
While this description does generally fit the development of the 2003 President’s Day Weekend Storm, much, if not most, of the snow we got in N DE came early Sunday as a result of the tremendous amount of warm moist Gulf air being forced over the intense cold air mass associated with the strong high to our north. While a good coastal storm did eventually develop and contribute to our high totals, we would have had a significant snowstorm even without the coastal low. The coastal storm was more of a player for areas to our north (New York and Boston).
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