Overall, weather this past weekend in Washington was quite fair – with temperatures in the low 70s and bouts of sunshine. But in some spots, a dark overcast hung heavy and some locations experienced patchy drizzle. The culprits: A humid, onshore flow off the ocean combined with a process called cold air damming. Both of these are quite unique to the Mid-Atlantic region.
Blame it on high pressure stalled over New England
A thick, low cloud deck first formed on Friday morning, as patches of light drizzle moved across the Baltimore metro. For the cause, let’s examine the big picture first – the U.S. surface weather map – below – valid Friday evening (8 p.m., September 28).
The map shows a large anticyclone (high pressure cell) anchored over New England (blue “H” on the map), remaining parked over this region Friday through Sunday. Around the high pressure cell, surface airflow spiraled clockwise, diverging outward from the center of pressure.
Over Washington-Baltimore, this large-scale flow established a low-level air trajectory off the cool North Atlantic. This is termed an onshore flow, with winds arriving from the east-northeast, shown by the blue arrow on the map.
The Appalachians, which trend southwest to northeast, helped to channel the flow toward the southwest, down the Piedmont and Coastal Plain. Note the maroon 1020 mb isobar (line of constant pressure) sagging toward the southwest, in the direction of the cool air flow. In other words, high pressure was building down the lee side of the mountains.
This overall pattern is termed Appalachian cold (cool) air damming. A shallow layer of cool, moist Atlantic air layer gets circulated toward the southwest and becomes trapped against the mountains, overspreading the metro region. The dense air layer cannot flow over the mountains, and hangs tight in valleys and low spots.
From Friday through Sunday, surface winds blew steadily from the northeast over Baltimore and D.C., and humidity (dew points) increased. As the morning sun warmed the surface, the air layer air ascended, cooled and condensed into a thick, low deck of cloud. Cloud bases were low because humidity was high. The clouds tended to disperse at night, after the shallow layer of convective overturning dissipated.
A type of weather pattern not well-forecast
Because cold air damming involves a very shallow layer, weather prediction models have a hard time forecasting its initiation and demise. That’s because the models have a limited number of grid points in the vertical, and cannot resolve fine air mass structure close to the ground. Additionally, there is little data over the ocean – so the models are essentially “blind” to the detailed structure of oceanic air masses approaching the East Coast.
When the models do predict cool air damming, they almost always dissipated it too rapidly. Cool, marine air is dense, ground-hugging air. So persistence is often the best forecasting rule of thumb. Intense solar heating can offset the influx of cool air during the day, but the marine layer re-establishes itself overnight. And long autumn nights allow the layer to cool further.
A shift in wind direction from the south or southwest is needed to “beat back” and scour out the cool air. This occurs once high pressure over New England moves east, or weakens. Then, warm and drier air arrives from the Southeast and slowly mixes out the cool, humid air.
Incidentally, when cold air damming becomes established in the dead of winter, extremely cold air is often funneled east of the Appalachians from due north. If a storm system approaches from the south, we have the classic setup for an ice storm or a heavy snowstorm.
The marine layer, best observed by balloons
During cold air damming, the best way to identify the presence of an ocean air layer is still via radiosonde, aka the humble “weather balloon”. Balloons are launched twice a day at about 100 locations across the U.S. The site closest to our metro region is Dulles airport. While Dulles is a ways from the coastline, marine layers often penetrate 100 or more miles inland and can thus be readily detected.
Data from weather balloons are used to construct soundings. These are graphs showing the vertical structure of temperature, moisture and winds in the atmosphere, in very fine detail. There are many more vertical layers in a sounding than in a weather prediction model (too many grid points means the model cannot be run fast enough, even on a supercomputer, to generate a forecast in short enough period of time).
The figure below shows the Dulles sounding from Saturday morning (September 28, 8 a.m). The red and green lines show how temperature and dew point (a measure of humidity) change with height, respectively. Altitude decreases from bottom of the graph (surface) to the top (12 miles).
To look for a marine layer, we identify a layer close to the ground with uncharacteristically cool temperatures (for the time of year), also near saturation. That is, the temperature and dew point (red and green lines) run very close together (when air is saturated, temperature must equal dewpoint). Additionally, low-level winds indicate that the flow from an oceanic source region, i.e. from the east.
In this sounding, the cool, nearly-saturated layer extending 5,000 feet above the surface is a classic marine layer. Right at 5,000 feet, the temperature equaled dew point, giving 100% saturation. Here, the balloon ascended through a thin overcast cloud deck (shown by the arrow).
Something interesting happened just above the cloud deck. Temperature began increasing sharply with altitude, and the dew point fell precipitously. This warm, dry layer is called an inversion. Inversions mark the top of a marine layer.
A warm air layer sitting on top of a cool surface layer means the atmosphere is thermodynamically stable; basically, it will not overturn or convect on its own (cool, dense air will not rise spontaneously into warm air above – it has no buoyancy). Thus, the stable inversion layer “caps” any further rising of air – meaning the cloud layer just below must remain very thin.
Note the red circle along the right side of the diagram. Here, wind barbs indicate direction of air flow and wind speed (only a few of the sounding’s hundreds of wind measurements are shown, for clarity). In the marine air layer, the winds blew from the east-northeast, off the Atlantic, at 15 kts (20 mph). Above the inversion layer, the winds turned more northeasterly, then northerly at high altitude.
The structures you see in this sounding persisted throughout the weekend.
So here you have the anatomy of a persistently cloudy weekend in Washington, courtesy of the cool northern Atlantic and the Appalachians. The effects may be subtle, but the manner in which geography and meteorology interact over the Mid-Atlantic make our regional climate unique.