A surprisingly early season snowstorm glazed our region with anywhere from an inch to four-plus inches of snow and sleet. We’ve mapped the snow coverage and explain how this bizarre, much-too-early storm unfolded.

It’s been a woefully compressed autumn: Leaves held off changing their color, then suddenly turned vivid hues. Then, just a couple weeks later, winter arrived with a multiple-choice day of snow, ice and cold rain. Oh, and by the way, it’s still mid-November.

At Reagan National Airport, Washington’s official observing station, 1.4 inches of snow fell, a record for the date. It was the most snow to fall in November in Washington since 3.5 inches in 1989, and the first measurable snowfall during the month since 1996. This was the first instance of an inch of snow so early in the season since Nov. 11, 1987, and among the top 10 earliest first-inch events on record.

Dulles International Airport, which received 3 inches, also set a daily snowfall record, as did Baltimore-Washington International Marshall Airport, which posted 1.7 inches.

The frozen fun began predawn, with a thump of snow across much of the region, arriving just in time for the heart of the morning commute — around 8 a.m. This was the leading edge of moist air arriving from the southeast, lifted by a gathering nor’easter and frozen by a deeply entrenched wedge of freezing air along and east of the Appalachians.

Volleys of sleet mixed with snow, moderately intense at times, came through the morning. By early afternoon, a surge of “milder” (translation: 1 to 2 degrees above freezing) air arrived from the southeast, changing the snow to plain rain along the Interstate 95 corridor. Meanwhile, snowfall topped half a foot by afternoon’s end in the high elevations far north and west of Washington.

The snow accumulation map below is based on data supplied by the National Weather Service. I’ve added generalized contour lines to orient the reader to the gradient in snowfall: Less than an inch fell along the Chesapeake Bay; almost eight inches coated the Catoctins. Those of us east of the mountains would have recorded even more snow, had significant sleet not mixed in and cut down the totals.

These coastal-type storms, which converge and lift vast quantities of Atlantic moisture, always get started at jet stream level. That’s 30,000 feet in the atmosphere. The weather map below shows this level at 8 a.m. Friday. There’s a lot going on here, but you can’t mistake the giant low pressure system swirling over the eastern third of the United States.

That intense, upper-level vortex distorted the jet stream airflow, causing it to “fan” or spread apart over the Mid-Atlantic. Notice how the red flow arrows diverge (meteorologists call this difluence). This action draws up large amounts of air from below. You’ll notice the zone of wintry precipitation (as mapped by radar, in a colored underlay) fits perfectly within the spreading zone of air aloft.

For snow, cold air is key. The next image (a ground-level map) shows how our region’s geography can funnel and trap cold airflow from the northeast. The Appalachians are shaded green.

A potent wedge of high pressure, sitting over New England, extended to the south and west, locking in a cold dome of freezing air against the mountains … and splaying out across the Piedmont of Virginia and Maryland. Cold air is dense air, and it became banked up against the terrain. Only a mighty, mild wind from an opposing direction (southeast) can scour out the cold; it usually takes lots of hours for this to happen.

The map shows this surge of mild air, from the southeast, directed onshore from the warmer Atlantic, by the coastal low’s windy circulation. As this low moved up the coast, the onshore flow got stronger. This map, at 8 a.m., also shows a purple dashed line, the air’s freezing line at the ground. In an animation, these maps would show the purple line slowly retreating deeper inland, nudging counties immediately west and north of Washington into the 33 degrees, above-freezing zone. That transition from snow/sleet to rain was well underway during the early afternoon.

We call the wedged-in, chilled air “Appalachian cold air damming,” and without our humble mountain range, we’d be subject to much less winter misery around here. These wedges become more frequent from December through February, as northern latitudes become more deeply chilled. It’s not uncommon to get wedges in mid-November, though, but a wedge this intense, so early, is quite rare.

These chilly wedges are fairly shallow, just a few thousand feet deep. This one extended to nearly twice that altitude, more than a mile up, well into the critical cloud layer where snow gets its start. Frankly, its depth, intensity and staying power surprised many meteorologists. This wedge was full-on, winter-strength, i.e., typical of late January or February, not mid-November.

Why was it so strong? At least three factors played a role. First, there was optimal positioning of the parent high-pressure cell over New England, setting up the perfect channel of northeasterly, cold flow — aligned exactly parallel to the eastern slopes of the Appalachians.

Second, very dry air at the onset of precipitation caused an efficient refrigeration effect: Trillions of water and ice particles evaporating (sublimating) into the dry air layer sapped the air’s heat content. The very intense precipitation rates during predawn ensured that the air layer extending from ground level to about 7,000 feet cooled several degrees in a few hours.

Third, recall the second image above, which showed the fan of rising air. Air cools as it rises and expands. The more vigorous this process, it’s possible for entire air layers to rise in the lower atmosphere. This “dynamic cooling” probably contributed to further chilling of the air mass overlying the Appalachians.

All of these complex variables made this a very challenging forecast for meteorologists. Capital Weather Gang will post a review of the forecast on Friday afternoon.

Capital Weather Gang’s Jason Samenow contributed to this story.