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Sunday’s atmosphere could be D.C.’s toastiest in December on record

A stream of air straight out of the tropics may boost atmospheric temperatures to unrivaled levels for December Sunday in the Washington, D.C. area.

NAM model shows stream of tropical moisture surging up the East Coast Sunday morning (

Some model simulations suggest the air temperature will reach 16 to 17+ degrees C (or 60-62 degrees F) at an altitude of 5,000 feet, which would  match or best the record at this elevation.  Since around 1950, weather balloons launched at either Reagan National or Dulles Airport have sampled the air temperatures at different altitudes, at 7 a.m. each morning.  The highest December temperature at 5,000 feet (850 mb) on record is 16.2 degrees C on December 1, 1977.

The European model, shown below, projects temperature Dulles’ temperature at 5,000 feet (or 850 mb) to be between 16 and 17 degrees C Sunday morning.

European model shows temperatures at 5,000 feet (850mb) near 16-17 C (60-62 F) at 7 a.m. Sunday morning. (

The GFS model, not shown, is slightly cooler and predicts the temperature to be around 15 C, just shy of the record.

Temperatures a mile high in the neighborhood of 60 degrees F may not seem that warm.  But they are actually astonishing when you consider they’re what we expect near the heart of summer at this elevation!

Remember, air warms as it compressed closer to the ground – so a temperature of 16 C (60 C) at 5,000 feet in the morning equates to about 30-31 C at the ground on a summer afternoon, assuming full sunshine. That’s 85-88 degrees F!

Of course, December is not summer and we’re not expecting full sunshine Sunday. So keep your shorts in the closet. But even if we cut the amount of warming from 5,000 feet to the ground in half compared to summertime, that still brings the temperature up to 23 C, which is 73 degrees F.  If it’s that warm, we’d break the high temperature records for the date at all three airports: 72 at Reagan National (from 1889, set at 24th and M, before observations moved to the airport), 69 at Dulles (from 1984), and 70 at BWI (from 1889) .

The GFS model – for what it’s worth – predicts a high of 74 on Sunday at Reagan National Airport.

GFS model output statistics show a high of 74 in D.C. on Sunday

The European model also simulates temperatures in that range.

European model forecast of surface temperatures Sunday afternoon (

As crazy warm as these high temperature sound, the low temperatures may even be more remarkable.  It’s possible that lows Sunday morning are only 55-60 (which is 10-15 degrees warmer than the normal high!).  The warmest low on record at Reagan National during the month of December is 59…we could flirt with that.

The bottom line is that the tropical nature of this weekend’s airmass is something special.  When you consider an end of the month warm spell may threaten previous records set early in the month, that’s impressive.

(Capital Weather Gang’s Ian Livingston contributed to this post)

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.
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Jason Samenow · December 19, 2013

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