Temperature difference from normal at high altitudes Tuesday night to Friday night.

A polar vortex blast may unleash the coldest December weather in Washington since 2004 later this week.

The Arctic air spills into the region late Wednesday, and Washingtonians will feel the sting by early Thursday when they awaken to temperatures around 20.


GFS model forecast lows Thursday.

But the air temperatures won't tell the whole story. Thanks to winds gusting to 30 mph, wind chills early Thursday will be near 10 degrees around the Beltway, and in the single digits to near zero north of town.


Wind chills forecast Thursday morning from GFS model.

Even through periods of afternoon sunshine Thursday, temperatures and wind chills will hardly budge from frigid morning levels, as winds continue to howl.


Forecast wind gusts Thursday afternoon from GFS model.

The most punishing combination of wind and cold will occur Friday morning. Air temperatures will range through the teens while wind chills range from the single digits to around zero.


Wind chills forecast on Friday morning from GFS model.

Winds should start to relent some Friday afternoon as high temperatures rise into the mid-to-upper 20s. While the cold will no longer be as biting, these temperatures are still 20 degrees below normal.


Forecast highs Friday by GFS model.

After some possible icy weather early Saturday, temperatures should rise above freezing in the afternoon and then rebound into the mild 50s on Sunday.

This week's cold snap will probably result in one day with highs in the 20s (Friday) and possibly two in a row if temperatures fall below 30 by 12 a.m. Thursday morning as the Arctic air arrives.

To put this cold snap into some perspective, we have to go back to 2010 to find the last December day during which highs held in the 20s in Washington. The last December to have consecutive days in the 20s was in 2000 (Dec. 25-26), and prior to that we have to go all the way back to Dec. 27-30, 1993, when there were four straight days in the 20s.

Given Friday's forecast for highs in the mid-to-upper 20s and lows in the midteens in Washington, the most recent days in history that cold were back on Dec. 22, 2008, when the high was 28 and the low 16, and Dec. 20, 2004, when the high was 24 and low 11.

As cold as it's going to be, this cold snap won't come close to the record-setting cold snap of December 1989.

The 1989 cold was remarkable for both its intensity and duration. Highs in Washington remained below freezing for 10 straight days from Dec. 16-25, and the mercury plunged to 5 degrees on Dec. 22, the second lowest temperature on record at Reagan National Airport for the month.