* Flash flood watch Thursday afternoon and night *

The weather in the 18 hours spanning Christmas Eve afternoon and Christmas morning will have it all: a surge of warmth, waves of rain that may produce strong winds, flooding and even a tornado, plummeting temperatures and then a chance of snow flurries.

The National Weather Service has issued a flash flood watch ahead of the Arctic cold front responsible for the turbulent weather. The watch, in effect Thursday and Thursday night, calls for up to 1.5 to 2.5 inches of rain, which may “result in flash flooding of small streams and creeks and possible river flooding.”

Scattered showers with locally heavy downpours could develop any time Thursday afternoon, but the most intense rainfall and the possibility of storms with strong winds is most likely to occur between 10 p.m. and 1 a.m.

As cold air spills into the region in the wake of the storm Christmas morning, some scattered snow showers and flurries are possible as temperatures plunge to near freezing with wind chills in the teens and 20s.

Discussion

The anticipated weather roller coaster centers on an approaching cold front and deepening area of low pressure moving north along the Appalachians late Thursday. An unseasonably mild and humid surge of air will get drawn over the D.C. region out ahead of these features. A secondary cold front — the true “Arctic front” — follows quickly on the heels of the main front, with a reinforcing surge of cold and dry air.

The mild air will contain a ribbon of very high moisture, as shown in the next image. This is a plume of tropical moisture, exceptionally large in humidity content for late December. Waves of heavy showers will develop periodically throughout the day tomorrow, tapping into this deep moisture.

The repeated movement of showers across the same areas has triggered a concern for flooding — hence the flash flood watch posted by the Weather Service. The ground is quite soggy from melted snow and recent rain across the region, so water will probably quickly pond in areas.

Forecast models predict the following rainfall amounts for the District: NAM: 0.8 inches; high-resolution NAM: 0.7 inches; American (GFS): 1.6 inches; European: 2.9 inches; Canadian: 1.4 inches; high-resolution Canadian: 1.4 inches. Most models show the heaviest rain focusing just west of Washington toward the mountains, although the European model places the bull’s eye close to the Interstate 95 corridor.

Within low levels of this moist current, winds will accelerate into the deepening low through the day and evening. Wind speeds will reach 60 to 70 mph just a few thousand feet above ground. Weak convective motions in the cloud layer will “stir” blobs of this higher momentum air to the surface. This will generate occasional gusts in the 30 to 40 mph range. It’s possible we could experience a few gusts upward of 45 to 50 mph, especially within storm cells, and the Weather Service may issue a wind advisory to cover this possibility.

As the front approaches, waves of showers may congeal into a squall line with heavy rain and gusty winds. The Weather Service Storm Prediction Center, at this time, foresees that the D.C. region has a marginal risk of any severe storms (Level 1 out of 5 on the threat scale). One possible realization of simulated radar is shown below, from the NAM model, showing a storm line pushing eastward of D.C., over the Chesapeake Bay, around 1 a.m. Friday (overnight Christmas Eve into Christmas morning).

Because the front is expected to move through around midnight, long after sundown, it won’t have as unstable an air mass to work with. But the low-level surge of mild, humid air on southerly winds will import enough instability to pose a bit of a concern. And the “dynamics” of the strong front (strong uplift of air, strong increase of winds with altitude or “shear”) and an energetic jet stream will add oomph to storm cells.

Brief, torrential downpours may accompany a late-night line of storms and potentially scattered wind gusts over 60 mph. Though these storms may be devoid of lightning and thunder, the Weather Service may issue severe thunderstorm warnings because of the potential for high winds, so folks should remain vigilant.

If any part of the line of storms develops an S-shaped kink or bow, we cannot completely rule out a brief tornado touchdown. This remains in play because of a warm front that is expected to be draped across the region. These boundaries often contain areas of enhanced wind shear (change in wind direction), leading to pockets of spin that can nucleate a tornado within stronger storm cells.

Once the Arctic front surges through early Christmas morning, expect a rapid plunge in temperatures and continued gusty winds. This 24-hour scenario certainly will create a bit of Christmas drama and weather whiplash.