The Washington PostDemocracy Dies in Darkness

Heavy rain and isolated flooding forecast for Washington region Wednesday into Thursday

Widespread rainfall amounts should reach two to three inches.

Predicted rainfall from the National Weather Service through Friday morning.
Placeholder while article actions load

After more than a week without rain, the Washington region is about to more than make up for it. Wednesday and Thursday promises a heavy rain event that could unload two to three inches across the region, with isolated higher amounts.

The heavy rainfall is related to a slow-moving cold front that will draw tropical moisture into the region, including some from Tropical Storm Eta in the Gulf of Mexico. The rain is expected to come in two waves — the first Wednesday afternoon (starting around midday) and the second Wednesday night into Thursday morning.

The National Weather Service is considering issuing a flood watch for the region. However, because the rain will be drawn out over an extended period and the ground is dry, flooding that occurs will probably be isolated rather than widespread. Zones most vulnerable to flooding are those near creeks and streams, and low-lying areas with poor drainage.

If Washington picks up two inches in one day from this event, it will mark the sixth such instance in 2020, tied for the most on record.

Here are rainfall amounts projected from different models through Thursday for the District:

  • American (GFS): 1.2 inches
  • Canadian: 3.5 inches
  • High-resolution Canadian: 3.5 inches
  • European: 1. 8 inches
  • NAM: 2.7 inches
  • High-resolution NAM: 3.6 inches
  • UKMet: 1.9 inches

Discussion

All meteorological factors point to a soaking rain event unfolding Wednesday afternoon through Thursday morning. In a nutshell, a slowly advancing cold front will team up with a tropical plume of moisture and a pocket of energy in the jet stream.

Let’s discuss the first of these three elements, the front, shown in the two forecast surface maps below. By comparing the two maps (the first is 7 p.m. Wednesday, the second is 7 a.m. Thursday) you can note how slowly the front advances in that 12 hours. This means more time for rain to accumulate.

Also, moist winds through the atmosphere will be streaming from the south-southwest, ahead of and along the front. When a front — which lifts up the air — aligns parallel to the flow like this, that is a recipe for “cell training,” where rain-bearing cloud elements continually pass over the same region, like a conveyor belt. The training effect is expected to contribute to hefty rain totals.

Now for the second element — a plume of deep tropical moisture. The figure below (valid 10 p.m. Wednesday) shows how much we are dealing with. The color scale shows a value integrated vertically through the atmosphere, with amounts around two inches This is uncharted territory for this time in November. For those familiar with statistics, it’s a value that is three standard deviations above average — translated as “extreme”!

In the figure above, you will notice Tropical Storm Eta embedded in this plume. Indeed, some of its upper-level moisture will probably stream over the Washington region, and it definitely will contribute to the “juice factor” of the larger plume. We may even observe surface dew points near 70 degrees, which would be truly exceptional for mid-November.

The final factor, jet stream energy, is shown below. This is a forecast chart at 10 p.m. Wednesday. The pocket of yellow-red color shows winds approaching 170 mph. Along the southern edge of this “jet streak” — over Maryland, Pennsylvania and West Virginia — the air will be ascending especially vigorously. We call this the “left entrance region” of the jet streak. This effect will stoke rain generation in deeper clouds.

We can expect a generally steady, moderate rain through the event, with pockets of heavy rainfall. There may be a burst of heavier rain Wednesday afternoon, and another late Wednesday night, as weak waves of low pressure develop along the front and move through.

As for the possibility of thunderstorms, we cannot rule out some rumbles of thunder in the heavier rain cells. Most of the unstable air will be “elevated” in nature, while the deeper, unstable air mass rooted to the ground should stay to our south, over the Carolinas. The Weather Service’s Storm Prediction Center does not presently predict severe thunderstorms in the region.

Loading...