Rainfall predicted through early next week by National Weather Service.

The rains this month have come relentlessly, and potential for another wave of torrents is on the rise as we conclude May and start June.

Slow-moving showers and storms, capable of unloading very heavy rain in a short time, enter the forecast Thursday and Friday. They won’t happen everywhere all the time, but they will be most numerous in the afternoon and evening and could produce pockets of flash flooding. The National Weather Service is likely to issue a flash flood watch for this period.

Then, over the weekend, the intensity of the rainfall should gradually ease, but it may become more persistent and widespread late Saturday into early Sunday. This may cause some rivers and streams to crest above flood levels for the second time in the past few weeks.

Even though we think any instances of damaging winds in thunderstorms will be isolated over the next several days, be aware that a few trees could come down because of heavy rain alone, as the soaked ground can disrupt their root systems.

This latest bout of rainy weather arrives after 7.41 inches have already fallen in Washington this month, the eighth wettest May on record.

While June will pick up rain Friday where May left off, the pattern should shift to a drier one early next week.

Discussion

As in the past two weeks, in which we experienced devastating flash floods in Frederick and Ellicott City, Md., a stubborn frontal boundary separating sultry summer air to the south and cooler Atlantic air to the north, will remain anchored across our region. As shown below, the front is expected to be positioned across far northeastern Maryland on Thursday afternoon.


Surface forecast chart for Thursday evening. (National Weather Service)

It’s possible that the front could drape anywhere between the District and the Mason-Dixon Line. Models suggest it will push farther north Friday, placing us deep in the warm sector of an approaching low pressure system and cold front.

On Thursday, regions near and along the boundary will experience the greatest triggering (uplift) potential for sustained showers and thunderstorms. On Friday, the potential becomes more area-wide, as a disturbance in the upper-level flow approaches the region, and the cold front moves through from the west.

With southerly flow and intermittent solar heating, the atmosphere will destabilize during the afternoon and evening of both days, providing the buoyant energy for convective storms. Atmospheric moisture will be plentiful, approaching or perhaps exceeding record levels (precipitable water of up to two inches), as shown below.


Plume of very high precipitable water (purple over our region late Thursday afternoon, sourced from both the Gulf of Mexico and Atlantic Ocean.)

As we remain under an upper-level ridge in a weak jet stream pattern, steering flow aloft will be on the weak side, meaning thunderstorm cells could linger over the same regions, increasing local rain totals. For regions that have received a soaking in the past few weeks, this means the soil will absorb less, and the potential for runoff increases.

For all these reasons, the Weather Service has placed our region in its slight risk (10 to 20 percent chance) zone of flash flooding, both Thursday and Friday.

Given the lack of strong wind shear, which intensifies storm wind speeds, we do not expect widespread severe thunderstorms either day. However, there remains the prospect of frequent lightning, and an isolated wet microburst or two in which a pocket of damaging winds hits a relatively small area.

As we get into the weekend, the atmospheric setup remains problematic. The cold front will have moved through, but a pesky feature termed a “cutoff low” is expected to develop over the region and persist for a couple days (shown below; note the swirl of air Sunday morning over the Mid-Atlantic). The air mass will remain quite moist. Rounds of slow-moving showers and thunderstorms may be frequent visitors Saturday. By Saturday night and early Sunday, the rains may become more moderate but widespread.


Forecast position of an upper level, cutoff low pressure center Sunday.

Models differ quite a bit on how much rain will fall through the weekend, and amounts may vary from location to location. They generally simulate one to two inches — on average — around Washington but will not be able to capture in advance the downpours unleashed in thunderstorm rains, which can rival such amounts in an hour alone. Here’s what the primary models project, specifically, for the District:

  • American (GFS): 2.0 inches
  • Canadian: 1.3 inches
  • European: 1.75 inches

Both the potential rainfall and soaked soils in some of our watersheds could spell larger stream flooding concerns in the Sunday-Monday time frame.