* Flash flood watch through Thursday afternoon *

Scattered to numerous showers and thunderstorms, heavy at times, are set to move across the Washington region this evening, overnight and into early Thursday. The turbulent weather is the result of a strong cold front crossing the area just as summer turns to fall, marking the autumn equinox.

Washington’s far western and northwestern suburbs, and the area to the northwest of that, are at greatest risk for flooding rain. Some of these locations already have received an inch or more through this morning.

However, a flash flood watch, previously in effect just for our western areas, has been expanded to cover the entire region through Thursday afternoon.

With most spots locally picking up only a few hundredths to tenths of an inch of rain so far, the heaviest is still on the way. An additional one to three inches, coming in at least two or three main bouts, is likely — most to the north and west — before the front driving the precipitation clears the area with a bang Thursday morning when strong thunderstorms may sweep through.

Main points

  • Several rounds of heavy showers or storms are possible: The first passes this evening, with another possible overnight. An intense closing round may come through early Thursday morning. Isolated flash flooding is a threat.
  • Severe thunderstorms are possible, especially early Thursday, as the front passes. As many as two brief tornadoes are not out of the question in the region.

Most likely hazards: Heavy rain, flooding.

Smaller chance: Damaging winds, isolated tornadoes early Thursday.

Rainfall potential: A widespread one to three inches of rain is a good bet. Some spots, probably to the north and west, could receive more than that.

All clear: Between midmorning and noon Thursday, first west and last east.

Confidence: Medium-high.

Discussion

The setup for the next day is a classic pattern for generating heavy rainfall across the broader region. Repeated bouts of rain and thunderstorm cells are generally moving from southwest to northeast until the cold front kicks moisture out of here Thursday.

More consistent heavy totals (up six to eight inches locally) are expected along and west of the Appalachians. Rain totals here of one to three inches are nonetheless impressive.

The main culprit is an unusually cold pocket of air aloft meandering through the Great Lakes region and extending into the Northeast. This jet stream dip is high-amplitude, meaning a very large dip to the south in the overall flow. Such an intense feature is triggering a broad area of rising air to its east, along and east of the mountains.

As the zone of cold air and wind shear aloft moves slowly northeastward, a zone of low pressure over the southeast Great Lakes will drag a cold front through the Mid-Atlantic. A swath of heavy rain with embedded thunderstorms — some possibly severe — is expected to regenerate several times.

Higher-end rainfall totals are a risk primarily because this is a very sluggish pattern, and the problem is that the flow through the entire atmosphere is from the south. This establishes a “conveyor belt” of sorts, in which periods of heavy rain are generated and potentially repeatedly track over the same spots.

Within that southerly current is a deep and anomalously humid air mass from the tropics, what we might call an “atmospheric river.” Two branches appear to be feeding the river — one from the Caribbean region, the other from the Gulf of Mexico.

These zones of enhanced moisture come together over the area. The heavy rain shield is likely to be at its most consolidated and intense tonight or early Thursday.

Weather modeling suggests that in the early morning, near and after sunrise, a line of more intense convective cells will develop right along the cold front and sweep through the Interstate 95 corridor. The likelihood of any severe thunderstorm cells, embedded in this line, hinges on the degree to which the air mass becomes destabilized.

The early timing is not optimum for destabilizing an air mass. However, two factors can compensate: The high humidity content of low-level air can add energy to storm updrafts, and very strong uplift along the front and ahead of the jet stream trough will add “oomph.”

Changing winds with elevation are also generally favorable for some rotating updrafts.

The Storm Prediction Center foresees a marginal risk, or level 1 out of 5, but is becoming increasingly concerned about the character of the wind shear Thursday morning. Any deeper storm cell may tap into the fast-flowing river of winds aloft, dragging down (in downdrafts) higher-momentum air to the ground — triggering isolated instances of damaging, straight-line wind gusts, some hail, or even a brief tornado.

It’s not out of the question that some portion of the Mid-Atlantic receives a severe thunderstorm watch or tornado watch, timed for the early-morning hours Thursday.