High resolution NAM model shows scattered showers and storms in the region around 10 a.m. Tuesday.

How weird is this weather? Weird enough that some gusty springlike thunderstorms may flare up Tuesday morning, potentially slowing the commute. Just five days removed from single-digit wind chills, there is the outside chance of a small and short-lived tornado in the region.

The National Weather Service has placed the area in a “marginal risk” zone for severe storms Tuesday as a cold front sweeps by. Heavy downpours, thunder and some locally strong wind gusts are possible as the front comes through between just before sunrise and midday. The National Weather Service cautions “perhaps a brief tornado” could spin up.

These storms should be hit or miss. Some areas may just deal with passing showers rather than severe weather.


National Weather Service risk assessment for severe thunderstorms Tuesday. (NOAA)

Chance of storms: 60 percent
Coverage: Scattered
Timing: 4 to 11 a.m. from southwest to northeast
Threats:

  • Heavy rain: Medium
  • Damaging winds: Low-Medium
  • Small tornadoes: Low-Medium
  • Hail: Low
  • Lightning: Low
  • Flash flooding: Low

Rainfall potential: 0.25 to 0.5 inches

The responsible front will be moving through quickly, and the rain and storms should be over in most places west of the Chesapeake Bay by midday.

Discussion

As a cold front approaches from the west early Tuesday, our region will be positioned within the stream of mild, humid, southerly flow ahead of the front — the “warm sector” of a large low-pressure system over the Great Lakes.

The flow of mild air at low levels will help destabilize the atmosphere a bit, although not to a significant extent. But the wind speeds will be increasing strongly through this layer, with height, creating a robust wind shear. The combination of shear, some weak instability and uplift triggered along the front — and as part of the larger weather system — will be sufficient to trigger bands of showers and some embedded thunderstorms during the morning.

What has us a bit concerned is the combination of these parameters, even though individually they are not excessive. A new tool under experimental use is called SHERB, short for “severe hazards in environments with reduced buoyancy.” It keys in on a combination of parameters that can lead to strong or severe, wintertime thunderstorms in the Southeast and Mid-Atlantic, under conditions of a weakly unstable atmosphere.

The image below shows the forecast SHERB parameter, using the high-resolution North American Model, for 10 a.m. Tuesday. The bull’s eye of robust SHERB values sit squarely over the entire Mid-Atlantic; however, the strongest indicator is south of Washington, over central and southeast Virginia.


(NOAA)

We are not saying this is a slam-dunk for severe thunderstorms, but we expect one or more lines of convective cells will develop. There may not be much lightning, but wind gusts could briefly exceed severe levels in isolated regions, and — as mentioned — a weak, short-lived tornado may also develop.