In the image above of radar at 9 p.m. this evening, we see two main lines of activity. The one west and southwest is what will impact us. The one near the coast is acting to rob some of the potential from the line to the west. Even so, there’s a good chance of a broken line of numerous showers and storms moving through from about 6 a.m. far west, then through the commute in the immediate area, and toward midday east and northeast.

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It is possible this timing could shift slightly later, as some weather modeling shows that. The best bet is it will be timed right to impact at least the middle and end of the commute if not the whole thing. Along with the downpours, thunder and lightning, there is still the risk of severe weather as outlined below.

From earlier…

A very powerful spring weather system will charge through the Mid-Atlantic on Thursday, setting off thunderstorms that could be severe. This is the same system triggering dangerous thunderstorms and tornadoes in the Southeast today.

The greatest threat of severe storms on Thursday is south and southeast of Washington, where some storms could produce large hail and even tornadoes. However, there is some chance of damaging storms in the immediate metro region.

Storms are possible beginning before sunrise Thursday, and several rounds of storms could pass through the region into the afternoon.

The window most favorable for severe weather, subject to change, is between about 8 a.m. and midday, meaning hazardous storms could affect the morning commute.

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Be weather-aware on Thursday and heed all watches and warnings.

Storm overview

Most likely timing? Multiple waves possible between 2 a.m. and 2 p.m. Thursday; intermittent showers could linger into evening.

Likely hazards: Torrential rain, lightning, gusty winds

Possible hazards: Damaging winds, hail, tornadoes

Rainfall amounts? Highly variable; average of 0.5-0.75 inches, but localized amounts of less than 0.25 inches and more than 1.0 inch possible.

The National Weather Service Storm Prediction has placed the Washington region in its slight-risk zone for severe storms, which is level two on its five-category scale. Areas just south of Washington, including Fredericksburg, Richmond and Norfolk, are in the enhanced-risk zone, which is level three.

“[G]reatest confidence in severe weather potential still appears focused along/east of the Interstate 95 corridor of southeastern Virginia into northeastern North Carolina during the late morning through midday hours,” the Storm Prediction Center writes. “However, given the current forecast of the track and rate of deepening of the surface low, this threat could develop northward into the Washington D.C and Baltimore metro areas.”

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While the main lines of storms are most likely to pass between the predawn and midday hours, intermittent showers could continue to cycle through the region into the late afternoon and early evening.

Technical discussion

The weather system predicted to affect the Mid-Atlantic region after midnight and into Thursday morning is expected to be very dynamic. A potent upper atmospheric wave and strong winds at jet stream level will cause surface low pressure to rapidly deepen east of the Blue Ridge.

There will be a brief window of time during which the D.C.-Baltimore region will be very close to the storm’s volatile warm sector, where strong to severe storms will erupt, before the system races off to the northeast.

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The warm sector is the zone of strong southerly flow, where a plume of warm, humid, unstable air is being drawn north from the Atlantic Ocean and Gulf of Mexico, and most conducive to severe weather. It is positioned south of the warm front and just ahead of the cold front.

In the image below, the warm sector is portrayed by the red-shaded region, the expected zone of severe storms, which encompasses southeastern Virginia, Tidewater and the lower Eastern Shore.

Washington is expected to be positioned very close to what’s known as the “triple point” mid- to late Thursday morning, where a warm front (red), a cold front (blue) and an occluded front (purple) intersect. Around this point, a new zone of low pressure is forecast to rapidly develop.

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Sometimes, severe weather can focus near the triple point — provided the air is sufficiently unstable. But the morning timing of the storms argues against significant destabilization for the immediate D.C. area.

You can see the effect of the timing of the storms and the amount of available unstable air in the figure below. It shows a time series of a measure of instability known as CAPE, convective available potential energy, over the District. These numbers are derived from ensemble forecasts made by the high-resolution models; the dark black line shows the average value of the ensembles.

The average CAPE value rises through the overnight hours and peaks just after sunrise, at around 1000 J/kg. This is a low-end value (usually, though not always, values greater than 2000 are needed for widespread severe storms), and note how rapidly it drops off by 2 p.m. Thursday.

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While CAPE will be relatively low, wind shear — another important ingredient for severe thunderstorms — will be significant. Wind shear is a measure of changing wind direction and speed with altitude.

Models predict a rapid increase in wind-speed difference between the surface and higher altitudes as shown in the diagram below.

The calculated shear of 50 knots (or 60 mph) is very significant for our region — exceptionally high, in fact. Any thunderstorms that develop and tap into this shear are likely to rotate, producing a type of storm called a supercell.

An additional facet of the shear (not shown) is a rapid change in the wind direction with altitude, namely because of the warm front so close to D.C. south-southeasterly flow on the south side of the front will “veer” or turn clockwise to out of the southwest and west close to the jet stream. The combination of strong speed increase and veering of wind with altitude is a potent one for tornadic supercells and large hail.

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The dynamic nature of the storm (rapid intensification) and strong wind shear profiles argue in favor of a widespread severe storm outbreak just to the south of D.C., but that’s assuming we remain NORTH of the warm front — in the cooler air mass, and probably under overcast conditions. In this case, our potential for strong destabilization would remain low. Let’s call this Scenario A.

Hence, there is some uncertainty. If the storm slows down and its track shifts northward, that nudges us closer to the storm’s unstable warm sector, fueling stronger thunderstorms that can tap the deep shear and develop rotation.

Also, sometimes there is enough “elevated instability” just north of the warm front, such that strong thunderstorms can get started above the sloping frontal zone, then develop roots that extend to the surface. Supercells can (and do) often develop right along the warm frontal boundary, and even slightly to the north. Under either of these situations, Scenario B, the Storm Prediction Center would probably upgrade our threat level to enhanced, and perhaps even moderate.

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Based on the available model guidance, we think Scenario A is the more likely one (60-70 percent), with the probability of Scenario B closer to 30-40 percent. However, there is non-negligible potential for significant severe weather in and around D.C.

There are a few take-aways from this discussion.

First, this will be a rapidly evolving storm, and frequent forecast updates may be required.

Second, severe weather may potentially affect our region at an unconventional time (i.e., the early morning through late morning hours).

Third, the impact could be widespread across parts of the Mid-Atlantic — including several tornadoes (a couple of which could be strong), large hail, dangerous lightning and downbursts (damaging wind gusts).

Stay tuned for updates and further discussions.