A massive, springtime storm is moving east with heavy snow and high winds across the Lower 48. When it reaches the Mid-Atlantic late Sunday, it may bring widespread rainfall totals exceeding 1 inch — very welcome news for our parched Washington region. The threat of severe thunderstorms is relatively low on Sunday, but a few storms overnight could generate gusty winds.

The forecast map shows a very large and dynamic low pressure system on Sunday morning. For at least the early part of the day, we will be in the system’s warm sector with abundant warm air and moisture. The threat of showers increases through Sunday afternoon as a cold front slowly approaches from the west.

The upper and lower levels of this storm are lined up to favor thunderstorms. It also an abundance of low-level humidity feeding into the warm sector — perhaps record-high levels for the day on Sunday.

Simulated forecasts from the American (GFS) and high resolution (NAM) models, shown below, reveal the potential for one to two inches of rain area-wide, with the heaviest showers Sunday night into early Monday morning. At this point, it appears that the heaviest rain may stay just east of the D.C. metro. Along a narrow corridor east of the city, 2.5 to 3 inches are possible. Heavy showers and ponding on roads could hamper the Monday morning commute.

We expect severe weather across the Southeast, but not the type of really intense outbreak threatening the Mid-South this afternoon. According to the Storm Prediction Center, the greatest concern for severe storms will be south of the D.C. region.

However, this is based on a scenario that assumes much cooler air will invade D.C. from New England on Sunday afternoon, thus keeping the warmest and most unstable air mass to our south. This is called a “backdoor front” (i.e. unlike a classic cold front, arriving from the west, a backdoor approaches from our north or even northeast). Not all of the model guidance suggests this. It’s possible D.C. will remain in the warmer air mass through Sunday evening. If this is the case, the risk of strong to severe storms may increase to Marginal, but timed for the overnight. CWG will monitor the situation and issue any updates as necessary.

Technical details

Computer models tend to struggle with the positioning and movement of these very shallow backdoor fronts. The higher resolution NAM has a higher resolution and, as you can see below, seems to better resolve the invading cold air from the northeast.

While southern Virginia basks in 75 degree air Sunday afternoon, the greater Washington region will have already slipped into the chilly mid-40s. The backdoor front lies in the zone of strong thermal gradient between northern and central Virginia. In this scenario, heavy rain is still possible but not thunderstorms.

The lower resolution GFS model, on the other hand, keeps the frontal boundary along the Mason-Dixon Line. D.C. and environs enjoy a 75-degree afternoon but now run the risk of thunderstorms in this unstable air mass.

If the unstable air persists through the night, as the cold front approaches, the stage may be set for a narrow line of squally showers and thunderstorms, right along the front, most likely sometime after midnight. Given the strong wind shear and dynamic nature of the jet stream trough, these storms may contain a few strong and even locally damaging downdrafts.

The odds may slightly favor the backdoor front scenario, as forecast by the high resolution guidance — perhaps 60-70 percent, with the warmer and potentially stormier scenario at 30-40 percent. Additionally, the late night passage of the cold front argues for a less unstable air mass than during the afternoon, which should help take the “edge” off any thunderstorms that develop. The type of storm that tends to develop in the less unstable period of day, and winter months, is termed High Shear Low CAPE — under conditions of strong dynamic forcing, but weaker thermodynamic or buoyancy forces.

Finally, it’s worth noting that amount of humidity being drawn northward into the warm sector is impressive for early April. The high humidity ribbon of air will take the form of a narrow plume, rooted in low latitudes. We look at “total precipitable water” as a measure of total humidity content in a vertical column of atmosphere. The high resolution NAM forecast for precipitable water early Monday morning is shown. Values of up to 1.6-1.7 inches stream into Northern Virginia. This is more than ample to deliver a widespread 1 to 2 inches of rain, with locally higher amounts.

Given the very dry soil, flash flooding is not likely a concern in our immediate region. Counties adjoining the Chesapeake Bay and the Eastern Shore, where rain totals may push beyond 3 inches, may have to contend with local flooding hazards.