A powerful and dynamic storm system, which spawned deadly tornadoes in Tennessee, is moving over the Mid-Atlantic, where it will bring turbulent and fast-changing weather.
In the D.C. area, the storm will have two phases. Into Sunday evening, mild and rainy weather will prevail. But into the pre-dawn hours Monday, it will turn wintry with possible wet snow. Temperatures will plunge from the 60s into the 30s between the phases.
The rainy phase will involve downpours that are expected to produce 1.5 to 3 inches of rain. The heaviest rain is probable between 6 p.m. and midnight, when flooding could occur near small streams and areas of poor drainage. An isolated gusty thunderstorm cannot be ruled out.
Rain could mix with and change to sleet and/or snow between 1 a.m. and 7 a.m. Monday, especially north and west of Interstate 95. Temperatures should remain above freezing during any wintry precipitation, but a coating of snow on grassy areas is possible in colder parts of the area.
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Most snow should end by sunrise Monday, so the morning commute probably won’t be too seriously affected. However, if you’re out in the pre-dawn hours, visibility could be reduced, and a little slush on the roads isn’t out of the question during any heavier bursts of snow or sleet.
The rainy phase of the storm
About 0.25 to 0.5 inches of rain had fallen through late Sunday morning; it was briefly heavy in some areas and even included rumbles of thunder and lightning. Periods of light to moderate rain are probable through the afternoon, with a brief pause or two possible.
Models simulate the heaviest rain coming through this evening. That’s when some pockets of flooding could occur; however, high water shouldn’t be widespread as the ground is relatively dry and can absorb considerable moisture.
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Despite the possibility of flooding, the rain should be mostly beneficial, because many parts of the region have a rainfall deficit of at least 5 to 10 inches for the year.
Precipitation should lessen in intensity after midnight, when it may transition more into the frozen form.
Here’s how much rain (and melted snow) the different models project (including Sunday morning’s rain) for the District:
- American model: 1.3 inches
- Canadian: 1.35 inches
- High-resolution Canadian: 1.9 inches
- German: 2 inches
- HRRR: 2 inches
- European: 2.1 inches
- High-resolution NAM: 2.15 inches
- NAM: About 3 inches
Models generally forecast the highest rainfall totals — over 2 inches — east and southeast of the District and near the Chesapeake Bay, and lesser totals toward the mountains.
Because this storm system is packing strong high-altitude winds, the National Weather Service Storm Prediction Center has placed much of the region in a Level 1 out of 5 (marginal) risk of severe storms. Downpours Sunday evening could be accompanied by a bit of thunder and lightning and perhaps gusty winds, especially south and east of the District; however, clouds and rain should reduce atmospheric instability — the fuel for more widespread intense storms.
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Regardless of whether thunderstorms develop, some wind gusts of 30 to 40 mph are probable late tonight as the storm system intensifies along the coast.
The snowy phase
After high temperatures near 60 Sunday afternoon, the storm system’s cold front will sweep across the area around sunset, causing temperatures to plunge. They’ll drop into the 40s by 10 p.m. and into the upper 30s by 1 a.m. That’s when the transition from rain to sleet and snow will begin.
As usual, our colder areas north and west of the Beltway will experience a changeover first. It may take until 3 or 4 a.m. until areas along and east of I-95 see frozen precipitation if enough cold air arrives.
Sleet and/or snow could be briefly heavy but should end between 4 and 7 a.m. from west to east across the region as drier air moves in.
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Throughout much of the region except for the mountains, temperatures are not predicted to fall below 33 or 34 degrees, limiting the potential for much, if any, snow accumulation.
However, because the frozen precipitation is coming through at night, it could leave behind a slushy coating on grassy areas, mulch, tree limbs, car tops and trash cans — especially if it falls heavily. It’s not totally out of the question that the snow could fall hard enough to briefly allow a little slush to form on roads, mainly in our colder areas west of the Beltway.
If you’re driving during the pre-dawn hours Monday, use caution, as any snow combined with gusty winds (over 30 mph) will also reduce visibility.
The snowfall accumulation map from the National Weather Service, above, makes sense to us. The best chances of any meaningful accumulating snow — where slick spots on roads are most probable — are in the Blue Ridge and Appalachian Mountains, generally at elevations above 1,000 feet. A winter storm warning is in effect for these areas; elevations above 3,000 feet could receive 4 to 8 inches.
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Some of the high spots in northern Maryland north of Interstate 70 could also pick up a slushy inch or so.
For most areas within a one- or two-county radius of the District, the Weather Service indicates there’s about a 40 percent chance of at least 0.1 inches of snow (or a dusting) and a 20 to 30 percent chance of at least an inch — which seems right to us.
“A slushy coating to an inch of snow will make it into some of the valleys, and a slushy coating may approach the metro areas,” the Weather Service writes. “However, the most likely scenario is for the boundary layer [area near the ground] to remain above freezing, so impacts should be minimized.”
It’s also possible that cold air will arrive more slowly than predicted by models, which would result in little or no snow, especially along and east of I-95.
Skies will gradually clear after sunrise Monday, but it will be blustery and cold with highs only reaching 40 to 45 in the afternoon.
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