Like a dripping sponge, the air above Washington is saturated in moisture and, where and when it’s squeezed out, intense rainfall is possible Thursday.
Pinpointing the timing and location of heavy rainfall is almost impossible. Some areas may get hit hard, while others see little. But models suggest downpours could occur anywhere, anytime Thursday, as early as the pre-dawn hours and the morning commute.
Fueled by daytime heating, the late afternoon and evening hours are most likely to produce the most numerous heavy rain cells — but the atmosphere will be more than moist enough to support a cloud burst at other times.
Models predict atmospheric moisture to approach record-high levels for July. Total precipitable water, a measure of the amount of water in a column of air between the ground and cloud level, is expected to climb to 2.1 to 2.5 inches.
The heaviest rainfall is generally predicted in Washington’s western areas, particularly in and close to the mountains, where the slopes help lift the air and intensify rainfall rates.
A deluge had already unfolded Wednesday morning east of Shenandoah National Park in west-central Virginia, where torrential rain cells parked themselves and unloaded more than five inches in just a few hours (and the rain was continuing). One personal weather station in Ruckersville, Va., reported about six inches of rain.
Greene County was hit hardest, with reports of roads and a bridge under water. Flood watches were posted for a large section of west-central Virginia.
— NWS DC/Baltimore (@NWS_BaltWash) July 5, 2017
The Washington region had seen only a few light showers through midday Wednesday, but the air was extremely moist, as indicated by dew points between 72 and 76 degrees.
The potential for heavy rainfall is expected to increase around Washington late Wednesday as a front lifts north over the area and could become a focus for storms.
The flow over the region Thursday will be somewhat weak, meaning heavy rain cells that set up over a given area may not move away quickly, promoting extremely high localized totals.
The rain is not expected to be continuous but rather to come in bursts. Extended dry periods are possible.
By early Friday, total rain amounts around the region are likely to be highly variable. Some areas may get missed by the soaking rains and pick up only a few tenths of an inch or even less. Other areas could get drenched and pick up a few inches in a short amount of time.
The potential for flooding will be somewhat mitigated by the dry ground in the region, although rainfall rates could become high enough to overcome that.
The rainfall forecast from the high-resolution NAM model (through Thursday night), below, shows how amounts are likely to vary dramatically over the region. But note that this model cannot reliably simulate what specific areas will get the most and least rain, so the specific numbers it projects should be disregarded.
A few additional showers and storms could pass through the region Friday, with locally heavy rain, but they should be hit or miss and less intense and numerous than on Thursday.