East Potomac Golf Course, partly flooded after heavy rain, on July 24 in Washington, D.C. (Salwan Georges/The Washington Post)

Through Saturday, the D.C. region will remain immersed in a steamy air mass. And a cold front is pushing toward the region. These ingredients tend to combine to create a shower and storm threat, which is the case Friday and Saturday.

Given that the soil remains saturated from all the recent rain, we’ll be watching for a risk of flash flooding as the main threat with any of this activity over the next day or two. A few severe thunderstorms with damaging wind gusts can’t be ruled out, either.

In terms of timing, the best chance for storms is between 5 and 9 p.m. both days.

Today, areas west and northwest of the District have the highest chance of storms — about 50 percent. Odds inside the Beltway and to the east drop to 30 to 40 percent.

On Saturday, the chance of showers and storms is about 60 percent areawide.

Discussion

The front triggering storm development has slowed down somewhat, compared with earlier forecasts. It is now set to remain to the west of the Appalachians on Friday and should pass through the D.C. region sometime Saturday night. The following animation shows forecast surface charts for 8 p.m. Friday night and 8 p.m. on Saturday.


Frontal positions Friday evening and Saturday evening, from the Weather Prediction Center. (NWS)

A wind convergence zone should develop east of the mountains both Friday and Saturday, perhaps setting up along the Piedmont and urban corridor. This weak zone of low pressure may help enhance the uplift of air east of the mountains, creating showers and storms.

Both Friday and Saturday, the air mass will be sufficiently unstable to warrant showers and thunderstorms. However, with a tropical-like air mass in place, the intensity of storm updrafts should generally remain below severe levels (this is what meteorologists refer to as “weak vertical lapse rates”). In other words, storms like to produce rain more than they like to grow rapidly upward to tap into higher winds aloft and/or start producing big hail.

Friday afternoon and evening

Wind shear — a key ingredient for severe thunderstorms — will be weak Friday afternoon and evening. This means winds change very little with respect to speed and/or direction as altitude increases. Storm cells that do pop up will likely be isolated to scattered, somewhat disorganized and generally short-lived.

The NWS Storm Prediction Center believes there will be enough instability for a “marginal risk” of severe storms, which is a 1 of 5 on their scale. It seems that any severe storms will be isolated.

As far as specifics go, the high resolution models such as NAM, WRF and HRRR have not done well in the moist, unstable regime in the summer. That said, for Friday’s activity, the HRRR (shown below) develops a small cluster of cells over the mountains and moves this over central Maryland around sunset. Other than that cluster, the high resolution models show little additional activity.


HRRR forecast radar between 3 and 7 p.m. Friday.

The WRF model presents several waves of storms: one around 5 p.m., the other closer to 10 p.m.


(weatherbell.com)

The NAM (shown below) is also in agreement that a wave of storms is more likely to affect Baltimore than D.C. early Friday evening.


(weatherbell.com)

Friday’s storms will move slowly, in the relaxed flow aloft, which could increase the time they linger over any one location. An inch or more of rain is possible for some areas.

Outlook for Saturday

On Saturday afternoon, as the front closes in, storms should be more widespread locally. Additionally, wind shear will tick upward, compared with Friday. This is courtesy of an upper-level disturbance passing to our north. Storms may congeal into longer-lived clusters, and a few may have a shot at reaching strong or severe levels.

The air mass remains very humid through the atmosphere in this period, which tends to favor rain output more than anything else.


Excessive rainfall outlook from the Weather Prediction Center for Saturday. (NWS)

Compared with today, the flow aloft will be stronger and will have an orientation that is parallel to the approaching front. This may increase the threat that storms will rain repeatedly over spots. Accordingly, the NWS Weather Prediction Center has placed our region in a “marginal risk” for excessive rainfall.

Interestingly, the models portray more of a general rain rather than convective showers and thunderstorms Saturday afternoon, even though the front is on our doorstep.

Relatively high instability is forecast, and wind shear will be stronger. One uncertainty is the degree of cloud cover ahead of the front, with more widespread and thicker cloud layers possible. It may be that diminished instability (compared with Friday) will take the edge off storm intensity. Many things to watch, as always.

Ian Livingston contributed to this post.