In Focus: Timeline and More for Two-Day Storm
*Flood Watch in effect through this evening*
For the second time this week a powerful March storm is set to barrel through the D.C. metro area. Like the Tuesday night storm, this one is expected to produce copious amounts of rain, with one inch or more a possibility. But this time the rain will come in two rounds over the course of a little more than 24 hours, rather than the single shot of precipitation we received earlier in the week in closer to nine hours.
Flooding is less of a concern with this storm since the rain will be spread out over a longer time period. Yet the threat of a period of heavy rain later this afternoon and early evening has prompted the National Weather Service to issue the Flood Watch linked to above. The main threat will be thunderstorms (slight chance today, better chance tomorrow morning) and strong winds (tomorrow afternoon and night).
Here's our best estimate on the storm timeline...
- 11 a.m. to 2 p.m. Fri: Rain spreads over area from southwest to northeast. Mid to upper 40s.
- 2 p.m. to 8 p.m. Fri: Rain, heavy at times. Slight chance of thunderstorms. Mid 40s to near 50.
- 8 p.m. to 11 p.m. Fri: Rain tapers from southwest to northeast. Mid to upper 40s.
- 11 p.m. Fri to 5 a.m. Sat: Lull in precipitation. Misty, foggy with some lighter showers. Mid to upper 40s.
Keep reading for the Round 2 timeline and more storm details. See Camden's post for the full forecast through the weekend and beyond.
- 5 a.m. to 8 a.m. Sat: Rain redevelops from southwest to northeast. Temperatures steady or rising.
- 8 a.m. to Noon Sat: Rain, heavy at times. Maybe thunderstorms, some with isolated damaging winds. Temperatures spike into the mid 50s, possibly upper 50s to near 60 south and east.
- Noon to 3 p.m.: Rain tapers from southwest to northeast. Increasingly breezy. Temperatures steady or falling through the 50s.
- 3 p.m. to 7 p.m.: Winds sustained at 15-20 mph, gusting to about 25 mph. Chance of lingering rain showers, or snow showers mainly north and west. Temperatures dropping through the 40s.
- 7 p.m. to 3 a.m.: Winds sustained at 20-25 mph, gusting to about 35 mph. Temperatures dropping into and through the 30s.
Will there be any snow?
Most of the precipitation will have probably departed the area to the northeast by the time air cold enough to make snow in the upper levels of the atmosphere arrives from the west. That said, some non-accumulating snowflakes aren't out of the question later in the afternoon or early evening, with the best chance being well north and west of the metro area.
When is the greatest chance for thunderstorms, and could they be severe?
While there's a slight chance of thunderstorms this afternoon and evening, the action really picks up as the cold front associated with the storm approaches and passes through tomorrow. The morning hours of Saturday hold the greatest potential for thunderstorms (especially south and east of town) that could produce heavy downpours and isolated high wind gusts, fueled by a surge of warm air from the south just ahead of the approaching front. Then, as and after the front passes through, the afternoon and evening are likely to bring a gradual end to the precipitation, but with the likelihood of gusty winds and falling temperatures.
The concern for severe storms is not as high as with the last storm because the atmosphere is expected to be more stable. Remember, before the storms hit on Tuesday night, daytime highs had reached the low 70s, the warmth providing enough energy to fuel severe weather that night. Still, there is the potential for some thunderstorms to produce isolated damaging winds.
Why such strong winds late tomorrow afternoon and night?
The explosive development of low pressure at the center of the storm will create a strong pressure gradient -- that is, a sharp change in pressure across a relatively short distance -- as indicated by the tightly packed black lines in the adjacent model forecast for 1 p.m. Saturday. Each line is called an isobar, or a line of constant pressure, and represents a four-millibar increase as you move outward from the center of the low. The sharper the pressure gradient, the stronger the wind.
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