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  • Heavy rain has cleared out of the D.C. area, though it continues to pour in southern Anne Arundel and Prince George’s counties.
  • BWI airport has blown past its rainfall record for the day, which was previously 4.91 inches in 1955. Today is now second wettest day on record at the airport, where 6.28 inches of rain has fallen as of 5 p.m.
  • A flash flood emergency is in effect for extreme northern Anne Arundel County and far southern Baltimore County.
  • Flash flood warnings are in effect for parts of Anne Arundel, Prince George’s, Calvert, Howard, Montgomery counties and Baltimore until as late as 8:15 p.m.
  • A flash flood watch is in effect for the D.C. metro through late tonight.

Update at 4:50 p.m.: The flash flood warning for Anne Arundel County has been extended until 8:15 p.m. as heavy rain continues to douse cities south of Baltimore. Over eight inches of rain has been logged at personal weather stations in northern Anne Arundel county, and radar estimates similar totals in the area.

Radar rainfall estimates. (NWS)

Update at 4:15 p.m.:  A flash flood emergency is in effect for Brooklyn Park, Linthicum, Glen Burnie, Dundalk, and other portions of  southeast Baltimore County in Maryland. Four to six inches of rain has already fallen in these locations, and the Weather Service warns that another three inches is possible.

Update at 3:50 p.m.: Rain has cleared out of the western and northern D.C. suburbs, though heavy rain continues to fall in northern Charles, Anne Arundel, and Prince George’s counties. These locations cannot seem to catch a break today.

Update at 3:10 p.m.: Radar estimates suggest that over six inches of rain has fallen south of Baltimore. Baltimore-Washington International Airport picked up another 0.78 inches in the 2 p.m. hour, bringing their total to 5.77 inches so far. Meanwhile, Reagan National had its wettest hour so far today from 2-3 p.m., with 0.61 inches, bringing the total there to 1.41.

Radar estimated precipitation accumulation as of 3:03 p.m. (NWS)

Update at 2:40 p.m.: A flash flood warning has been issued for Prince George’s and northern Charles counties, effective until 5:30 p.m.

Update at 2:05 p.m.: Heavy rain continues to move northeast through the area. As of 2 p.m., Baltimore-Washington International Airport has seen 4.99 inches of rain, and 2.7 of that fell in the 1 p.m. hour alone. Reagan National has logged 0.8 inches so far today. Personal weather stations across the D.C. region are reporting two to four inches of rain.

While downtown D.C. has mostly been spared from the strongest downpours, another wave of heavy rain is approaching from the southwest. South of D.C., a very strong batch of rain has entered Prince George’s and Charles counties. These echoes could eventually move across the southeast Beltway. The strongest rain in the region is falling south of Baltimore, where storms continue to build over the cities of Green Haven and Glen Burnie.

Update at 1:30 p.m.: A flood warning has been issued for urban areas and small streams in Fairfax and Prince William counties in Virginia. The Weather Service warns that one to three inches of rain is possible in these areas, and that streams may rise past their banks. Low lying areas and urban streets could also see flooding. Update at 1:15 p.m.: Within the 12 p.m. hour alone, 1.49 inches of rain fell at Baltimore-Washington International Airport. 2.27 inches have fallen thus far today at the airport. Three people have been rescued from their flooded car on Beach Drive near Connecticut Avenue, as Rock Creek is filling past its banks.

Update at 12:45 p.m.: The flash flood warning has been extended into Anne Arundel county and Baltimore, and further south inside the Beltway. This warning is now effective until 3:45 p.m. ET.

Flash flood warning in lime green, effective until 3:45 p.m. ET. (NWS advisories via Weather Underground)


Original post at 12:24 p.m.:

A weather system uncharacteristic of mid-August is unfolding across our region today.  Waves of moderate to heavy rainfall have already begun to pass through the region as a warm front approaches from the south, prompting flash flood watches and warnings. This evening, an occluded front moves through, increasing the risk of severe storms.

Some locations could see serious flooding as heavy rain is likely to continue for the next few hours.

A flash flood warning is in effect for far northern D.C., as well as southeast Montgomery, south central Howard, and north central Prince Georges counties until 2:45 p.m. In addition, a flash flood watch is in effect for the D.C. metro and counties to the north and south through late Tuesday night.

A flash flood watch (dark green) is in effect for much of the D.C. region until late Tuesday night. A flash flood warning (lime green) is in effect until 2:45 p.m. ET for areas north of D.C. (NWS advisories via Weather Underground)

While heavy rain and flooding will be the primary threat through the afternoon hours, severe storms are possible starting this evening. The Storm Prediction Center foresees a low chance of severe storms across the Mid Atlantic, with a 15 percent chance of damaging wind and 5 percent risk of tornadoes. However, the thick cloud cover and the cooling effect of rain should greatly limit atmospheric instability, and thus severe thunderstorms, across our region. Should severe storms erupt, the timing would be evening hours, from 6:00 p.m. until midnight.

Technical details of severe storm threat

A series of forecast weather maps for the surface is shown in figures 1 through 3, illustrating the predicted evolution of this weather system. We are dealing with a primary low pressure system moving through the Great Lakes (figure 1).

Ahead of the low, high pressure over the northwestern Atlantic has built in a wedge of cool, marine air over the Mid-Atlantic. Evaporation from bouts of heavy rain has further chilled this air, such that many locations struggled to reach 70 degrees this morning.

The warm front separating this cool air from much warmer air is located far to our south and west. A deep, moist current called a “warm conveyor belt” is lifting copious amounts of moisture over the gentle slope of the warm front, ensuring widespread, continuous rain.

Figure 1. Forecast surface analysis for 2 p.m. on Tuesday, showing areas of precipitation (green) and fronts. (NOAA)

By 8 p.m. (figure 2), the warm front and cold front have combined into an occluded front, and a weak wave of low pressure will likely form along the warm front. We will likely remain in the region of cooler, stable air, however, we might briefly break into the system’s “warm sector,” before the occluded front sweeps through around 2 a.m. Wednesday (figure 3).

Figure 2. Forecast surface analysis for 8 p.m. on Tuesday, showing areas of precipitation (green) and fronts. (NOAA)

Figure 3. Forecast surface analysis for 2 a.m. Wednesday, showing areas of precipitation (green) and fronts. (NOAA)

SPC’s portrayal of a slight risk for tornadoes over most of Maryland and central Pennsylvania is illustrated in figure 4.

Figure 4. Tornado risk on Tuesday into Wednesday morning. (SPC)

SPC’s concern for strong, rotating thunderstorms stems not so much from high instability in the atmosphere, but the potential wind shear configuration along the warm front. If low pressure develops along the warm front later today, to our south, this will enhance the curvature of winds in the lower atmosphere, actually intensifying the wind shear profile. Severe thunderstorms, however, also require minimal instability, and the chance of severe scales with the degree of instability.

Today’s setup reminds me more of an uncommon, wintertime type of severe storm scenario, termed a “high shear, low CAPE” event, in which small areas of rotation develop within a low-top line of convective storms, that might not even contain lightning.

The big question is whether enough instability will develop to support minimal growth of strong convective clouds. This morning’s Dulles balloon sounding reveals nearly zero CAPE (convective available potential instability). As the warm front approaches from the south, will breaks in the overcast develop, allowing the surface to warm into the 70s? Will the low-level winds turn more southerly, allowing warmer air to stream northward?

In these situations it’s not wise to discount SPC’s expertise, however, from local experience, a rain-chilled wedge of cool air lodged east of the Appalachians is tough to erode. The probability of severe hinges on whether we break into the warm sector, with southerly winds. This may occur only briefly, during the evening, but it may be just enough to set up a line of stronger storms along the approaching occluded front. On the other hand, the warm front may just be too stubborn to beat back the wedge of cool air. Movement of warm fronts is notoriously difficult to predict, and they sometimes show a tendency to “jump” discretely, rather than advance continuously.

In the next three figures, we show snapshots from the high resolution forecast models. First is the 4-km NAM (Figure 5), followed by the HRRR (figure 6) and WRF-ARW (figure 7), all valid for late this evening – the time when we are most likely to slip into the systems’s warm sector.

Figure 5. Future radar scenario from the high resolution NAM model. (Storm Vista Weather Models)

Figure 6. Future radar scenario from the HRRR model. (NOAA)

Figure 7. Future radar scenario from the WRF-ARW model. (NOAA)

In both the NAM and HRRR, there is the suggestion of a broken line of convective cells between the hours of 10 p.m. and 2 a.m. Coverage is spotty, at best, in the WRF. The models do not offer much support for an intense, cohesive line of storms along the occluded front.

Bearing all this in mind, there is some potential for instability to increase across our region late in the day, into evening. With wind shear in place, it is possible for a few stronger convective cells to develop, near the so-called “triple point” where the warm front, cold front and occluded front come together, which will be passing through our region this evening. I cannot rule out a few isolated, low-top supercells, and the possibility of a weak tornado or two. But the threat should remain low-end and not be widespread.

We will continue to monitor SPC’s outlooks and discussions through the day and post updates on the top of this article.

Heavy showers flooded streets and backyards in the Washington area on Tuesday, but forecasters said the weather should clear up by the end of the week. (The Washington Post)