The first of several significant flood events occurred on May 15th, 2018.  A line of thunderstorms moved southward near an approaching cold front into Maryland. The line stalled across southeastern Washington and southern Frederick Counties.  Torrential rain of 4 to 6 inches fell in a very short period of time, along with impressive baseball-sized hail.

A radar loop of the event can be viewed by clicking this link (warning: large file!).

The area was on the gradient of the highest precipitable water values, as indicated by the SPC Mesoanalysis Data:

The area was also in a region of strong moisture convergence:

Widespread flooding was reported in the city of Frederick, with numerous water rescues and flood damage to the YMCA building and other structures.
The heaviest rain was southwest of the city, where roads were washed out and some structures, especially in far southeastern Washington County, suffered flood damage. In one case, a small creek that usually barely flows turned into a raging river and inundated the lower level of one home, requiring a water rescue. With some rugged terrain in that area, some landslides also occurred.

The USGS streamgages on Little Catoctin Creek were right in the line of the heaviest rainfall, and one recorded a flood wave of at least ten feet in depth before being damaged, as shown in the observed graph below:

At this location, the gauge height rose from 2 feet to 11 feet in just 20 minutes. Floodwaters from the creek damaged Jefferson Pike (Maryland 180) in this area, closing the road for weeks.  The same floodwave undermined a section of train tracks near Brunswick, temporarily stranding dozens of MARC passengers.

Although instability and moisture had decreased somewhat, the Frederick area remained in the vicinity of another stalled front during the next afternoon/evening (May 16th) and the scenario repeated itself, with another band of torrential rain (3-6 inches) that affected the same areas as the night before, plus portions of Jefferson County, West Virginia.

Local officials estimate between the two events, several million dollars in damage occurred just to public infrastructure. However, one success story was Frederick's Carroll Creek/Baker Park flood mitigation project, which kept water within the designed floodplain, and spared downtown Frederick from significant flooding, despite this graph of the stream from USGS upstream of downtown, which showed rises of around 8 feet above base flow levels with each event:

As the Frederick (and nearby area) flooding was occurring, much of the rest of the region was seeing several consecutive days of moderate rainfall. The cumulative effects of these events caused some smaller stream flooding initially, and then flooded larger rivers as the flow moved downstream. Some locations experienced multiple peaks in flood during this time due to the multiple rounds of rainfall.

To view rainfall from this time period, visit http://water.weather.gov/precip and select the archive data section under the map.

The table below lists the forecast points which flooded during this time, and their observed crests.  Crest values are preliminary until verified by gauge operators:

The table below lists the non-forecast locations which flooded, and their observed crests.  Crest values are preliminary until verified by gauge operators:

A classic wedge of cooler air left yet another stalled front stretching across the vicinity of the Blue Ridge Mountains during the overnight hours of May 21st into the 22nd.

Heavy rain up to four or five inches fell in a fairly short period of time, mainly in northwestern Rockingham County.

Significant flooding was reported in the Fulks Run and Yankeetown areas, with vehicles stranded in the floodwaters and damage reported to some homes.
That portion of northwestern Rockingham County forms the North Fork Shenandoah River, which saw a flood wave move downstream through the course of the day on the 22nd.

The table below lists the forecast points which flooded, and their observed crests.  Crest values are preliminary until verified by gauge operators:

The table below lists the non-forecast locations which flooded, and their observed crests.  Crest values are preliminary until verified by gauge operators:

The NWS has already completed a thorough review of what happened on May 27th in the Ellicott City and southwestern Baltimore County/Baltimore City areas. To view that report, click here. As shown below, heavy rain also fell earlier in that same day in the eastern part of Baltimore County, near Perry Hall and White Marsh, causing flooding and water rescues in those areas. Somewhat heavy rain in Rockingham County, Virginia also sent small streams out of their banks.

The table below lists the non-forecast locations which flooded, and their observed crests.  Crest values are preliminary until verified by gauge operators:

As Tropical Depression Alberto moved northward through the Ohio Valley, a boundary set up across central and southern Virginia between cooler air to the north and the tropical warm air to the south and west, and remained stationary into and through the night.

Torrential rain fell north of this boundary, enhanced by easterly winds pushing up against the Blue Ridge Mountains. This led to widespread rainfall totals of 5-8 inches across western Madison and Greene Counties, and much of Albemarle County.

Dozens of roads were closed, with flooding of homes. At least a dozen water rescues occurred.

Tragically, three fatalities occurred during this flooding event.  In Madison County, Virginia, two women who were camping at Graves Mountain attempted to cross a flooded stretch of water in an ATV. Tasha Hardy of Culpeper County was swept away and perished. Her companion was stranded on an island for several hours. In Albemarle County, two vehicles were swept into Ivy Creek. One vehicle's occupant was rescued, but the occupants of the other vehicle, Robert and Carol Gilges, were swept away and could not be rescued.

Heavy rain later in the day on the 31st produced 1.5-2.5 inches of rain in portions of Rockingham and Culpeper counties in Virginia, and Berkeley County, West Virginia, causing flooding, including of streams, in those areas as well.

The table below lists the forecast points which flooded during this time, and their observed crests.  Crest values are preliminary until verified by gauge operators:

The table below lists the non-forecast locations which flooded, and their observed crests.  Crest values are preliminary until verified by gauge operators:

Over a multi-day period, a series of generally weak systems (and yet another stalled front for a portion of the time) moved through the mid-Atlantic. Heavy rains throughout the first three days of the month, combined with very wet soils from the relentless period of rain in late May, caused widespread, and in some areas, quite significant, flooding.

On the evening of the 1st, widespread flooding was reported in Washington and Frederick Counties in Maryland, and in Hampshire, Morgan, and Berkeley Counties in West Virginia. This included several rescues of people from cars. Some of that flooding continued into the 2nd, and by afternoon, new flooding was occurring throughout much of north central Virginia and the eastern West Virginia panhandle. Flooding remained significant into the 3rd, with larger rivers continuing to flood through the 4th.

The table below lists the forecast points which flooded during this time, and their observed crests.  Crest values are preliminary until verified by gauge operators:

The table below lists the non-forecast locations which flooded, and their observed crests.  Crest values are preliminary until verified by gauge operators:

On June 21-22, it happened yet again.  A front was stalled across Central Virginia with deep easterly flow to the north of it. This (as also occurred at the end of May) caused significant precipitation in Madison and Greene Counties, as well as surrounding areas, particularly those with upslope to mountains.

Moderate rain persisted into the next day, as the front moved northward as a warm front. 

Widespread flooding occurred in the Rappahannock and Shenandoah basins, with some minor flooding at the susceptible area of the Potomac near Edwards Ferry.

The table below lists the forecast points which flooded during this time, and their observed crests.  Crest values are preliminary until verified by gauge operators:

The table below lists the non-forecast locations which flooded, and their observed crests.  Crest values are preliminary until verified by gauge operators: