CASE STUDY: 27 June 1995
THE MESOSCALE ENVIRONMENT
OF AN EXCESSIVE RAINFALL EVENT
Learning Objectives:
(1) To consider the mesoscale forcing that determined the final location and
evolution of this major convective rainfall event.
(2) To relate the observed mesoscale features with similar features illustrated in
the classical climatological patterns of heavy rainfall events.
(3) To focus on the ingredients brought together by the pattern that turned this
from another heavy rain event into a historic rainfall and flash flood.
Materials Provided:
(1) Map of counties in Virginia.
(2) Analyzed maps from 0900 and 1200 UTC on 27 June 1995 for the surface
[isobars every 2 mb and isodrosotherms every 2 degrees F].
(3) Analyzed maps from 1200 UTC on 27 June 1995 for 850 mb [isodrosotherms
every 2 degrees C and height contours in conventional values] and for 250 mb
(streamlines).
(4) Flash flood forecast guidance.
(5) Refer to pages 43-60 in the Flash Flood Forecasting notes and consider
similarities with these western events.
Important Background Information:
Several backdoor cold fronts developed in advance of strong surface high pressure
centers that traversed eastern Canada during the week preceding June 27.
However, these fronts could push no further south than the Middle Atlantic states.
The first front moved slowly through the forecast area during the 20th and 21st
before dissipating. The front helped lift a tropical air mass ahead of it, causing
thunderstorms to develop. Meanwhile, an enlarging subtropical low over the
Mississippi Valley, whose circulation extended into the eastern Tennessee and
Ohio River Valleys, sent impulses rotating across the Appalachians and provided
added lift. One such impulse moved from northern Georgia into eastern
Tennessee late in the week of the 18th, producing locally torrential rainfall, which
caused isolated flash floods in western Virginia. Other areas received light to
moderate rains.
As the impulse moved first into western Pennsylvania, then towards the upper
Midwest, convection became more isolated. However, a small temperature-dew
point spread prevented significant drying from occurring; as a result, soils
remained saturated across the forecast area.
Early on the week of the 25th, the lead impulse had weakened after it had moved
to the upper Midwest. However, the overall circulation broadened, keeping weak
upper diffluence over much of the middle Atlantic region. At this time, a much
stronger backdoor cold front, in advance of a 1030 mb surface high over New
England, moved into the central forecast area. Notable flooding occurred in
northeast Virginia on June 26th.
Assignment: Following an updated "shift briefing", use the HP workstation to
view various charts and images between 0600 - 1215 UTC on 27 June 1995 and
the various model runs from the 1200 UTC cycle on 27 June 1995. Then:
(1) Which county or several-county area are you most confident will receive
regenerating, quasi-stationary convection during the period 1200 UTC 27 June
- 0000 UTC 28 June? Predict the maximum gauge report in this county or
counties that is likely during this period. Indicate this on the blank map and
hand in for compilation.
(2) Based on the ambient conditions, do you expect precipitation efficiency to be
higher or lower than average? What might you expect from the radar
guidance?
(3) Step ahead 2-hours at a time (1400 and 1602 UTC):
a. which county (or two counties) do you have the greatest confidence will
experience the most excessive rainfall?
b. what do you suppose is anchoring this storm?
c. where do you think the >50 dBZ echo associated with the “storm of
interest” will be at 1800 UTC? Mark this on your map and hand in.
d. Compare a radar and a satellite loop for the recent hours. How helpful
would the satellite be for the storm-scale detail relative to the radar?
CASE STUDY: 27 June 1995
THE EXCESSIVE RAINFALL EVENT OF 27 JUNE 1995:
What Happened
Narrative Summary of Events:
Heavy rain, falling on near-saturated soil, caused catastrophic flooding and flash flooding in
north central Virginia, leaving three people dead, at least 20 injured and $80 million in private
and public property damage and nearly $100 million in agricultural damage. Statewide (VA),
over 2000 homes were damaged (132 of which were destroyed), thousands of residents in small
towns were stranded, and 800 were evacuated from their homes. The President declared the
following eight counties Federal Disaster Areas: Madison, Greene, Albemarle, Culpeper,
Augusta, Warren, Orange, and Rappahannock. The estimated cost of recovery to get the farm
land suitable once more for crops may be as high as $1000 per acre.
The most severe flooding occurred in Madison County, where rainfall likely exceeded 20 inches
in a 12-hour period at higher elevations. One cooperative observer recorded 10 inches in two
hours! Other substantial flooding was reported in northern Greene, northwest Rappahannock,
northwest Culpeper, Orange, and Warren Counties during the day on the 27th. Rainfall at these
locations likely exceeded 10 inches.
River flooding was pronounced along the Rapidan and Rappahannock basins. With the ground
nearly saturated, heavy rains in Madison and Orange Counties became direct runoff and flowed
into the Rapidan. The Rapidan is a fast response river and had already exceeded flood stage on
the morning of the 27th. In fact, flooding along the Rapidan along the Greene/Madison County
line was deemed to be a 500-year event. Although the gauge was washed away, a water-mark
estimate at the Ruckersville site was 31.6 feet, smashing the previous record of 20.8 feet set in
October 1942. River flows were excessive. On the Rapidan alone, the flow of 37 billion cfs was
nearly 13 times the normal daily flow (290 million cfs). More than 500 landslides, debris flows
and debris avalanches occurred, making the storm comparable to the most severe in the central
Appalachian region.
Major highway arteries were closed for days after the floods due to washed-out bridges or
damaged road surfaces and supports. Federal Highway 29, a major thoroughfare between
Washington DC and Southwest Virginia, was cutoff when the bridge linking Madison and
Greene Counties was washed away.
The larger of two storm systems passed through the Rapidan River basin between 0500 -1000
UTC. A smaller storm system, which had horizontal dimensions less than 20 km by 20 km,
moved slowly through the upper Rapidan River basin from 1200 - 2000 UTC (but primarily
between 1425 - 1705 UTC) and produced the record flood peak. Truly devastating floods began
somewhere between 1400 -1600 UTC over southern Madison and northwest Orange Counties.
The operational WSR-88D rainfall products yielded underestimates of storm total rainfall for the
Rapidan storm by a factor of approximately 3. Underestimation of extreme rainfall has been
recognized as a problem for radar, especially for storms dominated by warm rain processes in
which the highest reflectivity values occur in the lowest portion of the cloud. A low height of the
echo-centroid on June 27 reflected the importance of warm rain processes -- features that were
distinctive in both the Big Thompson and Rapid City storms of the 1970s.
In Piedmont (Mineral County, WV), over 5.5" of rain fell with most of it falling between 1800 2000 UTC. The WSR-88D did not indicate a magnitude of heavy rain (in Mineral County) like
that which fell in Virginia. Hence, attention remained on central Virginia and no warnings were
issued for Mineral County.
Storm motion played a key role in the production of heavy rainfall for the Rapidan storm. Storm
motion and storm evolution for the Rapidan storm were closely linked to topographic features at
the scale of the ridges which extend southwest from the Blue Ridge and delineate the Rapidan
basin. The apparent storm motion was from the northeast to the southwest and generally
followed the peak of the Blue Ridge. It is important to note that the direction of storm motion
did not correspond with wind direction at any level. This is because net storm motion resulted
from propagation rather than advection (i.e., the location of growth of new components of the
storm dictated the apparent storm motion).
Key elements of the storm environment included strong boundary layer winds directed upslope
toward the Blue Ridge, weak upper level winds, high precipitable water values (52 mm at 1200
UTC on the Sterling sounding), and a near-saturated atmospheric column up to 6 km. In
particular, the WSR-88D VAD wind profile was instrumental in showing the strengthening of the
low-level easterly flow (note attached SFD issued at 3:45 AM EDT on June 27). The most
striking feature of the VAD analysis was the sharp peak in wind speed at approximately 600
meters AGL (peak wind speed at 1200 UTC was 12.5 meters per second). While the wind
direction veered from the east-southeast at the surface to southerly at 4 km, it remained steady
during the period of 0600 - 2000 UTC. As a result, the wind field contained a strong jet centered
vertically (i.e., above ground level) at approximately the elevation of the ridges bounding the
Rapidan basin and horizontally at the Rapidan section of the Blue Ridge.
SUMMARY OF IMPORTANT POINTS:
THE MADISON COUNTY FLASH FLOOD OF 27 JUNE 1995
1. The event was similar to the Maddox model for heavy rainfall events along the front range; it
bore only weak resemblance to the Maddox model of heavy rain events that involve fronts.
2. Negative tilting of the upper level ridge and its associated upper-level confluence held the
surface high pressure system in place over the northeast USA and created strong upslope flow
along the Blue Ridge Mountains of Virginia.
3. An upper level low well to the west of the Blue Ridge Mountains provided weak impulses of
energy that rotated northward around the low (much like in the Big Thompson and Rapid
City floods of the 1970s).
4. As a result, the low-level flow into the Blue Ridge Mountains was confluent and normal to
the mountain chain.
5. The depth of the warm layer over Virginia (3-4 km in depth) was similar to those found for
the Big Thompson, Rapid City, and Fort Collins (1997) floods.
6. Since the flow aloft was diffluent and weak, an environment was in place whereby
convective cells would be slow to move and possibly back-build; indeed, backbuilding
echoes were responsible for the catastrophic floods that did occur.
7. A close monitoring of mesoscale features in the surface moisture and in the low-level wind
field would be essential to provide an accurate and timely nowcast of a similar event that
might occur elsewhere across the USA.
Rainfall Duration and Intensity: Terrain-locked, backbuilding storm echoes enhanced
duration. A moist, tropical atmosphere resulted in a deep layer of very efficient precipitation
growth thus enhancing rainfall rates.
Hydrologic Response: Saturated soils from nearly a week of frequent heavy rains prevented
any significant infiltration. Near zero infiltration combined with steep terrain and excessive
rainfall rates resulted in massive amounts of runoff. Large amount of debris made for
complex runoff patterns.