THE SYNOPTIC-SCALE ENVIRONMENT OF A WIDESPREAD
EXCESSIVE RAINFALL EVENT:
associated with Hurricane Fran on 5-6 September 1996
Part I
Learning Objectives:
(1) To apply the QPF principles discussed in lecture by considering the synoptic-scale
forcing (if any) and the anticipated mesoscale contribution for this major rainfall
event.
(2) To develop skills in Quantitative Precipitation Forecasting.
(3) To better understand the strengths and limitations of QPFs.
Materials Provided:
(1) Maps of mid-Atlantic region.
(2) Analyzed maps from 0000 UTC on 6 September for:
a. the surface (isobars every 2 mb, isodrosothermns every 2oF)
b. 850 mb heights (every 30m) w/ isodrosotherms (every 2o C)
(3) Observed 24-h rainfall valid at 1200 UTC on 4 and 5 September.
(4) Quantitative precipitation guidance produced by NCEP.
(5) Map of site "IDs" and upper air stations.
Assignment => after receiving a "shift briefing," use the HP workstation to:
(1) View various products between 0000 and 0345 UTC on 6 September.
(2) Review the forecast model runs for the 0000 UTC 6 September cycle.
(3) Prepare a 24-hour QPF on one of your blank maps for the period ending 0000 UTC 7
September.
(4) Please record your team's scientific reasoning that supports your QPF. There will be a
class discussion after the lab. Among the questions you should be asking yourself
are:
a. Which state will receive the heaviest rainfall during the forecast period?
b. What is the maximum amount of rain likely to fall in any one gauge?
c. What feature will be the primary forcing mechanism for your QPF?
d. What type of geospatial features are likely to focus the runoff?
(5) Please do not go beyond 0345 UTC 6 September when reviewing the data; please do
not go beyond the 0000 UTC 6 September model run cycle. You can review all
forecast periods within the 0000 UTC model forecasts.
THE MESOSCALE ENVIRONMENT OF AN EXCESSIVE
RAINFALL EVENT IN CENTRAL APPALACHIA:
associated with Hurricane Fran on 5-6 September 1996
Part II
Learning Objectives:
(1) To consider the mesoscale contribution to the excessive amounts of convective
rainfall during this event.
(2) To relate mesoscale features accompanying this event with similar features illustrated
during classroom presentations.
(3) To learn how this flood event transitioned from a "forecastable" synoptic-scale setup
into a short-fused, mesoscale-driven hydrometeorological event.
(4) To begin understanding the performance issues of WSR-88D with excessive tropical
precipitation.
Materials Provided:
(1)
(2)
(3)
(4)
Map of county area of responsibility (VA,WV,MD).
Observed 24-hour rainfall ending 1200 UTC on 6 September.
Virginia IFLOWS reports for 12-hr period ending 7am EDT (1100 UTC) 6 Sept.
Flash Flood Guidance (1/6/12 hr).
Assignment => After receiving an "updated shift briefing" you will review any
graphics/images/forecast model runs that were made through 1300 UTC 6 September
(including any forecast period from the 0000 UTC 6 September model runs). Then,
answer the following:
(1) What is the maximum amount of rain that will fall in any one rain gauge for the 12hour period and the 24-hour period ending 0000 UTC 7 September? How does this
compare with what you thought in Part I.
(2) What meteorological feature or ingredients served as the primary forcing mechanism
to determine the location of the heaviest rainfall? Did this change during the course
of the event?
(3) What hydrologic feature or ingredients were most important for flash flooding?
(4) What was the maximum WSR-88D rainfall estimates and how did they vary from
radar to radar?
(5) Did "warm rain processes" possibly play a role in the precipitation production?
*NOTE: The radars from Virginia northward were using a tropical Z-R conversion in the
derivation of rainfall. The Raleigh, NC radar was using the default Z-R conversion.
A WIDESPREAD EXCESSIVE RAINFALL EVENT IN THE MIDAPPALACHIANS ASSOCIATED WITH LAND-FALLING
HURRICANE FRAN, 6 SEPTEMBER 1996.
Part III: Summary
What Happened: Hurricane Fran made landfall in southeastern North Carolina shortly
before 0000 UTC 6 September 1996. In the next 24 hours it moved northward through
North Carolina and Virginia, and then continued northward into the eastern Great Lakes
region on the 7th. Major main stem river flooding (historic in several cases) and
numerous reports of flash flooding were recorded in north-central North Carolina,
Virginia, extreme eastern West Virginia, and the Maryland panhandle. All four major
river systems in Virginia experienced major flooding. The most important 24-hour period
for QPF and short term flash flood forecasts was the period ending 0000 UTC 7
September.
The rainfall reports: Numerous rainfall reports of greater than 6 inches, and some over 12
inches occurred from north central North Carolina (in the Raleigh Durham area) to
extreme western Maryland. The mountains and valleys of western Virginia and extreme
eastern West Virginia received some of the greatest accumulations. Rainfall totals
exceeded 20 inches in Page County Virginia, with the 24- and 12-hour periods ending
7pm EDT (2300 UTC) 6 September receiving as much as 15.61 and 13.29 respectively
(see included maps of IFLOWS reports). Rainfall data can also be viewed at
http://www.nws.noaa.gov/er/lwx/lwxfld2.jpg.
Excessive rainfall production: Excessive rainfall in north-central North Carolina was
associated with eyewall convection. Urban flash flooding as well flash flooding along
Crabtree Creek were particularly noteworthy. The eyewall structure degenerated as the
storm moved northward into Virginia.
In Virginia and portions of West Virginia and Maryland, the most intense rainfall was
associated with the remnants of the feeder bands transporting moisture westward into
higher terrain. The main crest of the Appalachians (including the Shenandoah and
Allegheny mountains), and the Blue ridge further to the east had a major impact on the
distribution of heaviest precipitation. The Shenandoah Valley was particularly hard hit.
Page County, Virginia received the greatest amount of road damage from flash flooding.
Naked Creek in Rockingham County, Virginia was the site of particluarly severe flash
flooding. (Reports and summaries from the appropriate WFOs are attached).
Other things to consider: Although this was a land-falling tropical cyclone scenario,
some other meteorological features typically associated with flash flooding were in place.
A negatively tilted upper level ridge was producing mainly light steering flow along and
into the mountains ranges. Impressive low-level inflow air was supplying additional low
level moisture as well as enhancing low-level upslope flow. And a deep warm and
humid layer worked with the aforementioned ingredients to enhance the precipitation
efficiency of the clouds.
And how about those radars: As with just about all flood events involving convective
rains, the radar proved to be very valuable for excellent spatial and temporal detail. And
as with many events there are some questions about the quantitative accuracy (although
this appears to be one of the better events). All but the Raleigh Durham radar (KRAX)
were coordinated with the Mid-Atlantic RFC and went with the tropical Z-R. Based on
the ground reports it appears that the tropical Z-R resulted in derived rainfall amounts
that were representative of the processes occurring during this event. However, the Z-R
issue was not (and rarely is) the only important consideration. Beam blockage from
terrain can be seen in certain areas of the Blacksburg (KFCX) and Sterling (KLWX)
radars. In addition, the Sterling radar is apparently suffering from some miscalibration
during this period causing it to read cold, a problem that could easily counteract any
improvements to Z-R conversions. The Raleigh radar (KRAX) appears to have
overestimated rainfall with the tropical Z-R. But beware, strong winds which blew down
trees in the Raleigh area certainly had an impact rain gauge catchment. Rain gauge
measurements in the Raleigh area were likely at least 20% low. These were all
considerations that the forecaster needed to be mindful of when making optimal use of
radar guidance.