National Weather Service Flood Warning Operations and Associated Gage Networks


♪ [Opening music] ♪ ♪ ♪ ♪ ♪>>Chris Horne: Good morning.
Like we said, I’m the observing program leader down at
the Weather Service office in Greenville-Spartanburg, but, at
least for the time being, I’m also managing the Hydrology
program at the office due to the retirement of our long term
service hydrologist, Patricia Tanner, back in May, and she had
been doing much of the work over the past 20 years, with
respect to hydrology, and gage operation, and coordinating with
all our users across the Western Carolinas. My splash screen
here, or my title screen, is the photograph of the USGS gage down
at the Pearson St. Bridge, and that is the official forecast
point for the French Broad at Asheville. I’m going to talk a
little bit about kind of what we do down at the Weather Service,
and the latter half of the presentation will be kind of a
quicker review of the effects of the tropical remnants 20– 10
years ago. This is the mission statement of the Weather
Service, and our primary mission is to provide weather and flood
warnings for hazardous weather for the protection of life
and property, and our area of responsibility includes
46 counties in the Western Carolinas and extreme Northeast
Georgia. Our office is here, at the Greenville-Spartanburg
Airport. 13 of the North Carolina mountain counties are
which we have responsibility over, and I’d like to point
out that neighboring offices in Morristown, Tennessee and
Blacksburg, Virginia have similar responsibilities for the
balance of the North Carolina mountains and adjacent
areas of Tennessee as well. The Hydrologic Service Area for
the Greenville-Spartanburg Weather Service office
is interesting, because the divide, which is
on the southeastern kind of fringe of the
Northeast– the North Carolina mountains, roughly
bisects our forecast area, and the major basins across the
North Carolina mountains, or on the west side of the
divide, flow to the northwest, eventually working through the
Tennessee, Ohio, and Mississippi Valleys, and our Piedmont
basins flow southeast into the Atlantic. So, it’s very
important to which side of the divide excessive rainfall falls
for us to determine which basin will have the greatest rises.
In addition to the 122 Local Warning and Forecast offices
across the United States, like the one at Greenville-
Spartanburg, there are [Alarm] Is that it? [Laughter] Was that alright. That was
quicker than I thought. There are 13 River Forecast
Centers, which provide hydrologic support to our
warning operations, as far as hydrology
goes. On either side of the divide, which I just
mentioned, we have 2 servicing River Forecast Centers – for
the basins which drain into the Atlantic, the Southeast River
Forecast Center in Peachtree City, Georgia provides main stem
forecasts for these basins, and hydraulic support and forecast
for the mountain basins are provided by the Lower
Mississippi River Forecast Center, and that’s in Slidell,
Louisiana. Just a few of the things that the River Forecast
Centers assist the local WFO in, they are the providers of what
we call “Flash Flood Guidance.” On a fairly small scale, we
often in one of the tools we use in our office, we can scale them
as low as very small subbasins, and the idea behind Flash
Flood Guidance is to provide assistance in monitoring
excessive rainfall and the issuance of flash flood
warnings. Their primary job is for the issuance of river
and stage forecasts, and flood forecasts, for the main stem
rivers, and they also provide QPE and QPF. The QP is
“quantitative precipitation,” the E is “estimates” – what
has fallen – and the F is “forecast,” ’cause a big
component of their river forecast is how much rainfall is
expected. Here is a screenshot of an application that we use
at the weather office. It is the Hydroview application. I have it
zoomed in to the county warning and forecast area of the
Greenville-Spartanburg office outlined in red, and here is the
divide, which I just mentioned. The Lower Mississippi River
Forecast Center provides daily river forecasts for 9 locations
across the North Carolina mountains within the
Greenville-Spartanburg HSA. 6 of those are actually along
the French Broad Basin, from Blantyre all the way downstream
to Hot Springs before it exits into Tennessee. There’s a couple
of other forecast locations along the Pigeon River and parts
of the upper Little Tennessee – that’s the Tukaseegee and the
Birdtown River coming out of the Smokies. Over the past 20 years,
the Greenville-Spartanburg Weather Office has worked
with local users, emergency management to establish various
impacts, so that specific river forecasts can be related to
what actually would get affected during various flood levels. I
would also like to say that, in addition to those forecast
points, there is a smattering of data, or what we call “support
points” or “stream points,” that flow into our major tributaries.
A lot these – the majority of which are installed and
maintained by the USGS and the North Carolina IFLOWS, and I
just put some of the examples of some of the smaller basins,
which feed into our main stems, which we have stream gage
information for. In addition to the Doppler radar estimates,
it’s important to have ground truth, as far as how much rain
has fallen. This is kind of a snapshot of virtually every gage
in our database. It’s a mix of official cooperative weather
observers, several hundred automated gages from various
partners of the Weather Service, which, once again, includes
the USGS, includes the North Carolina IFLOWS, and we have
another network, which started in North Carolina about 2007,
and that is the CoCoRaHS. I kind of consider that our officially
sanction volunteer rainfall observer program, and I would
like to give props to them. This is the CoCoRaHS webpage, and we
are always looking for weather observers. They report once a
day with an official looking 4 inch plastic rain gage, and
there’s quite a bit of CoCoRaHS observers across the North
Carolina Mountains, and active observers in Buncombe County are
probably greater than 40, so And it’s kind of clustered
around population too, which is– I guess that makes sense.
The IFLOWS program is one I want to talk about next. Yeah, here
we go. This is the, once again, our Hydroview application,
filtered for the IFLOWS rain and stream gages across the
North Carolina mountains and foothills. You know, it is a mix
of state, county, and locally maintained rain and stream gage
networks, and here is a couple of snapshots of what the typical
IFLOWS gage looks like. This is an IFLOWS rain gage way out in
the deep creek area near Bryson City, and this is another,
different style rain gage. The stilling well is the type of
the long pipe, here, and the majority, if not all of the
IFLOWS gages are telemetered via radio. You can kinda see
the radio antennae to various repeaters and then bay stations
across the mountains. I didn’t put a snapshot of a stream
gage, but, essentially, the data logger and communications gear –
it looks very similar, minus the rain gage, right here, which
is the tipping bucket, and it’s typically located on a bridge or
a bank, and most of the IFLOWS maintained stream gages are of
the pressure transducer type to measure stream gage levels. For
the public to access data from the IFLOWS network, there is
the AFWS page – Automated Flood Warning Systems page – and, as
you can see, this mirrors the IFLOWS plot, and anybody who
accesses this webpage can click on a dot and see, you know,
rainfall intervals of the appropriate time period, and
my example gage was a rain gage kinda just upstream of here
along the Swannanoa, near Warren Wilson College. One of the most
important things that we use automated rain gage data for, or
near real-time data, is for the calibration of our rainfall
radar estimates. Here’s a screenshot of precipitation
estimates from our WSR-88D, and, especially in the North Carolina
mountains, where we have what we call “beam blockages,” the radar
is at the GSP Airport elevation – approximately 1,000 feet. So,
the further we try to estimate data away from the Blue Ridge,
many times that precipitation amounts are underestimated,
and it’s very important to have ground truth to kind of
recalibrate and set biases for what the radar has depicted.
We do have map overlays on the background, where all of our
gages are, for a quick reference to the forecaster to check the
rainfall with reality. IFLOWS stream gages, and USGS stream
gage information, is publically available in one spot on this
AHPS page – this is I forget what the A– Automatic
Hydrologic Prediction Services – and, just want to point out,
anybody who uses this webpage, the circles are where forecasts
are provided, and the squares are just data points, where you
get just observed data. When you click on one of those
circles, for example, you get a hydrograph. It is a trace of
the past river levels and the forecast. This is the Swannanoa
gage right down here in Biltmore. One more thing I wanna
point out about the AHPS page, is the ability to view
inundation mapping for select gages, and this is from the
Biltmore gage as well. Here is the gage location along the
Swannanoa, and the best thing about this is, you can mouse
over various levels of the river along this reach, and you can
actually see what the, you know, Lidar data, which was used to
map this, what would inundate, and to what depth. And, I
want to– the second half of my presentation – just wanna talk
briefly about the– show some rainfall maps and the effects on
the gages that this near-record, or record, rainfall showed.
You saw this slide on Doug’s presentation – the track of
Hurricane Francis, and it was about the 8th to the 9th of
September where the tropical remnants, probably still a
tropical depression at this point, passed just west of the
North Carolina mountains, and copious amounts of tropical and
upslope enhanced rainfall had this stripe of excessive
rainfall – 12+ inches in spots. About 9 days later, visited by
Ivan and his remnants. A very similar track – just to the west
of the North Carolina mountains. Once again, localized, upslope
enhanced rainfall – pockets of 10+ inches across the mountains
and adjacent foothills. One more thing I want to mention about
the Ivan event is, we had kind of one of our benchmark debris
flows, and, once again, there’ll be a later presentation on this.
This is a screenshot from the Peeks Creek debris flow, which
occurred during the evening hours of September 16th. There
were several homes destroyed, injuries, and fatalities with
that. This occurred right here, in southeast Macon County. By
this point in the month, between the two rainfall events, there
was probably about 18 inches of rain which had fallen in
southeastern Macon County. I do what to show you the Jeanne
track, which was interesting, because, unlike the previous 2
remnants, that actually passed over the central Carolinas –
across the Carolina and Georgia piedmont during the latter half
of the month, and you can see here, the more uniform
distribution of locally heavy rainfall, it was not just
localized across the upslope areas, but it was more uniform
from the Blue Ridge to the Piedmont, kind of along the
track of the tropical remnants, which I thought was, you know,
a little different than the previous 2 tracks. I’d like to
conclude with just a summary of what exactly the gages along the
main stem rivers reached during that period. This is a zoomed in
version of one the first maps I showed you with our forecast
points, and this is the French Broad Valley, Blantyre, down to
Marshall, and during Francis, the major flooding, obviously,
occurred along the Swannanoa and the French Broad River. The
forecast points at Asheville, at Biltmore and Blantyre during the
8th and 9th of September, all experienced their highest crests
since 1916. There was als o flooding along the Pigeon River,
where a record crest was set on September 8th, and, during Ivan,
there actually was a second, albeit slightly less crest,
but it still was above what we’d consider major flood stage along
the Swannanoa at Biltmore. Since 2004, giving props again to the
local efforts in expanding the IFLOWS network, new stream
gages were installed along the Swannanoa and various
tributaries, also the main tributaries which flow into the
French Broad and flow through greater Hendersonville were
added as well. So, this gives us a better representation of
actually the crests moving through the river systems than
before. In addition to stream gage expansion, the City of
Asheville added a slew of rain gages across the Swannanoa
Valley, and, once again, more representative rainfall coverage
was accomplished by the addition of these rain gages across the
Swannanoa Valley, and, one of the more important things as
well, the North Fork Reservoir advises the Weather Service.
When they release, they give us information about gate openings
and how much CFS is being added to the Swannanoa, so we can pass
that information along to the River Forecast Centers to kind
of tweak their forecasts along the Swannanoa River
at Biltmore. So – So, is that
enough time or – ? [Applause] ♪ [Closing music] ♪ ♪ ♪ ♪ ♪

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