Memorandum for: Robert Bell, MIC - NWSO BTV
Through: Stephen Hogan, WCM - NWSO BTV
From: Scott Whittier, Hydro Focal Point - NWSO BTV
"flash" flooding occurred in north-central and northeast
Preliminary storm damage
estimates were $10+ million (personal property losses not included).
Presidential disaster areas were declared for
This report summarizes the following aspects of this hydrologic event; rainfall, river responses and operational tools and guidance available for this event.
Hydrologic Service Area (HSA) includes all of
The terrain within NWSO
Burlington’s HSA is complex and diverse, consisting of two major mountain
ranges with elevations exceeding 4000 feet (
In the not so distant
past, flash flooding has occurred in portions of the same areas (
Since 1995, flooding has
resulted in five Presidentially Declared Disaster areas in NWSO Burlington’s
Other known previously declared presidential disasters due to flooding in Vermont included; June 1973, August 1976, June 1984, August 1989, July 1990 and March 1992. All of the above, except March 1992 were the result of "convective" flash flooding (due to convective weather).
Hydro-meteorology of the Event
An upper-level ridge was
centered across western
The combination of a
favorable upper-level jet stream, a weak short wave, a tropical moisture
connection (precipitable water values greater than 1.5 inches) and convergence
along the frontal boundary accounted for the development of numerous showers
and thunderstorms in southern Quebec.
These convective rains traveled southeast into north central and
Widespread rains of 3 to 5 inches with localized amounts in access of 6 inches occurred between 03z and 12z on the 15th as observed by official NWS cooperative observers (Attachment 1). The heaviest rains fell within a 6 to 8 hour period from 04z to 12z. During this period, WSR-88D estimated rainfall rates of 1 to 1.5 inches per hour were noted at times.
Some observed rainfall totals included:
5.68 inches Albany Orleans county
5.22 inches Greensboro Orleans county
4.87 inches Groton Caledonia county
4.75 inches Westfield Orleans county
4.51 inches Eden Lamoille county
3.08 inches St. Johnsbury Caledonia county
There was also an unofficial
rainfall report from
reports from Environmental Canada indicated an elongated area of 2.5 to 4
inches from southeast of
Much of the rainfall
occurred over the steep and complex terrain of north central and northeast
devastation occurred in and around
The inflows from these tributary sources was tremendous causing water levels of the larger rivers to rise up to 3 feet/hr at times (See Table 1). This caused flooding of the larger rivers, especially at the headwater and tributary points (Attachment 2).
Some of these larger
- the Missisquoi between Richford and Sheldon
- the Lamoille between Hardwick and
- the Black between
- the Passumpsic between East Burke and
- the Wells between
Table 1: Observed rises in water levels at the following USGS gages(LST...add 5 hours to get UTC):
Greatest Rises Observed (ft)
Gage Location 15 min 30 min 1 hour 2 hour 3 hour
Missisquoi at 0015-0030 0015-0045 0015-0115 0015-0215 0015-0315
E. Berkshire 1.01 1.88 3.15 5.11 6.28
Lamoille at 0615-0630 0600-0630 0545-0645 0545-0745 0545-0845
Johnson 0.78 1.52 2.68 4.76 6.59
Passumpsic at 0615-0630 0615-0645 0600-0700 0530-0730 0530-0830
Passumpsic 0.66 1.28 2.43 4.46 6.01
Table 2: NWSO BTV Flood Stage Report - NWS E3 for July 1997
Dates Above Prior to
Flood Flood Stage Rainfall Crest
River and Station Stage From To Stage Stage Date/Time
Hydrologic Tools and Guidance
The WSR-88D One Hour Precipitation (OHP) product estimated at least 1 inch/hr rates for several volume scans during the event and Storm Total Precipitation products (STP) showed an accurate position axis of heavy rainfall in the flood ravaged area (Attachment 3).
Limitations: WSR-88D Storm Total Precipitation algorithms under-estimated actual observed rainfall by as much as 30 to 50 percent. This problem may have been the result of two factors; beam blockage and invoked Z/R relationship.
Much of the area
impacted by heavy rainfall and flooding in
Table 3: WSR-88D Storm Total Precipitation estimates vs. Observed Rainfall
Location STP estimates (Inches) Observed
Lamoille 2-3, locally 3.5 3-5+
Jay Peak 3.0-3.5 6.58
St. Johnsbury 0.8-1.5 3.08
Another possible consideration for precipitation under-estimation by the WSR-88D during this event was the maintaining of the default Z/R relationship (Z=300R**1.4)for the precipitation algorithms.
A WSR-88D Operations Support Facility (OSF) Adaptable Parameter Change authorization memorandum has allowed a change in the Z/R relationship from Z=300R**1.4 to the Rosenfield-derived Z=250R**1.2 for convective "maritime" rain events. The documentation referenced several case studies from coastal radar locations during a tropical "maritime" event (tropical storm/ Hurricane), in which rainfall estimates increased to more representative values.
In this case, a tropical
air mass was present over NWSO Burlington’s forecast area with precipitable
water values in excess of 1.5 inches and a tropical connection noted on water
vapor imagery. However, there was no tropical maritime disturbance and the
tropical connection was arriving on northwest flow from
Currently, we are awaiting Archive II data from NCDC to compare Z-R relationships to see if utilization of the Rosenfield-derived relationship would have improved radar estimates.
At this time, I believe a change to the Rosenfield-derived relationship would have improved estimates, but that beam blockage problems represent a larger threat to the under-estimating of rainfall amounts by the WSR-88D.
For example, KCXX rainfall
estimates performed well within the Lake Champlain/St. Lawrence river basins of
The USGS has 5 telemetered river gages (3 satellite, 1 phone modem, 1 dual satellite and phone) in the flood affected area:
As shown in Table 1, rises along some of these rivers were over 3 feet/hr and over 6 feet in three hours during the height of the flood event.
Routine satellite telemetry interrogation every 4 to 6 hours at headwater points such as EBKV1 and JONV1 can allow rapid rises to go undetected between routine HADS product issuances. Meanwhile, hourly interrogation of phone telemetry like PASV1 allows for earlier detection of significant rises in river levels.
Fortunately, numerous phone calls from a variety of resources (spotters, police and Vermont Emergency Management) notified our office of the magnitude of flooding upstream from these gages and eventual receipt of HADS reports assisted in our warning operations.
We (NWSO BTV) need to continue to coordinate with USGS about setting benchmarks (specific stage height and rates of rises) for each satellite telemetered DCP river gage for the generation of automatic random transmissions.
However, due to the priority of NESDIS products on the DCP channel and possible transmission delays, it’s critically important that a dual system with phone telemetry capabilities be implemented. This would better serve our (NWS) flood warning and public safety programs.
Another option, would be to review current USGS telemetry of all gages and possibly transfer the type of telemetry based on river responses.
For example, current or
proposed phone telemetry on slow responding sites such as
Ground Truth Rainfall Reports
Rainfall in this event occurred during the overnight period, lending to limited ground truth rainfall observations until the normal observation time for our cooperative observers (7 am EDT).
Although usually an active SKYWARN spotter network, the lack of any prior notification of a possible heavy rainfall event (such as a flood watch) and the time of the event (late night) likely prevented any heightened awareness for a more active network.
The only Automated Surface Observing System (ASOS) available near the general area of rainfall, but outside the observed axis of heavy rainfall, was at Morrisville (Lamoille county) with 1.50 inches.
Otherwise, we severely
lack an automated rain gage network in the state of
Flash Flood Guidance
issued by the
Average Rainfall (inches) Needed Observed Rainfall
County 3 hour 6 hour 12 hour within 6-12 hours
Lamoille 2.8 3.0 3.2 3.0 - 5.0
Note: Observed rainfall pertains to portions of counties
affected by axis of heavy rainfall and experienced flooding. Also,
The NERFC’s River Forecast products (RVF), missed the rapid changes on the Passumpsic river at Passumpsic (PASV1) due to inaccurate initialization.
The NERFC’s initialized stage for PASV1 at 12z was at 4.1 feet (according to GOES interrogation at 0930z). Meanwhile, phone interrogation by NWSO BTV at 12z showed an observed reading of 8.21 feet.
At 15z, upon receiving RVF product with improper initialization, NWSO BTV contacted NERFC with the updated river stage information and a special RVF run for PASV1 was conducted with improved results. However, just looking at the initialization column within the updated BOSRVFBTV, it appears that the 15z stage information was not utilized, as the forecast for 18z was only 0.2 feet above the 15z reading (see Table 5).
In the afternoon, another special run was conducted by NERFC with additional QPF input for the 18-06z time frame. The additional QPF forecast for the PASV1 drainage basin was doubled what had been observed earlier in the day. This additional rainfall never materialized and the subsequent crest forecast of 19+ feet was over forecasted by 6 feet.
Table 5 (below) illustrates the different initialization stages and subsequent forecast mentioned above. Also worth mentioning was the inability of the model to depict accurate crest time. The observed crest occurred at 20z/15 (13.3 feet) while the modeled crest time ranged between 00 and 12z/16 (omitting 1842z RVF).
Table 5: PASV1 Observed and Forecasted Values (feet)
Initial 15z 18z 00z 06z 12z
NMCRRA(GOES at 0930) 4.1
NWS BTV(phone at 12z) 8.2
BOSRVFBTV at 1444z 4.1 10.5 10.9 10.8 9.8
BOSRVFBTV at 1525z 4.1 12.4 12.6 12.9 11.8
BOSRVFBTV at 1842z 12.7 (18z) 13.0 13.6 19.0
There were no Satellite Precipitation Estimates (SPENES) issued for this event. This may be in part due to the nature of the shower/thunderstorm activity. Several spotters reported heavy rain with infrequent lightning or thunder, indicating tropical showers with only a few embedded thundershowers. Further, infra-red satellite imagery showed warm-topped convection with WSR-88D echo top estimates consistently under 25,000 feet.
Flood producing rain
events have become a common occurrence and are clearly the leading cause of
declared presidential disasters in
Most flash floods occur in small, diverse basins of variable terrain. These basins include streams, brooks and small rivers which are generally tributaries to larger rivers. The larger basins are monitored by a limited number of gages along some of the rivers, but no such network exists on the smaller "flashy" waterways.
The characteristics of a "flash" flood, including the quickness and origin, requires the use of timely and accurate observing and remote sensing tools, such as WSR-88D precipitation estimates and automated rain and river gages.
provide timely and accurate warnings for these smaller basins, an automated,
real-time rain gage network needs to be seriously considered for development
and installation in
In order to accomplish this goal, a cooperative effort between local, state and federal agencies must occur. Some of those involved include, but are not limited to; local towns, Agency of Natural Resources (ANR), VT Agency Of Transportation (AOT), Vermont Emergency Management (VEM), FEMA, USGS and NWS (including the Office of Hydrology).
Here are a few options on how a collaborative effort could work to accomplish this goal.
1) Local towns with
"flashy" waterways (
2) Various state agencies (including AOT, VEM and ANR) procure and provide maintenance for automated rain gages and have information relayed via VLETS or other remote system terminal to NWSO BTV for real-time use in our warning and other public safety programs.
3) VEM/FEMA purchase automated rain gages (compatible with USGS telemetry platforms) for installation on current USGS river gage platforms. Phone telemetry for precipitation data would be preferred to best utilize data and provide NWSO BTV a warning tool. USGS has tentatively agreed to provide maintenance for current and future river gage telemetry using current funding formula (50/50 share between USGS and VT ANR).
4) National Weather Service’s Office of Hydrology (OH) and Operational Support Facility for the WSR-88D (OSF) should provide funding for acquisition and maintenance of automated rain gages to study and supplement WSR-88D radar estimates. A vision of the WSR-88D Precipitation post-processing system was to acquire real-time rainfall data to compare with WSR-88D estimates and make proper adjustments in various algorithms.
KCXX has a tremendous beam
blockage problem (Attachments 4,5). In fact, a representative from OSF during
KCXX acceptance commented that KCXX has one of largest beam blockage problems
within the continental
More likely, I believe
it will probably be a fragmented collection of options that will assist NWSO
BTV and the state of
cc: Sol Summer, W/ER2
cc via e-mail: Robert Kilpatrick, Sr. SH - NWSFO ALY
Mickey Brown, W/ER1
Gary Carter, W/ER3
Fig.1) Surface and Upper Air Analysis @ 12z
Fig 2) Water Vapor and Precipitable Water Analysis
Fig 3) Storm Total Rainfall Analysis
Fig 4) KCXX Storm Total Precipitation (BIG VIEW)
Fig 5) KCXX Storm Total Precipitation (Close-Up View)
Topography Maps of