Flash Flood Composite Analysis in Vermont and Northern New York John M. Goff Gregory A. Hanson NWS Burlington, VT
Impetus for study • Since 1990, 20 of 25 federally declared disasters in Vermont were attributed to flooding. • Flash flood detection remains a challenging process in the Burlington, VT CWA.
Overview • North American Regional Reanalysis (NARR) composites • Atmospheric sounding composites • Distribution analysis of key sounding parameters • Spatial distribution of events
NARR Analysis Methodology • Flash flood cases between 1981 and 2010 were analyzed (30-year period). - from archived AWIPS data and a local study (Breitbach 2009). • Data availability and temporal restraints narrowed dataset to 51 total cases for NARR analysis.
NARR Analysis - Isotachs * Weak flow less than 6 m/s below 850 mb. * Modest flow at mid and upper levels. * Coupled jet structure at mid and upper levels.
NARR Analysis – Height/MSLP * Modest front or trough evident at surface. * Broad weak troughing aloft from 850 mb through 250 mb.
NARR Analysis - Moisture * Distinct moisture plume extending north/northeast along surface trough into study area. * Enhanced boundary layer moisture convergence from eastern NY into central/northern New England.
NARR Analysis - Stability * Only modest instability present with LI values generally from -2 to 0°K and CAPE from 500-800 J/kg. * Little to no convective inhibition present. * Supports idea of modest though not excessive updraft strength.
Sounding Composite Methodology • Used RAOB software • 17 of original 51 cases • Limited by local archive of RUC Bufkit data • Events classified by discrete convective character. - Type A: isolated storm - Type B: classic training storms - Type C: MCS/MCC
Sounding Composite Methodology • Sounding composites for entire dataset and for each case type • Distribution analysis of key sounding parameters performed
Mean Composite Sounding * Light winds generally 20 kt or less in the boundary layer. * Modest winds aloft in the 25 to 45 kt range. * Modest veering/WAA boundary layer profile. * Light to modest values of shear.
Mean Composite Sounding * Deep WCD of 3.6 km indicative of warm rain processes at work. * Modest CAPE/N-CAPE values support only modest updraft strength. * Low LCL values suggest inhibition of moist downdraft production and limited cell movement.
Classification of Sounding Events * a) Isolated storm, b) classic training cells, c) MCC/MCS
Type B Profile • Classic training cells… Plainfield, VT 05.27.2011
Type B Profile * 71% of cases (12); sounding parameters thus similar to the mean sounding. * Slightly less wind but more marked veering in the boundary layer. * Deepest WCD value of the three plots indicating at least the potential for Type B events to produce the highest rainfall rates.
Type A Profile • The stationary single cell… Bristol, VT - 08.28.2004
Type A Profile • * 17% of cases (3); limited dataset limited overall conclusions. • * Highest CAPE/N-CAPE and LCL heights imply a deeper/warmer boundary layer. • * Lightest and most westerly low to mid-level wind profile of the three types.
Type A Profile * Mean westerly flow at mid-levels orthogonal to western slopes of Green Mountains, suggesting a mechanism by which Type A storms could become anchored to terrain.
Type A NARR Composite Heights * While limited, data suggests Type A cases arise when a weak surface trough is bridged aloft by weak to modest ridging.
Type C Profile • Mesoscale Convective Systems… Hancock, VT 08.06.2008
Type C Profile * 12% of cases (2). * Minimal instability and coolest thermal profile among the three types. * Light winds throughout the sounding, though most strongly veered implying more marked WAA signature in lower levels.
Type C Profile Type C * Larger meso-beta scale of Type C events suggest more widespread threat of flash flooding than in Type A or B events. Type B Type A
Distribution of Key Sounding Parameters • PWAT • WBZ height • WCD • LCL height
PWAT Distribution * PWAT values not excessive (1.5 -2.0”). * Mean value of 1.77” not all that unusual, only 117.3% of normal mid-summer value (1.51”).
WBZ Height Distribution * Relatively high values ranging from 10,077’ to 14,533’ AGL. * Non-normal distribution of data, though not surprising.
WCD Distribution * 15 of 17 cases above 3.0 km and a mean value of 3.5 km. * Supports prior research (Davis, 2004) suggesting deep WCD layers are a driving factor in many flash flood events. * Therefore it is hypothesized that WCD is a more important signal during potential flash flood days than PWAT or WBZ height.
LCL Height Distribution * All but 2 cases below 1,500’ AGL and 12 cases below 1,000’ AGL. * Low values in combination with high WCDs act to inhibit dry air entrainment and subsequent moist downdraft formation, thus limiting cell propagation. * Low LCL heights seemed to have the strongest signal during potential flash flood days of the 4 parameters analyzed.
Spatial Distribution of Events • Used entire 211 case dataset from Breitbach 2009 • Effort was done to determine spatial variability of events and to determine whether orography or other factors influenced location.
New York Events * Noticeable clustering of events along eastern slopes of Adirondacks highlighting orographical influence in this region. * Population bias evident in the Lake Placid area and to a lesser extent in the Saint Lawrence Valley near Potsdam and Canton. Also little to no reports in Adirondacks outside Lake Placid area. Lake Placid/Saranac Lake area
Vermont Events * Flash flood events more homogeneous in Vermont than in New York. * Some evidence of population bias in northeastern Vermont – low density.
Conclusions - NARR Analysis • NARR analysis indicated flash flood events characterized by: - relatively weak surface to mid-level troughing - modestly coupled mid to upper level jet - south to north oriented deep moisture axis - only modest instability
Conclusions - Sounding Composites • Composite mean sounding indicated events were characterized by: - light to modest flow throughout the column - distinct veering/WAA profile in the boundary layer - modest values of CAPE/N-CAPE • Subtle differences between composite mean and individual convective types - Type B - majority of cases -> classic training - Type A - greater instability and lighter winds - Type C - cooler though with more marked veering
Conclusions - Sounding Parameters • Sounding Composite Parameters indicated events were characterized by: - modestly high PWAT values near 1.75” - a deep WCD greater than 3.0 km and WBZ heights generally from 10-14 kft AGL - very low LCL heights below 1,500’ AGL
Limitations and Further Study • Study did not examine antecedent conditions such as prior 24-hour rainfall or soil moisture. • Sample size of sounding database. • Build a more comprehensive database of archived events.
References • Breitbach, M., 2009: Flash Flooding Climatology (1975-2009) for the WFO Burlington, VT County Warning Area. Poster, NWS Eastern Region Flash Flood Conference 2009. • Chappell, C. F., 1986: Quasi-Stationary Convective Events. Mesoscale Meteorology and Forecasting, P.S. Ray, Ed., Amer. Meteor. Soc., 289-310. • Chappell, C. F., 1993: Dissecting the Flash Flood Forecasting Problem. Post-Print Volume, Third Heavy Precipitation Workshop, NOAA Tech Memo. NWS ER-87, 293-297. • Cope, A. M., and L. R. Robinson, 2007: Composite Means and Anomalies of Meteorological Parameters for Summertime Flash Flooding in the National Weather Service Eastern Region. 22nd Conf. on Wea. Anal. And Fcstg., June 2007. • Davis, R. S., 2001: Flash Flood Forecast and Detection Methods. Severe Convective Storms, Meteor. Monogr., No. 50, Amer. Meteor. Soc., 481-525. • Davis, R. S., 2004: The Impact of Tropical Rainfall Rates on Flash Flood Detection. Preprints, 22nd Conference on Severe Local Storms, AMS, compact disk. • Federal Emergency Management Agency, cited 2011: Vermont State Disaster History. [Available online at http://www.fema.gov/news/disasters_state.fema?id=50.] • Hanson, G., 2004: 28 August 2004 Flash Flood in Addison County Vermont. 6th Northeast Regional Operational Workshop, November 2004. • Harnack, R., K. Apffel, M. Georgescu, and S. Baines, 2001: The Determination of Observed Atmospheric Differences between Heavy and Light Precipitation Events in New Jersey, USA. Int. J. Climatol., 21, 1529-1560. • Jessup, S. M., and A. T. DeGaetano, 2008: A Statistical Comparison of the Properties of Flash Flooding and Nonflooding Precipitation Events in Portions of New York and Pennsylvania. Wea. Forecasting, 23, 114-130. • LaPenta, K. D., and Coauthors, 1995: The Challenge of Forecasting Significant Rain and Flooding Throughout the Eastern Region of the National Weather Service. Part I: Characteristics and Events. Wea. Forecasting, 10, 78-90. • Maddox, R. A., C. F. Chappell, and L. R. Hoxit, 1979: Synoptic and Mesoscale Aspects of Flash Flood Events. Bull. Amer. Meteor. Soc., 60, 115-123. • Mesinger, F., and Coauthors, 2006: North American Regional Reanalysis. Bull. Amer. Meteor. Soc., 87, 343-360. • National Weather Service, cited 2011: Precipitable Water Plots. [Available online at http://www.crh.noaa.gov/unr/?n=pw] • Pontrelli, M. D., G. Bryan, and J. M. Fritsch, 1999: The Madison County, Virginia, Flash Flood of 27 June 1995. Wea. Forecasting, 14, 384-404.