CONVECTIVE STORM STRUCTURES AND AMBIENT CONDITIONS ASSOCIATED WITH SEVERE WEATHER OVER THE NORTHEAST UNITED STATES Kelly A. Lombardo and Brian A. Colle By Danielle LaFlamme and Erin Pratt

OUTLINE  Introduction  Data and Methods  Wind  Hail  Tornadoes  Cellular  Linear  Nonlinear  Comparisons  Summary

INTRODUCTION  Severe weather much less common over Northeastern U.S. than over Great Plains and Midwest  No studies have been done to identify convective structures associated with severe weather over the Northeast or over the coastal Northeast  Studies have shown how synoptic conditions support severe weather over the NE:  500 hPa flow, cyclonic directional wind shear, LL moisture, CAPE > 3000 J/kg  Elevated mixed layer  Topography

INTRODUCTION  “What is the type and frequency of severe weather associated with various convective organizational structures over the Northeast? How does this compare to the central U.S.?  How does the type of severe weather produced by specific convective organizational structures over the entire northeastern U.S. compare with that over the Northeast’s southern coastal region?  What are the lifting mechanisms and thermodynamic conditions associated with severe weather over the coastal region, and how do they vary between convective organizational structures (i.e., cellular, linear, nonlinear)?”

DATA AND METHODS  Two datasets – tornado and wind/hail  Wind/hail  May-August  NE wind, coastal wind: 276, 226  NE Hail, coastal hail: 195, 116  Tornadoes  May-August  NE tornadoes: 125  Coastal tornadoes: 27  Used 2-km NOWrad reflectivity imagery, SPC and NCDC storm report archives  For coastal regions, constructed spatial composites of ambient conditions using NARR  Generated box-and-whisker plots

DATA AND METHODS  Definitions:  Cellular convection – individual cells, clusters of cells and broken lines  Linear systems – bow echoes, squall lines with trailing stratiform rain/leading stratiform rain/parallel stratiform rain/no stratiform rain  Nonlinear systems  Area studied:

WIND EVENTS  NE:  Most common event, 68% of reports  Cellular: clusters  Linear: linear convection with no stratiform rain/with trailing stratiform rain  Nonlinear: 8 more events than clusters  Coastal:  Cellular: clusters  Linear: linear convection with no stratiform rain/with trailing stratiform rain  Nonlinear: 11% of wind events

HAIL EVENTS  1/3 of all events  NE:  Cellular: clusters  Linear: linear convection with no stratiform rain/with trailing stratiform rain  Nonlinear: slightly fewer events than clusters of cells  Coastal:  Cellular: clusters and individual  Linear: linear convection with no stratiform rain/with trailing stratiform rain  Nonlinear: least number of hail events

TORNADO EVENTS  NE:  Cellular and Linear types produce the same number of tornadoes, much more than non-linear  Coastal:  Equal chance from cellular, linear and non-linear (probably because of limited sample size)

CELLULAR EVENTS

LINEAR EVENTS

NONLINEAR SYSTEMS

COMPARISON BETWEEN ORGANIZATIONAL STRUCTURES

COMPARISON OF CONVECTIVE PARAMETERS IN NE TO CENTRAL U.S.  MUCAPE for NE coastal cellular events similar to MLCAPE for Great Plains nonsupercells  For linear events, NE MUCAPE half as large as Great Plains MLCAPE, however coastal MUCAPE is similar to Great Plains MLCAPE  Mean shear values for linear systems over the coast are less than shear values in the Plains for derecho-producing MCSs  NE severe events develop under weaker CAPE and vertical wind shear than the central U.S.  Synoptic-scale lift important to release more moderate instability in NE

COMPARISON OF ORGANIZATIONAL STRUCTURES IN NE TO CENTRAL U.S.  Over NE and Plains, nonlinear events produce 29% of severe weather events  Cellular convection responsible for more severe events over central U.S. than over NE  Linear convection responsible for more severe events over NE than central U.S.  Severe wind events over NE come from lines with trailing stratiform, while over Plains come from bow echoes  Small hail comes from bow echoes and broken lines over central U.S., while severe hail comes from broken lines  In NE, clusters of cells and lines with trailing stratiform produce more hail than bow echoes  Broken lines and lines with parallel stratiform produce tornadoes over Plains, bow echoes produce tornadoes over NE

SUMMARY  Northeast:  Severe wind: equally likely to be produced by all three convective structure types  Hail: cells produce more than linear or nonlinear  Tornadoes: primarily come from cells and lines  Coast:  Severe wind: typically from cells and lines  Hail: greater prevalence for hail events from cells  Tornadoes: no one structure is favored

SUMMARY  Understanding interactions between convective storms and Northeastern U.S. complex terrain/the Atlantic coastal boundary is important because forecasting severe weather over the NE coast is difficult due to subtle differences in conditions compared to the central U.S.

QUESTIONS?