Warm Season Climatology of Convective Evolution Over the Coastal Northeast U.S. Michael Charles and Brian A. Colle Institute for Terrestrial and Planetary Sciences SUNY at Stony Brook Jeffrey Tongue NOAA/NWS Upton, NY

Forecasting Northeast coastal convection is very difficult… False Alarm Rate (FAR) = Unverified warnings/total warnings (Svr tstm/torn.) ALY, BOX, OKX, and PHI from 1/1/1986 – 10/16/2003

Motivational Questions 1.What is the change in thunderstorm distribution in the Northeast from early to late summer? 2.How do thunderstorms evolve as they approach the coast? 3.Under what conditions does more significant convection occur at the coast even with cooler early summer SSTs?

NLDN sensor locations in the US

Objective Analysis Routine

Background Last year’s NROW talk:Last year’s NROW talk: –Northeast lightning climatology ( ) –Concluded that a coastal gradient existed in June, but not in August. –Hypothesized that the cooler June SST’s lead to a relatively cool marine boundary layer that weakens convection approaching the coast. This year’s research:This year’s research: –Included two more years ( ). –Investigate the inter-annual variability in the lightning distribution – Is SST the whole story?

June Density – strikes per 100 km 2

Sea Surface Temperatures 15 June August 2000

Average Lightning Distributions June August Strikes per 100 km 2 per month 800 m 50 m 600 m 200 m 400 m 100 m Terrain

June 2000 June 2004 June – Sharp coastal gradient Strikes per 100 km m 50 m 600 m 200 m 400 m 100 m Terrain

June 2002 June 2003 Strikes per 100 km 2 June – More significant coastal lightning 800 m 50 m 600 m 200 m 400 m 100 m Terrain

August – Sharp coastal gradient Strikes per 100 km 2 August 2002August m 50 m 600 m 200 m 400 m 100 m Terrain

August 2000 August 2001 Strikes per 100 km 2 August – More significant coastal lightning 800 m 50 m 600 m 200 m 400 m 100 m Terrain

Is synoptic forcing a factor in the lightning distribution?

Synoptic climatology Cases:Cases: Lightning cases defined as days that New Jersey, southeast New York or Connecticut received >10 lightning strikes.Lightning cases defined as days that New Jersey, southeast New York or Connecticut received >10 lightning strikes. Divided convection into two categories:Divided convection into two categories: –Near Front (Frontally maintained, referred to as frontal) Along surface cold front (< 100 km ahead)Along surface cold front (< 100 km ahead) Less than 200 km ahead of surface warm frontLess than 200 km ahead of surface warm front –Non-frontally maintained (referred to as pre-frontal) Propagates or develops > 100 km ahead of a cold frontPropagates or develops > 100 km ahead of a cold front > 200 km ahead of a surface warm front> 200 km ahead of a surface warm front –Both types of convection were summed into monthly totals for each year

Non-frontal: June 2000 case 00 UTC 12 th NCEP sfc. analysis00 UTC 12 th OKX sounding ~100 km storms CAPE=258 J/kg

11 June 2000 Lightning Strikes (per 100 km2) 800 m 50 m 600 m 200 m 400 m 100 m Terrain Non-frontal: June 2000 case

00 UTC 28 th NCEP sfc. analysis00 UTC 28 th OKX sounding ~100 km storms CAPE=793 J/kg Frontal: August 2001

21 August 2004 Lightning Strikes (per 100 km2) 800 m 50 m 600 m 200 m 400 m 100 m Terrain Frontal: August 2001

June average lightning distribution Frontal convection Strikes per 100 km 2 per month Pre-frontal convection 800 m 50 m 600 m 200 m 400 m 100 m Terrain

August average lightning distribution Strikes per 100 km 2 per month 800 m 50 m 600 m 200 m 400 m 100 m Terrain Frontal convectionPre-frontal convection

June - Little coastal lightning

June – Significant coastal lightning

August - Little coastal lightning

August – Significant coastal lightning

Conclusions There are large spatial gradients in lightning (convection) across the Northeast U.S. because of terrain (e.g. Wasula et al. 2002) and land-sea contrasts.There are large spatial gradients in lightning (convection) across the Northeast U.S. because of terrain (e.g. Wasula et al. 2002) and land-sea contrasts. The relatively cool SST’s in the early summer can lead to rapid weakening of convection near the coast, but there is a lot of inter-annual variability.The relatively cool SST’s in the early summer can lead to rapid weakening of convection near the coast, but there is a lot of inter-annual variability. The rapid weakening of coastal convection is favored for prefrontal (propagating) convection. Convection supported by a surface cold/warm front weakens less approaching the coast.The rapid weakening of coastal convection is favored for prefrontal (propagating) convection. Convection supported by a surface cold/warm front weakens less approaching the coast. Those months with significant lightning near the coast have more anomalous upper-level troughing around the U.S. Eastern Seaboard.Those months with significant lightning near the coast have more anomalous upper-level troughing around the U.S. Eastern Seaboard.

Future Work Convective mode:Convective mode: –Convection divided into two convective modes: Organized/long-livedOrganized/long-lived Disorganized/short-livedDisorganized/short-lived Determine the processes which favor more long- lived convection near the coast during warm season frontal passages. Is the convection more elevated?Determine the processes which favor more long- lived convection near the coast during warm season frontal passages. Is the convection more elevated? Mesoscale modeling of specific cases at < 5-km grid spacing using WRF. Mesoscale modeling of specific cases at < 5-km grid spacing using WRF.

The End