Unexpectedly Heavy Near- Coastal Precipitation Due to Mesoscale Features Induced by a Landfalling Tropical Storm Alan F. Srock, Lance F. Bosart, John Molinari University at Albany/SUNY NROW VIII – November 2006 Research supported by NSF Grant #ATM

Research Objectives Tropical cyclones can produce heavy precipitation both near and far from the storm center Mesoscale features which induce lift help account for this additional rainfall –Cold Air Damming (CAD) –Coastal Front (CF) These features can be caused or enhanced by the presence of a nearby TC

Data Sources NCEP’s NARR and ECMWF’s ERA-40 Rainfall maps provided by W. Junker (NCEP/HPC) and D. Roth (NCEP/HPC) NHC Best Track (HURDAT) GIBBS Satellite Archive Surface Data from NCAR –U.S.A.F. DATSAV3, NCEP ADP, I-COADS

Overview of TS Marco (1990) In NHC Best Track from 1200 UTC 9 October to 1800 UTC 13 October Short lifetime as a tropical storm (~36 h) Other tropical cyclones in proximity (Klaus, Lili) Induced / enhanced two distinct episodes of mesoscale features –We’ll examine both instances in this study

Image courtesy D. Roth

24 h total rainfall ending 1200 UTC 11 October h total rainfall ending 1200 UTC 12 October 1990 Images courtesy W. Junker

Upper-Level Summary 200-hPa jet positioned so region of upper- level divergence located over coastal SE U.S. 500-hPa mid-latitude vorticity maxima never reached close proximity with Marco First synoptic trough likely enhanced precipitation well north of Marco

901011/0000 Surface θ e gradient (K/100km) and wind (kt)

901012/1200 Surface θ e gradient (K/100km) and wind (kt)

GIBBS Satellite Image (Visible) from 21 UTC 10 October 1990

925-hPa Back Trajectories from 1200 UTC 10 October 1990

925-hPa Back Trajectories from 0000 UTC 11 October 1990

Mesoscale Plan View Summary Synoptic front associated with midlatitude trough stays west of the Appalachians Signatures of CAD and a CF in near- surface winds, though Marco was still far south LL coastal frontogenesis indicates region of enhanced lift – collocated w/ heavy rain Back trajectories suggest shift to tropical source air coincident with first CF/CAD episode

901011/0000 Surface θ e gradient (K/100km) and wind (kt)

Cross-Section Summary Highest θ air at the surface flows inland from offshore in both cases Cold air pocket inland advects in from the northeast while cooling, creating tight θ gradient near coast Strongest ascent throughout period located at and just inland of the CF Second CF moves inland more rapidly than first

Conclusions Cold-air damming and a coastal front induced/enhanced twice – once while Marco was still quite distant These mesoscale features led to greater precipitation totals inland of the coast, days before Marco arrived The CAD/CF appears to be dependent on a sufficient land/sea thermal contrast; thus, time of year can be important