The influence of the elevated mixed layer on record high temperatures and severe weather over the Northeast US in April and May 2010

Record surface and 850-hPa temperatures were either tied or set at several locations over the Northeast US in April and May 2010, contributing to the warmest March-April-May in the 116-yr record from the Great Lakes region to New England. For example, on 7 April widespread high temperatures >30°C were observed prior to “leaf-out” over the largely deciduously vegetated interior Northeast, whereas on 26 May widespread high temperatures >35°C and dew point temperatures ~20°C, produced favorable conditions for the maintenance of a southward-propagating high-impact mesoscale convective system (MCS) over western New England. A contributing factor to the record surface and 850-hPa temperatures and severe weather during both events was the presence of an elevated mixed layer (EML).

The EML is typically characterized by a region of mid-level high lapse rates (HLRs) that originate as a surface-based mixed layer over the semi-arid Intermountain West US. These surface-based mixed layers subsequently develop a low-level capping inversion as they are advected away from their source region as an EML. A 35-yr HLR climatology from 1974 to 2008 generated from the North American radiosonde network[, however, suggests that an early-season maximum in HLRs over the Mexican Plateau can expand poleward and periodically eastward to over the north-central US from March–June as EMLs. These EMLs are typically maintained via deep subsidence on the poleward-side of an upper-level anticyclone over the south-central US.]

Backward air parcel trajectories and synoptic-scale analysis demonstrate that the 7 April and 26 May 2010 events were associated with mid-level HLRs that originated over the Mexican Plateau. These mid-level HLRs were maintained via deep subsidence on the poleward-side of an upper-level anticyclone over the south-central US and subsequently traveled poleward and eastward as an EML to over the Northeast. The presence of the EML and strong surface sensible heating in the absence of significant evapotranspiration over the Northeast on 7 April favored early-season high temperatures >30°C. Conversely, the presence of the EML and sufficient low-level moisture on 26 May produced surface-based CAPE values ~4000 J kg-1 over Albany, NY and maintained the high-impact MCS over western New England.

Given the high-impact nature of these events, the 7 April and 26 May events will be placed into historical context in terms of the HLR climatology, 850-hPa temperatures, and record high temperatures. Complementary early-season events, such as an EML over Michigan on 2 April 2010 with a record-breaking 21.0°C 850-hPa temperature, and previous record high temperatures and significant severe weather events over the Northeast (e.g., 22 May 1996), will be considered.