The National Centers for Environmental Prediction's 29-km version Meso Era
Model and Weather Surveillance Radar-1988 Doppler base reflectivity data we
re used to diagnose intense mesoscale snowbands in three northeastern Unite
d States snowstorms. Snowfall rates within these snowbands were extreme and
, in one case, were close to 15 cm (6 in.) per hour. The heaviest total sno
wfall with each snowstorm was largely associated with the positioning of th
ese mesoscale snowbands. Each snowstorm exhibited strong midlevel frontogen
esis in conjunction with a deep layer of negative equivalent potential vort
icity (EPV). The frontogenesis and negative EPV were found in the deformati
on zone, north of the developing midlevel cyclone. Cross-sectional analyses
(oriented perpendicular to the isotherms) indicated that the mesoscale sno
wbands formed in close correlation to the intense midlevel frontogenesis an
d deep layer of negative EPV.
It was found that the EPV was significantly reduced on the warm side of the
midlevel frontogenetic region as midlevel dry air, associated with a midle
vel dry tongue jet, overlaid a low-level moisture-laden easterly jet, north
of each low-level cyclone. The continual reduction of EPV on the warm side
of the frontogenetic region is postulated to have created the deep layer o
f negative EPV in the warm advection zone of each cyclone. The negative EPV
was mainly associated with conditional symmetric instability (CSI). Each c
ase exhibited a much smaller region of conditional instability (CI) on the
warm side of the frontogenesis maximum for a short period of time. The CSI
and, to a lesser extent, CI are postulated to have been released as air par
cels ascended the moist isentropes, north of the warm front, upon reaching
saturation. This likely was a major factor in the mesoscale band formation
and heavy snowfall with each snowstorm.
The results indicate that model frontogenesis and EPV fields can be used to
predict the potential development of mesoscale snowbands. When a deep laye
r of negative EPV and strong midlevel frontogenesis are forecast by the mod
els, forecasters can anticipate the regions where mesoscale snowbands may f
orm. Inspection of saturation equivalent potential temperature in conjuncti
on with EPV is suggested to determine whether CI is present in a negative E
PV region. If CI is present in addition to CSI, then upright convection may
dominate over slantwise convection leading to heavier snowfall rates. The
region where the frontogenesis and negative EPV are forecast to persist the
longest (usually left of the 700-hPa low track) is where the heaviest stor
m total snowfall will occur. Once mesoscale bands are detected on radar, ac
curate short-term forecasts of areas that will receive heavy snowfall can b
e made.