A significant winter storm effected much of the central and eastern U.S. during the 12-14 February 2007.  A surface low pressure system moved from the southern plains to the Ohio valley producing heavy snow and near blizzard conditions through portions of the Midwest and Great Lakes.  By the morning of the 14th, a coastal low developed off the mid-Atlantic coast and moved northeast just offshore of the east coast. Areas along the coast received precipitation mostly in the form of ice pellets and freezing rain.  Further inland (portions of interior Pennsylvania, New York, and New England) received from 1-2 ft of snow (Fig. 1).  In portions of Vermont (e.g. Burlington), this was the 2nd largest snow storm and greatest 24 hr snowfall on record.  Estimates of 24 hr liquid equivalent precipitation from radar only and radar plus gauge are given in Fig. 2.
    This web page compares attributes of precipitation efficiency from GOES cloud top information, reflectivity profiles from WSR-88D radars, vertical air motion from RUC analyses, and divergence aloft from GOES water vapor winds.

• Moist ascent near the level of -15 C
  

    Ascending, saturated air near the -15 C temperature layer is an important condition for dendritic ice growth and formation of heavy snow.  The estimated pressure level of -15 C, P(-15 C), from GOES cloud top temperature and pressure are compared to measured levels from rawinsondes (Table 3) for this case study.  As seen in Table 4, the estimated levels from GOES are typically 10-50 mb too low (higher altitude).  These differences can be attributed to deviations from the moist adiabatic lapse rate (within the layer from cloud top to -15 C).  Although the upper portions of the sounding were often very close to moist adiabatic in regions of deep rising motion, the lapse rate was usually less steep a few hundred millibars above the strong inversion associated with the dome of polar/arctic air near the surface.  This error in P(-15 C) estimated from GOES is not expected to be significant for storms with intense vertical motion through a deep layer.  Note that estimated and observed pressure levels of -15 C are in the mid levels of the troposphere, close to the level of non-divergence, and are similar to heavy snow storms examined by Auer and White (1982).
    An indication of significant air ascent at -15 C can be obtained from the estimated omega variable (dP/dt) from the RUC analysis at the pressure level nearest to P(-15 C) at any grid point.  Movies of omega at 850, 700, 500, and 300 mb can be viewed in Table 1 (W(850) - W(300)).  Note that negative values of omega correspond to upward air motion.  Applying a threshold of omega at each point of the RUC grid, the values of P(-15 C) are shown only where omega is less than -5 microbar/s (background images: Table 1, column 2).  Hence, these images show only areas where significant rising motion is expected at the -15 C level. In addition, the value of the -15 C pressure level is color coded.  Note that significant rising motion at -15 C (roughly near 500 mb) covers a large area receiving snow and frozen precipitation from 12-23 UTC.  This area is to the north and west of the coastal storm as it moved northeast (surface reports can be overlayed by clicking "WX" on the figures in column 2, Table 1).  At times, there are reports of moderate snow rates outside of these areas.  This may be due uncertainties in the RUC omega fields, and/or that the threshold of omega is too constrained.
   The upper-level divergence from the GOES water vapor winds can be used as a supplement or alternative to the model omega fields (Table 1, last column).  Divergence near 300 mb is derived from motion vectors of water vapor and cloud features, and from a global model wind field (NOGAPS) used as a first guess in deriving the winds.  A strong divergent pattern is centered over the areas which received heaviest snow  (central New York to Vermont between 12-23 UTC).  While it is not possible to obtain vertical profiles of omega from the satellite winds, they do give an indication of where deep layered ascent may be occurring, with maximum ascent in the mid-levels (near the level of non-divergence).  Observations of strong divergence near 300 mb, combined with P(-15 C) near the mid-troposphere, suggest substantial ascent in the dendritic growth region.
    Another important factor in the production of snow is availability of moisture.  The figures in Table 1, column 2 contain total Precipitable Water (PW) as on overlay.  Note the very large values of PW in close proximity to the coastal storm (32 mm off the NJ coast), and moderate values in central New York state and Vermont (12 mm).

• Thermal structure in the lower troposphere and precipitation type.
  

    The precipitation type in winter storms is not only dependent on surface temperature, but also on the depth of freezing and subfreezing layers, and on the relative humidity through which precipitation falls before reaching the surface.  For example, the height of the 0 C wet bulb temperature is often used to determine rain versus snow at the surface.  Assessing the probability of rain, freezing rain, ice pellets, snow grains, or regular snow flakes, or a mixture of these elements is very challanging owing to complex microphysics and uncertainties in the detailed vertical temperature profile at any particular place and time.
    Monitoring the vertical temperature profile in the lower portions of the cloudy atmosphere requires additional information which cannot be provided by the GOES cloud top measurements. Other than in-situ samples from rawinsondes, radar can provide some information on the freezing level.  Table 2 links to movies of vertical reflectivity profiles from several WSR-88D radars in the area effected by the storm.  These profiles are derived from averages within 20-80 km of the radar at available elevation scans.  Temperatures obtained from the RUC analysis are shown as horizontal lines on these figures.  The plots (Vertical Profile of Reflectivity, VPR) were obtained from the NOAA/NSSL National Mosiac and Quantitative precipitation initiative (NMQ) web site.  A distinct maximum in the reflectivity profile exists in the Upton, NY and Mt. Holly profiles near 2 km AGL.  Such peaks in reflectivity are commonly observed where ice crystals fall into a layer of air with temperatures above 0 C and become wet.  The peak in reflectivity is quite close to the top of the elevated 0 C isotherm in the Upton, NY rawinsonde and in the RUC analysis.  Surface observations from near these sites experienced ice pellets rather than snow. 
    The Albany, NY reflectivity profiles show a peak near 1.5 km AGL which is most prominant between 16-18 UTC but is no longer visible by 20 UTC.  The 18 UTC rawinsonde at Albany shows a peak temperature near freezing (-0.1C at 1.75 km AGL) but no significant layer of above 0 C air is observed.  The 3-D reflectivity analysis at 17 UTC (Fig. 3) from NMQ reveals a band of enhanced reflectivity near 1.5 km AGL at Albany.  A vertical cross-section centered on Albany (Fig. 4) suggests that the maximum  reflectivity in the vertical profile may be associated with a convective band, with peak reflectivity in vicinity of the near-freezing level.  An extended bright band appears to the southeast of the core of highest reflectivity.  Moderate to heavy snow was reported at Albany during this period but is unknown if ice pellets were observed to the southeast.
     The Burlington, VT reflectivity profiles do not reveal a distinct peak.  However, there is some indication that a significant decrease of reflectivity with height occurs just above the -10 to -20 C level (e.g. 1615,  2115-2315 UTC).  Heavy snow was reported during this period at Burlington.
    It appears that vertical profiles of reflectivity, combined with observations of surface temperature and existing rawinsondes, would be useful in diagnosing the height of the zero degree isotherm, and possible impact on precipitation type at the surface.  In addition, the profiles may provide an indication of the height of the -15 C layer in addition to that diagnosed from the GOES data.