The November 26, 2014 banded snowfall case in southern NY Mike Evans NOAA / NWS Binghamton, NY NROW 2015

Outline Large-scale pattern Banding potential and movement High resolution model reflectivity Observed snow / liquid ratios

500 mb heights and vorticity The 500 mb flow pattern on the 26th featured a long wave trough over the eastern U.S., with a short wave lifting rapidly northward along the east coast up the east side of the long-wave trough.

250 mb heights and wind speed New York and Pennsylvania were in the right entrance region of an upper level jet streak over eastern Canada on the 26th. The right entrance region moved east of the area by around 00z on the 27th.

Sea-level pressure and 850 mb temperature At the surface, a 1006 mb low center near coastal South Carolina at 12z on the 26th deepened to 995 mb east of Cape Cold by 06z on the 27th.

80 km and 40 km NAM 18z 700-500 mb frontogenesis and temperature The 80 km resolution depiction of the NAM forecast valid at 18z on the 26th (left) showed a broad area of mid-level frontogenesis associated an with open wave. A closer look using a 40 km depiction of the NAM (right) showed two distinct frontal zones, one over eastern Pennsylvania and one to the northwest over central New York.

Frontogenesis and lapse rate valid at 18z A cross-section valid at 18z from near New York City to near Lake Ontario of frontogenesis and lapse rates is shown on this page. A southeast to northwest sloping frontal zone can be seen through the entire cross-section, along with a secondary front below the main front, over central New York. The band of heaviest snow at this time was over central New York, and appeared to be at least partly associated with the secondary front.

Schematic for a quasi-stationary snow band A schematic for the flow pattern most often associated with a stationary band of heavy snow is shown on this slide. Key features are confluent mid-level flow aligned with a band of mid-level frontogenesis in the right entrance region of an upper-level jet. Low-to-mid level warm advection is generally located southeast of the band, with cold advection to the north and weak temperature advection over the band.

Flow pattern on November 26 – 250 mb wind speed, 850-700 mb temperature advection, fgen and 700 mb heights The data on this slide is for comparison with the schematic shown on the previous slide. Many aspects of this event indicated the potential for a quasi-stationary snow band over central New York and northern Pennsylvania, including the location of the area in the right entrance of the upper jet, weak temperature advection over the area, and strong frontogenesis in a confluent mid-level flow pattern.

Radar reflectivity loop Radar loops for this event indicated the development of a pronounced band of heavy snow over central New York and northeast Pennsylvania on the 26th. The band was nearly stationary, drifting very slowly to the west through much of the day then shifting eastward toward 00z.

HRRR reflectivity forecasts Successive runs of the HRRR model valid at 18z indicated widespread moderate precipitation over central New York and northern Pennsylvania, however a clear banding signature was not indicated.

4 km NAM reflectivity forecasts Likewise, while the 4 km NAM may have indicated banding over southern Pennsylvania and New Jersey, no clear banding signal can be seen over central New York of northern Pennsylvania.

SPC storm-scale ensemble

Model QPF trends The data on these slides show the trend in model forecast liquid precipitation at Binghamton, NY (BGM), Avoca, Pennsylvania (AVP) and Syracuse, NY (SYR), with the earlier model runs on the left and later runs farther to the right. The right-most point on the graphic is the observed amount at the station. In general, models trended upward as the event approached. The NAM and GFS in particular were much to dry in early model runs, while the ECMWF appeared to have the best forecasts early in the period. The SREF forecasts appeared to be better than the NAM and GFS, but perhaps not quite as good as the ECMWF.

Liquid precipitation Observed liquid precipitation from cocorahs and NWS spotter networks is shown on this slide. Precipitation amounts decreased steadily from southeast to northwest, with amounts ranging from 0.5 to 0.75 inches in the northeast Pennsylvania, to around half an inch in central New York, to less than a quarter inch in the Finger Lakes region.

Snowfall Snowfall appeared to be maximized over the central part of the forecast area, in a band from near Cooperstown, NY southwest to Binghamton to Towanda, Pennsylvania. Amounts in that area ranged from 6 to 12 inches. Amounts in northeast Pennsylvania were slightly less, ranging from 4 to 10 inches. Amounts in the northwest from the Finger Lakes to Syracuse ranged from 2 to 5 inches.

Snow / liquid ratio The snow to liquid ratio for this event indicated that highest ratios were found in the quasi-stationary snow band from Cooperstown to Binghamton to Towanda. Ratios ranged from 15 to 1, to 20 to 1 in that band. Outside of that band, ratios ranged from 10 to 15 to 1.

Vertical motion and annotated -10 to -20 degree C layer at BGM The deep layer of upward vertical motion extending into the snow growth region with temperatures between -10 and -20 degrees C at around the time that the band was overhead (between 18z and 00z).

Temperature and vertical motion at 12z November 26th. A vertical cross section from south of New Jersey to Lake Ontario of upward vertical motion and temperature also indicated a deep layer of moderately strong upward vertical motion through the dendritic growth zone in the area of the snow band.

Summary: Interesting features associated with the November 26, 2014 snow event over central NY Large-scale environment matched the pattern previously identified for stationary snow bands (Kenyon 2013). Despite factors that indicated banding potential, high resolution models did not do a very good job depicting the development of the snow band over southern NY. This was an all snow event for all of northern Pa and central NY; snow / liquid ratios were maximized within the frontogenetic snow band over southern NY.