**[Overview of Dec. 11-12, 2002 Snow and Ice Event]{.underline}**

A mixed bag of precipitation developed across central New York and
northeastern Pennsylvania during the late morning and early afternoon
hours of the 11^th^. Precipitation took the form of sleet and freezing
rain at the onset. Freezing precipitation began to mix with and change
over to snow at times from late afternoon through the evening hours.
Some rain was even reported through early evening in valley locales and
more urbanized places like Wilkes-Barre/Scranton and Syracuse. A
transition to all snow occurred during the overnight hours and into the
early morning of the 12^th.^

The main focus of this discussion will be on the late afternoon and
evening period of the 11^th^ across northeastern Pennsylvania and the
Catskills when and where the heaviest precipitation fell. 24 hour liquid
equivalent totals from 12z on the 11^th^ to 12z on the 12^th^ were
commonly over an inch across this region, with a narrow band of greater
than 1.5 inches. A stripe of 6-10" snowfall totals also accompanied the
heaviest liquid equivalent amounts.

A deep, nearly isothermal layer near 0 degrees Celsius made
precipitation type quite variant as outlined above and difficult to
pinpoint at any given time. The Albany, NY (ALB) sounding from 00z,
12/12 illustrates this.

![](media/image1.png){width="5.993055555555555in"
height="3.029166666666667in"}

Figure 1 -- ALB Sounding from 00Z, 12/12.

Radar imagery (displayed in Figures 2 through 5) indicated that heavy
bands of precipitation rotated across much of northeastern Pennsylvania
and into the Catskills during the evening hours. It is theorized that
dynamic cooling associated with these heavier bands was just enough to
tip the scales towards snow and result in the higher accumulation totals
outlined above.

![](media/image2.png){width="5.752777777777778in" height="3.91875in"}

Figure 2 -- Radar composite image for 0100Z, 12/12. Note the east to
west oriented band with enhanced returns that developed from the
southern Poconos across the lower Hudson Valley and into southern New
England. Subsequent radar images shown below depict further
intensification of the banded feature as it lifted slowly northward
through the Poconos and western Catskills later that evening.

![](media/image3.png){width="5.993055555555555in"
height="4.082638888888889in"}

Figure 3 (Above) -- Radar composite image for 0200Z, 12/12.

Figure 4 (Below) -- Radar composite image for 0300Z, 12/12.

![](media/image4.png){width="5.993055555555555in"
height="4.082638888888889in"}

![](media/image5.png){width="5.993055555555555in"
height="4.082638888888889in"}

Figure 5 -- Radar composite image for 0400Z, 12/12.

Several factors contributed to an environment conducive to large scale
lift. First, a well developed short wave trough in the mid-levels
pivoted across eastern Pennsylvania and ultimately into southern New
England as the following figures show.

![](media/image6.png){width="5.993055555555555in"
height="3.165277777777778in"}

Figure 6 -- ETA 500 mb heights and vorticity for 18Z, 12/11.

![](media/image7.png){width="5.993055555555555in"
height="3.165277777777778in"}

Figure 7 -- ETA 500 mb heights and vorticity for 00Z, 12/12.

Secondly, mid and upper level jet configurations placed northeastern
Pennsylvania underneath a diffluent wind pattern. This fostered a deep
layer of upward motion.

![](media/image8.png){width="5.993055555555555in"
height="4.082638888888889in"}

Figure 8 -- ETA 300 mb isotachs for 00Z, 12/12.

A coupled jet structure was evident at upper levels, as shown above.
Northeastern Pennsylvania was within the entrance circulation of the
northern speed maximum over the Canadian Maritimes and near the exit
region of the southern maximum across the Carolinas. Thirdly, a
pronounced conveyor belt of moisture was pointed towards the Poconos at
850 mb during the evening hours with convergent flow at this layer.

![](media/image9.png){width="5.993055555555555in"
height="3.165277777777778in"}

Figure 9 -- ETA depiction of 850 mb winds and temperatures at 00Z,
12/12.

Within the broader area of synoptic scale forcing, atmospheric
parameters appeared to favor the development of smaller scale, more
intense banded features across the Catskills and Poconos, particularly
between 00Z and 06Z on the 12^th^.

![](media/image10.png){width="5.995138888888889in"
height="4.678472222222222in"}

Figure 10 -- ETA model cross section from Newark, NJ (EWR) to Rochester,
NY (ROC)

Of 2-D Frontogenesis (light blue) and Equivalent potential vorticity
(EPV) (purple) at 00Z, 12/12.

As depicted in Fig. 10, a sloped front was present roughly between 850
and 800 mb over the Poconos and Catskills. The frontogenetical
circulation appeared to be enhanced across this region by both an
unstable layer and divergence directly above at mid-levels.

The next figure adds divergence to the fields shown in Fig. 10.

![](media/image11.png){width="5.995138888888889in"
height="4.678472222222222in"}

Figure 11 -- ETA cross section from EWR to ROC with frontogenesis
(purple), divergence (light blue), and EPV (black).

![](media/image12.png){width="5.995138888888889in"
height="4.678472222222222in"}

Figure 12 -- Same ETA cross section with geostrophic momentum surfaces
(purple) and equivalent potential temperatures (black) at 00Z, 12/12.
The steeper slope of the theta contours further suggests the presence of
instability above 600 mb over northeast PA.

By looking at Fig. 11 in particular, one could envision a situation
where a nice convergence/divergence couplet in roughly the 700-600 mb
layer above the elevated front would accelerate parcels upward towards
the zone of negative EPV starting near 550 mb. This would, in turn,
release the potential instability and help produce localized and intense
banded structures. A look back at Fig. 1 also showed the instability
aloft with fairly steep lapse rates from about 550 mb upward.

Observed precipitation and snowfall totals (Figures 13 and 14), as well
as radar imagery (some displayed above in Figures 2-5), showed that
fairly intense bands did propagate across sections of northeastern
Pennsylvania and the Catskills for a period during the evening of the
11^th^. The main axis stretched from eastern Luzerne county across
central and northern Lackawanna county, northern Wayne county, and then
across the higher terrain of northern Sullivan and southern Delaware
counties.

![](media/image13.png){width="5.997222222222222in"
height="3.5590277777777777in"}

Figure 13 -- Event snowfall totals for 12/11 and 12/12.

![](media/image14.png){width="5.991666666666666in"
height="3.870833333333333in"}

Figure 14 -- Liquid Equivalent totals for 12/11 and 12/12.

Later that night into the early morning of the 12^th^, precipitation
type became mainly snow as colder air wrapped into the system. However,
precipitation intensity also slackened off as the best forcing
mechanisms translated towards the Hudson Valley and New England. The
last two figures (15 and 16) show coastal transformation taking place at

the surface, helping to cool the lower levels. At the same time, though,
the short wave is ready to kick out into southern New England, with NVA
forming locally.

![](media/image15.png){width="5.995138888888889in"
height="4.678472222222222in"}

Figure 15 -- ETA MSLP depiction at 06Z, 12/12.

![](media/image16.png){width="5.993055555555555in"
height="4.082638888888889in"}

Figure 16 -- ETA 500 mb heights and vorticity for 06Z, 12/12.