**WFO BGM LOCAL CASE STUDY -- MARCH 30, 2003.**

1)  **Introduction.**

A moderately heavy band of snow developed across central Pennsylvania
and central New York during the early morning on the 30^th^, and
continued through most of the day. Accumulations across central New York
ranged from 4 to 8 inches in a band from Elmira and Binghamton north to
Syracuse. The storm was particularly noteworthy for two reasons. First,
the surface low associated with this system was very weak and remained
well to the east of the area. The main forcing for upward motion for
this case was very strong mid-tropospheric frontogenesis. Secondly, the
models consistently forecast the heaviest snowfall to occur over the
Poconos and western Catskill mountains, yet less snow fell over those
areas than over central New York.

Sections 2 through 4 of this paper will examine this storm in more
detail. In section 2, basic weather maps associated with the case will
be shown, along with satellite, radar, snowfall and rainfall totals. In
section 3, the meteorological factors that resulted in the observed
distribution of snow and rain will be examined in greater detail. In
section 4, a hypothesis on the reason for the Eta's error in placing the
heaviest precipitation too far to the southeast will be given. Finally,
a brief summary will be given in section 5.

2)  **Basic Weather Maps and Observations**

Low pressure developed east of the mid-Atlantic coast during the morning
on the 30^th^, and tracked northeast along the coast during the day. An
associated area of precipitation developed over the central Appalachian
mountains early on the 30^th^, and continued into the evening hours. The
next two figures show the 6 and 12-hour mean sea-level pressure forecast
from the Eta model for the times verifying at a) 1800 UTC on the 30^th^
and b) 0000 UTC on the 31^st^.

![](media/image1.jpeg){width="5.9944444444444445in"
height="6.177777777777778in"}

*Figure 1a 6-hour Eta forecast mean sea-level pressure verifying at 1800
UTC on the 30^th^.*

![](media/image2.jpeg){width="5.998611111111111in"
height="6.238888888888889in"}

*Figure 1b. 12-hour Eta forecast mean sea-level pressure verifying at
0000 UTC, March 31^st^.*

These figures indicate that the surface low-pressure center was weaker
and farther off-shore than what would normally be associated with a
significant snowstorm for central New York. Figures 2a and b show the
associated Eta forecast 500 mb height fields.

![](media/image3.jpeg){width="5.99375in" height="6.270833333333333in"}

*Figure 2a. 6-hour Eta forecast 500 mb heights verifying at 1800 UTC
March 30^th^.*

![](media/image4.jpeg){width="6.0in" height="6.3in"}

*Figure 2b. 12-hour Eta forecast 500 mb heights verifying at 0000 UTC
March 31^st^.*

These figures indicate a neutrally tilted wave with moderately high
amplitude. The next two figures show the corresponding 250 mb wind speed
forecasts from the Eta model.

![](media/image5.jpeg){width="6.0in" height="6.35625in"}

*Figure 3a. 6-hour Eta forecast 250 mb wind speed verifying at 1800 UTC,
March 30^th^.*

![](media/image6.jpeg){width="6.0in" height="6.375in"}

*Figure 3b. 12-hour Eta forecast 250 mb wind speed verifying at 0000
UTC, March 31^st^.*

Pennsylvania and New York appear to be under the influence of coupled
jet streaks at 250 mb, with the area located in the left-exit region of
a southern maximum over the southeastern U.S., and the right entrance
region of an upper maximum over eastern Canada.

Figures 4a and b show infrared satellite imagery at 1200 UTC on the
30^th^ and 0000 UTC on the 31^st^.

![](media/image7.jpeg){width="5.998611111111111in"
height="6.256944444444445in"}

*Figure 4a. Infrared satellite imagery at 1200 UTC on the 30^th^.*

![](media/image8.jpeg){width="5.999305555555556in" height="6.21875in"}

*Figure 4b. Infrared satellite imagery at 1800 UTC on the 30^th^.*

At 1200 UTC on the 30^th^, a long-north south band of enhanced clouds
was evident on the infrared imagery. By 1800 UTC on the 30^th^, the
deepest moisture appeared to be pulling off to the east, however a south
to north band of lower to mid-level clouds was still evident across
Pennsylvania and New York.

The next series of images show radar imagery from the KBGM WSR-88D.

![](media/image9.jpeg){width="5.996527777777778in"
height="6.374305555555556in"}

*Figure 5a. KBGM 0.5 reflectivity at 1200 UTC on the 30^th^.*

![](media/image10.jpeg){width="5.991666666666666in"
height="6.346527777777778in"}

*Figure 5b. KBGM 0.5 reflectivity at 1600 UTC March 30^th^.*

![](media/image11.jpeg){width="5.988888888888889in"
height="6.346527777777778in"}

*Figure 5c. KBGM 0.5 reflectivity at 2000 UTC, March 30^th^.*

![](media/image12.jpeg){width="5.988888888888889in"
height="6.384027777777778in"}

*Figure 5d. KBGM 0.5 reflectivity at 2200 UTC, March 30.*

These series of images indicate that a band of moderately heavy snow
developed west of BGM by 1200 UTC. The band widened and moved east to
cover much of central New York by 1600 UTC, then began to decrease in
both width and intensity by 1800 UTC. By 2200 UTC, the band had again
begun to move east with a continued decrease in both width and
intensity.

The final two figures in this section show the observed snowfall and
rainfall associated with this system.

![](media/image13.png){width="6.0in" height="4.5in"}

*Figure 6a. Observed 24-hour snowfall through 11 pm on March 30^th^.*

![](media/image14.png){width="6.0in" height="4.5in"}

*Figure 6b. Observed rainfall from 1200 UTC on the 30^th^ through 1200
UTC on the 31^st^.*

In summary, this event was characterized by a moderately intense band of
snow, with a width varying from around 50 to 150 km. The band wobbled
back and forth during the day across central Pennsylvania and New York.
Snowfall amounts underneath the band ranged mostly from 4 to 8 inches.
Outside of the main band, lighter snows occurred, with amounts ranging
from 1 to 4 inches. The band remained west of the Poconos and Catskills
until the evening of the 30^th^, by which time it had become relatively
weak and narrow.

**3) A review of the forcing mechanisms that created the observed
snowfall distribution.**

Figure 7 shows the 6-hour Eta forecast large-scale forcing (q-vector
convergence) and omega verifying at 1800 UTC on the 30^st^.

![](media/image15.jpeg){width="6.0in" height="6.3in"}

*Figure 7. 6-hour Eta forecast large-scale forcing (300-500 mb q-vector
convergence) (green lines), and 700 mb omega (yellow lines, verifying at
1800 UTC on March 30th).*

The data in Figure 7 indicates that, while the large-scale forcing with
this event was widespread across a large portion of the mid-Atlantic and
northeast U.S. at 1800 UTC, the model omega was forecast to be much more
intense and narrow. The next figure shows the 6-hour Eta forecast 700 mb
2-D frontogenesis and 700 mb omega verifying at 1800 UTC on the 30^th^.

![](media/image16.jpeg){width="5.997222222222222in"
height="6.257638888888889in"}

*Figure 8. 6-hour Eta forecast 700 mb 2-D frontogenesis(image) and 700
mb omega (green lines).*

The data in Figure 8 show that the frontogenesis and omega were forecast
to be strongly correlated at 1800 UTC. The maximum of lift was forecast
to be just to the southeast (on the warm side) of the frontogenesis.
This would indicate that the band of heaviest snow could have been
anticipated to be located along a line from eastern Pennsylvania
northeast to western Massachusetts. The next figure shows the Eta
12-hour quantitative precipitation forecast for the period ending 0000
UTC on the 31^st^.

![](media/image17.jpeg){width="5.988888888888889in"
height="6.384027777777778in"}

*Figure 9. 12-hour Eta 12 hour precipitation forecast verifying at 0000
UTC on the 31^st^.*

The data shown in Figure 9 indicates that the Eta was forecasting an
axis of maximum precipitation of around 0.75 inches across eastern
Pennsylvania, the Catskill mountains and western New England. The axis
of maximum precipitation corresponded to the Eta's forecast axis of
maximum lift at 1800 UTC. The observed maximum of precipitation and
snowfall was approximately 100 km to the northwest of the Eta forecast.

The next figure shows a cross-section of frontogenesis and omega
verifying at 1800 UTC, taken along a southeast to northwest line from
near New York City to near Rochester, New York.

![](media/image18.jpeg){width="5.9944444444444445in"
height="6.233333333333333in"}

*Figure 10. 6-hour Eta forecast cross-section of 2-D frontogenesis
(shaded) and omega (yellow lines) verifying at 1800 UTC on the 30^th^,
taken along a line from near New York City to near Rochester, New York.*

This figure again demonstrates the close correlation between the
frontogenesis and the lift in this case. The strongest frontogenesis and
strongest lift appear to be in the layer from 900 mb to 700 mb. Again,
this corresponded to the area from northern New Jersey through the
Poconos, or around 100 km southeast of what was actually observed.

In summary, the precipitation distribution with this event appeared to
be governed primarily by the location and orientation of a strong region
of lower to mid-tropospheric frontogenesis. The Eta model appeared to
forecast the upward omega associated with this region of frontogenesis
too far to the southeast (by about 100 km).

**4. A Possible Source of Model Error.**

Identifying the reasons for error models is always difficult. In this
case, one hypothesis for the error in the Eta model forecasts may be
that the model had trouble initializing the strong temperature gradient
that was located over the northeast United States during the event.
Problems initializing the temperature gradient would have led to
problems initializing the frontogenesis, which appeared to be the key to
forecasting the distribution of lift and precipitation. The next figure
shows the Eta 00-hour 850 mb temperature forecast, along with the
observed temperatures from the RAOB network, both taken at 1200 UTC on
the 30^th^.

![](media/image19.jpeg){width="5.99375in" height="6.252777777777778in"}

*Figure 11. 850 mb RAOB data and the corresponding 00-hour Eta forecast
verifying at 1200 UTC, March 30^th^.*

In this case, the temperature gradient appears to be initialized
reasonably well across the mid-Atlantic region. The gradient may be
underestimated by one or two degrees C from SBY to IAD, and looks to be
accurate within a degree from ISP to ALB. The next figure shows the same
data, except at 700 mb.

![](media/image20.jpeg){width="5.99375in" height="6.252777777777778in"}

*Figure 12. 700 mb RAOB data and the corresponding 00-hour Eta forecast
verifying at 1200 UTC, March 30^th^.*

In contrast to the data shown in Figure 16, there appears to be some
problems with the model initialization at 700 mb. Note that, at IAD, the
observed temperature is +1 degree C, while the model is initializing the
temperature at around --3 degrees C. Meanwhile, at PIT, the observed
temperature is --15 degrees C, while the model is initializing with a

--13 degrees C. Therefore, it appears that the observed temperature
gradient from IAD to PIT is about 6 degrees C stronger than the 00-hour
forecast in the model. Farther north, the Eta appears to be better
initialized with the gradient from ALB to BUF.

In summary then, the Eta 00-hour forecast appears to be about right with
the temperature gradient across the mid-Atlantic region at 850 mb, but
too weak at 700 mb. It would follow then, that the frontogenesis may
have also been too weak at 700 mb. Stronger frontogenesis at 700 mb with
no change in the magnitude of frontogenesis at lower levels could have
resulted in the maximum of upward motion shifting to the northwest, to
be more co-located with the 700 mb frontogenesis (located northwest of
the 850 mb frontogenesis along the sloping frontal boundary). See the
figure below for an illustration:

*Figure 13. (a) Orientation of frontogenesis and associated omega as
forecast by the Eta. (b) Orientation of frontogenesis and associated
omega based on observations indicating stronger frontogenesis at 700 mb.
The maximum lift is shifted upward and toward the northwest in (b).*

Whatever the reason, the Eta model forecast lift and precipitation too
far to the northwest with this system. Areas outside of the main snow
band (such as the Pocono and Catskill mountains) were left in a zone of
relatively light precipitation.

A final aspect of this event that was likely a contributing factor to
the observed snowfall distribution was the thermodynamic profile across
the area. The next two figures show Eta forecast data from BUFKIT
forecast soundings at a) BGM where moderate snow fell, and b) Avoca,
where much less snow fell.

![](media/image21.png){width="6.626388888888889in"
height="4.879166666666666in"}

*Figure 14a. BUFKIT Eta forecast sounding at BGM, verifying at 1800 UTC
on March 30^th^.*

![](media/image22.png){width="6.747916666666667in"
height="5.1194444444444445in"}

*Figure 14b. BUFKIT Eta forecast at AVP verifying at 1800 UTC, March
30^th^.*

The data in Figure 14a indicates that the sounding at BGM exhibited a
deep layer with a temperature around --12 degrees C. This is near the
top end of the range considered favorable for snow flake production due
to growth of dendrites. By contrast, the data in Figure 14b indicates
that the forecast sounding at AVP exhibited a deep layer with a
temperature between --5 and --10 degrees C, much less favorable for
snowflake growth.

**5. Summary**

The snow event on March 30^th^, 2003 was noteworthy for two main
reasons. First, a significant snow event occurred despite the fact that
the accompanying surface low pressure center was weaker and farther to
the east than is typical for significant snowfall across central New
York. In this case, the main forcing for upward motion across central
New York was strong lower to middle-tropospheric frontogenesis.

The second interesting aspect of this case was that, despite model
forecasts to the contrary, heavier snow fell across central New York
than over northeast Pennsylvania and the western Catskills. Both the Eta
and Avn (which was not shown in this study) indicated that the heaviest
precipitation would occur over the Poconos and Catskill mountains in
association with strong lower-tropospheric frontogenesis. Evidence was
shown that the Eta model may have under-estimated the mid-tropospheric
frontogenesis in this case, which may have resulted in the associated
lift being focused farther to the northwest than what the model was
forecasting. Another factor which may have led to lower snowfall amounts
over the Poconos and Catskill mountains may have been a less favorable
thermodynamic environment for snow growth in those areas.