NWS State College Case Examples
Frontal Precipitation Event of 15-16 January 2013
By
Richard H. Grumm
National Weather Service State College, PA
1. Overview
A weak wave and a strong anticyclone (Fig. 1a-e) produced a precipitation event from the MidMississippi Valley into southern New England on 15-16 January 1996 (Fig. 1f). A strong frontal
boundary was present over the region with cold air to the north associated with the surface
anticyclone and warm air to the south in the region of the surface trough and to the south and
east (Fig. 2). The 850 hPa temperatures were above normal on the warm side of the boundary.
The heaviest precipitation fell over portions of the Ohio Valley and western Virginia, along and
mainly on the warm side of the 850 hPa frontal boundary. A moderate snowfall was observed on
the cold side of the storm. Central Pennsylvania received 1-4 inches of snowfall early on the 16th
of January.
There was a surge of moisture ahead of the frontal boundary (Fig. 3) with precipitable water
(PW) values in the 25 to 35mm range in the warm air. The anomalies were in the +2 to +3
range on the warm side of the southwest-to-northeast oriented frontal boundary. The 850 hPa
winds were relatively weak and out of the south-southwest during the period of precipitation
(Fig. 4). The southwesterly flow and modest 850 hPa winds produced higher values of moisture
flux and at times 3 to 4s above normal moisture flux anomalies in the warm air, in close
proximity to the region of the higher precipitation amounts (Fig. 5).
The larger scale pattern showed a deep trough over the western United States and strong ridge
over both the eastern Pacific and western Atlantic (not shown). The flow between the deep
trough and the western Atlantic ridge produced a strong 250 hPa jet (Fig. 6) with 250 hPa winds
near 100kts in the jet axis going over the implied strong Atlantic ridge. The 250 hPa wind
anomalies. Were +4 to +5s above normal near the ridge and implied a strong jet entrance region
over the eastern United States with the strong jet core on the cold side of the 850 hPa boundary
(Fig. 2).
The synoptic pattern suggested a strong frontal boundary and potential jet entrance moving over
the region. There were indications in the 700-500 hPa layer of a short-wave which was part of
the jet entrance circulation. The pattern was well suited to produce a precipitation event and had
the potential, with the sub-zero C air to the north to produce some snow. The winds at lower
levels were not indicative of a widespread high impact weather event (HIWE). Lacking strong
forcing, the event was not well predicted with significant lead-time, especially the northern edge
of the precipitation shield.
NWS State College Case Examples
This paper will document the pattern and standard anomalies associated with the eastern United
States precipitation event of 15-16 January 2013. The focus is on the standardized anomalies to
describe the pattern and on using guidance to include ensemble guidance to aid in the prediction
of this and similar events.
2. Data and Methods
The large scale pattern was reconstructed using the 00-hour forecast of the NCEP Global
Forecast System as first guess at the verifying pattern. The standardized anomalies were
computed in Hart and Grumm (2001). All data were displayed using GrADS (Doty and Kinter
1995).
The precipitation was estimated using the Stage-IV precipitation data in 6-hour increments to
produce estimates for various time periods. Snowfall data was collected from COOP and spotter
reports. The snowfall map in Figure 4 was produced from the NWS Eastern Region collective
dataset.
The NCEP global ensemble forecast system (GEFS) data were used to show the larger scale
forecasts. Overall, large scale models did not indicated a high probability of snow or mixed
precipitation in Pennsylvania.
3. Forecasts
The overall pattern was relatively well predicted. The significant issue was the exact location of
the baroclinic zone and thus where the frontal precipitation bands would set up. Thus, the focus
here is on GEFS and GFS precipitation forecasts.
The GEFS probability of 1mm of QPF (Figs. 7 & 8) showed a high probability of QPF over the
affected region and well into Pennsylvania from forecasts initialized on 10/0000 UTC through
11/000 UTC (Figs. 7a-c) but then shifted the boundary and thus the QPF threat to the south
(Figs. 7d-f). By 13/0000 UTC (Fig. 8a) the GEFS began shifting the axis of the QPF farther
north again. This see-saw movement was evident in the forecasts of 6.25 (Fig. 9) and 12.5mm of
QPF.
The NCEP GFS had the same basic issues with the axis of maximum QPF through the MidAtlantic region (Fig. 10) and no QPF into northern Maryland and Pennsylvania. But there was a
shift in the northern axis of the QPF in the 14/0000 UTC GFS , which reversed again on the
14/1200 UTC before finally correctly trending back to the north and west on 14/1200 UTC. The
GFS also introduced a 50 mm contour in the shorter range forecasts in KY and VA on 15
January. These GFS forecasts and the GEFS forecasts showed lots of variability and high
uncertainty.
NWS State College Case Examples
Table 1 shows the forecasts of QPF from the GFS and the ensemble mean from the GEFS from
10 to 15 January 2013 at a point near State College, PA, the black dot in all the Figures. The
changes in the probability of 1mm and 6.25 mm of QPF are significant and showed a farther
south than farther south solution. The GEFS provided some confidence in the forecasts but
provide no lead-time advantage over the GFS.
4. Summary
A frontal zone produced a precipitation event from the Mid-Mississippi Valley to southern New
England on 15-16 January. The northern edge of this precipitation event had 1-4 inches of snow
(Fig. 11) with some areas of sleet and freezing rain reported in southern Pennsylvania along and
within about 50 miles of the Maryland border (Table 2). Some snow reports contained sleet and
the highest snow report in Pennsylvania was 5 inches (Table 3). The heaviest precipitation was
in the warm air with over 50 mm of QPE in portions of KY and Virginia.
The forecasts of this pattern were generally good, however, the exact location of the frontal
boundary played a significant role in where and the type of precipitation that would occur. The
GFS and GEFS both initially had the precipitation to the north, close to the observed location but
over time, pushed the boundary and thus the QPF to the south. Only at shorter time lengths, on
the order of 1.5 days did the GEFS and GFS begin to pull the boundary and the QPF to the north,
closer to where it verified.
This relatively weakly forced event was not particular well predicted in term of where the higher
QPF amounts would occur and how far north the precipitation shield would extend. A forecast
none-event produced over 0.40 inches (~0.49) of liquid equivalent and 4 inches of snowfall in
State College where the GFS and GEFS predicted little significant QPF prior to 14/0000 UTC 14
January 2013 (Table 1). Clearly, the QPF amounts, precipitation types, and how far north the
precipitation shield would extend were serious forecast issues which limited the predictability
horizon of this event. This event lacked a strong cyclone and relied more on a frontal circulation.
Weakly forced events typically can have larger uncertainty than strongly forced events and this
event was dependent on a frontal zone and it is in gradients where the spread and thus
uncertainty is typically the largest.
.Acknowledgements
Local storms reports were gathered by Craig Evanego. Many of the snow and ice reports were
from FB and TWITTER.
5. References
Doty, B.E. and J.L. Kinter III, 1995: Geophysical Data Analysis and Visualization using GrADS.
Visualization Techniques in Space and Atmospheric Sciences, eds. E.P. Szuszczewicz and J.H.
Bredekamp, NASA, Washington, D.C., 209-219.
Miller, J.E. 1946: Cyclogenesis in the Atlantic coastal region of the United States. J. Meteor.,3,31-44.
NWS State College Case Examples
NWS State College Case Examples
CYCLE
GFS
GEFS
PROB12.5 PROB6.25
PROB1
00Z10JAN
0.00
1.94
4.76
4.76
28.57
12Z10JAN
0.00
1.81
0.00
9.52
33.33
00Z11JAn
0.00
1.12
4.76
0.00
9.50
12Z11JAN
0.00
0.16
0.00
0.00
4.70
00Z12JAN
0.00
0.08
0.00
0.00
0.00
12Z12JAn
0.00
0.01
0.00
0.00
0.00
00Z13JAN
0.00
0.00
0.00
0.00
0.00
12Z13JAN
0.00
0.04
0.00
0.00
0.00
00Z14JAN
1.10
1.16
0.00
0.00
28.57
12Z14JAN
9.70
5.04
0.00
33.33
85.71
00Z15JAN
12.40
6.67
0.00
71.42
100.00
12Z15JAN
15.30
8.98
4.76
90.47
100.00
Table 1. Forecasts of QPF (mm) by forecast cycles at 0000 and 1200
UTC 10-15 January 2013 from the GFS and GEFS including the
probability of 12.5,6.25 and 1 mm of QPF at a point near State College
where 12.35 mm of QPF was observed and 4.4 inches of snow was
reported. Return to text.
NWS State College Case Examples
Figure 1. GFS 00-hour mean sea-level pressure and pressure anomalies in 6-hour increments from a)0600 UTC 15 January through e) 0600 UTC
16 January 2013 and d) total liquid equivalent precipitation (mm) from 1200 UTC 15-16 January 2013. Return to text.
NWS State College Case Examples
Figure 2. As in Figure 1 except for GFS 850 hPa temperatures ( C) for 6-hour periods from a) 0600 UTC 15 January through f) 1200 UTC 16 January 2013.
Return to text.
NWS State College Case Examples
Figure 3. As in Figure 2 except for precipitable water (mm) and precipitable water anomalies. Return to text.
NWS State College Case Examples
Figure 4. As in Figure 2 except for 850 hPa winds (kts) and total wind anomalies. Return to text.
NWS State College Case Examples
Figure 5. As in Figure 1 except for 850 hPa moisture flux and moisture flux anomalies with accumulated precipitation. Return to text.
NWS State College Case Examples
Figure 6. As in Figure 2 except for 250 hPa winds (kts) and 250 hPa wind anomalies. Return to text.
NWS State College Case Examples
NWS State College Case Examples
Figure 7. GEFS probability of 1 mm or more QPF. Return to text.
NWS State College Case Examples
Figure 8. Return to text.
NWS State College Case Examples
Figure 9. Return to text.
NWS State College Case Examples
Figure 10. GFS QPF. Return to text.
NWS State College Case Examples
Town
Ice (in)
Report Time
CASHTOWN
0.3
915
Camp HILL
0.1
300
GREENCASTLE
0.2
400
Sporting HILL
0.1
800
WILLOW STREET
T
1052
JENNERSTOWN
0.8
1000
DALLASTOWN
0.1
815
Table 2. Ice accumulation reports in southern
Pennsylvania. Some values may include sleet and
freezing rainfall. Return to text.
NWS State College Case Examples
Figure 11. Eastern United States snowfall based on spotter reports as 16 January 2013. Data in inches. Return text.
NWS State College Case Examples
Town
Snow (in)
HUGHESVILLE
5.00
Point View
4.50
State COLLEGE
4.40
LEWISBURG
4.30
MIFFLINBURG
4.30
CENTRE HALL
4.20
MAHANOY
4.20
MAHANOY CITY
4.20
SELINSGROVE
4.10
LAUREL SUMMIT
4.10
Park Forest
4.00
LEWISTOWN
4.00
TAMAQUA
4.00
Pleasant Gap
3.80
ELIMSPORT
3.80
Bellwood
3.70
SUNBURY
3.60
LAPORTE
3.50
LAPORTE
3.50
ALTOONA
3.10
PRINCE Galliztizin SP
3.00
HOLLIDAYSBURG
3.00
LOCK HAVEN
3.00
BROOKLAND
3.00
COUDERSPORT
2.80
GALETON
2.50
WELLSBORO
2.40
WILLIAMSBURG
2.00
STEVENSON DAM
2.00
MAPLETON
2.00
NEWPORT
2.00
TIOGA
2.00
COWANESQUE
2.00
COVINGTON
1.70
Table 3. Snowfall (inches) from spotters and COOP
sites where snowfall was over 1.50 inches. Some
data may contain ice pellets and snow amounts.
Return to text.