11 January 2005 “First snow” winter storm
by
Richard H. Grumm
National Weather Service Office
State College PA 16803
1. INTRODUCTION
The middle of the week of
from 11 through 13 January
2005 was expected to be a
warm week. However, a
stronger than forecast
anticyclone to the north kept
the low-level cold air over the
region longer than anticipated.
Warm advection over the lowlevel frontal boundary
produced rain, snow, and
freezing rain over most of
Pennsylvania on Tuesday, 11
January 2005. Total
precipitation amounts ranged
from 0.15 in the extreme
southeast to around 2 inches in
southwestern portions of the State.
Figure 1 Total 24 hour precipitation for the 24-hour period ending at 1200
UTC 12 January 2005.
The resulting snowfall over central
sections was the first measurable and
first inch of snow in many locations.
State College received 2 inches of snow
and fell short of the old record for the
latest 1 inch snow fall set on 19 January
2000. Close, but no cigar.
This was the third in a series of marginal
winter storms to affect the region in 6
days. Unlike the previous two events,
there was considerably more snow
associated with this event than the two
previous events. The heavy rains in the
southwest produced flooding in both the
State College and Pittsburgh County
Warning areas.
This paper will serve to document the
winter storm of 11 January 2005. The
focus will be on the use of Ensemble
Prediction System (EPS) outputted data
in forecasting the event.
2. METHODS
All data were available in real-time via
the PSU-NWS data feed. All EPS output
images shown here were replicated using
the operationally available data and the
operationally deployed software. For ice
storms, determining the temperature of
the boundary layer is critical to
determine if the boundary will be cold
enough to cause rain to freeze. For this
purpose, EPS 2m temperature forecasts,
including spaghetti plots and probability
plots were used.
Figure 2 SREF forecasts initialized at 2100 UTC 10 January 2005 showing precipitation type and
amount forecasts. Forecasts valid at 1500 UTC 11 January 2005.
In addition to the temperature of the
boundary layer, it is critical to have an
estimate as to whether the precipitation
will be in the form of liquid or solid
upon entering the boundary layer. For
this approximation, the 850 hPa 0C
isotherm was used.
The SREF doe implicitly produce rain,
ice pellets, snow, and freezing rain data.
At this time these data are not shown.
Some fields were displayed showing the
mean of the EPS forecasts for that
variable verse the 30-year climatology.
In these instances, the departures are
shown as standardized anomalies in
standard deviations (SDs) from the 30year normal. The reproductions
software used the same data and scripts
used in real-time. All real-time graphics
changes are based on improvements
made during the case study process.
The term “winter storm” is used loosely
here and in fact this was more of a
winter precipitation event. There was no
significant surface cyclone associated
with this event.
3. RESULTS
For brevity emphasis will be on SREF
forecasts from 10 January 2005. The
Figure 3 SREF 2m temperatures from forecasts
initialized at 2100 UTC 10 January 2004 showing a)
spaghetti plots and spread and b) probability of 2m
temperatures less than 0C and the consensus -20, 0,
and +20C contour.
focus being on how to anticipate the
heavy rainfall and the precipitation
amounts.
The SREF precipitation type forecasts
valid at 1500 UTC 11 January from
forecasts initialized at 2100 UTC 10
January 2005 are shown in Figure 2.
These data show a high probability of
snow from central Pennsylvania
northward. The 2m and 850 hPa
temperature forecasts valid at 1200 UTC
are shown in Figures 3 and 4
respectively. These data show subfreezing temperatures to around State
College at both levels at 1200 UTC. In
Figure 4 As in Figure 2 except 850 hPa
temperatures.
fact, the 2m temperatures implied the
low-level cold air, east of the Alleghany
Plateau extended close to the Maryland
border. This may explain the low
probabilities, but a chance at least, of ice
pellets and freezing rain as far south as
the Maryland border in Figure 2.
Figures 5 and 6 show that the low-level
cold air would rapidly erode west of the
Allegheny Plateau and the 850 hPa
temperatures would warm up causing
mixed precipitation and rain issue across
central portions of the State.
Figure 5 As in Figure 1 except 2m
temperatures forecasts valid at 2100 UTC
11 January 2005.
The SREF precipitation forecasts for the
24-hour period ending at 1200 UTC 12
January are shown in Figure 7. These
data show that the SREFs forecast a high
probability of 0.5 inches or more QPF
over most of southern and central
Pennsylvania. The heaviest rainfall was
expected in the southwest part of the
State, in good agreement with Figure 1.
Figure 8 shows the 0.50 QPF SREF QPF
forecasts from 12 hours earlier (0900
UTC 10 January 2005). These data show
the precipitation potential was
considerably less of central and eastern
Pennsylvania than the latter forecasts.
The amounts were overall lighter and
forecast to be farther west. There was
Figure 6 As in Figure 1 except 850 hPa
temperatures forecasts valid at 2100 UTC 11
January 2005.
no consensus 1.0 inch area in these
forecasts.
Figures 9 and 10 show the 2m and 850
hPa temperatures from the 0900 UTC
SREF runs. These can be directly
compared to Figures 3 and 4. It appears
that there was more clustering toward
two solutions in the earlier forecasts, this
is most notable in the 2m temperature
forecasts. With a bimodal solution, the
2m temperatures provided conflicting
guidance and the probability distribution
function would have clearly been
bimodal. Forecasting precipitation at this
time with these data provided an
interesting forecast problem with
Figure 7 SREF forecasts initialized at 2100 UTC 10
January 2005 showing QPF probabilities of exceeding
0.5 inches and the consensus forecast with each
members 0.5 contour.
precipitation type over central and
southern Pennsylvania.
4. CONCLUSIONS
A strong east-west frontal boundary led
to a precipitation event over
Pennsylvania on Tuesday 11 January
2005. Over the northern half of the State,
most of the precipitation fell as snow,
freezing rain, ice pellets and ended
mainly as rains. Some pockets of
freezing rain persisted until 1200 UTC
12 January along the New York border.
Many locations, such as State College
had their first inch of snowfall (Fig. 11).
This event was well forecast based on
short-term SREF forecasts. However, the
Figure 8 As in Figure 7 except from forecast
initialized at 0900 UTC 10 January 2005.
lead-time potential was limited as the
models slowly converged on the solution.
This can be seen in the 0.50 QPF
contours between two SREF runs only
12 hours apart. This was third successive
minor winter storm in less than a week
where the probability and spaghetti plots
from the SREFs appeared to provide
valuable forecast information.
The resulting warm up is best illustrated
by figure 12. Its shows the pesky nature
of low-level cold air evacuating
Pennsylvania east of the Allegheny
Plateau. The fact that the SREFs are able
to simulate this slowly retreating cold air
during precipitation events is a rather
remarkable feat.
5. ACKNLOWEDGEMENTS
6. REFERENCES
Figure 10 As in Figure 9 except 850 hPa temperatures.
Figure 9 SREF 2m temperatures from forecasts
initialized at 0900 UTC 10 January 2004 showing a)
spaghetti plots and spread and b) probability of 2m
temperatures less than 0C and the consensus -20, 0, and
+20C contour.
Figure 11 24-hour snowfall from cooperative sites for the 24-hour period ending at 1200 UTC 12
January 2005.
Figure 12 1700 UTC 12 January 2005 surface temperatures of Pennsylvania.